Solar Power Technologies for Future Planetary Science Missions, JPL D-101316

Solar Power Technologies

for Future Planetary Science Missions

December 2017

Solar Power Technologies

for Future Planetary Science Missions

Strategic Missions and Advanced Concepts Office

Solar System Exploration Directorate

Jet Propulsion Laboratory

for

Planetary Science Division

Science Mission Directorate

NASA

Work Performed under the Planetary Science Program Support Task

December 2017

JPL D-101316

Assessment Team

Jet Propulsion Laboratory, Caltech

Rao Surampudi (Chair)

Julian Blosiu

Paul Stella

John Elliott

Julie Castillo

Goddard Space Flight Center

Thomas Yi

John Lyons

Glenn Research Center

Mike Piszczor

Jeremiah McNatt

Langley Research Center

Chuck Taylor

Johns Hopkins University Applied Physics

Laboratory

Ed Gaddy

Aerospace Corporation

Simon Liu

U.S. Army

Ed Plichta

NASA HQ

Christopher Iannello

Advisory Committee and Editors

Patricia M. Beauchamp

James A. Cutts

National Aeronautics and

Space Administration

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

Strategic Missions and Advanced Concepts Office JPL D-101316

Solar Power Technologies for Future Planetary Science Missions ii

Foreword

Future planetary exploration priorities envisioned by the National Research Council’s (NRC’s)

Vision and Voyages for Planetary Science in the Decade 2013–2022,

developed at the request of

the NASA Planetary Science Division (PSD), seek to reach targets of broad scientific interest across

the solar system. Power systems are required for all of these mission concepts, but which power

system is optimal for a particular potential mission depends on the mission’s scientific and

operational needs and, in some cases, constraints imposed by NASA. Radioisotope Power Systems

(RPS) are extremely important options for many planetary mission types, particularly to the outer

reaches of the solar system and beyond, and the current capabilities and future technological

pathways for RPS have been extensively discussed and previously documented.

2,3

However, solar

power is used for the majority of planetary spacecraft and, as a complement to recent RPS studies,

this report assesses the capabilities and limitations of state-of-practice solar power systems and the

status of advanced solar power technologies, and it documents innovations needed for upcoming

mission concept scenarios. Although solar power has been used on most planetary missions to date,

it has limitations as missions seek to operate further away from the Sun or in Sun-shadowed regions.

Thanks in part to the commercial sector, there have been substantial advances in solar cell and solar

array technologies that have enabled some outer planet missions, such as Juno, to be accomplished

with solar power, which were long thought to be out of the reach of such technologies. Now we see

that even some mission concepts to Saturn are possible with current solar power technology. A

companion report assesses energy storage technologies for planetary missions because, in some

cases, missions may need primary batteries for power.

Patricia M. Beauchamp

Chief Technologist,

Engineering and Science Directorate

Jet Propulsion Laboratory,

California Institute of Technology

Pasadena, CA 91109

December 12, 2017

National Research Council, “Vision and Voyages for Planetary Science in the Decade 2013-2022,” The National

Academies Press, Washington, DC (2011). https://solarsystem.nasa.gov/docs/131171.pdf

John Hopkins University, Applied Physics Laboratory. 2015. Nuclear Power Assessment Study, TSSD-23122.

http://solarsystem.nasa.gov/rps/npas.cfm

Jet Propulsion Laboratory, California Institute of Technology. June 2017. Next-Generation Radioisotope

Thermoelectric Generator Study Final Report, JPL D-99657.

Strategic Missions and Advanced Concepts Office JPL D-101316

Solar Power Technologies for Future Planetary Science Missions iii

Acknowledgments

This work was conducted as part of the Planetary Science Program Support (PSPS) task that the Jet

Propulsion Laboratory carries out for the National Aeronautics and Space Administration’s

(NASA’s) Planetary Science Division. The research was carried out at the Jet Propulsion Laboratory,

California Institute of Technology, under a contract with NASA. Gordon Johnston is the NASA

program executive responsible for PSPS Task and Leonard Dudzinski is the program executive for

this work funded under the Technology subtask.

The content of this report is pre-decisional information and is provided for planning and discussion

purposes only.

Reference herein to any specific commercial product, process, or service by trade name, trademark,

manufacturer, or otherwise, does not constitute or imply its endorsement by the United States

Government or the Jet Propulsion Laboratory, California Institute of Technology.

Special gratitude is extended to Joel Schwartz, Andreea Boca, and the many individuals who

contributed their knowledge and time in preparation of this report. Special thanks to Mary Young

for the technical publications support during the report preparation and to Richard Barkus for

development of the cover. The authors would also like to acknowledge the valuable technical

information provided by solar cell and array manufacturers and aerospace companies (Spectrolab,

SolAero, mPower, Microlink, Alta Devices, Orbital-ATK, Deployable Space Systems, Inc.,

Lockheed Martin Astronautics, Boeing Defense, Space, and Security, and Sierra Nevada

Corporation), as well as by the Department of Defense and National Laboratories (Army Research

Laboratory, Air Force Research Laboratory, Aerospace Corporation, Navy Research Laboratory,

and Applied Physics Laboratory).

Other Reports in this Series (which can be found on https://solarsystem.nasa.gov)

Power Technology

• Advanced Radioisotope Power Systems Report, Report No. JPL D-20757, March 2001.

• Solar Cell and Array Technology for Future Space Missions, Report No. JPL D-24454, Rev.

A, December 2003.

• Energy Storage Technology for Future Space Science Missions, Report No. JPL D-30268,

Rev. A, November 2004.

• Energy Storage Technologies for Future Planetary Science Missions, Report No.

JPL D-101146, December 2017.

Planetary Protection Technology

• Planetary Protection and Contamination Control Technologies for Future Space Science

Missions, Report No. JPL D-31974, June 2005.

• Assessment of Planetary Protection and Contamination Control Technologies for Future

Science Mission, Report No. JPL D-72356, January 2012.

Extreme Environments

• Extreme Environment Technologies for Future Space Science Missions, Report No. JPL

D-32832, September 2007.

Guidance Navigation and Control

• Guidance, Navigation, and Control Technology Assessment for Future Planetary Science

Missions: Part I. Onboard and Ground Navigation and Mission Design, Report No. JPL

D-75394, October 2012.

• Guidance, Navigation, and Control Technology Assessment for Future Planetary Science

Missions: Part II. Onboard Guidance, Navigation, and Control, Report No. JPL D-75431,

January 2013.

• Guidance, Navigation, and Control Technology Assessment for Future Planetary Science

Missions: Part III. Surface Guidance, Navigation, and Control, Report No. JPL D-78106,

April 2013.

Strategic Missions and Advanced Concepts Office JPL D-101316

Solar Power Technologies for Future Planetary Science Missions iv

Table of Contents

Foreword ........................................................................................................................................................................ ii

Acknowledgments ......................................................................................................................................................... iii

Executive Summary ....................................................................................................................................................... 1

1 Study Overview ....................................................................................................................................................... 8

1.1 Introduction .................................................................................................................................................. 8

1.2 Objectives .................................................................................................................................................... 9

1.3 Approach ...................................................................................................................................................... 9

1.4 Schedule ...................................................................................................................................................... 9

1.5 Assessment Team ..................................................................................................................................... 10

1.6 Participants ................................................................................................................................................ 10

2 Space Solar Power Needs of Future Planetary Mission Concepts ....................................................................... 11

2.1 Introduction ................................................................................................................................................ 11

2.2 Outer Planetary Mission Concepts ............................................................................................................. 11

2.3 Inner Planet Mission Concepts................................................................................................................... 13

2.4 Mars Mission Concepts .............................................................................................................................. 16

2.5 Small Body Mission Concepts .................................................................................................................... 18

2.6 Summary .................................................................................................................................................... 20

3 State-of-Practice Solar Cell and Array Technology ............................................................................................... 21

3.1 Introduction ................................................................................................................................................ 21

3.2 State-of-Practice Space Solar Cells ........................................................................................................... 22

3.2.1 Device Technology ........................................................................................................................ 22

3.2.2 Current Production Solar Cells ...................................................................................................... 22

3.2.3 Solar Cell Assemblies .................................................................................................................... 23

3.3 State-of-Practice Solar Arrays .................................................................................................................... 25

3.3.1 Body-Mounted Solar Arrays ........................................................................................................... 25

3.3.2 Deployable Rigid Solar Arrays ....................................................................................................... 26

3.3.3 Deployable Flexible Solar Arrays ................................................................................................... 27

3.3.4 Concentrator Solar Arrays ............................................................................................................. 29

3.4 Summary .................................................................................................................................................... 29

4 Advanced Solar Cell and Array Technologies ....................................................................................................... 30

4.1 Introduction ................................................................................................................................................ 30

4.2 Advanced Solar Cell Technologies ............................................................................................................ 30

4.2.1 Cell Efficiency ................................................................................................................................ 30

4.2.2 Planetary Mission Environments ................................................................................................... 35

4.3 Advanced Solar Array Technologies .......................................................................................................... 38

4.3.1 Flexible Arrays ............................................................................................................................... 38

4.3.2 Concentrator Arrays....................................................................................................................... 40

4.3.3 Specialized Planetary Mission Environments ................................................................................ 41

4.4 Infrastructure .............................................................................................................................................. 42

4.5 Summary .................................................................................................................................................... 43

5 Findings and Recommendations .......................................................................................................................... 44

5.1 Major Findings ............................................................................................................................................ 44

5.2 Recommendations ..................................................................................................................................... 45

5.2.1 Overall Recommendations ............................................................................................................ 46

5.2.2 Specific Recommendations ........................................................................................................... 46

6 Acronyms .............................................................................................................................................................. 47

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Solar Power Technologies for Future Planetary Science Missions v

List of Tables

Table 2-1. Solar power system needs of the outer planet missions. ........................................................................... 13

Table 2-2. Solar power systems needs of inner planet mission concepts. .................................................................. 16

Table 2-3. Solar power system needs of the future Mars mission concepts. ............................................................... 18

Table 2-4. Solar power systems needs required for small body mission concepts. ..................................................... 20

Table 3-1. Solar Arrays on NASA Planetary Science Missions. .................................................................................. 21

Table 3-2. Current Production Triple-Junction Space Solar Cells. .............................................................................. 23

Table 3-3. Overview of Current Solar Array State-of-Practice. .................................................................................... 25

Table 4-1. Comparison of Proposed LIHT Cell and SOP Triple Junction Solar Cell. ................................................... 37

List of Figures

Figure 1-1. Approximate relative applicability of power technologies to target body mission concepts as of

2015, updated in 2017, showing solar power in yellow (outer rings for Orbiters and Flybys and inner rings

for landers and probes) ........................................................................................................................................... 8

Figure 2-1. Outer Planet Mission Destinations. ........................................................................................................... 11

Figure 2-2. Inner Planet Mission Destinations. ............................................................................................................ 13

Figure 2-3. Potential Venus aerial and surface mission concepts under consideration. ............................................. 14

Figure 2-4. Variation of pressure and temperature at various altitudes at Venus. ....................................................... 15

Figure 2-5. Simulated Solar Spectrum in Venus Atmosphere. .................................................................................... 15

Figure 2-6. Notional future Mars missions under consideration for the next decade and beyond. .............................. 17

Figure 2-7. Asteroids orbit the Sun in a tightly packed belt located between Mars and Jupiter. ................................. 18

Figure 3-1. Illustration of a Triple-Junction Solar Cell and Solar Irradiance Spectrum. Three n/p junctions

convert bands of successively longer wavelengths into electrical current. ........................................................... 22

Figure 3-2. Solar Cell Assemblies. Two versions of a solar cell assembly are shown. At left is a SolAero cell

incorporating a discrete bypass diode mounted in one corner. At right is a Spectrolab cell with a discrete

bypass diode mounted on the cell back side. ....................................................................................................... 24

Figure 3-3. Body-mounted arrays. From left to right: GOES 7 (NASA/NOAA) spinning satellite (illustration),

CP8 IPEX (Cal Poly Intelligent Payload Experiment, app. 10 cm × 10 cm × 10 cm), RACE (Radiometer

Atmospheric CubeSat Experiment). ...................................................................................................................... 26

Figure 3-4. Deployable Rigid Arrays. From left to right: Mars Exploration Rover with 1.2 m

array, Dawn

spacecraft (mission to Vesta and Ceres) with 36 m

array (illustration), Juno spacecraft (mission to Jupiter)

with ~43 m

array (illustration). ............................................................................................................................. 26

Figure 3-5. Flexible Fold-Out Arrays. Left: Illustration of Terra spacecraft with APSA based array.

Middle: International Space Station with eight flexible arrays and ~2500 m

total area.

Right: Cygnus resupply vehicle with UltraFlex array. ............................................................................................ 28

Figure 3-6. Flexible Roll-Out Array. The ROSA comprises a flexible photovoltaic blanket unrolled using

composite booms. ................................................................................................................................................. 28

Figure 4-1. Solar cell efficiency limits versus semiconductor bandgap. The solid lines are semi-empirical

limits for AM0 and AM1.5 illumination; the dashed line is based on thermodynamic considerations for

black body solar cells under AM0 radiation.

....................................................................................................... 30

Figure 4-2. Shortcoming of SOP Solar Cells. The bottom junction generates excess current at a low potential

due to its low bandgap (0.7 eV). ........................................................................................................................... 31

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Solar Power Technologies for Future Planetary Science Missions vi

Figure 4-3. IMM Solar Cells. A 1.0 eV bandgap is added using the technique of inverted growth and

subsequent removal of the growth substrate. ....................................................................................................... 32

Figure 4-4. UMM Solar Cells. A 1.0 eV bandgap is added using lattice mismatched materials and buffer layers

to minimize propagation of crystal defects. ........................................................................................................... 33

Figure 4-5. Dilute Nitride Solar Cells. A 1.0 eV bandgap is added using materials that are lattice matched

to GaAs. Nitrogen in the 1.0 eV material is used to fine-tune the lattice constant. .............................................. 33

Figure 4-6. Semiconductor wafer bonding. Two separate wafers are bonded together to create single cell.

As a result, semiconductor quality is not affected by lattice mismatch. ................................................................. 34

Figure 4-7. Near-IR Absorbers. Quantum wells or quantum dots are used to extend the absorptance band of a

limiting junction. .................................................................................................................................................... 34

Figure 4-8. Flexible ultra-lightweight solar cells. Ultra-thin cells can be enabling for very large arrays, SEP

missions to the outer solar system and aero-vehicles.

........................................................................................ 36

Figure 4-9. Power versus temperature for GaInP

cell. Performance at temperatures above 300°C is

severely limited for SOP cells. .............................................................................................................................. 36

Figure 4-10. Measured Solar irradiance spectrum on Venus. Solar irradiance decreases at lower altitudes

and energy at wavelengths below 600 nm is particularly diminished. ................................................................... 37

Figure 4-11. MegaFlex array. At left, a 10-m diameter MegaFlex demonstration unit was deployed in

ground test. At right is an illustration of a MegaFlex array on a flight system. ...................................................... 39

Figure 4-12. A2100 spacecraft. Flexible fold-out arrays based on the heritage ISS arrays, are in development

for the Lockheed Martin A2100 spacecraft bus. .................................................................................................... 39

Figure 4-13. Mega-ROSA. The Mega-ROSA comprises multiple ROSAs deployed from a central structural

spine.

.................................................................................................................................................................. 39

Figure 4-14. COBRA. The COBRA array is designed to provide compact stowage for SmallSat applications. .......... 40

Figure 4-15. Reflective Concentrator Technologies. (a) At left is a Cell Saver demonstration figure.

(b) Below is a FACT demonstration figure. Both use ~2× concentration to reduce the required quantity of

solar cells. ............................................................................................................................................................. 40

Figure 4-16. Refractive Concentrator Technologies. At left is an SLA demonstration figure. At right is a

SOLAROSA demonstration figure.. Both use flexible Fresnel lenses to achieve ~7–10×, up to 25× for the EESP

program for point focus Fresnel concentration. ..................................................................................................... 41

Figure 4-17. Martian Dust Mitigation Technology. At left is a technology utilizing electric fields for dust removal.

At right is a technology using piezoelectric actuators and mechanical vibration. .................................................. 42

Figure 4-18. Solar Probe Plus (Parker Solar Probe). This is intended to reach a distance of 0.046 AU from the

Sun. ...................................................................................................................................................................... 42

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Solar Power Technologies for Future Planetary Science Missions 1

Executive Summary

Background

In order to plan effectively for the future, NASA’s Planetary Science Division requested an

assessment of the space solar power technologies required to enable/enhance the capabilities of

future planetary science mission concepts (>2025). The study report is organized into five major

sections: 1) study overview, 2) potential solar power system needs of future planetary science

missions, 3) capabilities and limitations of state-of-practice (SOP) space solar power systems,

4) status of advanced solar cell and array technologies, and 5) findings and recommendations.

Study Overview

The specific objectives of the study include: a) review the solar power system needs of potential

future planetary science missions, b) assess the capabilities and limitations of state of practice space

solar cell/array systems, c) assess the status of advanced solar cell/array technologies currently under

development and assess their potential capabilities and limitations, and d) identify and recommend

candidate solar cell and array technologies required for future planetary science mission concepts.

JPL assembled a technical assessment team to perform this study. The team consisted of subject

matter experts in the areas of mission planning, spacecraft power systems engineering, and space

solar power systems and technologies. The assessment team members were selected from NASA

(HQ, JPL-Caltech, GRC, LaRC, and GSFC), Aerospace Corporation, Johns Hopkins University

Applied Physics Laboratory (JHU-APL), and Department of Defense (DoD). The team met with

engineers and technologists from U.S. solar cell and array manufacturers, aerospace organizations,

and NASA mission centers to obtain information on the capabilities and limitations of the SOP

technologies. In addition, the team also met with several solar cell and array scientists and

technologists from universities, industry, NASA, DoD, and aerospace organizations to obtain

information on advanced solar cell and array technologies currently under development. The

assessment team held four meetings to: a) obtain information about potential next decadal planetary

science mission concepts and their power system needs, b) determine the capabilities of SOP space

solar power systems, c) assess the status and potential capabilities of advanced photovoltaic (PV)

power systems under development at various national laboratories, industry, and universities, and

d) summarize the findings and compile the recommendations.

Future Mission Concept Needs

Potential planetary science missions targeted for time period to be covered by the next decadal survey

(2023–2032) are grouped into four categories: a) outer planets b) inner planets, c) Mars, and d) small

bodies. The assessment team met with mission formulation study leads and power system engineers

from JPL, GSFC, Marshall Space Flight Center (MSFC), and JHU-APL to identify potential

planetary science missions that could be considered for implementation in the next 10–15 years and

determine the PV power system needs for solar powered mission concepts. The major findings of

the review team on the PV power system needs of these four groups of solar powered planetary

science missions are given below.

a) Outer Planet Missions

Radioisotope power systems are generally attractive for outer planet mission concepts because

RPS can be used in environments with limited or no sunlight. However, in some cases, solar power

systems are preferred compared to RPS due to performance, mass, or cost considerations. NASA’s

Strategic Missions and Advanced Concepts Office JPL D-101316

Solar Power Technologies for Future Planetary Science Missions 2

Juno mission is currently demonstrating the technical feasibility of using solar power at Jupiter

distance and the Europa Clipper mission has also baselined solar power.

Many planetary scientists are presently advocating two broad groups of outer planet missions for

future development: a) those to the Ice Giants—Neptune and Uranus, and b) those to Ocean Worlds

which include a number of moons of the outer planets with subsurface oceans of liquid water.

Potential Ocean World mission destinations include Enceladus, Europa, Titan, Ganymede, and

Callisto.

The major technical challenges for solar-powered outer planet missions are operation in extreme low

solar irradiance and low temperature environments. The solar irradiance at Jupiter (5.1 AU) is 3.7%

of that at 1 AU. At Saturn (9.5 AU) it is 1.1%, at Uranus (19.2 AU) it is 0.28%, and at Neptune

(30 AU) it is 0.1%. In view of these low solar intensities, missions would need solar arrays with high

power capability (>30 kW) at 1 AU to produce the required power (>500 W) at such large distances

(at >5 AU). In addition, Jupiter mission concepts require solar power systems that can operate in

high radiation environments. Other important requirements include long-life capability and high

reliability. Additionally, mission concepts using solar electric propulsion (SEP) would require high-

power solar arrays (>50 kW at 1 AU).

The major findings of the review team on the solar power systems required for outer planet mission

concepts to be considered in the next decadal survey are given below:

1. Ocean World missions require high efficiency (>38%), high voltage (>100 V) and high

power (>20 kW at 1 AU) solar power systems that can operate efficiently in low

irradiance, low temperature (LILT) environments. These missions would also require

solar power systems with low mass and volume. Missions to Jupiter and its moons

require solar power systems that can operate efficiently in high radiation environments.

2. The use of solar power systems for missions beyond Saturn (Ice Giants missions) is

highly challenging, and would require significant advances in solar cells and array

technologies to reduce mass and volume and improve operational efficiency and life

capabilities.

b) Inner Planet Missions

Inner planet missions to Venus and Mercury present quite different challenges for solar power

systems, because the power systems would need to operate in very close proximity to the Sun. The

Venus mission concepts under consideration for the next decade include: a) orbital missions,

b) variable altitude aerial platforms, and c) long-duration surface probes. The technical challenges

of the inner planet missions vary depending on the type of spacecraft (flyby, orbital, aerial, and

surface) and destination (Venus, Mercury). No missions to Mercury are presently under

consideration for the next decade or two.

Venus exploration mission concepts pose several challenges for solar power systems and they

depend significantly on the type of mission (orbital/surface/aerial). The temperature and pressure on

Venus ranges from 460°C and 90 bars at the surface, to a benign 0°C and 1 bar at an altitude of

55 km. In addition, very little sunlight reaches the Venus surface. There is very little atmospheric

motion near the surface. However, at 55 km altitude, the winds are strong enough to enable aerial

mission concepts, such as those with balloons.

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Solar Power Technologies for Future Planetary Science Missions 3

The major findings of the review team on the solar systems required for next decadal Venus mission

concepts are given below:

1. Venus orbital missions can be implemented with existing solar power systems, as these

environmental conditions are relatively benign and are similar to those of Earth orbital

missions. However, future missions concepts would benefit from the use of high-

efficiency solar cells and low-mass solar arrays.

2. High-altitude aerial missions where solar fluxes are high and temperatures are benign

would require few solar power innovations except protection from the sulfuric acid

environment.

3. Low-altitude Venus aerial missions would require solar power systems capable of

operating in low solar irradiance (50–300 W/m

), high temperature (200–350°C), and

corrosive environments.

4. Solar cells required for these missions need to be optimized to operate efficiently under

the altered Venus surface solar spectrum.

Mercury orbital missions, such as NASA’s MESSENGER and the European Space Agency’s

(ESA’s) BepiColombo mission, required solar power systems that could operate in extremely high

solar intensities (1000–14000 W/m

) and high-temperature environments (~270°C).

c) Mars Mission Concepts

Although the NASA Mars mission roadmap is unclear after the 2020 launch of the next Mars rover,

Mars science mission concepts under consideration for the next decade include: 1) multi-functional

next-generation Mars orbiters, 2) potential Mars sample return missions (includes Mars ascent

vehicles, landers, and sample-fetching rovers), 3) Mars helicopters and other forms of proposed

aerial vehicles, and 4) human Mars precursor missions (large landers, rovers, demonstrations for in-

situ resource utilization). The major solar power system challenges for Mars surface missions are:

1) efficient operation of solar arrays under the Mars solar spectrum, 2) the complexity of deploying

and operating large photovoltaic arrays on rovers and landers, and 3) efficient operation of solar

arrays in Mars dust environments.

The major findings of the review team on the solar power systems required for next decadal Mars

mission concepts are given below:

1. Mars orbital missions do not present major challenges for solar power technologies and

can be implemented with SOP systems. However, future missions would benefit from

the use of high-efficiency solar cells and low-mass solar arrays.

2. Future Mars surface landers and rovers require solar cells capable of operating

efficiently under Mars solar spectral conditions. Since the effective solar spectrum at

the surface of Mars is depleted at short wavelengths, a cell designed to maximize the

efficiency in the red-shifted spectrum on Mars would optimize solar power for Mars

surface and aerial missions. Additionally, surface missions using solar arrays could

benefit greatly and reduce operational costs by incorporating dust removal capabilities.

3. Mars aerial missions would require high efficiency solar cells and low-mass solar arrays

since mass would be at a premium for helicopters and airplanes.

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Solar Power Technologies for Future Planetary Science Missions 4

4. Future human precursor missions likely require low mass and high power arrays with

autonomous deployment capability. These missions also would require solar arrays

capable of operating efficiently in dusty martian environments.

d) Small Body Mission Concepts

Small bodies in our solar system include asteroids, comets, and dwarf planets, such as Ceres and

Pluto. Science priorities and potential mission recommendations, provided through community

white papers and the Small Body Assessment Group (SBAG), include the following: a) Near-Earth

Objects: Mega-multi-flyby, Multi-rendezvous, and Sample Return, b) Main belt asteroids and

Jupiter Trojans: Main Belt Sample Return, Multi-asteroid Rendezvous, and Jupiter Trojan

rendezvous, c) Comets: Comet Surface Sample Return and Comet Nuclear Sample Return, d) Small

Satellites: Phobos and Deimos Sample Return, e) Dwarf Planets: Flyby (rendezvous preferred), and

f) Centaurs and Trans-Neptunian Objects: Flyby (rendezvous preferred). Of particular interest is

solar electric propulsion, which is an attractive option for some, but not all small body mission

concepts.

The major technical challenges of the solar power systems required for small body missions are:

a) large solar arrays with low mass and low stowage volume (~2× lower than SOP), and b) long

operational life (>10 years).

The major findings on the solar power systems required for next decadal small body mission

concepts are given below:

1. Solar electric propulsion missions to small bodies would require high voltage (>100 V),

high power solar arrays (20–100 kW at 1 AU), with low mass and low stowage volume;

2. Missions to small bodies beyond 3 AU are similar to outer planet missions and would

require solar cells capable of operating in LILT environments.

State of Practice Solar Power Systems

This assessment team met with engineers and technologists from U.S. solar cell and solar array

manufacturers and user organizations, such as NASA mission centers, and from aerospace industry

to obtain information on the SOP solar cell array capabilities and their limitations. The major findings

on the capabilities and limitations on SOP space solar power systems cell and arrays are:

Solar Cells: Through the 1980s, spacecraft used primarily silicon solar cells with efficiencies

increasing from less than 10% to over 15%. During the 1990s, gallium arsenide (GaAs) solar cells

began to replace silicon solar cells, and progressed from single junction to dual junction cells that

were grown on germanium substrates (replacing the more expensive GaAs substrates). During the

2000s, triple junction solar cells became the standard for most space missions. Today’s space solar

cells offer efficiencies of ~30% at 1 AU along with resilience to radiation (electrons and protons).

Future solar cells are expected to reach efficiencies of ~38% at 1 AU by the mid-2020s.

Solar Arrays: The types of solar arrays currently in use are: a) body-mounted arrays, b) deployable

rigid arrays, and c) flexible fold out arrays. During the past 25 years, the specific power of solar

arrays has improved from 30 W/kg to 100 W/kg. In the past decade, these advances have enabled

several orbital and surface missions at Mars, as well as flyby and orbital missions to small bodies

and inner planets.

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Solar Power Technologies for Future Planetary Science Missions 5

Limitations: In spite of these advances, SOP solar power systems are not attractive for the following

future planetary mission concepts:

1. Outer planetary missions beyond Saturn, because of limited performance capabilities at

low solar irradiance and low-temperature environments;

2. Low-altitude Venus aerial and surface missions, due to their limited operational

capabilities at high temperatures, high/low solar irradiance, and corrosive environments;

3. Long-duration Mars surface solar powered missions, because of dust accumulation on

solar arrays;

4. High-power, solar electric propulsion missions to small bodies and outer planets,

because such solar arrays would be heavy, bulky, and could not function in LILT

environments.

Advanced Space Solar Power Technologies

The assessment team met with several solar cell and array scientists and technologists from

universities, industry, NASA, DoD, and aerospace industry to obtain information on advanced solar

cell and array technologies currently under development. The major findings of the review team on

the status of advanced solar cell and array technologies are given below.

Solar Cells: High-efficiency solar cells are under development at several companies and universities

with support from DoD and private funding. NASA supports cell development through the Small

Business Innovation Research (SBIR) and other programs. The advanced solar cells architectures

under development include a) inverted metamorphic multi-junction (IMM), b) dilute nitride,

c) upright metamorphic, and d) semiconductor wafer bonding technologies (SBT). SBT refers to the

mechanical connection of one semiconductor wafer on top of another. Significant improvements in

solar cell performance are envisioned: a) near-term (1–2 years): >33% efficient, and b) mid- to far-

term (5–10 years): >37% efficient.

The IMM solar cells under development have an efficiency of approaching 35% at beginning-of-life

(BOL), 28°C, under the standard spectrum outside the Earth’s atmosphere (Air Mass 0 [AM0])

conditions. Space qualification of these types of cells is currently in progress. Development of dilute

nitride cells for space applications is also underway and efficiencies from 30–31% have been

reported

under the same conditions. To date, upright metamorphic multi-junction (UMM) solar

cells have an efficiency from 29–30% at AM0. SBT cells under development have an efficiency of

34–35% at AM0.

Limited work is currently in progress on the development of solar cells that can function: a) at low

solar irradiance and low temperatures (outer planet environments) and b) at high temperature,

high/low solar irradiance and corrosive environments of Venus. No identified research projects are

currently underway on the development of solar cells that can function effectively under Mars

spectral conditions, although research has been done in this area in the past.

Research into martian

dust removal has also been previously been studied but is not currently being pursued.

Suarez, Ferran, et al., “High Efficiency Multijunction Solar Cell Based on Diluted Nitrides”, Presented at 33rd

Space Power Workshop, Manhattan Beach CA (2015).

Boca, Andreea, et al., “Advanced-Architecture High-Efficiency Solar Cells for Low Irradiance Low Temperature

(LILT) Applications”, Proceedings of 44th IEEE-PVSC (2017).

Stella, Paul, et al., “Mars optimized solar cell technology (MOST)”, Proceedings of 33rd IEEE-PVSC (2008).

Strategic Missions and Advanced Concepts Office JPL D-101316

Solar Power Technologies for Future Planetary Science Missions 6

Solar Arrays: Several types of advanced solar arrays are under development with support from

DoD, commercial funding and NASA. Advanced solar arrays under development include: a) flexible

fold-out, b) flexible roll-out, c) concentrator, and d) solar arrays for extreme environments. Major

advances in solar array performance are envisioned: a) near-term: 150–200 W/kg, b) mid- to far-

term: 200–250 W/kg. A summary of the status of development of these advanced solar arrays is

given below.

Flexible Fold-out Arrays: UltraFlex is a flexible fold-out solar array from Orbital-ATK, Inc.

MegaFlex, currently under development in the same company, is an extension of their current

UltraFlex array to larger diameters and higher power. The MegaFlex deploys as a flexible fold-out

array with a circular geometry, similar to the UltraFlex. Deployment of a 10-m diameter MegaFlex

has been demonstrated in a ground test and is intended to reach diameters as large as ~30 m.

Flexible Roll-out Arrays: Roll-out solar arrays (ROSA) have been recently unfurled and

successfully tested at the International Space Station (ISS). Mega-ROSA is a flexible roll-out solar

array under development at Deployable Space Systems, Inc. (DSS). It represents an extension of the

ROSA to higher power. The Mega-ROSA comprises a set of multiple ROSAs deployed from a

central structural spine. The Mega-ROSA is intended to reach power capability exceeding 100 kW

at 1 AU, BOL.

Concentrator Arrays: Concentrator arrays offer a potential approach for mitigating the losses

associated with LILT conditions. Specifically, increasing the effective irradiance using concentrating

optics would allow solar cells in the outer solar system to operate as if they were much closer to the

Sun. Concentrator arrays that have undergone some development over the past decade include

a) Cell Saver Solar Array, b) Flexible Array Concentrator Technology (FACT), and c) Stretched

Lens Array (SLA). For outer planet mission concepts, one has to be careful that the amount of power

generated in Earth- or Venus-assisted trajectories to the outer planets does not overheat the arrays

and associated hardware. This is usually mitigated by feathering the arrays to reduce the solar

irradiance within the inner solar system. Novel ideas for concentrators are emerging, including

gossamer or very large collectors, and may have potential that could substantially alter the ability to

use solar power in the distant reaches of the solar system.

Solar Arrays for Extreme Planetary Environments: Solar arrays that can survive and operate in

high-temperature environments and are actively cooled by a pumped fluid loop have been developed

for Solar Probe mission concepts, which fly close to the Sun. Some limited work in the early 2000s

has also been carried out on the development of dust-tolerant solar arrays for Mars. Technical

feasibility of the dust removal has been demonstrated

but further work is needed to demonstrate this

at the system level. Limited work is currently in progress on the development of arrays for low-

irradiance, high-temperature conditions on Venus.

Recommendations

The review team formulated the following overall and specific recommendations to NASA-PSD.

These recommendations were formulated after reviewing the solar power system needs of future

planetary science mission concepts and after examining the capabilities and limitations of SOP

Calle, C. I., et al., “An Active Dust-Mitigation Technology for Mars Exploration,” Proceedings of Concepts and

Approaches for Mars Exploration (2012).

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Solar Power Technologies for Future Planetary Science Missions 7

solar power systems, and the status of the advanced energy storage technologies currently under

development.

Overall Recommendations

1. Targeted investments should be made in the specific solar cell and array technologies

required for unique planetary environments.

2. Partnerships with the NASA Human Exploration and Operations Mission Directorate

(HEOMD) and the Space Technology Mission Directorate (STMD) and/or other

government agencies such as Department of Energy (DoE) and DoD (Air Force

Research Laboratory [AFRL], Aerospace Corporation, Naval Research Laboratory

[NRL], and Army Research Laboratory [ARL]) should be established and maintained to

leverage/tailor the development of advanced cell and array technologies to meet future

planetary science mission concept needs.

3. Existing infrastructure for PV technology development, testing and qualification at

various NASA centers should be upgraded to support future planetary science missions,

as needed.

Specific Recommendations

Specific recommendations on solar cell and array technologies required for future planetary science

mission concepts are that PSD should leverage the DoD investment in higher-efficiency solar cells

(~38%) and array technologies to enhance options for future planetary space science missions and

develop:

1. High power (>100 kW) and low mass (200–250 W/kg) solar arrays operable up to

10 AU (for outer planet missions);

2. Higher efficiency LILT solar cells and low mass, radiation resistant arrays for potential

orbital missions to Jupiter, Saturn, and Ocean Worlds (Europa, Titan, etc.);

3. Low irradiance, high temperature (LIHT) cells and arrays tolerant of the sulfurous

environment required for Venus aerial and surface mission concepts;

4. Solar cells tuned to the Mars solar spectrum and solar arrays with dust mitigation

capability for future Mars surface mission concepts.

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Solar Power Technologies for Future Planetary Science Missions 8

1 Study Overview

1.1 Introduction

Most of the planetary science missions conducted to date have used solar power systems, including

some Mars missions, and all of the inner planet and small body missions. However, outer planet

missions, such as Voyager, Cassini, and Galileo, have typically used radioisotope power systems.

But, this is changing. For the first time, Juno, a mission to Jupiter, is powered by a solar power

system and a planned NASA mission to Jupiter’s moon Europa, the Europa Clipper, has baselined

the use of solar power. Figure 1-1 illustrates the current status of solar power missions in the solar

system.

Figure 1-1. Approximate relative applicability of power technologies to target body mission concepts as of 2015, updated in

2017, showing solar power in yellow (outer rings for Orbiters and Flybys and inner rings for landers and probes)

In order to plan for the future, NASA’s Planetary Science Division requested an assessment of the

space solar power technologies required to enable/enhance the capabilities of future planetary

science mission concepts (>2025). The study report is organized into five major sections:

1) overview, 2) potential solar power system needs of future planetary science missions,

3) capabilities and limitations of SOP space solar power systems, 4) status of advanced solar cell and

array technologies, and 5) findings and recommendations.

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1.2 Objectives

The purpose of this assessment was to identify candidate advanced space solar power technologies

that would enable/enhance the capabilities of future Planetary Science mission concepts. The

specific objectives were:

• Review the space solar power system needs of future planetary science mission concepts

• Assess the capabilities and limitations of state of practice space solar cell/array systems

to meet the needs of future planetary science missions.

• Assess the status of advanced solar cell/array technologies currently under development

at NASA, DoD, DoE, and in industry, and assess their potential capabilities and

limitations to meet the needs of future planetary science missions.

• Identify and recommend candidate solar cell and array technologies required for future

planetary science missions.

1.3 Approach

A technical assessment team was assembled to support this study. The team consisted of experts in

the areas of mission planning, spacecraft systems engineering and space solar power subject matter

experts. The team members were selected from NASA (HQ, JPL-Caltech, GRC, LaRC, GSFC),

Aerospace Corporation, APL, and DoD.

Three multi-day meetings were held to: a) obtain information about potential next decadal planetary

science missions and their power system needs, b) determine the capabilities of SOP space solar

power systems, and c) assess the status and potential capabilities of advanced photovoltaic power

systems under development at various national labs, industries, and universities. A fourth meeting

was held to finalize the findings and recommendations.

To make the study tractable, the technology needs of a large number of potential future missions

were distilled into four generic mission concept types. For each generic mission type, we analyzed

the power needs, assessed the PV capabilities of SOP technologies, and identified the gaps between

current capabilities and mission needs. The team also reviewed advanced PV technologies currently

under development at various national laboratories, industries, and universities. The assessment team

examined each PV technology to answer the following questions:

• How does it function?

• What is the present status of the technology?

• What is the future potential of the technology in terms of performance parameters such as

specific power and efficiency under various conditions?

• What would be the impact of such improvements on future mission concepts?

• What technical challenges remain?

These results were analyzed to identify the most promising advanced solar power technologies with

the greatest potential impact on enabling and enhancing future planetary science missions. The final

results are documented in this report along with findings and recommendations.

1.4 Schedule

The assessment team conducted three multi-day meetings between March and September 2016. The

first meeting was held at JPL, the second was held at NASA GSFC, and the third meeting was at

NASA GRC. A fourth meeting was also held at JPL in October 2016. The final report was prepared

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Solar Power Technologies for Future Planetary Science Missions 10

as a draft in February 2017 for review by the assessment team and was revised to final form by

November 2017.

1.5 Assessment Team

The Space Solar Power Technology Assessment Team members are:

1. Rao Surampudi, NASA JPL (Chair)

2. Julian Blosiu, NASA JPL

3. Paul Stella, NASA JPL

4. John Elliott, NASA JPL

5. Julie Castillo, NASA JPL

6. Thomas Yi, NASA GSFC

7. John Lyons, NASA GSFC

8. Ed Gaddy, JHU-APL

9. Mike Piszczor, NASA GRC

10. Jeremiah McNatt, NASA GRC

11. Ed Plichta, U.S. Army

12. Simon Liu, Aerospace Corporation

13. Chuck Taylor, NASA LaRC

14. Christopher Iannello, NASA HQ

1.6 Participants

This assessment required detailed technical information on: a) next decadal planetary science

mission concepts and their projected solar power system needs, b) SOP solar power systems

currently being used in various planetary space science missions and their capabilities, and

c) advanced solar power technologies currently under development by other government agencies

and their potential capabilities. The information was obtained from various NASA centers, aerospace

companies, companies involved in the development and manufacturing of solar cells and arrays, and

National Laboratories. The names of the organizations that supported this study are given below:

Solar Cell R&D/Manufacturers

1. Spectrolab

2. SolAero

3. mPower

4. Microlink

5. Alta Devices

Array R&D/Manufacturers

1. Orbital-ATK

2. DSS

3. Lockheed-Martin

4. Sierra Nevada Corporation

5. Boeing

NASA Centers

1. Glenn Research Center (GRC)

2. Jet Propulsion Laboratory-California

Institute of Technology (JPL-Caltech)

3. Langley Research Center (LaRC)

4. Goddard Space Flight Center (GSFC)

DoD & National Laboratories

1. Army Research Laboratory (ARL)

2. Air Force Research Laboratory

(AFRL)-Philips Laboratory

3. Aerospace Corporation

4. Navy Research Laboratory (NRL)

5. Applied Physics Laboratory (APL)

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2 Space Solar Power Needs of Future Planetary Mission Concepts

2.1 Introduction

The NASA Planetary Science Division is considering a number of ambitious missions to various

destinations in our solar system, including outer planets, inner planets, Mars and small bodies. The

power systems required for these mission concepts have several unique needs compared to Earth-

orbital missions, and the needs vary based on the destination and mission type. We invited mission

formulation study leads and power system engineers from JPL, GSFC, MSFC, and APL to identify

potential next decadal planetary science missions and their possible PV power system needs.

2.2 Outer Planetary Mission Concepts

The outer planet destinations consist of four planets: Jupiter, Saturn, Uranus, and Neptune, as well

as their satellites (Figure 2-1). These planets combined have over a hundred moons orbiting them.

In the past, Pluto was originally included in the outer planet category; however, it is now categorized

as a dwarf planet. In the past, all outer planet missions have been powered by Radioisotope

Thermoelectric Generators (RTGs). These include Pioneer 10, Pioneer 11, Voyager 1, Voyager 2,

Galileo, Cassini, and Ulysses, and they were mostly flyby missions except for Galileo, which was

the first Jupiter orbital mission (it also deployed an atmospheric probe to Jupiter), and Cassini, the

first mission to orbit Saturn. RTGs were chosen to power these spacecraft after an assessment of

their mission needs as compared to the often limited capabilities of earlier generation solar power

systems.

Outer planet missions currently in operation include New Horizons [NH] (Pluto flyby) and Juno

(Jupiter orbiter). Like the past outer planet missions, New Horizons is RTG-powered. Juno is the

first solar-powered outer planet spacecraft. Juno, which has mission requirements less demanding

than prior flagship missions to Jupiter, such as the RTG-powered Galileo, benefits from advanced

solar cells that are 50% more efficient and radiation tolerant than silicon cells used in earlier space

missions. The Europa Clipper will be the second NASA solar-powered outer planet mission and

ESA is also developing a solar-powered orbiter bound for Jupiter (the Jupiter Icy Moons Explorer

Figure 2-1. Outer Planet Mission Destinations.

Outer Planets

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Solar Power Technologies for Future Planetary Science Missions 12

[JUICE]). In addition, several New Frontiers outer planet mission concepts currently in the proposal

stage are considering the use of solar power systems to destinations as far from the Sun as Saturn.

The most recent planetary science decadal survey, Vision and Voyages,

recommended the following

outer planet mission concepts for development: a) Europa Multiple Flyby Mission, b) Uranus

Orbiter, c) Enceladus Orbiter, d) Saturn Probe, and e) Io Observer. Among these, a Europa mission

(Europa Clipper) was selected for development and is currently scheduled for launch in 2022/2023.

The NRC has not yet initiated the next planetary decadal survey (2023–2032) and the outer planet

missions recommended in the past decadal survey that were not able to be funded by NASA may

again be considered for development in the next decade. Currently, scientists are predominately

advocating two groups of outer planet mission concepts for future development: a) missions to

Ocean Worlds and b) missions to the Ice Giants.

Ocean Worlds known to have subsurface oceans—determined from measurements by the Galileo

and Cassini spacecraft—include Enceladus, Europa, Titan, Ganymede, and Callisto, although

several other planetary bodies may also fall under this category. The overarching goals of the Ocean

Worlds missions

are: a) identify Ocean Worlds in the solar system, b) characterize the oceans,

c) characterize the habitability of each body, d) understand how life might exist at each Ocean World

and search for life.

Orbital and probe missions to the Ice Giants, Neptune and Uranus, were considered as a high priority

in Vision and Voyages.

In fact, the Giant Planets panel ranked Ice Giants as their #1 priority and

recommended a Uranus orbiter/probe mission concept for development. Although this mission has

undergone further study,

it has not been selected for development in this decade as yet, because of

competing priorities and reduced funding. However, it may result in being one of the higher priority

outer planetary missions for the next decade. Additionally, a Neptune System Orbiter with a probe

could be another consideration for the next decade.

Radioisotope power systems are generally attractive for outer planet missions because they can be

used in environments with limited or no sunlight. However, in some cases, solar power systems are

more cost effective compared to radioisotope power systems, even when the total power system mass

is higher. In addition, SEP is attractive for many outer planetary missions because it has the potential

to significantly reduce risk and/or the cruise time required to reach the outer planets, and/or increase

the payload mass. SEP to an outer planet might be in the form of an SEP stage containing solar arrays

and electric propulsion elements that could be jettisoned, if desired, after use in the inner solar

system.

The major technical challenges for solar-powered outer planet mission concepts are operation in

extreme low solar intensities and low-temperature environments. The solar irradiance at Jupiter

(5.1 AU) is 3.7% of that at 1 AU. At Saturn (9.5 AU) it is 1.1%, at Uranus (19.2 AU) it is 0.28%,

and at Neptune (30 AU) it is 0.1%. In view of these low solar intensities, mission concepts need solar

arrays with high power capability (>20 kW) at 1 AU to produce the required power (>500 W) at

such large distances (at >5 AU). In addition, Jupiter missions would require solar power systems that

can operate in high radiation environments. Other important requirements include long-life

capability and high reliability. SEP missions also require high power solar arrays (>50 kW at 1 AU).

http://www.lpi.usra.edu/opag/ROW/

http://www.lpi.usra.edu/icegiants/mission_study/Full-Report.pdf

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Solar Power Technologies for Future Planetary Science Missions 13

Higher-power solar arrays will benefit from high efficiency solar cells (>38%) to minimize the solar

array size and mass. Such an advance would reduce the mass and area of the array by almost 20%

without lowering power output. Additionally, reducing the solar array size improves the

maneuverability of the spacecraft.

Solar power system needs of outer planet mission concepts are summarized in Table 2-1.

Table 2-1. Solar power system needs of the outer planet missions.

Mission Type

Mission

Performance Capability Needs

Orbiters/Flyby

Jupiter*

Saturn

Europa*

Titan

Enceladus

• LILT Capability (>38% at 10 AU and <−140°C)

• Radiation Tolerance (6e15 1 MeV e-cm

)

• High Voltage (>100 V)

• High Power (>50 kW at 1 AU)

• Low Mass (3× lower than SOP)

• Low Volume (3× lower than SOP)

• Long Life (>15 years)

• High Reliability

*Radiation tolerance is critical for Jupiter system, including Europa, missions.

2.3 Inner Planet Mission Concepts

The inner planets include Mercury, Venus, Earth, and Mars. These rocky planets are nearer to the

Sun and are much more closely spaced to each other than their outer planet counterparts. Here only

Mercury and Venus are classified as inner planet destinations (Figure 2-2), while Mars missions are

considered separately.

Past U.S. missions that explored Mercury are Mariner 10 and Mercury Surface Space Environment

Geochemistry and Ranging (MESSENGER). NASA’s Mariner 10 was the first U.S. spacecraft to

fly by Mercury. MESSENGER was the first spacecraft to orbit Mercury. Both Mariner 10 and

MESSENGER were solar powered. MESSENGER used a solar power system that was designed for

operation at 0.31 AU. Two important features were incorporated in the design and operation of the

MESSENGER solar power system. One feature involved the replacement of a significant fraction of

the solar cells by optical solar reflectors (OSRs) to control the array temperature near the Sun; the

Figure 2-2. Inner Planet Mission Destinations.

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Messenger design used about 2/3-cell coverage. The second feature was that the array was off-

pointed from the Sun to avoid overheating.

All orbital and flyby missions to Venus have been solar powered and no current proposed mission

to Venus envisages using RPS. The past planetary science decadal survey (2013–2022)

recommended a Venus In-Situ Explorer (VISE) for development during 2013–2023. However, no

Venus proposals were selected for development. Several proposals are currently being developed in

response to the New Frontiers (NF)-4 mission call, with Step 2 selections expected late in 2017.

NASA is also in discussions with Roscosmos, the Russian Space Agency, on a potential role in their

Venera D mission, tentatively planned for 2025 launch.

The highest-priority science objectives (as defined by the Venus Exploration Analysis Group

[VEXAG]) for the next decadal Venus exploration mission concepts are: 1) understand atmospheric

formation, evolution, and climate history on Venus, 2) determine the evolution of the surface and

interior of Venus, and 3) understand the nature of interior–surface–atmosphere interactions over

time, including whether liquid water was ever present. Other potential Venus exploration missions

under consideration include: a) orbital missions, b) constant and variable altitude aerial platforms,

c) long-duration surface missions, and d) Venus sample return missions.

The technical challenges

of Venus missions vary

depending on the type of

spacecraft (flyby, orbital,

aerial, and surface) and

destination. Venus or-

bital missions can be

implemented with SOP

solar power systems, as

Venus orbital envi-

ronmental conditions are

relatively benign. Some

potential Venus atmos-

pheric and surface

missions under consid-

eration are given in

Figure 2-3.

Venus aerostats (balloons) operating at high altitudes (>50 km) can be implemented with SOP solar

cells coated with materials for protection from the acidic environment. Venus airplanes and hybrid

vehicles, such as the Venus Aerial Mid-Altitude Platforms (VAMP), require lightweight solar arrays

resistant to the Venus corrosive environment. However, medium- to low-altitude Venus aerial

missions impose several technical challenges. These include operation in low solar intensities (300–

50 W/m

), high temperature (200–350°C), and corrosive environments. Variation of pressure and

temperature at various altitudes is given in Figure 2-4. Further, solar cells required for these mission

concepts need to be optimized to operate efficiently under a filtered Venus solar spectrum

(Figure 2-5).

Figure 2-3. Potential Venus aerial and surface mission concepts under consideration.

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Solar Power Technologies for Future Planetary Science Missions 15

The major technical challenges of

Venus surface missions are

operation in very low solar

intensities (<5 W/m

), high tem-

peratures (>450°C) (Figure 2-4),

and corrosive environments. The

atmosphere is an amalgam of

gases, composed primarily of

carbon dioxide, with a 92 bar

pressure and 460°C temperature

at the surface. Short-duration

Venus surface missions of a few

hours were implemented using

SOP primary batteries enclosed

in an environmental chamber

equipped with a complex thermal

management subsystem. These

past Venus surface missions did not consider the use of solar power systems because SOP solar cells

could not function under the severe Venus surface environments, and also cannot function efficiently

because Venus’ solar spectrum is deficient in shorter wavelengths. However, long-duration Venus

surface missions would require a rechargeable power system, which could be achieved with

advanced solar cell and array technology. Solar power systems needs of the inner planet mission

concepts are summarized in Table 2-2.

Figure 2-4. Variation of pressure and temperature at various altitudes at Venus.

Figure 2-5. Simulated Solar Spectrum in Venus Atmosphere.

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Solar Power Technologies for Future Planetary Science Missions 16

Table 2-2. Solar power systems needs of inner planet mission concepts.

Mission Type

Mission

Needs

Orbiters

Venus and Mercury

Low- Medium Temperature Operation

High Solar Intensities at Mercury

Aerial (high- to mid-altitude)

Venus

Medium-High Temperature Operation (0–300°C)

Venus Solar Spectrum Operation

Operation in Corrosive Environments

Aerial (mid- to low-altitude)

Venus

Medium Temperature (0–300°C) operation. High Temperature (460°C)

survival

Landers/Probes

Venus

High Temperature Operation (460°C)

Low Solar Irradiance Operational Capability

Venus Solar Spectrum Operation

Operation in Corrosive Environments and Super-critical CO

2.4 Mars Mission Concepts

Since 1965, NASA (and now ESA and the Indian Space Research Organisation [ISRO]) have sent

several robotic space missions to Mars to understand whether Mars was, is, or can be, a habitable

world. The major goals of the NASA Mars Exploration Program are: 1) determine if Mars ever

supported life, 2) understand the processes and history of climate on Mars, 3) understand the origin

and evolution of Mars as a geological system, and 4) prepare for human exploration. Several types

of spacecraft have been used for the exploration of Mars, including flybys, orbiters, landers, and

rovers. All of the flyby and orbiting missions and several landers have been solar powered but

radioisotope power systems were used to power some of the long-lived Mars landed missions

(Viking 1–2 and Curiosity) and the upcoming Mars 2020 rover.

Mars exploration mission concepts being studied for the next decade include: 1) multi-functional

next-generation Mars orbiters, 2) potential Mars Sample Return missions (includes Mars ascent

vehicles, landers and sample fetching rovers), 3) a Phobos lander mission, 4) Mars helicopters and

other forms of aerial vehicles, 5) subsurface explorers, and 6) human Mars precursor missions (large

landers, rovers, in-situ resource utilization [ISRU] demonstrations missions). Some of the missions

under consideration for the next decade and beyond are given in Figure 2-6.

It is envisioned that a Mars Sample Return (MSR) effort could be implemented with a series of three

steps. The Mars 2020 rover mission will collect and cache surface samples for possible future return

to Earth. It could be followed by an SEP-powered orbiter that would include a system designed to

retrieve the samples from Mars orbit. The third element could be a fetch rover that would land,

retrieve the cached samples, and inject them into Mars orbit, where the sample cache could be

collected by the orbiter.

Mars subsurface missions are also under consideration for the next decade to provide information

about the geology of the planet, the presence of water, and maybe even clues about whether Mars

was ever a habitat for life. Mars aerial vehicles could enable the study of Mars from a perspective

that has never been achieved before: aerial views from the martian sky where the spatial resolution

is much better than can be achieved from orbit and the range of observation is much greater than

is possible from the mast on a rover. NASA is also planning for human exploration of Mars in the

mid- or late-2030s, and several robotic precursor missions to Mars are being considered, with a

mixture of both scientific and human mission preparation objectives.

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Solar Power Technologies for Future Planetary Science Missions 17

Among the potential future Mars mission concepts under consideration, the most challenging

missions from the power system point of view are: 1) long-duration Mars landers and rovers required

for potential Mars sample return missions, 2) Mars subsurface missions, 3) Mars aerial missions,

4) and Mars human precursor missions (large landers and rovers).

The major solar power system challenges for future Mars surface missions are: 1) efficient operation

of solar arrays under Mars solar spectrum, 2) complexity of deploying and operating large

photovoltaic arrays on rovers and landers, and 3) efficient operation of solar arrays in Mars dusty

environments. In addition, human precursor missions require low mass and high-power arrays with

autonomous deployment to demonstrate technical feasibility of human missions. Since the effective

solar spectrum at the surface of Mars is depleted at short wavelengths, a cell designed to maximize

efficiency in the red-shifted spectrum on Mars would be valuable for Mars surface applications. The

other issue has to do with dust accumulation on the arrays. Dust accumulates on arrays and partially

obscure them, thus reducing their power output. However, periodically, the observed martian dust

devils and wind clean off the arrays and power levels are partially restored. For longer missions,

overt dust removal techniques may be beneficial, since the naturally occurring dust removal

processes may not be sufficiently reliable and repeatable, which would result in increased operational

costs.

Solar cells maximized for martian surface operations are important to future aerial missions. With

current technology for Mars helicopters, flight times are limited to a few minutes before the vehicle

must land to recharge its batteries. With faster recharge times, flight repetition could be improved.

For airplane concepts that do not descend to the surface, improvements in efficiency and specific

power are needed for extended mission operations.

Figure 2-6. Notional future Mars missions under consideration for the next decade and beyond.

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Solar Power Technologies for Future Planetary Science Missions 18

Solar power system needs of the future Mars missions are summarized in Table 2-3.

Table 2-3. Solar power system needs of the future Mars mission concepts.

Mission Type

Mission

Needs

Orbiters

Mars Orbiter

Low Mass (>3× lower than SOP)

Low Volume (>3× lower than SOP)

Long Life (15 years)

High Reliability

Landers/Rovers

Robotic Precursor

Mars Solar Spectrum Operation

High Efficiency Cells

Low Mass (>3× lower than SOP)

Low Volume (>3× lower than SOP)

High Power Density (50% higher than SOP)

Dust Removal Capability

High Reliability

Aerial Vehicles

Helicopter

Low Mass (>4× lower than SOP)

High Power Density (50% higher than SOP)

Mars Solar Spectrum Operation

Dust Tolerance

2.5 Small Body Mission Concepts

Small bodies in our solar system include asteroids, comets, and dwarf planets. Asteroids and comets

are considered remnants from the giant cloud of gas and dust that condensed to create the Sun,

planets, and moons some 4.5 billion years ago. Today, most asteroids orbit the Sun in a tightly packed

belt located between Mars and Jupiter (Figure 2-7). Comets ablate and shed ice and dust as they

approach the Sun in the course of

their highly elliptical orbits.

Dwarf planets are celestial

bodies resembling a small

planet, but lack certain technical

criteria to be classified as

planets. Dwarf planets, e.g.,

Ceres, Pluto, Eris, Haumea,

Makemake, share their orbits

around the Sun with other

objects such as asteroids and

comets. There have been

multiple solar-powered missions

to small bodies such as New

Millennium Deep Space 1

(NM-DS-1), the first solar

electric propulsion mission that

passed by the near-Earth asteroid

9669 and comet Braille; Wide-

field Infrared Survey Explorer

(WISE), a solar-powered space-

craft with an infrared-sensitive

telescope; Stardust; and, Deep

Impact. Stardust was a solar-

Figure 2-7. Asteroids orbit the Sun in a tightly packed belt located between Mars

and Jupiter.

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Solar Power Technologies for Future Planetary Science Missions 19

powered spacecraft that collected interstellar dust from the nucleus of comet Wild-2 during its closest

encounter and returned it back to Earth for analysis. Deep Impact was a solar-powered mission that

studied the composition of the comet Tempel 1.

Recent/ongoing comet and asteroid missions include: ESA’s Rosetta, NASA’s Origins-Spectral

Interpretation-Resource Identification-Security-Regolith Explorer (OSIRIS-REx), and Dawn.

Rosetta was the first spacecraft to orbit a comet (67P/Churyumov-Gerasimenko) and carried a lander

(Philae) developed by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt

e.V. [DLR]) that was the first to land on the surface of a comet. Both the orbiter and the lander were

solar powered. Rosetta was launched in 2004 and reached the comet almost ten years later in August

2014 after reaching distances as far as 5 AU. Flight system design required that the spacecraft enter

a hibernation mode to conserve power when it was beyond 4.4 AU and Rosetta remained in that

condition for 2.5 years. After surveying the comet and selecting a landing site, it deployed Philae to

the surface in November 2014 but the vehicle bounced and landed on its side in a crevice. Power to

the Philae lander solar arrays was occulted and it was only able to operate briefly until the mission

ended.

Dawn is a NASA solar-powered spacecraft with solar electric propulsion requiring large solar arrays

that targeted the giant asteroid Vesta and dwarf planet Ceres. Dawn entered Vesta orbit on July 16,

2011, and completed a 14-month survey mission before leaving for Ceres in late 2012. Dawn entered

Ceres orbit on March 6, 2015, and may remain in orbit after the conclusion of its mission. OSIRIS-

REx is also a solar-powered mission, but uses chemical propulsion. It was launched to a near-Earth

asteroid called Bennu (formerly 1999 RQ36). OSIRIS-REx will orbit the asteroid beginning in

August 2018 and is expected to return a sample to Earth in September 2023. NASA has also recently

approved two new Discovery Program missions, Psyche and Lucy, to explore asteroids in early

2020s. Lucy is a solar-powered flyby mission with chemical propulsion and will visit six Trojan

asteroids at close range from August 2027 through March 2033. Psyche is a planned solar powered,

solar electric propulsion mission that would orbit the large metal asteroid of the same name.

Science priorities and mission concept recommendations for small bodies as provided by Vision

and Voyages

and through community white papers and the SBAG are:

a) Near-Earth Objects: mega-multi-flyby, multi-rendezvous, sample return,

b) Main belt asteroids and Jupiter Trojans: main belt sample return, multi-asteroid

rendezvous, Jupiter Trojan rendezvous,

c) Comets: comet surface sample return and comet nucleus sample return (flagship),

d) Small Satellites: Phobos and Deimos sample return,

e) Dwarf Planets: Haumea flyby (rendezvous preferred),

f) Centaurs and Trans-Neptunian Objects: flyby (rendezvous preferred).

SEP is an attractive option for some, but not for all small body missions. SEP missions require high

voltage and high-power solar arrays (20–100 kW). The chemical propulsion missions also require

solar power systems, but with low–medium power capability (<10 kW). The major technical

challenges for solar electric propulsion missions are: 1) high voltage arrays (>100 V), 2) high power

arrays (20–100 kW), 3) low mass, 4) low stowage volume, 5) radiation tolerance for some missions,

6) operational capability in LILT environments for some missions, and 7) high reliability.

Higher-power solar arrays would benefit from high efficiency solar cells (>38%) to minimize the

solar array size and mass. The mass of the solar array is inversely proportional to the specific power

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Solar Power Technologies for Future Planetary Science Missions 20

of the array. For low values of the specific power, the array is very massive, and SEP is less mass-

efficient than chemical propulsion. As the array specific power increases, a point is reached where

SEP provides positive mass benefits to small body missions. For very large values of array specific

power, the mass of the array becomes small compared to the balance of the spacecraft mass, and

further increases in specific power produce only diminishing returns in mass saving. Reduced solar

array size also improve the maneuverability of the spacecraft. Further, high-power solar arrays

require low mass array structures with high reliability deployment capability. In addition, the

missions to small bodies beyond 3 AU require solar cells capable of operating in low irradiance and

low temperature environment. Table 2-4 summarizes the needs of the solar power systems required

for small body missions.

Table 2-4. Solar power systems needs required for small body mission concepts.

Mission Type

Needs

Flyby/Orbiter

High Efficiency Solar Cells (>38%)

High Voltage (>100–200V)

High Power (>20 kW)

Low Mass (>3× lower than SOP)

Low Volume (>3× lower than SOP)

Long Life (15 years)

High Reliability

Surface Missions

Low Mass (>3× lower than SOP)

Low Volume (>3× lower than SOP)

High Power Density (50% higher than SOP)

High Reliability

2.6 Summary

NASA is considering a number of exciting planetary science mission concepts for the decade of

2023–2032. The solar power system characteristics required for future potential planetary missions

are given below:

• Outer planet missions could require high-power solar power systems that can function

efficiently in low solar irradiance, low temperature, and high radiation environments

• Inner planet mid/low altitude aerial and surface missions could require solar power

systems that can survive and function in high temperatures, low solar intensities, and

corrosive environments.

• Mars surface missions would benefit from solar cells tuned to the Mars spectrum and

require solar arrays with dust mitigation capability.

• High power SEP at small bodies and asteroids would require high voltage, low mass, and

low volume solar array systems.

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3 State-of-Practice Solar Cell and Array Technology

3.1 Introduction

Space solar power technology has advanced significantly since the first solar-powered satellite,

Vanguard I, was launched in 1958. The first space solar array comprised six silicon solar cells that

powered a 5 mW transmitter. Since that time both solar cell technology and array structure and

deployment technology have undergone many changes leading to significant progress in power

capability, mass and cost effectiveness. Spacecraft primarily used silicon solar cells through the

1980s, with cell efficiency increasing from less than 10% to over 15%. During the 1990s, GaAs solar

cells began to replace silicon and progressed from single junction to dual junction cells grown on

germanium substrates (replacing the more expensive GaAs substrates). During the 2000s, triple

junction cells became the standard for most space missions. Today’s space cells offer efficiencies of

~30% at 1 AU along with improved performance under electrons and protons radiation.

The earliest solar arrays comprised solar cells mounted on the body of a spacecraft, limiting the area

available for solar cells and consequently available electrical power. Deployable structures were then

developed enabling significantly larger arrays and hence higher power generation capability. Power

output exceeding 20 kW is readily available on today’s commercial satellites. To achieve even

higher power, flexible blanket technology was developed so that a significantly larger area of solar

cells could be stowed compactly for launch and unfolded or unrolled in space. The International

Space Station (ISS) is the largest space solar power installation today, providing up to 120 kW using

silicon solar cells on flexible blankets. Implementation of current triple junction cells on large

flexible arrays could provide substantially higher capability.

A summary of the solar array technologies demonstrated on NASA planetary science missions is

shown in Table 3-1. Each technology is discussed in the sections below.

Table 3-1. Solar Arrays on NASA Planetary Science Missions.

Mission

Class

Mission

Destination

Launch

Date

Solar Cell

Technology

Solar Array

Technology

Power

Capability

at 1 AU (W)

Outer planets

Juno

Jupiter

5-Aug-11

Triple junction

Deployable rigid

14000

Inner

planetary

systems

Messenger

Mercury

3-Aug-04

Triple junction

Deployable rigid

450

LCROSS

Moon

18-Jun-09

Triple junction

Body-mounted

600

Lunar Reconnaissance

Orbiter

Moon 18-Jun-09 Triple junction Deployable rigid 1850

Grail

Moon

10-Sep-11

Triple junction

Deployable rigid

763

LADEE

Moon

6-Sep-13

Triple junction

Body-mounted

295

Mars

Mars Global Surveyor

Mars

7-Nov-96

GaAs/Ge and Si

Deployable rigid

2100

Mars Odyssey

Mars

7-Apr-01

GaAs/Ge

Deployable rigid

2092

Mars Exploration Rover

(2 rovers)

Mars surface

10-Jun-03

7-Jul-03

Triple junction Deployable rigid 390

Mars Reconnaissance Orbiter

Mars

12-Aug-05

Triple junction

Deployable rigid

6000

Phoenix

Mars surface

4-Aug-07

Triple junction

UltraFlex

1255

MAVEN

Mars

18-Nov-13

Triple junction

Deployable rigid

3165

Asteroids/

comets

Deep Impact/EPOXI

Tempel-1

Hartley-2

12-Jan-05 Triple junction Body-mounted 620

Dawn (with solar electric

propulsion)

Vesta

Ceres

27-Sep-07 Triple junction Deployable rigid 10300

OSIRIS-REx

Bennu

8-Sep-16

Triple junction

Deployable rigid

3000

LCROSS

—Lunar Crater Observation & Sensing Satellite; LADEE—Lunar Atmosphere Dust & Environment Explorer; MAVEN—Mars Atmosphere &

Volatile Evolution; EPOXI—Extrasolar Planet Observation & Characterization Investigation (EPOCh) + Deep Impact Extended Investigation (DIXI)

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The following discussion of the technologies currently in use is divided into two sections, “State-of-

Practice Space Solar Cells” and “State-of-Practice Space Solar Arrays” where SOP is defined as the

technologies currently in space or in production for flight missions.

3.2 State-of-Practice Space Solar Cells

3.2.1 Device Technology

Current space solar arrays predominantly use triple junction III-V solar cells. These cells comprise

three n/p junctions, grown using metal-organic vapor phase epitaxy (MOVPE) in a lattice-matched

monolithic stack on a germanium substrate. The three junctions, or “subcells”, include the III-V

materials GaInP

and GaInAs, and a germanium substrate with an active junction. Each subcell is

optimized to convert a different portion of the solar spectrum to electrical current, in particular, those

photons with energy above the bandgap of the subcell material. The subcells are connected

electrically in series by tunnel junctions, which are also part of the monolithic stack. Multi-junction

cells provide higher efficiency than a single junction because higher energy photons can be converted

to current at a higher potential than with a single junction device at a lower bandgap, minimizing

thermal energy losses.

A simplified illustration of the SOP triple junction cell is shown in Figure 3-1, along with the solar

irradiance spectrum. As shown in the figure, the highest bandgap material serves as the top junction

and each successive subcell comprises a lower bandgap material. Hence, lower energy photons (i.e.,

light with longer wavelength) pass through the higher bandgap material and are converted to

electrical current in the subcells below.

Figure 3-1. Illustration of a Triple-Junction Solar Cell and Solar Irradiance Spectrum.

Three n/p junctions convert bands of

successively longer wavelengths into electrical current.

3.2.2 Current Production Solar Cells

A summary of space solar cells currently in production is provided in Table 3-2. The principal

manufacturers include Azur Space (European), SolAero Technologies (formerly Emcore) and

Spectrolab, Inc.; the latter two both are USA manufacturers. Each offer similar products based on

https://www.greentechmedia.com/articles/read/arpa-e-award-caltechs-harry-atwater-aims-for-50-solar-efficiency

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the GaInP

/GaInAs/Ge structure. A range of different sizes is available, from ~20 cm

up to ~75 cm

The efficiencies listed in Table 3-2 refer to cell sizes in the 20–30 cm

range.

Azur Space also lists a silicon space solar cell on its website. The average efficiency at AM0, 28°C

is 16.9% and the bare cell mass 32 mg/cm

. These cells include a monolithically integrated Zener

bypass diode. The cells are of the same type (referred to as “high efficiency silicon”), which were

once common on space systems, and also previously produced by Sharp Corporation and Spectrolab.

SolAero Technologies also offers a 4-junction IMM solar cell. The average efficiency, as listed on

its website, at AM0, 28°C is 33.0%. Although this cell is not fully qualified for general use, it

provides higher performance capability and is a candidate for future missions. The IMM cell is

discussed in detail below, under “Developing Cell Technologies”.

The degradation due to radiation exposure is a critical parameter for many space missions. The

summary data in Table 3-2 include two different values, representing two different test methods

contained in published specifications for radiation testing. The two standards are published by the

American Institute for Aeronautics and Astronautics (AIAA) and the European Cooperation for

Space Standardization (ECSS), respectively. The ECSS standard (ECS-ET-20-08C) includes both

photon and temperature annealing subsequent to irradiation and generally results in higher measured

performance than the AIAA standard (AIAA-S111). The selection of the most accurate test method

is an area of current research. For example, work is currently underway to compare the two methods

and investigate which provides a more accurate measurement of actual in-orbit performance.

Table 3-2. Current Production Triple-Junction Space Solar Cells.

Characteristic

Value/Description

Manufacturer Azur Space

SolAero

Technologies

Spectrolab

Manufacturer’s designation

3G30C

ZTJ

XTJ-prime

Efficiency at 28°C, AM0

29.8%

29.5%

30.7%

Voltage at maximum power, 28°C, AM0 (V)

2.41

2.39

Typical areal mass density (mg/cm

)

Temperature coefficient at 28°C, un-irradiated (% Pmax/°C)

−0.23%

−0.22%

Typical cell thickness

(µm)

150

140

Normalized maximum power degradation at 1E15 1 MeV

e/cm

per AIAA-S111

Not reported 0.85 0.85

Normalized maximum power degradation at 1E15 1 MeV

e/cm

per ECSS-ET-20-08C

0.90 Not reported 0.87

Solar absorptance

0.91

0.92

0.88

Source data:

azurspace.com, solaerotech.com, spectrolab.com, September 7, 2016

Reported efficiencies assuming a solar irradiance of 135.3 mW/cm

Values represent Ge wafer thickness. Azur Space and Spectrolab have offered cell thickness down to 80 µm; 140–150 µm has been the

standard in flight production.

The ECSS test standard includes photon and temperature annealing subsequent to irradiation.

3.2.3 Solar Cell Assemblies

Space solar cell assemblies comprise the solar cells described above with cover-glass, interconnects

and, typically, a bypass diode installed. A simple illustration is shown in Figure 3-2.

The cover-glass is used for radiation shielding and improved thermal and optical characteristics.

Ceria-doped borosilicate glass is used predominantly, although other materials such as fused silica

have been used. Three common versions of ceria-doped borosilicate cover glass material, each

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Solar Power Technologies for Future Planetary Science Missions 24

manufactured by Qioptiq Space Technologies, include Qioptiq CMX, CMG, and CMO. CMG and

CMO are formulated specifically for use with germanium-based cells. CMG is used most commonly

while CMO offers improved optical transmission characteristics particularly at greater thickness

(>200 µm). The cover-glass is attached to cells using optically transparent adhesive; the most

common is Dow Corning 93-500 silicone.

Cover-glasses are normally coated to enhance optical properties. Common coatings include the

following:

• Anti-reflective coating – one layer of magnesium fluoride (MgF

) on front side

• Enhanced anti-reflective coating – multi-layer front side coating with slightly higher

optical transmission that simple MgF

• Ultraviolet (UV) reflective coating – multi-layer anti-reflective front side coating with

reflection band at wavelengths under ~350 nm under normal incidence; enables slightly

lower operating temperature

• Static dissipative coating – typically a layer of indium-tin-oxide (ITO) under the anti-

reflective coating; enables electrical charge flow to the cover-glass edge, which has a

metallic coating for bleeding to ground

• Infrared reflective coating – multi-layer coating typically on the cover-glass back side

that reflects wavelengths longer than the cell can use and also enables slightly lower

operating temperatures. This coating is not presently in common use.

Solar cell interconnects are used for connecting solar cells electrically in series. Interconnect

materials include both Kovar and molybdenum. Molybdenum is used for missions requiring

magnetically clean arrays. In both cases the interconnect materials are clad or plated with silver.

Interconnects generally require strain relief to provide survivability against excessive stress during

thermal cycling (caused by different thermal expansion of panel components). Electrical connection

is achieved by welding or soldering the interconnect to the cell n and p metallic contacts.

The bypass diodes shown in Figure 3-2 are connected electrically in parallel with the solar cell, but

with opposite polarity orientation. The diode provides a current path around the cell in the event that

the cell is in shadow, under-illuminated or damaged; in the absence of the bypass diode, current

http://solaerotech.com

http://spectrolab.com

Figure 3-2. Solar Cell Assemblies. Two versions of a solar cell assembly are shown. At left is a SolAero cell incorporating

a discrete bypass diode mounted in one corner.

At right is a Spectrolab cell with a discrete bypass diode mounted on the

cell back side.

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would be forced through the cell by other cells in the series string. III-V cells are susceptible to

permanent damage under such conditions from reverse bias breakdown. Bypass diodes can consist

of discrete silicon chips connected to the cell with interconnects, as shown in Figure 3-2, or can be

grown monolithically in the III-V material. Cells with monolithically integrated bypass diodes are

offered by Azur Space and SolAero Technologies. These generally have slightly lower efficiencies

than cells with discrete diodes but eliminate the need for interconnecting discrete diodes.

3.3 State-of-Practice Solar Arrays

An overview of the current state-of-practice for array level technologies is shown in Table 3-3. The

values in the table are approximate and based on data in the public domain. Values for specific

missions will depend on design details such as output voltage, wire harness design, geometrical

layout and thermal environment.

In Table 3-3, maximum power is reported for arrays in orbit or, in the case of flexible roll-out

technology, estimated from ground measurements. Values for specific power and areal power

density are based on the assumption that all arrays utilize current production triple junction solar

cells (this enables a consistent comparison of different architectures).

Table 3-3. Overview of Current Solar Array State-of-Practice.

Array Technology

Maximum power at 1 AU (current

state-of-practice), approximate*

Specific power at

1 AU, BOL (W/kg)**

Areal power density at

1 AU, BOL (W/m

)**

TRL

Body-mounted array

2 kW

N/A

314

Deployable rigid array

25 kW

330

Flexible fold-out array

120 kW

150

338

Flexible roll-out array

25 kW

150

338

Based on demonstrated capability

** Assuming all arrays have

SOP triple junction cells

A description of each technology is given below. Solar arrays can be classified using the following

primary categories.

3.3.1 Body-Mounted Solar Arrays

Body-mounted arrays comprise solar panels installed directly on the body of a spacecraft or space

platform. These arrays generally do not require deployment mechanisms and do not include Sun-

tracking mechanisms. Orientation with respect to the Sun depends on the orientation of the space

platform. The primary advantage of body-mounted arrays is simplicity because of the lack of

deployment and tracking mechanisms. A second advantage, in the case of spinning spacecraft, is a

lower operating temperature (and higher cell efficiency) compared with Sun-tracking arrays. The

primary disadvantage is limited power capability due to limited size. In addition, spinning spacecraft

do not illuminate all cells on the spacecraft, eliminating power from cells shadowed from the Sun by

the spacecraft body. Hence, body-mounted arrays have generally been used on smaller platforms

and missions with low power requirements (less than 2 kW). These include spinning satellites and,

more recently, CubeSats. Figure 3-3 provides examples of each, incorporating low power body-

mounted arrays.

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Figure 3-3. Body-mounted arrays. From left to right: GOES 7 (NASA/NOAA) spinning satellite (illustration), CP8 IPEX (Cal

Poly Intelligent Payload Experiment, app. 10 cm × 10 cm × 10 cm), RACE (Radiometer Atmospheric CubeSat Experiment).

3.3.2 Deployable Rigid Solar Arrays

The vast majority of space solar arrays currently deployed in space use rigid panels with strings of

solar cells installed on a single side. The panels are stowed against the spacecraft or space platform

during launch and subsequently unfolded upon deployment. Power levels (at 1 AU) vary over a wide

range, from tens of watts to tens of kilowatts, depending on the mission. Figure 3-4 provides three

examples: the Mars Exploration Rover, with 1.2 m

of array area; the Dawn spacecraft, with two

deployable wings; and the Juno spacecraft, with three wings and a total array area of ~43 m

Rigid arrays generally use a structure comprising honeycomb sandwich panels with composite face-

sheets, such as graphite/epoxy, and aluminum honeycomb core. Solar cell strings are bonded on one

side and wiring is installed on both front and back sides. Multiple panels are connected by hinges

which deploy after release in orbit. Deployments are coordinated with damping or other hinge

sequencing mechanisms. Single- or dual-axis tracking can be provided to maintain optimum pointing

towards the Sun. In this case, power can be transferred across the rotating mechanism to the

spacecraft or space platform.

Beginning-of-life specific power using current state-of-practice solar cells at 1 AU is typically up to

~80 W/kg. Areal power density is typically ~330 W/m

. These values vary due to design details that

may include the specific thermal environment, end-of-life (EOL) conditions for which the cell strings

are optimized, wire harness design and specific solar cell type. Power output degradation throughout

mission life depends on multiple factors, such as radiation environment, shielding design (e.g.,

cover-glass thickness), contamination environment and mission duration.

Numerous missions utilizing the rigid deployable architecture have incorporated designs to address

specific mission requirements. Design elements that are particularly relevant to future missions

include the following:

Figure 3-4. Deployable Rigid Arrays. From left to right: Mars Exploration Rover with 1.2 m

array, Dawn spacecraft (

mission to

Vesta and Ceres) with 36 m

array (illustration), Juno spacecraft (mission to Jupiter) with ~43 m

array (illustration).

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Electrostatically clean arrays. Electrostatically clean arrays are designed to prevent accumulation

of electric charge on solar array surfaces, either to control electric fields or prevent electrostatic

discharge (ESD). ITO coatings can be used on solar cell cover-glass to bleed charge from the

dielectric surface. Conductive tape can also be used to shield dielectric surfaces, such as adhesives,

from space plasma.

High temperature arrays. Arrays operating at less than 1 AU are subject to high temperatures

which reduce solar cell efficiency and can jeopardize survival of the hardware. For example, the

MESSENGER mission operated at 0.31 AU and incorporated rows of mirrors between rows of solar

cells to reflect light and reduce the operating temperature (similar to the Magellan Venus orbiter

launched in 1989). The approach also included deliberate off-pointing of the array from the Sun.

Even so, the nominal operating temperature was ~130°C, compared with 40 to 70°C that is typical

at 1 AU.

The Solar Probe Plus mission (renamed Parker Solar Probe) is scheduled for launch in 2018 and will

approach the Sun at less than 0.046 AU. Hence, the thermal environment is even more severe and

an active cooling system is used to control the array temperature.

3.3.3 Deployable Flexible Solar Arrays

Deployable flexible arrays replace the rigid panel substrate described above with a flexible blanket,

such as a mesh or polyimide sheet. As a result, the system mass can be reduced and a large array can

be stowed in a smaller volume for launch. The specific power typically ranges from 100 to 175 W/kg

for larger arrays, depending on the solar cell mass and deployment structure. The areal power density

is slightly higher than for rigid panels (~338 W/m

) using current state-of-the-art cells, due to slightly

lower operating temperatures. Flexible arrays are capable of providing higher deployed strength and

stiffness than traditional rigid arrays, by incorporating a highly stiff deployment boom or frame

structure (see discussion of Cygnus array, below). Deployment mechanisms are generally more

complex than for rigid panels. Current deployment architectures are described as follows:

Flexible fold-out arrays. Recent flexible fold-out arrays include the following:

• ISS arrays (manufactured by Lockheed-Martin)

• Terra (EOS AM-1) array, manufactured by Northrup-Grumman for the Terra Earth-

observation spacecraft, based on the Advanced Photovoltaic Solar Array (APSA)

developed with NASA

• UltraFlex array (manufactured by Orbital-ATK). The UltraFlex was used on the Mars

Phoenix Lander and the Cygnus cargo resupply vehicle. The Cygnus completed four

missions to the ISS using the UltraFlex, in December 2015, March 2016, October 2016

and April 2017. Several additional missions are planned. Each Cygnus wing has a

diameter of 3.7 m and is designed to withstand 5 g’s acceleration, highlighting one of the

advantages available with a flexible fold-out architecture.

The UltraFlex is also

providing power for the Mars InSight Lander, scheduled for launch in 2018.

Figure 3-5 shows each type of flexible fold-out array. As shown in the figure, the APSA (on the left)

and ISS (in the middle) arrays comprise rectangular, flexible blankets (or semi flexible in the case

of Terra), which are packed together for launch and are deployed with a boom (or booms), unfolding

the blanket, similar to rigid panel arrays. The UltraFlex deploys in a circular manner, resulting in a

http://www.orbitalatk.com/space-systems/space-components/solar-arrays/docs/FS007_15_OA_7463%20UltraFlex.pdf

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disc-shaped structure (shown on the right in Figure 3-5). Fold-out arrays generally require motorized

deployment. Current developments include a flexible fold-out array using state-of-practice solar

cells, developed by Lockheed-Martin, available for commercial spacecraft,

and larger versions of

the UltraFlex, such as the MegaFlex, developed by Orbital-ATK. The MegaFlex is discussed in more

detail below under “Developing Array Technologies”.

Figure 3

-5. Flexible Fold-Out Arrays. Left: Illustration of Terra spacecraft with APSA based array.

Middle

: International Space

Station with eight flexible arrays and ~2500 m

total area.

Right: Cygnus resupply vehicle with UltraFlex array.

Flexible roll-out arrays. Flexible roll-out arrays comprise a photovoltaic blanket that is rolled

around a cylinder for launch and unrolled by a deployment boom(s) in orbit. Roll-out arrays were

used on the Flexible Rolled-Up Solar Array (FRUSA) in 1973 and the Hubble Space Telescope in

1990, the latter subsequently replaced during a servicing mission in 1993.

More recently, the ROSA was developed by DSS using roll-out composite booms and a successful

flight demonstration occurred on the ISS in July 2017.

The booms unroll without the assistance of

a motor. Work on qualification of the ROSA for use on commercial satellites was reported by DSS

and Loral Space Systems in 2015 and Figure 3-6 shows the ROSA under development. Further

developments on the ROSA, including the MegaROSA, intended for higher power (>100 kW)

capability, are described below under “Developing Array Technologies”.

http://www.lockheedmartin.com/us/news/press-releases/2014/september/0908-ss-a2100.html

https://terra.nasa.gov/

https://www.nasa.gov/mission_pages/station/main/index.html

https://en.wikipedia.org/wiki/Cygnus_(spacecraft)

http://www.dss-space.com/products_solar_array.html#flexible_blanket_arrays

https://www.nasa.gov/feature/roll-out-solar-array-technology-benefits-for-nasa-commercial-sector

Figure 3-6. Flexible Roll-Out Array. The ROSA comprises a flexible photovoltaic blanket unrolled using composite booms.

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3.3.4 Concentrator Solar Arrays

Although concentrator arrays are not currently in use or in production for flight systems, concentrator

technology previously developed for flight and technologies currently under development have

potential importance for planetary exploration. Concentrator arrays refer to arrays in which sunlight

from a given area is directed onto a smaller area of solar cells. Concentration of sunlight on solar

cells alleviates the performance losses associated with LILT operation at the outer planets, since

concentration increases the effective solar irradiance on the cells. At the same time, care in the design

must be made to avoid high concentrated illumination intensities and temperatures when near the

Earth. Hence, a brief overview of concentrator technology is provided here and new developments

are discussed under “Developing Array Technologies”.

Concentrator arrays can be either body-mounted, deployable rigid or deployable flexible arrays and

the basic structure can be described by those three categories described earlier. The optics needed

for concentration include the following:

Refractive optics. Refractive optics were used on the Deep Space 1 array, referred to as SCARLET

(solar concentrating array with refractive linear element technology) and launched in 1998. The solar

array was manufactured by Orbital-ATK. The array comprised rigid panels with Fresnel lenses

having a line focus and ~8× concentration factor. The lenses were constructed from silicone on the

back side of ceria-doped glass. Other approaches to refractive optics include point focus lenses and

lenses constructed with alternate materials.

Reflective optics. Reflective optics designed for space include planar reflectors, such as the Orbital-

ATK Cell Saver, DSS FACT (Functional Advanced Concentrator Technology), and Hughes 702

designs, which provide ~2× concentration. The latter suffered from higher than expected degradation

which would need to be considered and mitigated in similar designs. In principle, reflective optics

can utilize parabolic cylinders, paraboloids and non-imaging reflective surfaces.

Compound reflective/refractive optics. Compound optics have been developed for terrestrial

concentrators but are not currently in use for space applications.

3.4 Summary

Space solar cell and solar array technology has advanced significantly since the first solar-powered

satellite in 1958 with solar cell efficiency has increasing from less than 10% to over 30%. Arrays have

grown in power from milliwatts to over 20 kW on spacecraft with the International Space Station

arrays producing 120 kW. The efficiency of SOP triple junction cells, designed and optimized for

Earth orbital missions, is typically 29.5 −2/+1% under standard test conditions (1 AU, 28°C). The

specific power of the solar arrays has also improved from 30 W/kg to 100 W/kg during the past 25

years and have enabled several Mars (orbital and surface), small body (flyby and orbital), and inner

planetary (flyby and orbital) missions during the past decade.

In spite of these advances, SOP solar arrays have limited operational capabilities in extreme

environments. These include low solar irradiance and low temperature environments at the outer

planets, high temperature, high or low solar irradiance at the inner planets, corrosive environments

at Venus, and dusty conditions on Mars. In view of these limitations, the SOP solar power systems

need improvements to achieve future outer planet, inner planet, and Mars missions under

consideration for the next decadal planetary science missions. SOP solar arrays need to be reduced

in mass and volume to power the next decadal solar electric propulsion missions to small bodies and

outer planet destinations. Some, but not all, of this can be achieved with solar cell and solar array

optimization for the particular environment.

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4 Advanced Solar Cell and Array Technologies

4.1 Introduction

This section describes the various solar cell and solar array technologies currently under

development in industry and at various national laboratories, universities, NASA, and JPL.

4.2 Advanced Solar Cell Technologies

Space solar cell technologies currently under development are focused on increasing solar cell

efficiency and enabling operation in specific mission environments. A great deal of effort is being

devoted to improving efficiency. As discussed below, substantial gaps are still present in addressing

the key environments required for future planetary science missions.

4.2.1 Cell Efficiency

Background. The maximum theoretical efficiency, using thermodynamic considerations for black-

body solar cells under terrestrial solar illumination (1-sun, AM1.5 spectrum), for a single junction

solar cell with a bandgap of 1.1 eV

is 30%

. For a triple junction cell,

that limit is 49%.

For an unlimited

number of junctions, the limit

approaches 68%. Under extrater-

restrial (AM0) illumination, the

theoretical efficiency limit for a

single junction GaAs solar cell is

~25%, as shown in Figure 4-1.

This is 17% lower than the

terrestrial limit of 30%. Assuming

similar behavior for a triple

junction solar cell, its theoretical

efficiency limit would be ~40%

under AM0 illumination.

The highest reported efficiency for

a single junction solar cell under

terrestrial illumination (1-sun,

AM1.5 spectrum) is 28.8%,

reported by Alta Devices, Inc. for a

GaAs cell.

For a triple junction solar cell, the highest reported efficiency is 37.9%, provided by

Sharp Corp. The highest efficiency for a non-concentrator multi-junction cell is 38.8%, reported by

Spectrolab, Inc., for a 5-junction cell. For concentrator cells, the highest reported efficiency is 46.0%,

from Fraunhofer Institute for Solar Energy (ISE)/Soitec.

Shockley, W., H. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells”, J. of Appl. Phys.

32, 510 (1961).

De Vos, A., “Detailed Balance Limit of the Efficiency of Tandem Solar Cells”, J Phys. D: Appl. Phys. 13, 839–46

(1980).

Green, M., “Solar Cells – Operating Principles, Technology, and System Applications”, Prentice-Hall, 1981.

https://www.nrel.gov/news/press/2013/2226.html

Figure 4-1. Solar cell efficiency limits versus semiconductor bandgap.

The solid lines are semi-empirical limits for AM0 and AM1.5 illumination; the

dashed line is based on thermodynamic considerations for black body solar

cells under AM0 radiation.

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Two trends are apparent in these results. First, actual cell efficiencies tend to approach closer to the

theoretical limits as technologies mature. Second, the theoretical limits increase as new junctions are

incorporated into cell designs.

Since space cells currently in large scale production provide ~30% efficiency, it is clear that there

are significant opportunities for continued efficiency improvements. These improvements can be

expected to include narrowing the gap between research and production cells, increasing the number

of junctions, and developing new materials that can further approach the maximum possible

efficiency.

Limitations of SOP cells. The primary approach to increasing cell efficiency is to optimize the

bandgap of each subcell in the multi-junction stack. The greatest source of energy loss is the

difference between the photon’s energy and the junction bandgap. Hence, subcells must convert each

photon in a material with a bandgap as close as possible to the photon’s energy to maximize

efficiency. At the same time, cell current in a series stack of subcells is limited by the junction with

the smallest photocurrent. Hence, subcells must also be current-matched to maximize efficiency.

Ongoing developments are focused on growing materials and developing devices that can

simultaneously address both goals.

The limitation inherent in SOP cells is shown in Figure 4-2. As shown in the figure, the three subcells

are not current-balanced. The maximum current obtainable from each subcell is represented by the

area under each colored section of the spectral response curve. Specifically, the bottom junction (Ge)

generates excess photocurrent due to its low (0.7 eV) bandgap. As a result, a substantial number of

photons converted to current at 0.7 eV could be converted more efficiently at a higher energy level.

Most approaches under development include adding one or more junctions to the 3-junction structure

and including a subcell with a bandgap ~1.0 eV (corresponding to 1,200 nm in Figure 4-2). Target

efficiencies for next generation cells are ~36–37% under AM0 illumination at 28°C. In principle,

higher efficiencies, above 40%, are possible in the longer term. Key approaches include the

following technologies.

Figure 4-2. Shortcoming of SOP Solar Cells. The bottom junction generates excess current at a low potential due

to its low bandgap (0.7 eV).

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Inverted Metamorphic Multi-junction (IMM) Cells. IMM solar cells include a 1.0 eV bandgap,

as shown in Figure 4-3. The term “metamorphic” refers to a mismatch is the crystal lattices of

different materials in the structure. In this case, the crystal lattice of the 1.0 eV material is not

matched to the top two junctions (GaInP

and GaInAs). This would generally introduce significant

crystal defects if the top two subcells were grown on the 1.0 eV materials. However, the IMM

approach addresses this difficulty by growing the structure on Ge or GaAs starting from the top

subcell (i.e., in inverted order). The growth substrate is then removed, for example, by an etching

process, as illustrated in Figure 4-2. As a result, the crystallinity of the top two subcells is preserved.

IMM solar cells have demonstrated up to 35% BOL efficiency (at AM0, 28°C). Space qualification

is currently in progress. Production 4-junction IMM assemblies (with cover-glass and interconnects)

are offered by SolAero Technologies with an average AM0 efficiency of 33.0%.

Five- and six-

junction versions of the IMM technology are also under development, intended to achieve 36–37%

efficiency. The key challenge for this technology has been achieving cost parity with SOP cells.

Figure 4-3. IMM Solar Cells. A 1.0 eV bandgap is added using the technique of inverted growth and

subsequent removal of the

growth substrate.

Upright Metamorphic Multi-junction (UMM) Cells. UMM materials generally include a 1.0 eV

bandgap and at least four junctions but, in contrast to IMM cells, are fabricated by growing the

structure in the same order as SOP cells, starting from the bottom subcell. The difficulty of lattice

mismatch between the 1.0 eV material and the top two subcells is addressed by growing transparent

buffer layers between the mismatched layers, as shown in Figure 4-4. Development of this

technology is aimed at finding the buffer layer structure that minimizes propagation of defects into

the metamorphic junctions. For example, buffer layers can be graded; i.e., stoichiometry can be

varied as a function of depth. Efficiency from 29–30% at AM0 has been reported by Azur Space.

Five- and six-junction structures are also possible in principle providing higher efficiency. The key

challenge for this approach has been achieving sufficient crystal quality in the higher bandgap

junctions.

Dilute Nitride Materials. The challenge of finding a material with a 1.0 eV bandgap with a crystal

lattice matched to the SOP structure is solved by adding a small amount of nitrogen to the 1.0 eV

material. Hence, these cells provide optimized bandgaps without sacrificing crystal quality or

introducing inverted growth techniques. The cell structure is illustrated in Figure 4-5.

https://solaerotech.com/products/space-solar-cells-coverglass-interconnected-cells-cic/

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Figure 4-4. UMM Solar Cells. A 1.0 eV bandgap is added using lattice mismatched materials and buffer layers to minimize

propagation of crystal defects.

Figure 4-5. Dilute Nitride Solar Cells. A 1.0 eV bandgap is added using materials that are lattice matched to GaAs.

Nitrogen in the 1.0 eV material is used to fine-tune the lattice constant.

Growth of these materials has been successful to date using molecular beam epitaxy (MBE) rather

than MOVPE. As a result, growth rates have been substantially slower than SOP cells and scale-up

for manufacturing has been a key challenge. Dilute nitride cells manufactured by Solar Junction, Inc.

have set the record efficiency is 43.5% at 925 suns

for terrestrial concentrator cells grown on GaAs

substrates. Development of cells for space application is underway; AM0 efficiencies from 30–31%

have been reported. Five- and six-junction structures are also possible, in principle providing higher

efficiency and meeting the goal of 37% at AM0.

Semiconductor Wafer Bonding. Semiconductor wafer bonding technology (SBT) refers to

mechanical connection of one semiconductor wafer on top of another. This approach enables two

wafers that are grown separately, with different lattice constants, to be combined into a multi-

junction stack. As a result, difficulties associated with defects from metamorphic growth are avoided.

Sabnis, V., et al., “High-Efficiency Multijunction Solar Cells Employing Dilute Nitrides”, Proceedings of AIP

Conference 1477, 14 (2012).

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For example, a Ge or GaAs-based wafer can be bonded on top of an InP-based wafer. The resulting

cell structure is illustrated in Figure 4-6. In general, removal of one growth substrate (e.g., Ge) is

performed to provide a transparent optical path, as shown in the figure.

AM0 efficiencies from 34–35% have been reported by Spectrolab, Inc. Five- and six-junction

structures are also possible, in principle providing higher efficiency and meeting the goal of 37% at

AM0. The key challenge with this approach has been achieving cost parity, due to the need to grow

two wafers to fabricate a single cell, the high cost of InP substrates, and the cost of substrate removal

and inverted processing as in the case of IMM.

Figure 4-6. Semiconductor wafer bonding. Two separate wafers are bonded together to create single cell.

As a result, semiconductor quality is not affected by lattice mismatch.

Near-IR Absorbers. Multiple approaches to increasing solar cell efficiency have employed

techniques for generating current from photons with energy below the material bandgap. These

techniques include introduction of quantum wells and quantum dots. Typically, these absorbers are

added to the middle (GaInAs) subcell of the SOP solar cell, to address the shortcoming of excess

photocurrent in the bottom (Ge) subcell. The principle of these approaches is illustrated in Figure 4-7.

Figure 4-7. Near-IR Absorbers. Quantum wells or

quantum dots are used to extend the absorptance band of a limiting junction.

AM0 efficiencies from 26–27% have been reported by Rochester Institute of Technology (RIT). In

principle, these techniques could be applied to multiple junctions and cells with more than three

junctions. The key challenge with this approach has been reaching the performance of current

SOP cells.

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4.2.2 Planetary Mission Environments

It is one thing to compare cell designs that can perform under terrestrial or AM0 conditions, but

planetary science missions call for cells that perform under varying environmental condition. Not all

cells technologies can be optimized for such a wide variety of environments and, therefore, cell

technologies for operation in several specialized mission environments are in development or under

study. Missions of interest include the inner planets, planetary surfaces, outer planets, and SEP

missions. The relevant environments and developing technologies are described below.

Low Irradiance, Low Temperature (LILT) Conditions. LILT conditions degrade solar cell

performance at the outer planets and the degradation varies significantly within a population of SOP

cells. The primary mechanism for LILT-induced efficiency loss arises from low resistance current

paths, or shunt paths, from base to emitter within the semiconductor. These paths can be caused by

crystalline defects. Current losses through these shunts are generally proportional to the cell voltage

and are negligible under 1 AU conditions as long as the shunt resistance is on the order of KOhms,

However, at low irradiance, these losses become a larger fraction of the photocurrent and, hence,

significantly impact efficiency. Cell voltage increases at low temperature, which partially

compensates for the shunt-related losses.

Current missions such as Juno and the planned Europa Clipper address this issue using a screening

process to select the subset of cells with acceptable LILT behavior. A significant quantity of cells do

not pass the screening and are unusable. Furthermore, the usable cells are not optimized for LILT

conditions; in principle, higher performance could be achieved by designing for the expected current

and temperature.

Research is underway to better understand the LILT phenomena and develop cells that are optimized

for LILT conditions. For example, research at JPL is focused on developing high-efficiency solar

cells with LILT capability at Saturn.

The project employs advanced device architectures (such as

upright and inverted metamorphic structures) with potential for high AM0 efficiency, in combination

with LILT-optimized cell designs that reduce or eliminate performance-limiting features for the

Saturn environment (9.5 AU, −165°C). The design optimization techniques being studied include:

1) eliminating any rectifying semiconductor-to-semiconductor interfaces that limit the I-V curve fill

factor at LILT; 2) modifying the subcell base epitaxial layer thicknesses to ensure optimal current

balance and maximal EOL/BOL efficiencies at low temperature; 3) taking advantage of the lower

series resistance losses at low irradiance to implement low-obscuration grid designs for improved

current production at LILT; and 4) identifying and mitigating the source of any low-current shunts

that are insignificant at 1 AU but performance-limiting at LILT.

Ultra-Lightweight Arrays and SEP missions. Flexible, ultra-lightweight solar cells can be

enabling for very large solar arrays and SEP missions to the outer solar system. In order to produce

~1 kW at these distances from the Sun, solar arrays are needed that produce ~100 kW at 1 AU. The

dense packaging for stowage of these arrays allows for a mass reduction that has a dramatic system

impact.

Aero-vehicles for exploration in the Mars and Venus atmosphere or high-altitude aircraft on Earth

all would benefit from the development of thin, flexible, ultra-lightweight cells. The cells are

typically installed directly on the wings or fuselage of these vehicles. The wings can themselves be

flexible membranes and the cells must be flexible to survive. Minimum mass is also essential to the

successful flight of these vehicles.

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Cells fabricated using a substrate-removal processes, such as IMM cells, can be ultra-thin (e.g.,

<40 µm) and, therefore, flexible and extremely lightweight. Examples of ultra-thin cells are shown

in Figure 4-8. These include ultra-thin GaAs cells from Alta Devices, Inc., epitaxial liftoff (ELO)

cells from Microlink Devices, Inc., and IMM cells from Sharp Corp.

Figure 4-8. Flexible ultra-lightweight solar cells. Ultra-thin cells can be enabling for very large arrays, SEP missions to the

outer solar system and aero-vehicles.

26,27,28

High Temperature. Missions to the inner

planets result in high temperature condi-

tions. Landers or near-surface operations are

unable to mitigate these conditions through

off-pointing or active cooling. For example,

a rover on Venus would need cells capable

of operation at ~460°C. Temperature at an

altitude of 25 km would be ~300°C. These

temperatures affect survivability because

they induce diffusion of materials, such as

metallic contacts, into the semiconductor;

diffusion of metals can cause internal

shorting and culminate in failure. These

temperatures also impact performance by

decreasing cell bandgaps, and increasing

bandgap-to-voltage offsets, reducing the cell

voltage. Power versus temperature for a

GaInP

cell is shown in Figure 4-9. As

shown in the figure, operation would not be

feasible on the venusian surface with

existing cells due to the low light levels and

high temperatures; however, operation at

higher altitudes appears more feasible if survivability can be improved.

Initial development of technology for high temperature operation was initiated by the ARPA-E Full-

Spectrum Optimized Conversion and Utilization of Sunlight (FOCUS) program. Development of

altadevices.com/technology

mldevices.com

Takamoto, T., “Reliability of space solar sheet with inverted metamorphic triple-junction cells,” Proceedings of

Space Power Workshop, 2016.

Landis, G., NASA GRC

Figure 4-9. Power versus temperature for GaInP

cell.

Performance at temperatures above 300°C is severely limited for

SOP cells.

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cells specifically for high temperature operation in the Venus atmosphere is underway under the

NASA Hot Operating Temperature Technology (HOTTech) program. The HOTTech program is

focused on low irradiance, high temperature (LIHT) conditions as well as the corrosive environment

and solar spectrum on Venus (discussed in the subsections below). The technical approach for

HOTTech is to implement contact materials that are stable at high temperatures and optimize

bandgaps for the operating temperatures (and solar spectrum) on Venus. The projected performance

advantages of the proposed LIHT solar cells are that they: a) operate efficiently (>16%) at high

temperatures (i.e., 300°C), b) operate effectively at low solar intensities characteristic of Venus

environments, c) survive and operate in Venus corrosive environments, d) provide long operational

capability (>6 months), and e) survive at Venus surface temperature for more than a month.

Comparison of the advantages of the proposed LIHT solar cells with SOP triple junction solar cells

are given in Table 4-1.

Table 4-1. Comparison of Proposed LIHT Cell and SOP Triple Junction Solar Cell.

SOP Cell

LIHT Cell Performance Goals

Advantages of LIHT Cells

Operating Temperature −140°C to 150°C 25°C to 300°C

Operation at Venus

temperature

Efficiency

2.75% at 300°C and solar

intensities of 2600 W/m

(AM0 at Venus)

16% at 300°C and at solar

intensities of 200 W/m

5 times higher efficiency at

300°C

30% at 25°C

25% at 25°C

Solar Irradiance

1,365 W/m

200 W/m

Improved operational capability

at low solar intensities and low

temperature

Lifetime

15 years at 25°C

Few hours at 300°C

15 years at 25°C

6 months at 300°C

Survivability

~1 h at 460°C

1 month at 460°C

Planetary Surface Spectra. The solar

irradiance spectrum and the overall

solar irradiance are modified by the

presence of a planetary atmosphere for

missions involving surface or near-

surface operations. For example, the

effect of the venusian atmosphere at

various altitudes on the solar spectrum

is shown in Figure 4-10. Solar cells

optimized for operation on Venus are

only conceptual at present. Solar cells

optimized for the martian surface have

been demonstrated

and can provide

substantial benefits. Approximately 7%

improvement in power output can be

achieved for a typical point in the day,

with the Sun at a 60° elevation angle;

slightly reduced improvements can be achieved at lower Sun elevations.

Corrosive Atmosphere. Clouds in the venusian atmosphere contain sulfuric acid (H

) and can

corrode SOP solar cell assemblies. For example, metallic electrical contacts are susceptible to

Geoffrey, A.L. and H. Emily, Analysis of Solar Cell Efficiency for Venus Atmosphere and Surface Missions, in

11th International Energy Conversion Engineering Conference. 2013, American Institute of Aeronautics and

Astronautics.

Figure 4-10. Measured Solar irradiance spectrum on Venus. Solar

irradiance decreases at lower altitudes and energy at wavelengths below

600 nm is particularly diminished.

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corrosion by H

. Development of the solar cells for this environment would enable future solar

powered missions to Venus, such as atmospheric measurements using high altitude balloons.

Corrosion resistant cells are likely going to be covered with coatings, e.g., Al

and an additional

protective coating will cover the entire cell and contacts to provide the required resistance. These,

along with novel metal contacts, are expected to be tested as part of the HOTTech program discussed

above.

High Radiation. High radiation environments of interest include the jovian magnetosphere and

Earth’s Van Allen belts. The Van Allen belts are also significant for SEP missions that pass through

these belts en route to planetary destinations. Electron and proton radiation degrades cell

performance by creating defects in the semiconductor material. These defects become recombination

sites for minority carriers, reducing the fraction of photons that are successfully converted to current

in each junction.

Solar arrays with SOP solar cells incur a large mass penalty for shielding and/or a limited lifetime

when exposed to these environments. For example, solar cells on the Juno mission utilize cover-

glass that is three times the thickness of typical cover-glass (300 µm versus 100 µm). Future missions

will incur either greater mass penalties or performance degradation. The Europa Clipper mission is

preparing for a more severe radiation environment, approximately 2.7 times the radiation exposure

of Juno (an equivalent 1 MeV electron fluence of 3.63E15 versus 1.35E15 e/cm

), due to its orbit

through the jovian magnetosphere. The Europa Lander mission is expected to be subject to an even

more severe environment, approximately 4.4 times the exposure of Juno (5.9E15 e/cm

Development of cells optimized for high radiation involves modifications to the doping profile and

layer thicknesses within the semiconductor. The technology for optimization for extremely high

radiation doses is available in principle, but has not been implemented at present. Juno used, and the

Europa missions are planning to use, SOP cells which are not optimized for the extremely harsh

radiation environment at Jupiter.

4.3 Advanced Solar Array Technologies

Solar array technologies in development for space flight missions emphasize increasing specific

power, reducing cost, and novel methods of deploying higher power arrays given existing constraints

on launch volume and deployed stiffness. Flexible arrays, concentrator arrays, and array technology

for specific environments are discussed below.

4.3.1 Flexible Arrays

A key trend in developing array technology is the growth in power capability for flexible arrays and

a reasonable target for specific power of the next generation flexible array is 200 W/kg at BOL and

1 AU, assuming next generation solar cells. The following flexible array technologies are under

development.

MegaFlex. The MegaFlex array is manufactured by Orbital-ATK and represents an extension of the

UltraFlex array, described in the section on state-of-practice, to larger diameters and higher power

capability. The MegaFlex deploys as a flexible fold-out array with a circular geometry, similar to the

UltraFlex. However, to achieve a larger diameter, the MegaFlex deploys in two stages; the first stage

extends the radius of the circle and the second stage unfolds along a circular path (same as UltraFlex).

Deployment of a 10-m diameter MegaFlex, shown in Figure 4-11 has been demonstrated in ground

test. The MegaFlex is intended to be scalable to diameters as large as ~30 m.

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Figure 4-11 also shows a conceptual illustration of a large MegaFlex array on a flight system. The

MegaFlex is intended to reach power levels up to 300 kW at BOL, 1 AU.

Figure 4

-11. MegaFlex array. At left, a 10-m diameter MegaFlex demonstration unit was deployed in ground test.

At right is an

illustration of a MegaFlex array on a flight system.

A2100 Spacecraft. Lockheed Martin Corp.

announced development of flexible solar

arrays for an upgraded version of the

A2100 spacecraft bus. The configuration,

shown in Figure 4-12, uses the flexible

fold-out technology similar to the ISS.

Mega-ROSA. Mega-ROSA is manufac-

tured by DSS, and represents an extension

of the ROSA, described in the section on

state-of-practice, to higher power. The

Mega-ROSA is shown in Figure 4-13 and

comprises a set of multiple ROSAs deployed

from a central structural spine. The Mega-

ROSA is intended to reach power capability

exceeding 100 kW at 1 AU, BOL, and is also

shown to be extensible to 300 kW.

Composite Beam Roll-Up Solar Array

(COBRA). The COBRA is manufactured by

SolAero Technologies and represents a new

approach to roll-out arrays. Solar cells are

unrolled on a composite blanket and structural

stiffness is provided by the blanket’s curvature.

The configuration is shown in Figure 4-14. The

primary applications for this technology are

http://www.orbitalatk.com/space-systems/space-components/solar-

arrays/docs/FS008_15_OA_7463%20MegaFlex%20Solar%20Array.pdf

https://www.nasa.gov/offices/oct/home/feature_sas.html#.WAmcEzKZOqA

http://www.lockheedmartin.com/us/ssc/commspace.html

Figure 4-12. A2100 spacecraft. Flexible fold-

out arrays based on the

heritage ISS arrays, are in development for the Lockheed Martin

A2100 spacecraft bus.

Figure 4-13. Mega-ROSA. The Mega-ROSA comprises multiple

ROSAs deployed from a central structural spine.

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presently small satellites (SmallSats) with power output up

to ~600 W at 1 AU. The COBRA configuration provides

compact stowage for these applications.

4.3.2 Concentrator Arrays

As noted in Section 3.3, concentrator arrays offer a potential

approach for mitigating the losses associated with LILT

conditions. Specifically, increasing the effective irradiance

using concentrating optics would allow solar cells in the

outer solar system to operate as if they were much closer to

the Sun. As discussed in Section 3.3, several challenges with

concentrating systems must be overcome. These include avoiding excessive illumination during

portions of a mission that are closer to the Sun, sensitivity of the optics to pointing, and sensitivity

of optics to degradation in the space environment.

Nevertheless, concentrating arrays provide an intriguing approach for planetary exploration at farther

distances, such as those beyond Saturn. Hence, further development of this technology may be

significant in the long term. The following paragraphs briefly describe developmental work on this

technology.

Two developmental reflective concentrators are shown in Figure 4-15.

(a)

Figure 4-15. Reflective Concentrator Technologies.

(a) At left is a Cell Saver demonstration figure.

(b) Below is a FACT demonstration figure.

Both use ~2× concentration to reduce the required quantity of

solar cells.

(b)

Cell Saver Solar Array. The Cell Saver provides ~2× concentration using the deployable optical

reflectors shown in Figure 4-15. The Cell saver is manufactured by Orbital-ATK and is intended

primarily for cost reduction. A flight experiment of a small module is currently in Earth orbit.

http://www.solaerotech.com/wp-content/uploads/2016/08/COBRA-Datasheet-August-2016.pdf

http://www.orbitalatk.com/space-systems/space-components/solar-

arrays/docs/FS002_15_OA_3862%20CellSaver.pdf#search=%22Cell%20Saver%20Solar%20Array%22

http://www.techbriefs.com/component/content/article/ntb/tech-briefs/manufacturing-and-prototyping/15070

Figure 4-14. COBRA. The COBRA array is

designed to provide compact stowage for

SmallSat applications.

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Flexible Array Concentrator Technology (FACT). The FACT array is under development by DSS

and is a ~2× reflective concentrator, similar to the Cell Saver. FACT incorporates the reflective

concentrator into the ROSA deployment architecture, as shown in Figure 4-15.

Two developmental refractive concentrators are shown in Figure 4-16.

Stretched Lens Array (SLA). Development of the SLA has been conducted by Entech and Orbital

ATK. Deployable Fresnel lenses provide ~7–10×, up to 25× for the Ecole d’Etudes Sociales et

Pedagogiques (EESP) program for point focus Fresnel concentration, as shown in Figure 4-16

(left). The approach is similar to the array on the DS-1 spacecraft, which utilized rigid Fresnel

lenses and was launched in 1998. The design for DS-1 was configured to operate at lower

temperatures than planar arrays normally operate thereby avoiding degradation induced by high

temperatures. The SLA replaces the rigid lens with a flexible lens that can be stowed more

compactly for launch.

Solar Optical Lens Architecture on Roll-Out Solar Array (SOLAROSA). Development of the

SOLAROSA has been conducted by DSS. This approach also uses a flexible Fresnel lens to provide

~7–10×, up to 25× for the EESP program for point focus Fresnel concentration. The flexible lens is

incorporated into the ROSA architecture, as shown in Figure 4-16 (right).

Figure 4-16. Refractive Concentrator Technologies. At left is an SLA demonstration figure.

At right is a SOLAROSA demonstration figure.

. Both use flexible Fresnel lenses to achieve ~7–10×, up to 25× for the EESP

program for point focus Fresnel concentration.

Novel ideas for concentrators are emerging, including gossamer or very large collectors, and may

have potential that could substantially alter the ability to use solar power in the distant reaches of the

solar system.

4.3.3 Specialized Planetary Mission Environments

Developing array technologies for special mission environments include dust mitigation for

operation on Mars’ surface and arrays for high irradiance conditions.

Dust Mitigation for Mars Surface Operations. Solar arrays on Mars are highly sensitive to

accumulation of dust, as well as periodic removal of dust by martian winds. However, the

unpredictable power output of arrays impacts the ability to perform surface operations. Hence,

https://spinoff.nasa.gov/spinoff2002/er_7.html

http://dss-space.com/products_solar_array.html

Strategic Missions and Advanced Concepts Office JPL D-101316

Solar Power Technologies for Future Planetary Science Missions 42

technology to control or mitigate dust accumulation is potentially extremely advantageous and could

reduce surface operational costs.

Two approaches that formed the basis of previous research efforts are shown in Figure 4-17. At left

is a technology that uses electric fields to remove dust.

39,40

At right is a technology that uses

piezoelectric actuators to remove dust via mechanical vibration.

Both of these technologies require

further development to enable implementation on flight solar arrays.

Figure 4-17. Martian Dust Mitigation Technology. At left is a technology utilizing electric fields for dust removal. At right is a

technology using piezoelectric actuators and mechanical vibration.

High Irradiance High Temperature (HIHT) Environ-

ments. The Solar Probe Plus mission (renamed Parker Solar

Probe) is intended for operation as close as 0.046 AU from the

Sun. The solar irradiance will be roughly 500 times the solar

irradiance in Earth orbit. As mentioned earlier, to survive and

operate in this environment, the array is actively cooled by a

pumped fluid loop. The array area is 1.6 m

and comprises two

wings. An illustration of the spacecraft is shown in Figure

4-18. The ESA BepiColombo mission to Mercury is using

high temperature solar arrays capable of operating at

temperatures as high as 190°C. Further, the solar arrays

contained solar cells and optical solar reflectors.

4.4 Infrastructure

It is necessary that NASA maintain facilities and other

required resources for solar cell/array measurement, analysis, and characterization. Special

photovoltaic facilities are needed to develop, test, and assess PV technologies for the various NASA-

unique mission requirements. In addition, NASA needs to maintain the required calibration methods

and standards for these measurements. Special test facilities are required for testing and

characterization of PV systems under special conditions, such as LILT (outer planets and their

Calle, C. I., et al, “Dust Particle Removal by Electrostatic and Dielectrophoretic Forces with Applications to

NASA Exploration Missions”, Proc. ESA Annual Meeting on Electrostatics (2008).

Mazumder, M., “Performance Restoration of Dusty Photovoltaic Modules using an Electrodynamic Screen”,

IEEE Photovoltaic Specialists Conference (2015).

“Solar Array Dust Removal System (SADRS) for Long Life Mars Surface Missions Phase 2 Final Report”, ATK

Space, JPL Contract No. 1264237, 2005.

Figure 4-18. Solar Probe Plus (Parker

Solar Probe). This is intended to reach a

distance of 0.046 AU from the Sun.

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Solar Power Technologies for Future Planetary Science Missions 43

moons), HIHT (inner planets), LIHT (Venus aerial and surface), dusty environments (Mars), and/or

high radiation environments (Jupiter and its moons).

Some limited capabilities exist at JPL, GRC, and GSFC. These capabilities should be maintained to

conduct the required measurements and characterization of PV systems under unique planetary

environments. NASA should augment simulation testing under combined space environments

(thermal, radiation, plasma/charging, chemical, solar irradiance) for the characterization of solar

cells, panels, and array materials. In addition to test capabilities, NASA needs to update the analytical

tools for mission modeling under conditions unique to NASA spacecraft. GRC provides solar cell

measurement, characterization, and standards. GSFC conducts array design exercises, costing

estimates, and spacecraft integration trade studies. JPL maintains a world-class radiation effects

laboratory. Relying on cell and array manufacturers for accurately predicting cell and array

performance is costly and often lacks reliability. Early detection of problems for a given mission is

extremely cost effective. Without the use of long-standing expertise and facilities, NASA cannot

make the most intelligent mission design choices.

4.5 Summary

Taking into account the development status of ongoing solar cell and array technology programs at

NASA, DoD, DoE, and in industry, the assessment team agreed on the following major findings:

a) Several types of advanced solar cells are under development at several companies and

universities with support from DoD and private funding. NASA supports cell

development through SBIR and, recently, other programs as well.

− These include 4–5 junction cells, inverted metamorphic, dilute nitride, upright

metamorphic and semiconductor wafer bonding.

− Significant improvements in solar cell performance are envisioned:

○

Near-term (1–2 years): >33% efficient

○

Mid- to far-term (5–10 years): >37% efficient

b) Several types of advanced solar arrays are under development with support from DoD,

commercial funding, and NASA:

− Flexible fold-out, flexible roll-out, and concentrators

− Major advances in solar array performance are possible:

○

Near-term: 150–200 W/kg

○

Mid- to far-term: 200–250 W/kg

c) The largest technology investments are focused on Earth-orbiting satellites.

d) Limited work is in progress currently on solar cells and arrays required for operation in

the extreme environments that future planetary missions will incur. These include low

solar irradiance and low temperature environments at the outer planets, high temperature,

high- or low-solar irradiance at the inner planets, corrosive environments at Venus, and

dusty conditions on Mars.

e) Currently, there is limited NASA STMD funding for the development of large solar

arrays, particularly for SEP mission concepts requiring high power and for missions

requiring large areas to provide power at the outer planets.

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5 Findings and Recommendations

This section gives a summary of major findings of the assessment team on space solar power

technologies for future planetary science mission concepts.

5.1 Major Findings

Findings are grouped into three major areas: a) Solar Power System Needs of Future Planetary

Science Missions, b) Capabilities and Limitations of SOP Space Solar Power Systems, and c) Status

of Advanced Solar Cell and Array Technologies.

a) Solar Power System Needs of Future Planetary Science Mission Concepts

The solar power systems required for solar system exploration missions have several unique needs

compared to Earth orbital missions and their needs vary based on the destination and mission type.

The major findings of the assessment team on the solar power systems required for future planetary

science missions are:

1. Outer planet missions likely require high power solar power systems that can function

efficiently in low solar irradiance, low temperature, and high radiation environments

2. Venus mid/low altitude aerial and surface missions generally require solar power

systems that can survive and function in high temperatures, low solar intensities, and

corrosive environments.

3. Many Mars surface mission concepts require solar cells tuned to the Mars spectrum and

solar arrays with dust mitigation capability.

4. Many SEP mission concepts to small bodies and Asteroids require high voltage, high

power solar power systems with low mass and volume.

b) Capabilities and Limitations of SOP Space Solar Power Systems

The major findings on the capabilities and limitations on SOP space solar power systems cell and

arrays are:

1. Significant advances in solar power systems have occurred in the last twenty-five years

mainly due to DoD funding.

a. Solar cell efficiency has increased from less than 10% to over 30%.

b. Solar array specific power has improved from 30 W/kg to 100 W/kg.

c. Solar array power has increased from milliwatts to over 20 kilowatts.

2. The above advances have enabled some outer planetary (Jupiter orbital), inner planetary

(flyby and orbital), Mars (orbital and surface), small body (flyby and orbital) missions

over the last two decades.

However, SOP solar power systems have limited performance capabilities at low

irradiance and low temperature environments, and these limitations need to be overcome

for future outer planetary missions beyond Saturn. PSD needs to concentrate its efforts to

develop PV systems that can function under these extreme planetary environments.

3. SOP solar power systems are attractive for Venus orbital missions, but challenges exist

for low altitude Venus aerial and surface missions due to their limited operational

capabilities at high temperatures, high/low solar irradiance, and corrosive environments.

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4. SOP solar arrays for long duration Mars surface missions would require dust removal

capabilities.

5. SOP solar power are also not attractive to power the next decadal solar electric

propulsion missions to small bodies and outer planet destinations, as they are heavy,

bulky, and cannot function in LILT environments.

c) Status of Advanced Solar Cell and Array Technologies

The major findings of the review team on the status of advanced solar cell and array technologies

are given below:

1. Several types of advanced solar cells are under development at various companies and

universities, with support from DoD and corporate funding. NASA supports cell

development through SBIR and other programs as well. These include 4–5 junction

cells, inverted metamorphic, dilute nitride, upright metamorphic, and semiconductor

wafer bonding. Significant improvements in solar cell performance are envisioned:

− Near-term (1–2 years): >33% efficient

− Mid- to Far-term (5–10 years): >37% efficient

2. Several types of advanced solar arrays are under development, with support from DoD,

private corporate funding, and NASA. These include flexible fold-out, flexible roll-out,

and concentrators arrays. Major advances in terrestrial and Earth orbiting solar array

performance are envisioned:

− Near-term: 150–200 W/kg

− Mid- to Far-term: 200–250 W/kg

3. The largest technology investments in this area are mainly from DoD and they are

focused on improving the performance capabilities of solar power systems for Earth-

orbiting satellites only.

4. Limited research (funded by PSD) is in progress on the development of solar cells and

arrays required for operation in low irradiance and low temperature environments of

outer planets. However, more funding is required to advance these technologies to

TRL 5.

5. Limited research (again funded by PSD) is also in progress to development (to TRL4)

solar cells that can operate at the high temperature, high/low solar irradiance, and

corrosive environments of Venus. However, more funding is required to advance these

technologies to TRL 5/6.

6. No R&D projects are currently underway to develop solar cells optimized for the

martian surface conditions and solar arrays that can operate in Mars dusty environments.

7. Some research is in progress to develop large high power solar arrays, required for SEP

missions to small bodies.

5.2 Recommendations

From examining the current state of the art, state of practice and upcoming developments in solar

power as well as the future mission needs, the assessment team formulated the following

recommendations.

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5.2.1 Overall Recommendations

1. Targeted investments should be made in the specific solar cell and array technologies

needed to withstand the unique planetary environments.

2. Partnerships with HEOMD and STMD and/or other government agencies such as DoE

and DoD (AFRL, Aerospace Corporation, NRL, and ARL) should be established and

maintained to leverage/tailor the development of advanced cell and array technologies

to meet future planetary science mission needs.

3. Existing infrastructure for PV technology development, testing and qualification at

various NASA Centers should be upgraded to support future planetary science missions,

as needed.

5.2.2 Specific Recommendations

Specific recommendations on solar cell and array technologies required for future planetary science

missions are:

1. Develop high power (>100 kW) and low mass (200–250 W/kg) solar arrays for future

solar electric propulsion missions operable up to 10 AU (for outer planet missions).

2. Develop higher efficiency LILT solar cells and low mass, radiation resistant arrays for

orbital missions to Jupiter, Saturn, and Ocean Worlds (Europa, Titan, etc.).

3. Develop LIHT cells and arrays tolerant of the sulfurous environment required for Venus

aerial and surface missions.

4. Develop solar cells tuned to the Mars solar spectrum and solar arrays with dust

mitigation capability for future Mars surface missions.

5. Leverage the DoD investment in higher efficiency solar cells (~38%) and array

technologies to enhance next decadal planetary space science missions.

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6 Acronyms

AFRL

Air Force Research Laboratory

AIAA

American Institute of Aeronautics and Astronautics

AM0

air mass zero

AM1.5

air mass 1.5

APL

Applied Physics Laboratory (John Hopkins University)

APSA

Advanced Photovoltaic Solar Array

ARL

Army Research Laboratory

ATK

Alliant Techsystems

astronomical units

BOL

beginning-of-life

CMX, CMG,

CMO

three types of ceria-doped borosilicate cover-glass material used in optical solar

reflectors, made by Qioptiq Space Technology

COBRA

Composite Beam Roll-Up Solar Array

CP8 IPEX

Cal Poly 8 Intelligent Payload Experiment

DAVINCI

Deep Atmosphere Venus Investigation of Noble Gases, Chemistry, and Imaging

DLR

German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt e.V.)

DoD

Department of Defense

DoE

Department of Energy

DSS

Deployable Space Systems

ECSS

European Cooperation for Space Standardization

EESP

Ecole d’Etudes Sociales et Pedagogiques

ELO

epitaxial liftoff

EOL

end-of-life

EOS AM-1

Earth Observing System satellite; formerly named Terra

EPOXI

Combination of two extended mission components: Extrasolar Planet Extrasolar

Planet

Observation and Characterization Investigation (EPOCh) + Deep Impact

Extended Investigation (DIXI)

ESA

European Space Agency

ESD

electrostatic discharge

FACT

Flexible Array Concentrator Technology (pages 6, 42, 43);

Functional Advanced Concentrator Technology

(page 31)

FOCUS

Full-Spectrum Optimized Conversion and Utilization of Sunlight

FRUSA

Flexible Rolled-Up Solar Array

GOES 7

geosynchronous satellite; also known as GOES H before becoming operational

GRC

Glenn Research Center

GSFC

Goddard Space Flight Center

sulfuric acid

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HEOMD

Human Exploration and Operations Mission Directorate

HIHT

high irradiance high temperature

HOTTech

Hot Operating Temperature Technology

headquarters

IMM

inverted metamorphic multi-junction

ISE

Institute for Solar Energy

ISRO

Indian Space Research Organisation

ISRU

in-situ resource utilization

ISS

International Space Station

ITO

indium-tin-oxide

JPL

Jet Propulsion Laboratory

JUICE

Jupiter Icy Moons Explorer

LADEE

Lunar Atmosphere Dust and Environment Explorer

LaRC

Langley Research Center

LCROSS

Lunar Crater Observation and Sensing Satellite

LIHT

low irradiance, high temperature

LILT

low irradiance, low temperature

MAVEN

Mars Atmosphere and Volatile Evolution

MBE

molecular beam epitaxy

MESSENGER

Mercury Surface Space Environment Geochemistry and Ranging

MgF

magnesium fluoride

MOVPE

metal-organic vapor phase epitaxy

MSFC

Marshall Space Flight Center

MSR

Mars Sample Return

NASA

National Aeronautics and Space Administration

Near-IR

near-infrared

New Frontiers

New Horizons

NM-DS-1

New Millennium Deep Space 1

NOAA

National Oceanic and Atmospheric Administration

NRC

National Research Council

NRL

Navy Research Laboratory

OSIRIS-REx

Origins-Spectral Interpretation-Resource Identification-Security-Regolith

Explorer

OSR

optical solar reflector

PDR

Preliminary Design Review

PSD

Planetary Science Division

PSPS

Planetary Science Program Support

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photovoltaic

R&D

research and development

RACE

Radiometer Atmospheric CubeSat Experiment

RIT

Rochester Institute of Technology

ROSA

Roll-Out Solar Array

RPS

Radioisotope Power Systems

RTG

Radioisotope Thermoelectric Generator

SBAG

Small Body Assessment Group

SBIR

Small Business Innovation Research program

SBT

semiconductor wafer bonding technologies

SCARLET

solar concentrating array with refractive linear element technology

SEP

solar electric propulsion

SLA

Stretched Lens Array

SmallSat

small satellite

SMD

Science Mission Directorate

SOLAROSA

Solar Optical Lens Architecture on Roll-Out Solar Array

SOP

State-of-practice

STMD

Space Technology Mission Directorate

TRL

Technology Readiness Level

UMM

upright metamorphic multi-junction

ultraviolet

VAMP

Venus Aerial Mid-Altitude Platforms

VERITAS

Venus Emissivity, Radio Science

VEXAG

Venus Exploration Analysis Group

VISE

Venus In-Situ Explorer

WISE

Wide-field Infrared Survey Explorer