Pierce E.C. Gordon

The University of Michigan, Ann Arbor, Michigan

Anthony J. Colozza

QinetiQ NA, Cleveland, Ohio

Aloysius F. Hepp

Glenn Research Center, Cleveland, Ohio

Richard S. Heller

Massachusetts Institute of Technology, Cambridge, Massachusetts

Robert Gustafson

Orbital Technologies Corporation, Madison, Wisconsin

Ted Stern

DR Technologies, Inc., San Diego, California

Takashi Nakamura

Physical Sciences, Inc., Pleasanton, California

Thermal Energy for Lunar In Situ Resource 

Utilization: Technical Challenges and 

Technology Opportunities

NASA/TM—2011-217114

October 2011

AIAA–2011–704
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Pierce E.C. Gordon

The University of Michigan, Ann Arbor, Michigan

Anthony J. Colozza

QinetiQ NA, Cleveland, Ohio

Aloysius F. Hepp

Glenn Research Center, Cleveland, Ohio

Richard S. Heller

Massachusetts Institute of Technology, Cambridge, Massachusetts

Robert Gustafson

Orbital Technologies Corporation, Madison, Wisconsin

Ted Stern

DR Technologies, Inc., San Diego, California

Takashi Nakamura

Physical Sciences, Inc., Pleasanton, California

Thermal Energy for Lunar In Situ Resource 

Utilization: Technical Challenges and 

Technology Opportunities

NASA/TM—2011-217114

October 2011

AIAA–2011–704

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

49th Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition

sponsored by the American Institute of Aeronautics and Astronautics

Orlando, Florida, January 4–7, 2011
Acknowledgments

We acknowledge support of this effort by the ISRU project of NASA’s Exploration Technology Development and Demonstration 

program. We thank Dr. Kurt Sacksteder, Charlie Castle, Bob Macosko, Mike Piszczor, and Diane Linne at NASA Glenn and 

Dr. Mark O’Neill of Entech Solar, Eric Cummings of Cool Earth Solar Inc., and Harvey J. Gibson, Jr. of Edmund Optics for 

technical discussions. Support of Pierce Gordon by the NASA Science and Technology Institute (NSTI) program supported by the 

UNCF Special Programs Corporation is acknowledged. Richard Heller is grateful to NASA Glenn for support from the Graduate 

Student Researcher Program (GSRP). 

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Hanover, MD 21076–1320

National Technical Information Service

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Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identification 

only. Their usage does not constitute an official endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-217114 1

Thermal Energy for Lunar In Situ Resource Utilization: 

Technical Challenges and Technology Opportunities

Pierce E.C. Gordon

The University of Michigan

Ann Arbor, Michigan 48109

Anthony J. Colozza

QinetiQ NA

Cleveland, Ohio 44135

Aloysius F. Hepp

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Richard S. Heller

Massachusetts Institute of Technology

Cambridge, Massachusetts 02139

Robert Gustafson

Orbital Technologies Corporation

Madison, Wisconsin 53717

Ted Stern

DR Technologies, Inc.

San Diego, California 92126

Takashi Nakamura

Physical Sciences, Inc.

Pleasanton, California 94588

Abstract

Oxygen production from lunar raw materials is critical for 

sustaining a manned lunar base but is very power intensive. 

Solar concentrators are a well-developed technology for 

harnessing the Sun’s energy to heat regolith to high 

temperatures (over 1375 K). The high temperature and 

potential material incompatibilities present numerous technical 

challenges. This study compares and contrasts different solar 

concentrator designs that have been developed, such as 

Cassegrains, offset parabolas, compound parabolic 

concentrators, and secondary concentrators. Differences 

between concentrators made from lenses and mirrors, and 

between rigid and flexible concentrators are also discussed. 

Possible substrate elements for a rigid mirror concentrator are 

selected and then compared, using the following (target) 

criteria: (low) coefficient of thermal expansion, (high) 

modulus of elasticity, and (low) density. Several potential

lunar locations for solar concentrators are compared; 

environmental and processing-related challenges related to 

dust and optical surfaces are addressed. This brief technology 

survey examines various sources of thermal energy that can be 

utilized for materials processing on the lunar surface. These 

include heat from nuclear or electric sources and solar 

concentrators. Options for collecting and transporting thermal 

energy to processing reactors for each source are examined. 

Overall system requirements for each thermal source are 

compared; system limitations, such as maximum achievable

temperature are discussed. 

Nomenclature

Aconc concentrator solar collector area

Aspot spot size

CR concentration ratio

CRmax,CPC maximum concentration ratio of a dielectric 

compound parabolic concentrator (CPC)

CR2Dmax,Th maximum concentration ratio of a line-based 

trough concentrator

CR3Dmax,Th maximum concentration ratio bounded by thermodynamic limit of solar concentration

ISun intensity of Sun

Lo initial length
NASA/TM—2011-217114 2

∆L change in length

n index of refraction of dielectric (lens) material

nsurr index of refraction of surroundings

Pconc collected solar power

Tspot temperature of spot

T∞ ambient temperature

∆T change in temperature

α coefficient of thermal expansion

ε (regolith) emissivity

θmax acceptance angle of CPC

θSun Sun’s divergent half-angle

ηconc concentrator efficiency

σ Stefan-Boltzmann constant (5.6704·10–8 Wm–2

K–4)

Background

Solar concentrators use optical media, such as lenses and 

mirrors, to focus incident (solar) light. Concentrated light can 

then be focused to produce thermal or electrical energy via 

photovoltaic (PV) cells. Solar concentrator technologies date 

back to Leonardo da Vinci, who suggested making a parabolic 

mirror four miles across in 1515 as a means of melting metals 

for industrial processes (Ref. 1). Solar concentrators have been 

applied in industry, consumer products, and numerous 

advanced technologies. Concentrator technologies have 

applications across the electromagnetic (EM) spectrum; 

paraboloid shaped antennae dishes used for satellite 

communications use similar geometries for radio waves that 

mirror-based concentrators use for visible light. Parabolic 

trough technologies are applied for cutting-edge terrestrial 

solar power plants and in space via a Stretched-Lens Array 

(SLA) Fresnel concentrator design (Ref. 2). Concentrator 

technologies enable a higher mass-specific power rating than 

planar PV; the promise of higher efficiencies and higher 

power output for lower mass and cost are enticing goals

continually spurring further technological developments.

Introduction to Solar Concentrators 

A solar concentrator collects radiation and concentrates 

energy into a reduced spot of light. This concentrated sunlight 

can be used to heat up materials, such as lunar regolith. 

Surface temperature of material at the spot of concentrated 

sunlight is governed by an energy balance between power 

flux, density of concentrated sunlight and losses to the 

surroundings. Heat losses will be a combination of:

• Conduction to surrounding solid material(s)

• Convection to the atmosphere or other fluids 

• Radiation to the surroundings. 

If the material is in a vacuum, such as on the lunar surface 

and has a low thermal conductivity, as does lunar regolith, 

then conduction and convection methods of heat transfer can 

be ignored. Radiative heat transfer, as expressed by the StefanBoltzmann Law (given in Eq. (1)) will determine the ultimate 

achievable material temperature within an illuminated spot.

( ) 4 4 Pconc = σεAspot Tspot −T∞ (1)

Concentrator power, Pconc is determined by solar intensity, 

ISun, assumed to be the average solar intensity of 1353 W/m2

, 

area of the concentrator and concentrator efficiency (Ref. 3)

(ηconc) as given in Equation (2). For the lunar surface, 

temperature of the surroundings T∞, is assumed to be 270 K. It 

should be noted that ambient temperature is wholly dependent 

upon lunar surroundings, and can only be calculated through a 

precise energy balance of the operational geography and Sun

angle. Due to radiative losses of the heated material, as spot 

size increases there is a decrease in achievable steady state 

material temperature within the illuminated spot. 

Sun P A I conv = ηconc conc (2)

This relationship between solar flux captured by the 

concentrator and concentrated spot size is expressed as

concentration ratio (CR) of the concentrator. It is determined 

by equating Equation (1) and Equation (2) and solving for the 

ratio of concentrator area over spot area (Aconc/Aspot). This ratio 

is given by Equation (3). 

( )

conc I

T T

CR

η

εσ − = ∞

Sun

4 4

spot (3)

There is a theoretical upper limit on concentration ratio for 

sunlight. This is because the Sun is not a point source of light. 

The Sun’s disk has a half angle of 4.653 mrad (θs) when 

viewed from Earth’s distance. The theoretical limit on 

concentration ratio is given by one over sine squared of the 

solar half angle (Ref. 4), as expressed in Equation (4) (Ref. 1).

46,188

sin (0.004653)

1

sin ( )

1

2 2 max = = θ = s

CR (4)

The relation between regolith temperature within the spot of 

concentrated sunlight and concentrator concentration ratio at 

differing emissivities are displayed in Figure 1. For processing 

materials on the lunar surface, temperatures up to the melting 

point of lunar regolith are of interest. Because regolith is a 

mixture of materials it melts over a temperature range of 1475 

to 1775 K (Refs. 5 to 7). 

The corresponding CRs needed to achieve these 

temperatures are approximately 180 and 400 respectively. 

When other losses of the system are considered, such as 

reflectivity of mirrors, end angle losses, losses due to heat pipe 

or optical cable use, and deterioration of mirror performance

due to lunar environmental effects, the required CR becomes 

much higher. Maximum CR of each concentrator is limited by 

overall system design as well other specific limitations that are

discussed below. 
NASA/TM—2011-217114 3

Figure 1.—Minimal Concentration Ratio versus 

temperature model at different emissivity levels of 

a solar concentrator. The sink temperature was 

estimated to be 270 K, but actual lunar sink 

temperatures are dependent on lunar energy 

balance. Graph only considers radiative heat 

transfer from surface. 

While it may seem logical that a larger concentrator, 

capable of capturing more solar energy, can produce a higher 

spot temperature this is not the case because as concentrator 

size increases so does spot size as given by concentration ratio 

expressed in Equation (2). This larger spot size in turn 

increases heat losses to the surroundings, thereby limiting the 

temperature. This observation implies an important 

consideration about solar concentrators; a large solar flux does 

not mean a higher temperature. Typically, with larger 

concentrators, CRs are lower due to larger shape errors; thus, 

spot temperatures are less likely to result in significant oxygen 

production. This does not mean, however, that the aim is 

always to forego capturing less solar energy for a higher CR; a 

certain amount of thermal energy must be captured to reach 

set goals for the desired material process such as oxygen 

production, in this case. For example, a preliminary study 

previously performed determined that 15,241 W of thermal 

power was required to produce 1000 kg/year of oxygen from 

the hydrogen reduction of ilmenite (Ref. 3). It is the 

combination of maximum achievable temperature as well as 

total thermal power supplied that dictates sizing and required 

applicability of a concentrator design to a specific task. 

Some characteristics desired in a concentrator for a lunar 

surface mission, such as large size, low mass, low cost and

deploy-ability, tend to limit achievable concentration ratio and 

therefore limit process temperature. One method to enhance 

concentration ratio is to utilize a secondary and possibly 

tertiary concentrator within the system. This type of staged 

concentration of sunlight provides some key benefits to the 

concentrator system. It allows geometry tolerances of the 

primary concentrator to be relaxed, facilitating stowage and 

deployment. The secondary concentrator can then be 

fabricated with high precision to efficiently transport

concentrated light from the primary concentrator and focus it 

down further to achieve the desired concentration ratio. 

Another benefit of staging concentration of sunlight is that 

thermal energy can be redirected to the desired process 

location as it is being further concentrated. Utilizing a 

secondary and possibly a tertiary concentrator within the 

system can enable the primary concentrator to track the Sun

while keeping concentrated sunlight focused on a specific 

location relative to the surface. Utilizing multiple 

concentrators will decrease overall system efficiency. There 

are losses associated with each mirror or lens within the 

concentrator system. These losses are due to geometry 

inaccuracies, light absorption and scattering. The following 

design features help minimize these effects:

• Short ray length, minimizes impact of mirror errors.

• For mirrors, high reflectivity, specularity and minimal 

geometrical errors.

• For lenses, high transmissibility, and minimal losses from 

groove shadowing or spectral shifting.

The Application of Concentrators to Lunar 

Oxygen Extraction

Concentrators have been extensively researched for In-Situ 

Resource Utilization (ISRU), specifically for production of 

oxygen from lunar regolith. Oxygen obtained from regolith 

could be used for extended human habitation on the Moon, 

rocket propellant, or for fuel cells. Lunar regolith is 

approximately 45 percent oxygen by weight, chemically 

locked up in a complex mixture of oxygen-containing 

minerals with balance of silicon, magnesium, calcium, and 

other metals. Table 1 shows major composite metal oxide

compositions in lunar highland and mare regions (Ref. 6).

TABLE 1.—LUNAR SURFACE REGOLITH

COMPOSITE OXIDE COMPOSITIONS

Compound Highland 

(wt %)

Mare 

(wt %)

SiO2 44.5 41.0

Al2O3 26.0 12.8

FeO 5.77 16.2

CaO 14.9 12.4

MgO 8.05 9.2

TiO2 ------- 7.3

To obtain oxygen, various materials processing methods have 

been developed requiring temperatures ranging from 1375 to 

1875 K for extended periods of time. The limiting temperatures 

of this range refer to two specific processes: hydrogen reduction 

and carbothermal reduction processes, respectively. The 

carbothermal and hydrogen reduction processes are two of over 

20 such processes examined for extracting lunar oxygen. The 

other processes are not further considered here; examples are 

listed below and in References 6 to 8.

• Ilmenite Reduction by Carbon Monoxide

• Ilmenite Reduction with Methane
NASA/TM—2011-217114 4

• Glass Reduction with Hydrogen

• Reduction with Hydrogen Sulfide

• Extraction with Fluorine

• Carbochlorination

• Chlorine Plasma Reduction

• Molten Silicate Electrolysis

• Fluxed Molten Silicate Electrolysis

• Caustic Solution and Electrolysis

• Magma Partial Oxidation

• Li or Na Reduction

• Vapor Phase Reduction

• Ion (Plasma) Separation

• Plasma Reduction of Ilmenite

• HF Acid Dissolution

• H2SO4 Acid Dissolution

• Hydrogen/Helium/Water Production from Soil

Mirror Design Considerations

Various concentrator geometries have inherent advantages 

and disadvantages depending upon the application. Each

design would be used for a different specific task or 

environment. In the following section, an overview of mirror 

geometry specifics, advantages and disadvantages, possible 

design opportunities, and design comparisons are discussed. A 

chart with literature values relating concentrator types and 

maximum CR is shown in Table 2 (Ref. 9 to 13).

TABLE 2.—CONCENTRATOR TYPE AND APPROXIMATE 

MAXIMUM CONCENTRATION RATIO (CR)

Concentrator type Concentration ratio range

Fixed Cassegrain concentrator 8000 max.

Offset parabola 4000 max.

Compound parabolic concentrators 

(CPC)

6.5 to 10 (line-based), 

81 (spot-based)

Inflatable offset 2400

Trough (line-focus) 8 to 30

Cassegrain Reflectors

A Cassegrain reflector, shown in Figure 2, is a reflectingmirror assembly where incident solar light is focused by a 

collection of two or more mirrors towards a desired focus

behind the primary mirror. A primary concave mirror focuses 

incident light in the direction of the light source; a secondary 

convex mirror refocuses that light towards a hole in the 

primary mirror, as illustrated in Figure 3 (Ref. 14). This

technology is used primarily with telescopes; both the Hubble 

Space Telescope and its replacement, the James Webb Space 

Telescope, are Cassegrain assemblies (Ref. 15).

Types of Cassegrains normally differ in the shape of 

primary and secondary mirrors. For telescopes, which are 

imaging devices, mirror designs are used to counteract effects 

of coma, spherical, and chromatic aberrations; in most cases, 

the more aspheric the design and the more accurate the desired 

geometry, the more complex and hence, costly the 

manufacturing process. One possible way to for reduce cost 

would be to use a primary spherical mirror and an aspherical

secondary mirror to correct aberrations. Researchers at 

Physical Sciences Incorporated (PSI) conducted simple raytracing analyses of two configurations; one with two spherical 

mirrors, and one with a spherical primary and aspherical 

secondary, and found that the aspherical secondary mirror 

sufficiently corrects spherical aberration to meet the 

requirements of their application (Ref. 16).

Because a Cassegrain’s secondary focus can be positioned 

just behind the primary mirror, the ray length is usually much 

smaller than that of other mirror geometries of similar size.

Thus, scattering effects caused by mirror aberrations are less 

likely to cause a decrease in spot size or lower solar flux 

efficiency. Rigid Cassegrains typically have the highest CRs

of any concentrator design. However, setting up a complex 

array of Cassegrains, or a particularly large single Cassegrain 

mirror in a lunar environment, is still a complex engineering

Figure 2.—Pierce Gordon standing next to a 

Cassegrain concentrator manufactured for Glenn.

Figure 3.—Cassegrain light-ray path.
NASA/TM—2011-217114 5

task involving a considerable design effort. A significant issue 

when utilizing a Cassegrain concentrator, or any geometry 

concentrator, for that matter, is packaging and deployment. 

One possible folding maneuver is the one used by the James 

Webb Space Telescope, where edges of the telescope fold 

back like leaves of a table during launch and deployment, and 

spread back into place during operation (Ref. 15). Other 

deployment possibilities can also potentially be applied and 

are being developed, as discussed below. Multiple 

concentrator units have demonstrated their utility, as 

demonstrated below by concentrator assemblies designed and 

assembled by PSI (Ref. 16).

Although theoretical performance of a Cassegrain

concentrator can be very high, there are several issues with 

Cassegrain technology that could impact their use. They must 

be capable of tracking the Sun very accurately to work 

properly; because of its high CR; if the Sun’s rays were no 

longer parallel with the mirror’s normal, the spot could focus 

onto the primary mirror instead of into the mirror cavity, 

possibly causing catastrophic mirror damage. 

Most Cassegrain designs also have supports that hold the 

secondary mirror in place blocking some solar energy from 

reaching the first mirror; thus the entire mirror is not used to 

its full potential. Because Cassegrains have two mirrors, they 

have two sources of reflectivity loss. Deterioration of coatings

within the lunar environment must also be considered.

Because the Cassegrain’s focus moves along with the tracking 

mirror, a process is needed to transfer solar energy to the 

stationary reactor; each energy transfer option has inherent

heat losses. As with most concentrator designs, Cassegrain

technologies can be assembled in modular fashion thereby 

enabling them to be effectively utilized for most lunar 

environments.

Offset Parabola

Circles create a slight aberration of light rays at the focus; the 

best geometry for focusing parallel light rays is in fact, a 

parabola. Though the Sun’s rays diverge at a small angle, and 

some concentrators do account for such aberration, parabolic 

geometries are sufficient approximations for optimum focusing 

technology. Parallel light rays converge at the parabola’s focus. 

In fact, most advanced non-imaging optics technologies utilize a 

parabola as the cornerstone of the geometrical design. The

offset parabola configuration is a reflecting mirror whose shape 

represents a portion of a whole paraboloid. If the Cassegrain 

primary mirror is a paraboloid located at the center, an offset 

parabola is a portion of the paraboloid’s sidewall, as seen in 

Figure 4 and Figure 5 (Ref. 17).

Figure 4.—Concentrator with off-set parabolic geometry.

Figure 5.—Diagram of Ground Test Demonstrator at Glenn using an offset 

parabolic solar concentrator.
NASA/TM—2011-217114 6

Parabolic mirrors, like Cassegrains, must track the Sun. 

Because the focus of off-set parabolas is in front and below 

the concentrator, there is no need for a secondary concentrator. 

Therefore light is only reflected once before it reaches its 

focus, thus decreasing reflectivity losses. The ability to design 

a concentrator with the focus on or near the surface provides a 

significant benefit in integrating this concentrator geometry 

into an oxygen reactor design. Because currently designed 

reactor models are immobile, solar flux can be easily pointed 

towards a window in the reactor, directly illuminating regolith.

This configuration would work most effectively at the Moon’s 

poles, such as the lunar South Pole, where the lunar surface is 

nearly in constant illumination by the Sun (Ref. 6). In fact, the 

spot could be focused in any direction perpendicular to the 

direction of incident sunlight, e.g., to the side or above the 

concentrator. This opens up the possibility for novel design 

opportunities for optical steering by secondary concentrators.

Another advantage to this geometry is that the parabolic 

mirror can be configured so its entire mirror surface is sunlit, 

unlike Cassegrains, where some of the sunlight is blocked by 

the mirror-support structure. Because the parabola is 

essentially a small section of a much larger paraboloid mirror, 

the ray lengths of the offset parabola will always be longer 

than those of a similarly sized Cassegrain mirror. Thus, offset 

parabolic mirrors will either always have lower CRs, or 

smaller captured solar flux, than Cassegrains. Because of its 

ability to direct the concentrated sunlight off of its central axis, 

the geometry of an offset parabola lends itself to a number of 

applications such as illuminating the surface to provide

‘thermal wadis’ to store thermal energy to protect rovers and 

other assets from harsh thermal cycling effects (Ref. 18).

Compound Parabolic Concentrators (CPCs)

Compound parabolic concentrators (CPCs) are designed to 

capture light at a much larger acceptance angle than other 

concentrators, reducing the need for highly accurate tracking. 

Spot-based CPCs have the cross section of two deep 

parabolas; the edge of one parabola is the focus of the other, 

and vice versa. The geometry is rotated about the axis of 

concentration; incoming light is internally reflected and 

eventually focused at a spot near the base. Tracking does not 

need to be as precise or accurate with these concentrators, 

because they can have a fairly large acceptance angle of up to 

45°, depending upon the design (Ref. 19). Use of a CPC 

primary concentrator would require some degree of tracking to 

keep incoming solar radiation within the concentrator’s 

acceptance angle.

CPCs are manufactured as both hollowed mirrors and

dielectrics (lenses). Examples of each type are shown in 

Figure 6 and Figure 7 respectively. The largest theoretical CR

of all CPC’s, mirror-based or dielectric, is defined by 

Equation (5) (Ref. 19):

sin ( max ) 2

2

θ = n CR (5)

In a mirror-based CPC, the refractive index n is assumed to 

be one. As shown in Equation (5), the CR is inversely 

proportional to maximum acceptance angle; higherconcentration CPCs have a small outer diameter and a long 

length. Consider a CPC used as a primary concentrator; the 

diameter of the CPC would need to be a similar to the 

diameter of the solar flux. The length of the CPC would be 

considerably longer; both the volume and weight of such an 

instrument are not feasible for use as a primary concentrator. 

Dielectric CPCs are also limited by another restriction; 

propagating light is assumed to be totally internally reflected 

by concentrator walls. The maximum concentration due to this 

restriction is shown by Equation (6) (Ref. 19):

(6)

Figure 6.—Hollow-mirror CPC. 

Figure 7.—Dielectric (lens) CPC.

2

2

max,CPC

2 1

1





 



 −

=

n

CR
NASA/TM—2011-217114 7

This equates to a maximum concentration of CRmax,CPC of 81 

for an example refractive index of n = 1.5 for crown glass

(Ref. 11). Such concentrations are too small for achieving 

temperatures necessary for melting lunar regolith by itself. 

However, there is still a possibility for CPCs to be used as 

secondary concentrators.

Trough (Line-Focus) Designs

As discussed previously, the most common concentrator 

shape is a parabolic dish, or cone in which incident light is 

directed towards a spot focus of a specified area. Another 

approach is to utilize a trough or cylindrical configuration, 

which sweeps out a parabola along a line perpendicular to its 

two-dimensional shape. Incident light will then focus onto a 

line. Two main types of line-focus concentrators have been 

developed; one, well known for its terrestrial applications, is a

parabolic trough. This technology is used in large-scale solar 

plant designs (Ref. 20); it could also be adapted for use in 

water purification, for example. The SLA, another line-focus 

technology developed in recent years, uses arched Fresnel lens 

concentrators to direct light towards a desired focus as 

displayed in Figure 8 (Ref. 2). The SLA technology offers 

state-of-the-art space or surface operational performance in the 

following metrics: aerial power density (W/m2

), mass-specific 

power (W/kg), stowed power density (kW/m3

), operating 

voltage (V), and array power capacity (kW to multi-MW) 

(Ref. 2). Arrays can be manufactured using a roll-to-roll 

process, making it easy to rapidly mass-produce any desired 

quantity (Ref. 10).

However, in thermal applications where high temperatures 

are necessary for processes such as oxygen production, a linefocus design falls short. The maximum possible CR of a line 

concentrator, as bounded by thermodynamic limits for solar 

concentration (Ref. 1), is described by Equation (7):

Sun

2Dmax, sin

1

θ

CR Th = (7)

Because the half-angle of the Sun’s incident light (θSun) is 

0.275°; maximum concentration of a linear, or trough 

concentrator, as defined by its thermodynamic limit is 

approximately 208, and realistic linear concentrators aim for a 

maximum CR of about 30 (Refs. 1 and 13). From Figure 1 it is 

apparent that trough configurations do not have sufficient 

concentration ratios to achieve temperatures required to 

process lunar regolith for oxygen production or similar 

processes.

Fresnel Reflector 

In order to achieve high temperatures for ISRU processing 

while minimizing re-radiative losses, high concentration ratio 

solar concentrators are needed. To achieve this reflector 

configuration in a composite without complex curvature 

surfaces, DR Technologies developed a point focus Fresnel 

reflector configuration under a small business innovative 

research (SBIR) program managed by Glenn. The reflector 

uses an array of simple curvature parabolic strips, each with a 

small line focus, and each oriented to overlay the line focus 

into a central focal area, thus simulating a point focus 

concentrator, as shown in Figure 9.

Figure 8.—The Stretched-Lens Array (SLA) 

focusing incident light onto PV collector.

(a)

(b)

Figure 9.—(a) Model 1.5 by 1.5 m Fresnel Reflector using 60 

strips. (b) Edge view showing individual strips.
NASA/TM—2011-217114 8

Figure 10.—Intercept efficiency as a function of focal length for 

a 150 cm aperture, leading to an F/d of 1.5, see Figure 11.

Figure 11.—Flux distribution predicted by SolTRACE for the 

optimized F/d of 1.5.

This design results in several significant advantages. An 

array of simple curvature strips can be fabricated without 

using an accurately machined mandrel as would be needed for 

a parabaloidal dish, with each strip cut from a replicated 

mirror flat and then mounted on an accurately machined 

structure. The Fresnel reflector has a low cross-section, which 

makes packaging for shipment and launch considerably more 

efficient; independently mounted strips are less likely to incur 

global thermal distortion errors.

Concentrator design synthesis used SolTRACE code

provided by National Renewable Energy Laboratory. The 

model drove design trade-offs that varied width, length and 

number of Fresnel strips in the concentrator, focal length 

(Figure 10), overall aperture, and then determined the number 

Figure 12.—Implementation of Fresnel mirror on composite rib, 

and assembled on a positioning tool using a frame that fixed 

the mirror strip position.

and distribution of rays falling within a selected aperture size 

by ray tracing, while considering realistic mirror slope and 

specularity errors. These trade-offs drove design parameters to 

achieve high intercept efficiency with a maximum 

concentration ratio of 2100, as shown in Figure 11, showing 

flux distribution for an optimized Fresnel concentrator. 

A Fresnel reflector model was developed using mirror strips 

of Graphite Fiber Reinforced Composite (GFRC) fabricated 

using a proprietary mirror replication process for high 

specularity. Each mirror strip was mounted on a composite rib 

structure, as shown in Figure 12 and Figure 13, and assembled 

into a frame using a placement tool. Illumination tests on the 

concentrator using a heliostat resulted in temperatures in 

excess of 1575 K. Higher temperatures should be achievable 

with an optimized absorber and secondary optic configuration.
NASA/TM—2011-217114 9

Figure 13.—Mirror test article and illumination testing using fog machine for visual feedback.

Secondary Concentrators

Secondary concentrators are optical devices used to further 

concentrate focused light after being directed by an initial 

concentrator, or to move focused light to a desired location. 

Such technology has been designed and tested for 

effectiveness; a sapphire refractive concentrator, see

Figure 14, has demonstrated some potential promise (Ref. 21).

Although most secondary concentrators developed are lenses, 

mirrors could also be used. Secondary concentrators could 

capture solar energy obtained from large flexible or inflatable

concentrators, which typically have lower CRs due to 

inaccuracies in their shapes, projecting it into a smaller spot

thereby enhancing their CR. As optical media affects light, 

there are inherent losses; the tested sapphire concentrator has a 

calculated transmission of 87 percent (Ref. 21). Losses could 

be minimized and CR increased, however, with effective 

design and testing. As discussed above, a CPC’s most 

effective use could be as a secondary concentrator to a 

Cassegrain or offset parabola. Secondary concentrators could 

also be used to direct solar flux into a designated position for 

prolonged time periods; the primary concentrator’s moving 

spot could be pointed towards a secondary concentrator, that

points towards regolith or an energy transfer device. Such

technology, however, would require complex control systems,

and is likely to increase losses induced by Sun tracking and 

inhibits its ability to capture end angle light rays. An example 

of a secondary lens system for focusing and further 

concentrating sunlight is shown in Figure 15. This system 

utilizes a combination of lenses and mirrors to be able to 

concentrate and move the focus. 

One significant issue with secondary concentrators, 

however, is likely to be material selection. Though sapphire 

was chosen for its ability to be used under extreme 

temperatures, a recent test at Glenn resulted in cracking and 

failure once a prototype reached 1550 K (Ref. 21). The 

primary concentrator chosen will focus (solar) light at least 

hundreds, preferably thousands, of times incident sunlight, and 

as such, the secondary concentrator must be much more 

resistant to EM radiation and intense heat for prolonged 

periods of time. Slight imperfections in secondary 

concentrators are likely to produce high operating 

temperatures due to high light flux from a primary 

concentrator. Therefore, secondary concentrators should not 

be considered an as afterthought, but as an integral component 

of the overall system for effective concentrator design. 

Figure 14.—Refractive Secondary Concentrator.

Figure 15.—Cassegrain Concentrator with OpticalConcentrating and Focusing System.
NASA/TM—2011-217114 10

Reactor Designs

Producing oxygen in-situ on the Moon can provide 

significant benefits and overall cost savings to future missions. 

Earth has a relatively large gravity well with an escape 

velocity of 10.4 km/s (Ref. 7). This escape velocity requires 

tremendous resources to launch any significant object into 

orbit, and even more to bring along fuel necessary to 

maneuver, once in orbit. The Moon on the other hand has onesixth gravity of Earth. A launch from the Moon to Earth orbit 

would require much less propellant than launching a similar 

vehicle from Earth. The ability to effectively and efficiently 

produce oxygen from lunar surface materials must be realized 

to effectively harness these benefits. Both carbothermal and 

hydrogen reduction processes have shown potential for 

producing oxygen from lunar regolith and therefore can 

achieve that mission-enabling goal.

Hydrogen Reduction 

Hydrogen reduction makes use of ilmenite (FeTiO3) in lunar 

regolith as well as H2 gas (likely from water splitting) to 

produce pure iron (Fe), TiO2 and H2O. The temperature 

required to carry out this reaction is approximately 1300 K. A 

simplified design, shown in Figure 16, of a hydrogen 

reduction reactor utilizes an auger to move regolith through 

the reactor as well as heating regolith with the blades (Refs. 22

and 23). A heat pipe would be used to evenly heat the auger 

by distributing the energy from the solar concentrator,

focusing light onto the heat pipe. The auger blades could also 

be used to introduce hydrogen by providing a fluidized bed of 

regolith. There is great potential for this process, but there still 

remain a number of challenges in reactor design and 

operation. For example, hydrogen embrittlement caused by 

hydrogen gas permeating a metal structure can cause certain 

metals to crack, promoting premature failure; some metals and 

alloys are more susceptible to hydrogen embrittlement.

Carbothermal Reduction 

Carbothermal reduction relies on temperatures of at least 

1800 K to extract oxygen from all of the major composite 

constituents of lunar regolith as shown in Table 1. A system 

being tested by Orbital Technologies Corporation (ORBITEC)

is detailed in Figure 17 (Ref. 24). A carbothermal reactor must 

melt regolith to initiate the reaction; this melt is achieved by 

concentrated solar energy heating a small amount of regolith

and using the surrounding regolith as thermal insulation. 

Excess space above the regolith is used to flow methane. 

Methane reacts with molten regolith to eventually produce 

water, which is then split (electrolyzed) to produce oxygen 

and hydrogen. Hydrogen gas is reused in the process, 

generating methane from carbon monoxide, the product of 

methane reduction of metal oxide(s) (Ref. 24).

Figure 16.—Simplified diagram of Current 

Hydrogen Reduction Reactor Design.

Figure 17.—Diagram of current ORBITEC carbothermal reduction reactor with features identified.
NASA/TM—2011-217114 11

Solar energy is focused through a window into the reactor 

chamber. There are other methods that could be implemented 

to concentrate solar energy onto regolith, including heat pipes 

or fiber optic lines. There are design concerns for each of 

these approaches. For example, using a window to concentrate 

sunlight in the chamber could result in that window being

obscured by clinging of regolith particles in the reactor. Also, 

the chamber has free flowing gas, which could pick up dust 

and interfere with focusing of light. It should be noted that a 

sufficient gap is required between the regolith and window, 

otherwise, a temperature gradient across the window can cause 

excessive thermal stresses potentially causing failure.

The ORBITEC carbothermal regolith reduction module 

(CRRM) was initially tested with a CO2 laser to simulate 

concentrated solar energy. The CO2 laser energy passed 

through a zinc selenide window and illuminated a bed of lunar 

regolith simulant located at the bottom of the sealed 

processing chamber within the CRRM. Laser energy absorbed 

by regolith simulant caused rapid, localized heating. Given a 

sufficient laser energy flux, a pool of molten simulant would 

form surrounded by unmelted simulant due to low thermal 

conductivity of regolith simulant. Surface temperature of 

molten simulant was determined by the laser energy flux; this 

heating approach worked well as long as the zinc selenide 

laser window remained completely clean during operation. 

Surface temperatures of molten regolith simulant in excess of 

2075 K were achieved and maintained. 

If any deposits or particulates accumulated on windows, 

some laser energy would be absorbed causing window heating 

and eventual failure. A solar concentrator system was recently 

developed by PSI and integrated with ORBIRTEC’s CRRM. 

The integrated system was successfully operated for nearly 

two weeks during the 2010 International Lunar Surface 

Operations and ISRU Analog Test on Mauna Kea in Hawaii 

shown in Figure 18. Concentrated solar energy was delivered 

into the processing chamber through a quartz rod, as shown in 

Figure 19. Since regolith surface temperature was determined 

by solar energy flux, varying the distance of the quartz rod 

above the surface controlled regolith temperature. 

As with the zinc selenide laser window, the quartz rod 

efficiently transmitted concentrated solar energy into the 

processing chamber when it was clean. However, transmission 

performance of the quartz rod degrades rapidly if any deposits 

or particulates accumulate on its surface. Deposits on the end 

of the quartz rod will absorb concentrated solar energy, both 

attenuating the solar energy delivered to the regolith and 

quickly heating the quartz rod until failure. Unfortunately, 

keeping the quartz rod clean within the processing chamber 

proved to be more difficult than expected.

The end of the quartz rod needed to be located 2.5 to 4 cm 

above the surface of the regolith simulant to maintain the 

temperature of the molten regolith at 1975 to 2075 K. The 

carbothermal reduction of regolith produces silica vapor that 

will coat interior surfaces of the processing chamber. In 

addition, rapid production of carbon monoxide in molten 

regolith causes molten regolith to splatter on nearby surfaces 

as CO gas bubbles break. To prevent potential silica vapor 

deposits or molten simulant splatter from accumulating on the 

end of the quartz rod, ORBITEC enclosed the end inside a 

clear quartz window. A thin layer of gas blowing across the 

bottom of the quartz window was used to keep the quartz 

window clean during processing. 

Figure 18.—Photograph of the Solar Concentrator Array (left) 

with the Carbothermal Regolith Reduction Module hardware 

(right).

Figure 19.—Cross-section view of the processing 

chamber within the Carbothermal Regolith Reduction 

Module (CRRM).
NASA/TM—2011-217114 12

Figure 20 shows the interior of the processing chamber after 

a carbothermal reduction-processing batch. Note the layer of 

silica vapor that extends from processed regolith simulant. 

When a sufficient gas flow rate was used, silica vapor was 

blown away from processed regolith by gas flow across the 

bottom of the quartz window. If the gas flow is not optimized, 

silica vapor accumulates on the quartz window and damages it 

as seen in Figure 21. Although the layer of gas blowing across 

the quartz window was very effective at preventing any silica 

vapor from accumulating on the window, glass splatter from 

molten regolith remained a problem if the bottom of the quartz 

rod was moved less than 3 cm above the regolith surface 

during processing. Figure 22 shows a photograph of the quartz 

window after a carbothermal reduction-processing batch 

where the end of quartz rod was located 2.5 cm above the 

regolith surface; note numerous glass beads on the quartz 

window surface.

Figure 20.—Example of silica vapor produced during 

carbothermal reduction processing of JSC-1A lunar 

regolith simulant with solar energy.

Figure 21.—Damage to a protective quartz window 

due to accumulated silica vapor.

ORBITEC measured improvement in delivered solar energy 

due to the presence of a protective quartz window. Quartz 

windows with and without anti-reflection coatings were tested. 

The energy loss with an uncoated quartz window was 

~7.5 percent due to Fresnel reflection loss. When a quartz 

window with a VIS-IR anti-reflection coating was used, the 

energy loss was reduced to less than 1 percent. However, 

when a coated quartz window was used for a carbothermal 

reduction test, the anti-reflection coating was damaged and 

began to flake off. It appears that the window became too hot 

and the anti-reflection coating failed. An uncoated quartz 

window was used in subsequent tests of the CRRM.

There are several important lessons learned from this 

project. First, a quartz rod is an effective and efficient method 

to deliver highly concentrated solar energy into a carbothermal 

reduction-processing chamber. Using this delivery approach,

Figure 22.—Magnified view (top) of small glass beads 

accumulated on bottom of protective quartz 

window, normal view on the bottom.
NASA/TM—2011-217114 13

molten regolith surface temperatures in excess of 2075 K were 

easily achieved. Second, varying the distance between the end 

of a quartz rod and regolith surface will impact the 

temperature of the molten regolith. Third, special precautions 

must be taken to protect any optical surfaces within a 

carbothermal reduction-processing chamber, such as the end 

of the quartz rod. Any deposits or accumulation of particulates 

will lead to rapid heat and degradation of optical surfaces. 

Finally, anti-reflection coatings can be very effective at 

reducing Fresnel reflection losses. However, care must be 

taken to ensure that coating can survive thermal stresses. 

Design Implementation

Consideration of a complete system for material processing 

on the lunar surface requires that numerous other design issues 

be addressed in addition to the concentrator geometry. In order 

to operate on the lunar surface, a concentrator must survive 

launch and be resistant to lunar environmental challenges. The 

presence of a hard vacuum, solar wind, micrometeorites, EM

radiation, and constant thermal cycling of the range of ~300 K 

are primary issues to consider. Available technology options, 

such as lenses vs. mirror concentrators, and rigid vs. flexible 

concentrators, must be compared and contrasted to determine 

the suitability of each technology. Solar thermal component 

options available to harness and transfer concentrator energy 

to heat regolith must be addressed as well. What follows is not 

an exhaustive list of every necessary design consideration, but 

provides an overview of design issues that must be considered 

when developing effective lunar-based solar concentrators.

Mirror Versus Lens Concentrators

Typically, mirror-based concentrators are developed for 

thermal applications and lens-based concentrators have been 

associated with PV applications. These specific applications

seem to be based largely on history and tradition (Ref. 1). The 

environment, design, manufacture, and operation of these 

systems for lunar processing are inherently challenging and 

complex. Design heritage, cost, launch vehicle capabilities, 

packaging, mass environmental compatibility and operational 

lifetime factor into optimizing and matching specific 

concentrator technology application(s). In addition, because 

mirror concentrator designs for thermal applications have a 

legacy of research, testing, and development, mirror 

concentrators are likely to continue to be developed and 

studied for these applications. The same logic applies to lens 

concentrators for PV applications.

From a theoretical perspective, lenses are actually a much 

better choice for concentrators. The maximum CR of a solar 

image on the absorber of a three-dimensional mirror 

concentrator, bounded by thermodynamic limits for solar 

concentration, is described by Equation (4) and is roughly 

equal to 46,000 (Ref. 1). Theoretical optical constraints dictate 

that a solar concentrator made from a refractive material can 

reach higher concentrations than reflective concentrators, as 

defined by Equation (8) (Ref. 1).

97,700

sin( )

2

3Dmax, ≈ 















θ = surr s

Th

n

n CR (8)

The index of refraction nsurr in air equals 1 and n for crown 

glass equals 1.5 (Ref. 1). However, lens effects such as 

spectral shifting and groove darkening reduce the actual light 

concentrating capability of a lens. Because of intrinsic 

physical deficiencies of lenses, during actual usage, mirror 

reflectors usually have a higher effective CR than most lens 

concentrators. However, lenses do have several advantages 

over mirrors: angle incidence errors caused by errors in Sun

tracking or concentrator shape distortions are drastically less 

with lenses than with mirrors. Fresnel lenses with the unique 

arch shape of the SLA actually provide more than two orders 

of magnitude better shape error tolerance than reflective 

concentrators, or than conventional flat Fresnel lens 

concentrators; when SLA materials are damaged, these 

devices continue concentrating light effectively (Ref. 2). In 

fact, for mirror-based thermal concentrators, accuracy of 

tracking mechanisms need to be approximately one hundred 

times better than for lens-based concentrators (Ref. 1).

Consideration of manufacturing techniques and possible 

materials for lunar surface operation are significant concerns 

for utilizing lenses. Lens materials usually come in either

glass, or transparent plastics. When glass lenses are used as 

primary lens material, concentrators become very heavy.

Previous ISRU oxygen production studies estimate the 

concentrator or concentrator assembly area needs to be at least

14.38 m2 to capture sufficient solar flux; leading to use of

large, heavy lenses as primary concentrators (Ref. 3).

Furthermore, glass is a brittle material that has potential to 

crack or shatter due to intense stresses experienced by launch. 

Plastics, such as space-qualified silicone DC 93-500 used for a

SLA, must be held in place by an external skeletal structure

(Ref. 2). Plastics can also be degraded by space environment 

effects such as atomic oxygen, UV, micrometeoroids, and 

must be protected by oxide films. Metals and composites used 

in mirror materials are much less likely to be affected by 

environmental effects. Scaling of technologies is also a factor; 

it is traditionally easier and lower cost to make larger mirrors 

with high CRs than larger lens concentrators with similar CRs. 

Because tradition has linked mirror concentrators and 

thermal applications, and lens concentrators and PV 

applications, companies specializing in manufacture of such 

technologies have been formed which reinforce such 

traditional links. However, concentrator options are widely 

available for multiple functions by different companies: 

Optiforms, Optical Mechanics, and Hardric Laboratories 

specialize in manufacturing mirrors meant for a spot focus. 

Minnesota Mining and Manufacturing (3M) produces sheets 

of space-rated silicone for line-focus concentrators of any size; 

there are few if any companies who specialize in the 
NASA/TM—2011-217114 14

production of commercial point-focus Fresnel lens 

concentrators (Ref. 2).

Rigid Versus Inflatable/Flexible Concentrators

Rigid concentrators are self-supporting; they have enough 

structural rigidity to maintain their shape without external 

structural support. Flexible concentrators (Figure 23), 

however, forego structural rigidity for very low weight and 

small storage volume (Ref. 12); they must, however, be 

maintained in a semi-permanent shape by some external 

structure during operation or rigidized after deployment. One 

approach is that of an inflatable concentrator kept rigid by 

trapped gas inside. Such technology has a much higher 

specific power rating, or power per unit mass, than rigid 

concentrators (Ref. 22). This means inflatable concentrators 

can be made much larger than rigid concentrators of the same

mass and capture much more solar flux.

Inflatable solar concentrators have been produced that 

capture an effective solar flux diameter of over 4 m (Ref. 22). 

Flexibles and inflatables, however, usually maintain their 

shape by a combination of some supporting back structure 

along with the inflating gas. Such technologies however, have 

a much higher shape error than a polished rigid mirror 

concentrator or a finely milled Fresnel lens concentrator.

Thus, the spot size of inflatable concentrators is much larger 

than similarly sized fixed concentrators. Low-weight rigid 

concentrator assemblies are possible, like the rigid 

concentrator hexagonal assembly made for the James Webb 

Space Telescope (Ref. 15). Employment of such designs,

however, is likely to drastically increase manufacturing costs.

Figure 23.—Flexible 4 m by 6 m offset parabolic concentrator.

Although inflatable concentrators have advantages of lower 

mass and stowage volume over rigid concentrators, there are 

other issues that limit their applicability. The inflated area that 

actually focuses light has to be inflated from both sides; 

therefore, for mirror concentrators, the initial material that 

incident light travels through must be transparent. Most 

materials that are not gas permeable and can take extended 

operation in a vacuum are not completely transparent; potential 

losses over the usable EM spectrum increase. Another important 

issue with flexible concentrators is the effective life of such 

technology in the hazardous space environment; most usable 

plastics are less resistant to space and lunar environmental 

effects than metals and glass. Micrometeoroid puncture analyses 

have been performed for flexible structures in low-Earth orbit, 

but lunar projectile fluxes have yet to be studied in detail. 

Atomic oxygen causes a chemical reaction with space 

inflatables that can erode or build up a growth on the polymer 

surface, causing material degradation. Ultraviolet radiation and 

solar wind serve to break chemical bonds and thereby decrease 

strength in exposed materials. Silicones are some of the most 

resistant materials to harmful radiation, and silicon oxides 

(SiOx) and related glasses are particularly resistant to atomic 

oxygen effects (Ref. 25). UV darkening is known to occur for 

many plastics; UV-resistant coatings to protect space-rated 

silicones are being developed with promising results, but such 

technologies must undergo sufficient testing (Ref. 2). 

Transparency of specific materials with coatings and their wear 

over prolonged periods of time must be further studied.

Rigid Mirror Concentrator Material Overview

Material requirements for a rigid mirror concentrator 

technology are as follows: 

• High Reflectivity

• Low Coefficient of Thermal Expansion (CTE)

• Strong (High Tensile Stress or Modulus of Elasticity)

• Light (Low Density)

• Inexpensive, if possible

• Reliability over prolonged periods of operation during 

harsh lunar exposure

Many of the material requirements are straightforwardly 

related to effective lunar surface operations. For example, a 

high ultimate strength and modulus of elasticity means the 

material is strong and resistant to deformation. Low-density 

materials mean lighter weight structures and lower cost to 

launch. Reliability means the material will not break, creep, 

falter, or wear in any of a number of ways in which material 

breakdown occurs. A low CTE means that during large 

temperature swings on the Moon, materials are resistant to 

effects of thermal stresses and deformation. 

Most materials considered for mirrors do not have all of 

these qualities; copper, silver, gold, platinum, and rhodium, 

which have the highest reflectivity, have high CTE’s, are 
NASA/TM—2011-217114 15

Figure 24.—Graphical comparison of candidate elemental substrate materials chosen for optimal values of melting 

temperature, coefficient of thermal expansion (CTE), density, and bulk modulus of elasticity.

heavy (dense), and the final three are quite costly (Refs. 26

and 27). Aluminum, which can produce a mirror finish with a 

reflectivity of 91 percent, is light and strong, but has a high 

CTE (Ref. 16 and 28). Composite mirror technologies are 

being developed where a light and strong substrate is used for 

the structure and a reflective material is deposited on its 

smooth surface. Atomic layer deposition (ALD) processes are 

being considered for mirror film application; the deposited 

film is thin enough that there are minimal thermal stresses 

between substrate and mirror coating. Mirror deposition for 

optical thin films has been shown to be effective for deposited 

layers on the sub-micrometer level. More research must be 

conducted on substrate-mirror composites for lunar-surface 

solar concentrator operating simulations to demonstrate that 

they can be made with sufficient surface quality and that highperformance reflective coatings can be deposited (Ref. 29).

A preliminary consideration of potential elemental materials 

that could be used as simple mirror substrates is summarized 

in Figure 24; five candidate elements are considered, and four

physical properties, that will impact their effectiveness are 

compared: CTE, density, melting temperature, and module of 

elasticity. Silicon, boron, carbon, magnesium, and titanium are 

all potential substrate candidates due to their high strength, 

relatively low density and low CTE. Silicon has the lowest 

CTE; titanium, known for its aerospace applications, has the 

highest. A detailed study of advanced materials such as 

metallic alloys, ceramics, or composites as mirror substrates 

should also be considered (Refs. 26, 27, and 30 to 33). The 

unique properties of these engineered materials would need to 

be evaluated to access their applicability to lunar and/or space 

environments. 

More advanced technologies and concepts include use of 

electro-active functional materials, such as ionic polymer 

metal composites (IPMC’s) as mirror substrates. Originally 

conceived as an alternative for prosthetic muscle material, the 

mirror surface itself could deform to adjust to different 

conditions and a means of actively focusing the mirror. The 

polymer accomplishes this by deforming when exposed to an 

electric field. This technology holds significant promise in 

providing the ability to fine tune mirror focus during operation 

as well as redirecting the focus and potentially aiding in 

tracking.

Lunar Environment

The lunar environment provides several design challenges 

as well as potential opportunities. The Moon has a 

gravitational attraction of 1/6 Earth. Reduced gravity can 

make it awkward for humans to walk. However for structures 

and some processes, the lower gravity could be advantageous.

Solar concentrators on Earth require bulky and heavy support 

structures to keep mirrors from distorting due to gravity and 

wind. The majority of deflections in parabolic systems on 

Earth are due to forces exerted by wind especially during a 

storm. On the Moon, with a hard vacuum of ~10–12 torr

(Ref. 34), except for solar wind, there are no forces to cause 

any additional deflections. Another potential advantage from

vacuum is heat transfer; there are no convection or conduction 

processes, save conduction through the lunar surface. This 

leaves radiation as the only appreciable heat exchange 

mechanism. This is advantageous for heating and processing 

of lunar materials since heat losses are minimized. A

significant negative, however, is that reactions must take place 

in a pressure vessel to maintain internal gas pressure and 

provide a means of capturing reactants for processing. With 

radiation as the only means of heat transfer; temperature 

gradients across devices and components can become extreme 

because there is no atmosphere to help even out heat transfer 

and reduce these gradients. 

A quite significant environmental challenge on the Moon is 

lunar regolith or dust. Average particle size of constituents of 

lunar regolith is about 70 μm, with 20 percent by weight of 

regolith being less than 20 μm (Ref. 35). Because of the small 

size and shape of regolith, there are three primary challenges 

to solar concentrator and oxygen production systems: dust 

clinging, abrasiveness, and clogging (Refs. 34 to 36).

i. Dust Clinging.—The main concern with dust clinging is 

adhesion to optical surfaces. Dust becomes electrically 

charged allowing it to stick to a variety of surfaces

(Ref. 35). A mirror lens, or window will transfer less 

light while coated in dust, which will greatly diminish 

system efficiency. Some technique must be devised to 

either keep dust off of concentrators and other optical 

components or be able to clear away dust that will 

inevitably coat optical surfaces during operation. 
NASA/TM—2011-217114 16

ii. Abrasiveness.—The abrasiveness of dust can obscure 

and damage optical surfaces. During Apollo 17, 

scratching on Harrison Schmitt’s Sun-shade prevented 

him from seeing in some directions (Ref. 34). Extensive 

scratching of solar concentrator mirrors would cause 

light to scatter due to small changes in the surface, which 

would once again negatively affect system efficiency.

iii. Clogging.—Dust clinging can clog pressure vessels. This 

can affect seals as well as inlets and outlets, gears and 

motors. This issue will be very difficult for oxygen 

reactors. Both carbothermal and hydrogen reduction 

methods use gases for their reactions, which will create a 

pressure inside reactors. With a hard vacuum outside the 

reactor, some pressure seals will need to be in place.

However, dust cannot be avoided because it is to be used 

as a reactant. Care will need to be taken for any moving 

part or opening, to avoid dust clogging.

A final environmental issue to be discussed is vacuum 

welding of moving parts. When two parts move relative to 

each other, there is friction and the surfaces of the two parts 

abrade each other. Earth’s atmosphere has significant partial 

pressure of oxygen (21 percent), which reacts with exposed 

materials to create chemically inert coatings. In a vacuum, 

since there are no oxidation reactions that occur, parts of the 

moving system may bond to each other, potentially seizing up 

mechanisms. It will be a challenge to ensure that the tracking 

system on a lunar solar tracker or the moving parts in the 

reactor do not vacuum weld together.

Geographical Location

Selection of a specific concentrator technology will also be 

greatly influenced by lunar location. On the lunar surface there 

are two main types of regolith: older, brighter soils at high 

topographies, termed highlands, and darker basaltic lava flows 

of mare located at lower topographies (Ref. 3). A display of 

lunar topography is shown in Figure 25 (Ref. 37). The regolith 

of highlands and mare contain different minerals with varying 

elemental compositions; therefore different oxygen production 

processes are applicable in each of these areas. The hydrogen 

reduction process for producing oxygen can yield 1 to 

5 percent of oxygen per unit mass of ilmenite (Ref. 38).

For comparison, the carbothermal oxygen production 

process requires higher temperatures to melt the regolith 

(~1900 K), but can be used with most of the minerals and 

other materials found in the regolith. The oxygen yield by 

weight, of the process, is approximately 15 percent (Ref. 3). 

For locations within mare regions, either hydrogen reduction or 

carbothermal reduction can be used as an oxygen production 

process. Since ilmenite is not found in usable amounts in 

highland regions (Ref. 39), carbothermal processing would need 

to be utilized. Higher temperatures and more precise concentrator 

optics would be required. 

Figure 25.—Topographic map of the Moon determined by the Clementine 

mission of 1999.
NASA/TM—2011-217114 17

Figure 26.—Percentage of time with solar 

illumination during lunar winter. The arrows 

are pointing at the two regions that are 

illuminated >70 percent of the time during 

lunar winter; near the Shackleton Rim.

Though the mare and highlands have different regolith 

compositions, they both share one significant issue; for 

approximately 14 Earth days, almost the entire lunar surface is

enshrouded in darkness (Ref. 40). Because the Moon’s axis is 

tilted 1.5° from the ecliptic, the South Pole is almost 

constantly bathed in sunlight; during lunar summer, the entire 

South Pole is illuminated, and during lunar winter, the 

Shackleton crater and other small southern regions are 

illuminated by the Sun for more than 70 percent of the time

(Ref. 40). Figure 26 shows percentage of time areas near the 

South Pole are illuminated during lunar winter. These locales

would be ideal for a constantly operating solar concentrator 

that could generate approximately twice the amount of oxygen 

than an ISRU reactor anywhere else on the Moon. However, 

there are minimal deposits of iron and titanium located at the 

poles (Ref. 39). Less than 1 percent of ilmenite is present, 

hydrogen reduction would not be the preferred oxygen 

production method; carbothermal reduction or other nonmineral-specific processes must be utilized.

Additionally, long lunar days and nights in a hard vacuum

result in significant thermal cycling; as discussed previously,

this can cause thermal stresses to build up in materials. The 

average temperature range on the lunar surface is 

approximately 100 K during the night and 400 K during the 

day. The equation for simple linear thermal expansion is:

∆L = αLo∆T (9)

Change in length is ∆L; thermal expansion coefficient of a

material is α; Lo is starting length; and ∆T is change in 

temperature. For a 1 m-long aluminum strip, length would 

increase 6.9 mm during the transition from night to day.

Consideration must be made during design to ensure that 

different parts of operating systems do not expand differing 

amounts, causing stresses, breakages, or unwanted gaps.

Energy Transfer Options 

Most concentrators are required to Sun-track to be effective, 

and as the Sun “moves,” so does a concentrated spot of solar 

energy needed to heat regolith. Models of regolith heating 

cycles require that regolith be illuminated and heated by 

stationary spots for 2.5 hr before it can be effectively 

processed (Ref. 41). Reactor assemblies being used for 

research and production are too large to move with a

progressing solar spot, so in most cases, energy needs to be 

transferred to the stationary reactor. There are currently 

multiple concepts for thermal energy transport.

Direct illumination of regolith within a reactor is one option 

that can be utilized under certain operational conditions. In

such a configuration, the focused concentrator solar spot 

would strike the regolith directly. For example, the South Pole 

offset parabola configuration would track the Sun’s movement

by rotating and would ideally have a stationary spot, making

direct illumination possible for hours at a time (Ref. 9). Direct 

illumination would have minimal energy losses due to 

inefficiencies as additional optical components used to direct 

and channel concentrated sunlight would not be needed. Other 

than the efficiency of the concentrator mirror itself, the only 

other source of loss would be the window in which the 

concentrated sunlight would pass to enter the reactor chamber. 

For operating locations other than the North and South poles, 

concentrator configurations with additional optical components 

would be required to keep the spot in one place for extended 

periods of time. A secondary concentrator possibly made from a

CPC or a refractive concentrator lens is a plausible alternative

(Ref. 21).

Because of its low thermal conductivity, direct heating of the 

surface does not provide a good means of uniformly heating a 

batch of regolith. Only the top several microns would be 

effectively heated by direct illumination. For certain reactor 

designs this may be acceptable. For example, in a carbothermal 

reactor, reaction takes place at the regolith surface; regolith 

below the surface is used to insulate the chamber from the 

intense process heat. In other designs such as those for 

hydrogen reduction, uniform heating is more desirable and 

surface heating would not be an optimal approach.

Another form of energy transfer to be considered is the 

optical cable, as outlined above for a solar concentrator system 

for CRRM/oxygen production assembled by PSI (Ref. 16). A 

secondary concentrator located at the primary concentrator’s 

focus would guide concentrated sunlight to a fiber optic cable 

bundle; this in turn would transfer and focus sunlight onto a 

stationary position within the reactor. The flexibility of fibers 

makes it possible for a concentrator to be tracking while 

illuminating a fixed spot within the reactor. Great strides 

towards effective optical cable technology have been realized

through PSI’s efforts, an assembly is shown in Figure 27.

There are inherent losses in this type of light transfer system 

due to inefficiencies of optical cables. The complete 

concentrator configuration assembled by PSI had an efficiency
NASA/TM—2011-217114 18

of 37 percent, though use of higher quality components would 

increase system efficiency (Ref. 16).

Figure 27.—Optical cable assembly designed and 

utilized by Physical Sciences Inc. (PSI).

Figure 28.—(a) Demonstration heat pipe-test set up 

and (b) heat pipe demonstration operating at 

1325 K using a Kanthal Heating Element.

Heat pipes are also being considered as an alternative 

technology for heat transfer; see Figure 28 (Ref. 41). Incident 

solar radiation is focused onto a heat pipe, using solar energy 

to evaporate the working fluid, the heat pipe would transfer 

heat to the regolith as the fluid condenses. Heat pipes provide 

isothermal heat transfer into the reactor, which increases lunar 

regolith processing efficiency (Ref. 41). Heat pipes also make 

it possible to transfer heat throughout the reactor thereby 

providing more uniform heating than that achieved by direct

surface illumination (see Figure 16). Unlike an optical 

component, solar energy can be incident upon a heat pipe at 

very large acceptance angles, up to 90°. Fiber optic cables 

require solar energy to be incident at an angle less than 20°. 

This enables heat pipes to accept incoming sunlight from a

concentrator over a range of angles, simplifying tracking and 

transmission of solar energy from the concentrator. 

There are issues with use of heat pipes at high temperatures, 

for example, with the carbothermal process. For the heat pipe 

to effectively transfer heat to regolith it must be in direct 

contact. In the carbothermal process, high reaction 

temperatures and molten regolith provide a harsh and 

corrosive environment for heat pipes. Material buildup on the 

heat pipe is a significant concern as well as corrosion and/or

oxidization of the surface. This would lower the thermal 

conductivity of the heat pipe, and thus the effectiveness as 

well as potentially lead to its failure. Heat pipes are better 

suited for processes that remain below the melting temperature 

of regolith, such as hydrogen reduction. 

Heat pipes are rigid; designs include heat pipes attached to a

tracking concentrator or heat pipes inside the reactor as a heat 

collection cavity. Many issues with keeping the spot stationary 

that apply to direct illumination also apply to heat pipe

technology. Mobile or flexible heat pipes might be an option 

for such a technology, however, and should be further studied. 

More research needs to be conducted on materials that enable 

effective operation of heat pipes at temperatures in excess of 

1900 K, needed for carbothermal reduction (Ref. 24).

Conclusions

Solar concentrator technology for ISRU-based oxygen 

production requires high temperatures for regolith processing.

Several different designs, materials, and energy transfer 

options have been surveyed, each has inherent advantages and 

disadvantages. Some technologies are adaptable for multiple 

oxygen reduction sites (lunar poles or equator) and can be 

used for multiple oxygen production processes (i.e., hydrogen 

reduction or carbothermal process), while other designs are 

much more specific. A Cassegrain concentrator system with 

an optical cable assembly, for example, could be placed 

anywhere on the Moon, while an offset parabolic concentrator 

using direct illumination could be designed specifically for 

operation on the South Pole (Refs. 9 and 16). It appears that a

more highly modular concentrator design lowers the efficiency 
NASA/TM—2011-217114 19

of the system; flexibility in design may be preferable or even 

necessary for specific missions and oxygen production rates.

Recent development of solar concentrators for both space 

and terrestrial applications along with development of oxygen 

production technologies has established the viability of 

producing oxygen from lunar resources. However, further

development is needed for both concentrator and reactor 

technologies to enable successful operation on the lunar 

surface. Some of these developments would include:

composites with a low CTE, high-strength, lightweight 

materials, materials able to withstand the harsh lunar 

environment, and highly-reflective coatings bonded to these 

advanced substrate materials. 

More research needs to be conducted on the effects of lunar 

dust on concentrators. Plasma sprayers, compressed air, or 

simple window wipers are all being considered as dust 

mitigation alternatives for large mirror based concentrators. 

Integration of multiple concentrator technologies such as 

parabolic mirrors, secondary concentrators, heat pipes and 

fiber optic cables can be combined to provide a system that is 

optimized for a specific mission or task. It is entirely feasible,

for example, to have a primary Cassegrain, a secondary 

concentrator, and a heat pipe working in conjunction to 

provide thermal energy to an oxygen production reactor. Each 

mission or application will require a unique technology 

solution that employs some combination of these technologies.

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Thermal Energy for Lunar In Situ Resource Utilization: Technical Challenges and 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Oxygen production from lunar raw materials is critical for sustaining a manned lunar base but is very power intensive. Solar concentrators 

are a well-developed technology for harnessing the Sun’s energy to heat regolith to high temperatures (over 1375 K). The high temperature 

and potential material incompatibilities present numerous technical challenges. This study compares and contrasts different solar 

concentrator designs that have been developed, such as Cassegrains, offset parabolas, compound parabolic concentrators, and secondary 

concentrators. Differences between concentrators made from lenses and mirrors, and between rigid and flexible concentrators are also 

discussed. Possible substrate elements for a rigid mirror concentrator are selected and then compared, using the following (target) criteria: 

(low) coefficient of thermal expansion, (high) modulus of elasticity, and (low) density. Several potential lunar locations for solar 

concentrators are compared; environmental and processing-related challenges related to dust and optical surfaces are addressed. This brief 

technology survey examines various sources of thermal energy that can be utilized for materials processing on the lunar surface. These 

include heat from nuclear or electric sources and solar concentrators. Options for collecting and transporting thermal energy to processing 

reactors for each source are examined. Overall system requirements for each thermal source are compared and system limitations, such as 

maximum achievable temperature are discussed. 

15. SUBJECT TERMS

Thermal energy; Solar energy; In situ resource utilization; Fresnel reflectors; Regolith; Oxygen production; Cassegrain optics 

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