A.F. Hepp, B.A. Palaszewski, G.A. Landis, and D.A. Jaworske

Glenn Research Center, Cleveland, Ohio

A.J. Colozza

Vantage Partners, LLC, Brook Park, Ohio 44142

M.J. Kulis

Glenn Research Center, Cleveland, Ohio

R.S. Heller

Sandia National Laboratories, Albuquerque, New Mexico 

In-Situ Resource Utilization for Space Exploration: 

Resource Processing, Mission-Enabling Technologies, 

and Lessons for Sustainability on Earth and Beyond

NASA/TM—2015-218867

December 2015

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A.F. Hepp, B.A. Palaszewski, G.A. Landis, and D.A. Jaworske

Glenn Research Center, Cleveland, Ohio

A.J. Colozza

Vantage Partners, LLC, Brook Park, Ohio 44142

M.J. Kulis

Glenn Research Center, Cleveland, Ohio

R.S. Heller

Sandia National Laboratories, Albuquerque, New Mexico 

In-Situ Resource Utilization for Space Exploration: 

Resource Processing, Mission-Enabling Technologies, 

and Lessons for Sustainability on Earth and Beyond

NASA/TM—2015-218867

December 2015

AIAA–2014–3761

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Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for

Propulsion and Energy Forum 2014

sponsored by the American Institute of Aeronautics and Astronautics

Cleveland, Ohio, July 28–30, 2014
Acknowledgments

The following students are acknowledged for their contributions to this work during summer fellowships at NASA Glenn 

Research Center (GRC): Pierce Gordon, Karen Guerrero-Medina, Blain McLeod, Thomaz Santana, and Innocent Udom. We also 

acknowledge technical discussions with, and valuable input from our GRC colleagues, Ms. Diane Linne, Mr. Michael Piszczor, and 

Dr. Kurt Sacksteder, and from our numerous co-authors from previous publications.

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NASA/TM—2015-218867 1

In-Situ Resource Utilization for Space Exploration: Resource Processing, 

Mission-Enabling Technologies, and Lessons for 

Sustainability on Earth and Beyond

A.F. Hepp, B.A. Palaszewski, G.A. Landis, and D.A. Jaworske

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

A.J. Colozza

Vantage Partners, LLC

Brook Park, Ohio 44142

M.J. Kulis

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

R.S. Heller

Sandia National Laboratories

Albuquerque, New Mexico 87123

Abstract

As humanity begins to reach out into the solar system, it has 

become apparent that supporting a human or robotic presence 

in transit and/or on station requires significant expendable 

resources including consumables (to support people), fuel, and 

convenient reliable power. Transporting all necessary expendables is inefficient, inconvenient, costly, and, in the final analysis, a complicating factor for mission planners and a 

significant source of potential failure modes. Over the past 

25 years, beginning with the Space Exploration Initiative, 

researchers at the NASA Glenn Research Center (GRC), academic collaborators, and industrial partners have analyzed, researched, and developed successful solutions for the challenges 

posed by surviving and even thriving in the resource limited environment(s) presented by near-Earth space and nonterrestrial 

surface operations. In this retrospective paper, we highlight the 

efforts of the co-authors in resource simulation and utilization, 

materials processing and consumable(s) production, power systems and analysis, fuel storage and handling, propulsion systems, and mission operations. As we move forward in our quest 

to explore space using a resource-optimized approach, it is 

worthwhile to consider lessons learned relative to efficient utilization of the (comparatively) abundant natural resources and 

improving the sustainability (and environment) for life on 

Earth. We reconsider Lunar (and briefly Martian) 

resource utilization for potential colonization, and discuss next 

steps moving away from Earth.

Nomenclature

C1 generic term for simple one-carbon (CO, CO2, CH4) 

compounds

CH4 chemical symbol for methane

CO chemical symbol for carbon monoxide

CO2 chemical symbol for carbon dioxide

CRRM carbothermal regolith reduction module

ECLSS Environmental Control and Life Support System

∆G Gibbs free energy change

∆H enthalpy change, related to Gibbs free energy change: 

∆G = ∆H – T∆S

3,4He helium-3 or -4, an isotope of He determined by total 

of protons + neutrons

F-T(S) Fischer-Tröpsch (synthesis)

H2 chemical symbol for molecular hydrogen

H2O chemical symbol for water

Isp Specific impulse, a measure of effectiveness of a fuel

ISRU in situ resource utilization

ISS International Space Station

LEO low Earth orbit

LRR Logistics Reduction and Repurposing

N2 chemical symbol for molecular nitrogen

NTP nuclear thermal propulsion

O2 chemical symbol for molecular oxygen

PAG plasma assisted gasification

Re Reynolds number of a fluid flow, related to amount 

of turbulence
NASA/TM—2015-218867 2

Sabatier exothermic reaction that reduces CO2 with molecular 

H2 to CH4 and H2O

SBIR Small Business Innovative Research Program

syn-gas synthesis gas: H2/CO gaseous mixture, typical ratio 

is 2-3:1

ΔV Delta-V is related to the relative velocity needed to 

change trajectories 

WTE waste-to-energy technologies

1.0 Introduction and Background

As NASA moves forward with plans to support human 

exploration of the solar system, a critical need arises to supply 

basic materials such as oxygen (O2) and water (H2O), food, propellants, and other materials (radiation shielding, clothing, etc.)

(Ref. 1). Thus far, exploration has relied on materials brought 

from Earth; this conservative approach, while quite costly, 

minimizes risk to crews. As mankind ventures farther from 

Earth and for greater periods of time, it becomes imperative to 

develop technologies and mission architectures that utilize local 

resources such as lunar regolith or Martian atmosphere, referred 

to as in-situ resource utilization or ISRU (Ref. 2). 

Therefore, ISRU is an overall approach for human exploration based upon utilization of scarce resources (and anthropogenic material(s)) derived from extraterrestrial bodies (surface 

and extant atmosphere) including planets, moons, and asteroids. 

GRC has a 25 year legacy in developing technologies to contribute to this technically challenging exploration approach. 

The work pioneered and supported by GRC includes concept 

proposals, mission studies, hardware (including contractorprovided hardware) development, and technology demonstrations to produce propellants (and other expendables) using extraterrestrial resources for the exploration of the Moon (Refs. 3

to 5), Mars (Refs. 6 to 8), and beyond (Refs. 9 and 10).

For sustainability considerations on Earth, efficient utilization of raw materials and energy often involves recycling: 

re-use or recovery of hydrocarbons (or syn-gas, vide infra) from 

waste plastics (Ref. 11); another critical concern is production 

of clean water from salt or wastewater. These can be related 

back to long-term in-flight or surface-operating utilization of 

waste and trash to produce essential materials such as H2O, propellants, and O2 (Ref. 12). On a rocky surface, lighter elements 

such as O2, nitrogen (N2), and particularly carbon and hydrogen 

(H2) are either not readily available or strongly bonded to metal 

or metalloid atoms in loose regolith and rocks (especially O2) 

requiring significant energy for extraction (Refs. 3 to 5).

For a variety of economic, logistical, and technical reasons,

transportation fuels production (including aerospace propellants) from non-traditional sources (gases, waste materials, and 

biomass) has been pursued for decades. These reasons include 

reducing terrestrial waste streams simultaneous with energy 

conversion, plentiful biomass, new low-cost methane (CH4)

sources made available by aggressive new extraction methods, 

and/or ISRU of limited resources found in space (or extraterrestrial surface); they each present significant technological and 

business opportunities being realized by a new generation of 

visionary entrepreneurs (Ref. 13).

From an operational perspective of ISRU, we have explored 

processing of available resources (Refs. 3 and 7), use of solar 

concentrator technologies to process raw materials (Refs. 4 and 

5), and compared methods of production for green aerospace 

fuels processing (Ref. 13) including a preliminary comparison 

of methods using a two-stage production approach (Ref. 12).In 

this report, we provide a brief retrospective while looking 

towards the future. The ISRU rationale and paradigm are 

described; fundamental considerations and mission-enabling 

technologies are highlighted. Finally, we will discuss a series of 

issues to be addressed while developing technologies relevant 

to sustaining life on the Earth, Moon, and beyond.

2.0 Processing Technologies: Terrestrial 

Beginnings and ISRU Applications

The concept of “living off the land” imposes severe constraints for utilization of power, propellants, and expendables. 

The normal rigors (mass, space limitations and environmental 

challenges) imposed by space travel are magnified by a resource-limited situation. On the other hand, enhanced energy 

efficiency and minimal launch mass will simplify missions (and 

increase successful outcomes) by resultant limitations to 

planned activities. Thus the technical hurdles and challenges alluded to above will stimulate development of technology solutions for space exploration that can be spun off to solve 

terrestrial problems for defense, dual-use, and commercial 

transportation, power generation, and efficient resource utilization (Ref. 14). This section introduces new technologies and/or 

applications for materials processing, hardware engineering, 

and systems integration to enable solutions for the challenging 

technical hurdles of space exploration and green energy conversion.

2.1 Fuel Production From Alternative 

Feedstocks: Thermodynamics and Reactors

An important consideration for fuel production from non-petroleum raw materials begins with the energy balance of the 

chemical reactions at the heart of the various unit operations 

that comprise the processing steps and related hardware. The 

enthalpy (ΔH) change is an indication of whether a transformation releases energy (exothermic, negative ΔH) or requires 

energy input (endothermic, positive ΔH). Depending upon the 

feedstock and/or desired products, combining endothermic and 
NASA/TM—2015-218867 3

exothermic reactions minimizes the energy that must be added 

to the overall system. Several endothermic reactions are available to produce syn-gas (carbon monoxide (CO) and H2)

(Ref. 15) or producer gas (includes other gases such as H2O, O2, 

CH4 and carbon dioxide (CO2)) (Ref. 16) from steam reforming 

of coal (1) or biomass (2), or CO2 reforming of CH4 (3), for 

example.

C + H2O  CO + H2 ΔH = +31 kcal/mol (1)

C6H12O6 + 6 H2O  6 CO2 + 12 H2 ΔH = +145 kcal/mol (2)

2 CH4 + CO2  2 CO + 4 H2 ΔH = +59 kcal/mol (3)

This step can be followed by an exothermic reaction to break 

C-O and H-H bonds, making C-C bonds and C-H bonds such 

as the Sabatier (Eq. (4)) (Ref. 17), methanation (5) (Ref. 18) or 

thermodynamically equivalent Fischer-Tröpsch (6) (Ref. 19)

reactions. 

CO2 + 4 H2 CH4 + 2 H2O ΔH = –40 kcal/mol (4)

CO + 3 H2  CH4 + H2O ΔH = –49 kcal/mol (5)

(6)

Figure 1 shows such an operational system developed by Pioneer Astronautics combining a steam reformer and Sabatier 

reactor (Ref. 13). Alternatively, a one-step approach can be pursued, typically relying on endothermic processes that can be 

fueled by heat supplied by combustion of volatile low-carbon 

by-products (C1-C4) that are not suitable as propellants; see for 

example Figure 2 showing an operational thermal cracking or 

pyrolysis unit developed by RES Polyflow (Ref. 13). These tertiary “cracking” processes are also referred to as chemical recycling (Ref. 11) and are much simpler from a reaction 

engineering and system perspective but produce relatively large 

amounts of solid waste or char and are not suitable for a nonterrestrial or minimally-attended environments relevant for 

space exploration (Refs. 1 to 8). 

A third process that captures elements of steam reforming 

and pyrolysis is flash cracking of plastic waste into tunable 

molecular weight fuels. This technology, being developed by 

Aerodyne Research, Inc., “flashes off” desired hydrocarbon 

products as they form, thus preventing the over-cracking of the 

polymers into more volatile hydrocarbons (Ref. 20). Key 

advantages of this innovation are 1) improved selectivity for 

low vapor-pressure hydrocarbons, which are easier to store as 

fuel in large quantities at low pressures; 2) tunable molecularweight products by changing operating conditions for multiple 

applications. Promising initial results suggest that this technology can be utilized to produce useful liquid fuels with tunable 

product distributions. It can also be easily modified for FischerTröpsch wax cracking during Fischer-Tröpsch synthesis

(F-T(S)) to improve its overall conversion (Refs. 15 and 19).

Figure 1.—Steam reformer/Sabatier reactor system 

delivered to GRC from SBIR Phase II contract.

Figure 2.—Commercial pilot-scale reactor for chemical recycling.

n CO + (2n+1) H2  CnH(2n+2) + n H2O

ΔH = –49 kcal/mol (n = 1)
NASA/TM—2015-218867 4

2.2 Energy Efficiency Issues and Solar 

Concentrator Testing in a Simulated Lunar 

Setting

Energy must be considered at a system level, where heat is 

added to the system for driving endothermic reactions ((1) -

(3)); heat is subsequently harvested from exothermic reactions 

((4) - (6)) for improving overall system efficiency. Overall heat 

rejection must also be considered. Electric power derived from 

photovoltaics is a likely source of process energy, including the 

electrical energy needed for running pumps and splitting water. 

In certain cases there may be merit in considering solar thermal 

to augment the process energy, particularly for endothermic reforming reactions.

Given a Sun-tracking requirement, solar thermal augmentation 

may best be suited for treatment facilities that are anchored to the 

lunar or Martian surface rather than part of a spacecraft rotating 

about its axis for thermal control. Numerous studies have been 

completed on the various types of solar thermal concepts available for possible use, ranging from rigid structures having facets 

with high quality optical surfaces and concentration ratios of the 

order of 8000:1, to concepts that tout light weight inflatable structures, light pipes, and trough systems with lesser efficiency and 

lesser concentration ratios (Refs. 5, 21, and 22). 

A recent successful demonstration of a solar concentrator for 

an ISRU-based application involved running a carbothermal 

reactor in a simulated lunar environment (Ref. 5). The carbothermal reduction method uses a temperature of at least 1800

K to extract the O2 from all the major constituents of the lunar 

regolith. The carbothermal reactor must melt the regolith to 

start the reaction; this melt is achieved by heating a small 

amount of regolith and using the surrounding regolith as insulation. Concentrated solar energy is used to melt the regolith.

Excess space above the regolith is used to flow the reacting gas, 

CH4. The gas reacts with the molten regolith to produce water, 

which is then electrolyzed to produce O2 and H2. The H2 gas is 

reused in the process to regenerate CH4.

The solar energy is focused through a window into the reactor 

chamber. There are other methods that could be implemented 

to focus concentrated solar energy onto the regolith, including 

heat pipes or fiber optic cables. There are some potential design 

concerns with each of these approaches. For example by using 

a window to let concentrated sunlight into the chamber the window could be obscured by regolith particles in the reactor clinging to the window. Also the chamber has free flowing gas, 

which could pick up dust and interfere with the focusing of 

light. It should be noted that a sufficient gap is required between 

the regolith and window otherwise, the temperature gradient 

across the window can cause excessive thermal stresses potentially causing failure. Deposits on the end of the quartz rod will 

absorb some of the concentrated solar energy, both attenuating 

the solar energy delivered to the regolith and quickly heating 

the quartz rod until failure.

An Orbital Technologies Corporation (ORBITEC) Carbothermal Regolith Reduction Module (CRRM) was initially tested 

with a CO2 laser to simulate concentrated solar energy (Ref. 23). 

The CO2 laser energy passed through a zinc selenide (ZnSe) window and illuminated a bed of lunar regolith simulant located at 

the bottom of the sealed processing chamber within the CRRM. 

The laser energy absorbed by the regolith simulant caused rapid, 

localized heating. If laser energy flux was high enough, a pool of 

molten simulant would form surrounded by unmelted simulant 

due to the low thermal conductivity of regolith simulant. The surface temperature of molten simulant was determined by the laser 

energy flux. This heating approach worked well as long as the 

ZnSe laser window remained completely clean during operation. 

Surface temperatures of the molten regolith simulant in excess of 

1800 °C were easy to achieve and maintain.

A solar concentrator system developed by Physical Sciences 

Inc. (PSI), was integrated with the ORBITEC CRRM (Ref. 24). 

The integrated system was successfully operated for nearly two 

weeks during the January 2010 International Lunar Surface 

Operations and ISRU Analog Test on Mauna Kea in Hawaii 

shown in Figure 3. Concentrated solar energy was delivered 

into the processing chamber through a quartz rod; since the temperature of the regolith surface was determined by the flux of 

the solar energy, varying the distance of the quartz rod above 

the regolith controlled the regolith temperature. Similar to the 

ZnSe laser window, the quartz rod efficiently transmitted the 

concentrated solar energy into the processing chamber when it 

was clean. Further development is required to optimize the 

efficiency of this unit operation.

Figure 3.—Integrated Carbothermal Regolith Reduction reactor 

integrated to a solar energy collection and delivery module 

developed on SBIR contracts—successfully demonstrated 

on a mountainside in Hawaii.
NASA/TM—2015-218867 5

While use of concentrated solar power for processing of insitu resources is attractive for lunar installations, Martian applications are less attractive, owing to the greater distance from 

the Sun, the prevalence of dust storms, and wind loading on 

structures (Ref. 25). Use of concentrated solar power on a deep 

space habitat is problematic, given the anticipated rotisserie 

motion of the spacecraft needed for thermal control. Autonomous mining of near Earth asteroids is another potential use for 

concentrated solar power, where core samples carved from an 

asteroid are directed into a high temperature reactor to recover 

volatiles. Recovering iron (Fe) or precious metals may be 

another possible use for concentrated solar power.

Finally, as we reconsider systems designs, shuttling of excess 

heat from exothermic reactions to power endothermic reactions 

may be possible utilizing conventional heat pipes or heat exchangers designed to deliver such energy from one location to another

(Refs. 5 and 21). However, a temperature gradient is needed in 

such systems, the processing temperatures may limit such a concept, and losses are inevitable. Heat pumps are available to shuttle 

excess heat from one location to another, with the added feature of 

increasing the temperature at the outlet. Of course, heat pumps represent an electric load on the system and must be incorporated into 

any energy balance study. Perhaps the ideal concept is one where 

sufficient electrical power is provided to the reactor into which 

reactants are provided to overcome the sensible heat of the reactor 

mass and the energy needed to initiate endothermic reactions during transient startup operations, and electrical power is throttled 

back as new solar thermal heat sources are brought on line during 

steady state operation. Repeated transient and steady state operation should be expected for either batch or continuous processing, 

though far fewer cycles would be anticipated for a continuous 

reactor (Ref. 5).

2.3 Transformation of Raw Materials: Cross 

Fertilization of Terrestrial and ISRU 

Technologies

There have been numerous excellent reviews that summarize 

and analyze in detail the various technologies, products, energy 

balance, and economics of several mainstream terrestrial wasteto-energy (WTE) technologies; relevant reactions can be considered as tertiary (includes longer-chain products: typically C5 

or higher) or quaternary processes (shorter-chain, typically C1 

to C4 products) (Ref. 11). For space exploration, NASA’s Logistics Reduction and Repurposing (LRR) program addresses a 

similar technical problem of waste utilization but on a much 

smaller scale. On a deep space mission, each astronaut will generate a waste stream of about one kg/per day, consisting primarily of organics suitable for decomposition into carbon, O2, and 

H2. In-flight utilization of waste and trash to produce essential 

materials such as water, fuel(s), and O2 is an anthropogenicwaste application of an ISRU approach for sustainability. For 

prudent and efficient utilization of terrestrial raw materials and 

energy for transportation fuel production, re-use or recovery of 

hydrocarbons from waste materials or gas flares is increasingly 

integrated into process design(s) (Refs. 5, 11, and 26).

The LRR program considered a number of processes that 

have been explored for potential use as reactions for the conversion of the waste into useful products, including incineration, steam reforming, pyrolysis, catalytic wet air oxidation, and 

ozonation (Ref. 12 and 20). A comparison of relevant technical 

details as well as energy utilization for several representative 

processes and suitability for potential use in mobile platforms, 

terrestrial applications or space missions is detailed in Table I.

The list of processes considered includes primary (more energy 

TABLE I.—COMPARISON OF RELEVANT METRICS FOR WASTE HANDLING PROCESSES

Process

(type)a

Temperature; pressure 

(MPa) range(s)

Product Output Technical

complexity

Energy efficiency or utilization Appropriate 

application(s)

Thermal cracking (T) 400 to 450 °C

4 to 6.9

C1-C5

Mostly > C6

Hydrocarbons

Low Burning C1-C5 fraction supplies 

80% energy

Chemical recycling 

plant

Flash cracking (T or Q) 400 to 600 °C

0.1 to 1.0

Flexible: C1-C10 depending upon T, 

P, catalyst

Medium Designed to be low energy; 

potential solar energy use

Recycling or mobile/ISRU

Photocatalysis (Q) 25 °C

0.1

Oxygenated C1-C5 depends upon 

time, light energy and photocatalyst

Lowb Low energy; relies on solar energy 

use

Municipal water or 

auxiliary for ISRU

Catalytic wet air oxidation

(Q)

200 to 350 °C

2.0 to 20

Syn-gas (CO +H2), CO2, CH4 Mediumb Designed to be low energy; 

potential solar energy use

Auxiliary process for 

ISRU

Steam reforming (Q) ~ 850 °C

0.3-0.7

Syn-gas (CO +H2), CO2, CH4 Mediumb Balancing endothermic and 

exothermic reactions

Mobile/ISRU

PAG (Q) ~ 5,000 °C

0.1

Primarily

Syn-gas

Highb Recovery of Thermal energy from 

> 1000 °C syn-gas stream

Municipal power

a Chemical Recycling (T/Tertiary) or Energy Recovery (Q/Quaternary). 

b Needs to be combined with a secondary process to produce fuels.
NASA/TM—2015-218867 6

intensive) processes such as thermal cracking, flash cracking, 

steam reforming, and plasma assisted gasification (PAG) technologies; for comparison, auxiliary processes—photocatalysis 

(Ref. 27) and catalytic wet air oxidation (CWAO) (Ref. 28) are 

included.

An example of a thermal cracking or pyrolysis chemical 

recycling technology being developed by (among others) an 

Akron, Ohio-based, start-up company (RES Polyflow) is shown 

above in Figure 2. The RES Polyflow pyrolysis process is quite 

simple but scalability may be an issue for space utilization. 

However, an interesting potential application of this technology 

could be as the key process to enable a fleet of sea-going vessels 

to collect and process islands of plastic waste or marine litter

(Ref. 29) that are currently gathering in various places on the 

oceans and are thought to pose a potential long-term pollution 

issue. Using solar thermal and solar electric energy along with 

selling the chemical feedstocks and fuels produced would defray costs and possibly produce a near break-even or profitable 

enterprise. 

As discussed above, a flash cracking reactor is quite flexible 

from a process perspective and hence is somewhat similar to 

FTS, the most flexible of the secondary processes of synthesis 

or producer gas conversion. The final product mix can be tailored depending upon the temperature and pressure as well as 

the presence of a catalyst (Ref. 30). The lower energy demand 

for this process can be met by solar thermal and photovoltaic 

sources; this aspect of system design was addressed in a generic 

sense above.

The steam reforming process can handle a broader (dirtier) 

feedstock and includes an integrated Sabatier process to produce CH4 but requires a H2 source. Pioneer Astronautics has 

combined two unit operations that work in tandem to produce 

CH4 and O2. The process is quite scalable and was delivered to 

GRC in Summer 2013 as a prototype unit (Figure 1) from a 

Phase II Small Business Innovative Research (SBIR) Program. 

One issue is the need for an external H2 source for a Sabatier 

reactor: this would most likely come from splitting water

(Ref. 13). But on the whole, steam reforming offers the prospect 

of significantly reducing the waste plastics derived from LRR.

The rather complex plasma-assisted gasification process relies 

on high temperatures to recover thermal energies; it is included 

for the sake of comparison, and is currently only practical for 

larger-scale (municipal) use. While it is difficult to make direct 

comparisons regarding scale of infrastructure required, a typical 

plasma system requires significant balance-of-system hardware 

in order to enable self-sustaining electrical power. An energyefficient system will include reusing otherwise wasted thermal 

energy from stage-one products (i.e., syn-gas at > 1000 °C) to 

drive turbines to generate electricity. This would then be followed by an F-T operation to convert syn-gas (CO and H2) into a 

product soup of saturated and unsaturated hydrocarbons as well 

as some oxygenated products such as alcohols of C5 to C20 or so 

with some waxy materials. The balance-of-system technology 

requirements drive up the complexity (and cost),thus minimizing 

suitability for space applications (Ref. 31).

Finally, the Sabatier reaction (4), a century-old CO2 reduction 

process (Ref. 17), is considered an important part of the architecture for the human exploration of Mars (Ref. 25). A Sabatier 

reactor is already part of the Environmental Control and Life 

Support System (ECLSS) equipment on the International Space 

Station (ISS) to process CO2, primarily for its removal and 

water recovery. Utilizing a Sabatier reactor saves considerable 

mass in water resupply by diminishing the need for water upmass and substituting a significantly less massive resupply of 

H2. For ISS, the CH4 created is jettisoned overboard. However, 

for a deep space habitat, one could easily envision an additional 

treatment process where H2 is recovered from CH4 via pyrolysis 

at 1000 °C. The byproduct of this additional step is pyrolytic 

graphite that could be utilized as filler or a conductivity-enhancing agent in the 3D printing of composite tools and hardware 

during a 400-day mission.

3.0 Saltwater Desalination Using Solar 

Concentrators: A Terrestrial Case 

Study

In an effort to provide fresh water through desalination in a 

clean, economical, and environmentally friendly way, a solar 

powered water desalination facility was considered. Selling the 

fresh water produced as well as the left over salt could pay for 

this facility. With a water production rate of 475,000 gal/day 

the possible profit could be $9,000,000 a year. However, the 

current costs of the solar concentrators alone were estimated to 

be over $260,000,000. The process could become feasible if the 

solar concentrator price per unit were to be reduced.

3.1 The Oceans as a Potential Resource

Fresh water shortages are a major concern in many places 

around the world. With growing human populations, these 

sources are being severely taxed. People are now looking towards other possible sources for potable water. One major 

source could be the oceans. Consisting of approximately 

97 percent of Earth’s water, the oceans could provide all the 

water needed by expanding populations(Ref. 29). However, before seawater can be usable, it must first be desalinated. This is 

naturally done by evaporation due to the solar energy on the 

surface of the water sources such as the oceans. The evaporated 

water later falls back to the Earth as rain. Increasing the heat 

energy to the water and then capturing evaporation can accelerate this process. The problem with this approach is in the 

amount of energy required to quickly evaporate water. 
NASA/TM—2015-218867 7

However, it is possible to utilize solar energy through solar concentrators to provide the energy required. Furthermore, sea salt 

has recently become a desirable commodity. After the water has 

been evaporated, the remaining salt can be collected and sold to 

help offset costs of the desalination process. This helps to resolve an issue related to dumping concentrated brine into the 

ocean: the ecosystems will not be affected since isolated salt is 

collected for sale. Furthermore, using solar concentrators provides a clean renewable energy source (Ref. 5).

3.2 Process Overview

The desalination process discussed in more detail below had 

two objectives. The first was to provide fresh water. The second 

was to extract the sea salt for the added profit. A diagram of the 

process can be seen in Figure 4. The steps in the process are as 

follows:

• Seawater was transported from the ocean to the desalination site.

• The seawater entered a heat exchanger to raise its temperature.

• The warmed seawater would then enter the boiler (powered by solar concentrators) where 95 percent would be 

evaporated.

• The evaporated steam would pass through the heat 

exchanger to condense back into water, while the salt and 

remaining water would be transported to the salt evaporation beds.

• The condensed steam would be stored until it was transported to its final location.

3.3 Location Selection

The ideal location for the desalination plant would be on the 

coast in an area with an abundance of sunlight. Several locations were considered depending on their distance from the 

coast and the amount of sunlight received. The main consideration for the distance was the amount of power needed to move 

the water. Only two aspects of the flow were considered for water transportation (the change in height and the headloss). The 

headloss depended on the pipe material and the flow through 

the pipe. The first step to calculate the headloss was to determine the Reynolds number of the flow (Re), which can be found 

by Equation (7)

μ

ρ Re vD = (7)

where ρ is the density of the water, v the velocity of the flow, 

D the diameter of the pipe, and μ the dynamic viscosity of the 

water. 

The Reynolds number along with the roughness of the pipe 

(assumed polyvinyl chloride (PVC)) was then used to determine the fouling factor of the pipe ( f ), see Equation (8), where

ε was the roughness of the pipe. The flow was assumed to be 

turbulent, since the turbulent flow was more power intensive 

and in most cases was indeed turbulent. The head-loss (hl) was 

then determined by Equation (9). The power required to move 

the water was dependent on the headloss and the height the 

water needed to be lifted to reach the site. The relation used, 

assuming no change in pressure and a constant flow is (10).

















 + 

 



 ε −

=

Re

6.9

3.7

1.8log

1

1.11 D

f (8)















 = g

v

D

L h f l 2

2

(9)

w p = (z + hl )m g (10)

The chosen location would therefore dictate the amount of 

power needed to transport the saltwater, which would depend on 

the distance and the height relative to the water source. The other 

major consideration was the amount of sunlight received. Since 

solar concentrators were used to provide the energy to boil the 

water, an area that is nearly free from clouds year round was preferable. Therefore, the best locations that were identified are found 

in Table II. To compare the locations equally, the same amount 

of water was considered for each. Table III gives the initial 

assumptions in order to produce about 475,000 gal/day.

TABLE II.—LOCATIONS FOR A SOLAR CONCENTRATOR

POWERED WATER DESALINATION FACILITY

Location Sunshine,

%

Average 

number 

of clear 

days

Approximate 

distance from 

ocean,

km

Elevation,

m

Yuma, Arizona 90 242 120 43

Fresno, California 79 194 190 90

Sacramento, California 78 188 100 8

Salton Sea, California ~80 N/A Inland salt lake –69

TABLE III.—ASSUMED PIPE FLOW CONDITIONS

Variable Value

Pipe diameter...................................................................0.6096 m

Pipe length ......................................................................120000 m

Elevation of site ......................................................................43 m

Velocity of water ................................................................ 0.2 m/s

Pump efficiency.........................................................................0.5

Pipe roughness (PVC)................................................ 0.0000015 m
NASA/TM—2015-218867 8

Figure 4.—Solar concentrator-powered Saltwater Desalination diagram.

Based on only the distance and elevation of the location with 

respect to the ocean, the best choice would appear to be the Salton Sea. However, the Salton Sea has been considered an important wetlands area and also has a limited supply of water

(Ref. 32). If this water source were used, it could slowly be 

depleted. The best location turned out to be Yuma, Arizona. 

This was due primarily to the cost of the solar concentrators 

compared to the cost of pumping the water. Since Yuma had the 

most sunshine, fewer concentrators were needed to 

desalinate the same volume of water. Therefore, Yuma was 

selected as the location for further study.

3.4 Power Requirements for Desalination

The power requirements for desalination were calculated 

based on the power needed to bring the saltwater to the boiling 

point along with the heat of evaporation of the water. The 

assumed values used to calculate the power requirements can 

be found in Table IV. Since salt would increase the boiling point 

of water and the concentration of salt would increase as water 

boils, it was assumed that the boiling temperature was 115 °C. 

However, not all of the water was removed from the salt. The 

remaining water could be used to help flush out the salt. Since 

there was remaining water, the exiting water was a mixture of 

liquid and gas. The ∆H of the mixture (h) was determined by

Equation (11); using the calculated ∆H of the mixture, the thermal power required (Q) was calculated by Equation (12).

h = hf + x(hg − hf ) (11)

Q = m c pw( − xs )(Ti −To )+ m xsc ps(Ti −To )+ m(h − hf )  1 (12)

TABLE IV.—CONSTANTS FOR WATER EVAPORATION

Constant Value

Water temperature (Ti) .................................................................... 25 °C

Temperature saturated saltwater boils (Tf)..................................... 115 °C

Composition of salt (xS)....................................................................0.035

Percent water to steam....................................................................... 95%

∆H of sat liquid (hf) ............................................................. 419.17 kJ/kg

∆H of sat vapor (hg) ............................................................ 2675.6 kJ/kg

Specific heat of water (cpw).................................................4.186 kJ/kg•K

Specific heat of salt (cps).....................................................0.854 kJ/kg•K

Dynamic viscosity at 30 °C (μ)......................................0.000798 N•s/m2

Density of water..................................................................... 1000 kg/m3

Density of salt ........................................................................ 2160 kg/m3

The total thermal power comes from the power required to 

heat the water, to heat the salt, and to evaporate the water up to 

obtain the desired weight-percentage of salt. This power came 

directly from the solar concentrators. The left over saltwater 

mixture would be transported to a series of evaporator beds, 

which would allow natural evaporation to remove the remaining water.

3.5 Solar Concentrators

The main concept of a solar concentrator was to take a large 

area irradiated by solar energy and compress it to a smaller area.

This directly makes use of the thermal energy available from 

the Sun to heat the water. The thermal energy depends on a few 

factors such as the location and time of year. A location in the 

desert south of Yuma was chosen for the availability of sunlight. The location was approximately latitude 32.5° N and longitude 114.5° W. These were used with an insolation program 

available online to give the available insolation depending on 

the day, month, and year (Ref. 33). The value for each day in 
NASA/TM—2015-218867 9

2009 was used to determine the average insolation, which was 

used for calculations. This means in winter less water would be 

desalinated; in summer more water would be desalinated. The 

average insolation was 331.6 W/m2

, and the total power needed 

was 146.5 MW. The solar concentrator considered was one 

designed and build for NASA (Ref. 5). The concentrator was 

approximately 11 m2 and can track the Sun’s movements 

throughout the day. 

3.6 Estimated Costs

The unit costs to buy or sell different items can be found in 

Table V. The costs were broken up into three categories: one 

time startup cost, yearly expenditures, and yearly income. A 

breakdown of each can be seen in Figure 5 to Figure 7. The 

biggest expense was in the cost of the concentrators. The estimated cost ($5000 per concentrator) was assumed to be achievable with mass production. However, the price may need to 

drop further for the setup cost to be reasonable. The pump cost 

is the total cost of electricity needed to transport the seawater.

The remaining was the assumed operation and maintenance 

costs.

It can be seen in Figure 7 that the bulk of the income comes 

from the sale of sea salt due to its high selling price. This could 

actually cause the desalination facility to become a sea salt producing facility with fresh water being a byproduct, instead of a 

water facility with sea salt as a byproduct. The estimated yearly 

operating cost is completely covered by the income generated 

from selling the water (operating expense = $107,328 and water 

income = $220,886). This would leave the entire sale of sea salt 

as profit year to year. However, this profit must first go towards 

covering the startup costs. Assuming an extremely low interest 

rate of 1 percent yearly, it would still take 30 years to pay off 

the initial debt, which leaves no room for expansion or replacement of worn out concentrators for that 30 year period.

TABLE V.—ESTIMATED UNIT COSTS

Unit Cost,

$/unit

Number of 

units

Total,

$

Concentrators 5,000 52,196.6 260,983,140

Pipe, m 34.48 120,000 4,137,931

Water, gal 0.0015 147,257,380 220,886.07

Sea salt, kg 0.5 10,268,444 5,134,222

Electricity, kWh 0.0865 331,377.6 28,664.16

Evaporator beds, m2 10 2,499.46 24,994.61

Land, acre 406.25 162.86 66,162.10

Figure 5.—Breakdown of startup expenses.

Figure 6.—Breakdown of yearly operating costs.

Figure 7.—Breakdown of yearly income.

concentrators

land

beds

pipe

pump costs

operation

water

sea salt
NASA/TM—2015-218867 10

3.7 Pipe Flow

There were two costs associated with transporting seawater.

One was the reoccurring pumping costs, and the other was the 

cost of the actual pipe. The piping used was PVC, since any 

metal pipe would have heavy corrosion problems due to the 

saltwater. In order to allow for expansion and to reduce pumping costs, a large diameter (0.6096 m) pipe was chosen.

However, this pipe may not be large enough to carry the 

128,000,000 gal/day that some of the larger desalination plants 

process (Ref. 34). To produce 128,000,000 gal/day of fresh 

water would require over $2 billion in electricity per day just to 

pump in the water. This is obviously impractical, but the optimum point has yet to be calculated. To reach a production rate 

of 128,000,000 gal/day would therefore require multiple pipelines to be within reasonable limits. 

3.8 Case for Solar Concentrator Technology to 

Produce Clean Water

A water desalination facility powered by solar concentrators 

would be able to fund itself during operation, assuming public 

financing covered costs for the plant. However, unless the capital equipment costs of solar concentrators decrease, the facility 

would not be able to recoup the initial startup cost. Nevertheless, the system would still save money in the long run. Using 

the average cost of electricity in Arizona (0.0865 $/kWh), the 

total cost of the solar concentrators would equal about 6.6 years 

worth of electricity running off the power grid for the same 

amount of power (not accounting for losses in converting electrical power to thermal). Furthermore, this process makes use 

of a clean renewable resource (the Sun) to provide needed 

freshwater.

4.0 Resource Utilization on the Moon

Oxygen is the highest-value resource to be produced on 

extraterrestrial bodies such as the Moon (Refs. 1 and 2), since 

it is the oxidizer component of rocket propellant, and is, for 

most chemical rocket types, the most massive component of the 

fuel used for propulsion. Thus, production of O2 from local material fuels the transportation system that supports the lunar 

base, as well as serving as a possible product for use in propulsion systems either commercially or to support exploration of 

other locations. Replacement O2 is also a necessary product for

life support, since even closed-ecological life support systems 

will not have perfect efficiency of recycling. 

In addition to the oxidizer, it would be desirable to produce 

the fuel as well. This is more difficult to produce from lunar 

material, since hydrocarbons are absent. The polar regions of 

the Moon are now understood to have permanently shadowed 

craters, in which water ice is present; this ice could be mined 

and then electrolyzed to produce H2 and O2, which can be used 

for fuel, although LH2 is difficult to store, requiring cryogenic 

tankage. The high latitudes of the Moon also contain water in 

the form of hydrated minerals. An alternate technology would 

be to develop rockets that do not utilize H2 or hydrocarbons.

One alternative is metal propellants, such as aluminum (Al) or 

magnesium (Mg) (Ref. 3).

However, it will also be desirable to refine raw materials for 

other purposes, including structural materials and materials for 

industrial applications, such as production of solar arrays

(Ref. 35). Many possible applications for lunar-produced materials are summarized by Benaroya (Ref. 36). The most desirable 

processing sequence therefore would focus on an initial goal of 

O2 production, but using a technique that could be expanded to 

produce and refine other raw materials.

Technologies for refining raw materials out of lunar regolith 

and rocks will also have applications on Earth. Terrestrial refining technologies used today require significant use of resources

such as coal, energy, and water, and produce large amounts of 

waste byproduct such as CO2 and particulate pollution. Lunar 

resource extraction, on the other hand, must of necessity minimize the use of expendable resources, will recycle reactants and 

exhaust gasses, and will be optimized to be efficient in the use 

of energy. Thus, pilot production sequences for lunar resource 

utilization will serve as models for highly efficient, not-polluting technologies that can be utilized on Earth.

Workers at GRC have examined several methods of materials 

production from lunar regolith (Refs. 3, 35, 37, and 38). The 

same process sequences could be used at other locations. Stony 

asteroids typically have regolith similar to that of the Moon, and 

refining of asteroidal material could use the same techniques, 

adapted for microgravity. Other types of asteroids would allow 

processing possibilities not considered here. For example, 

Type-C asteroids contain carbon and can contain some amount 

of chemically bound water, which can be used for different purposes. Metallic asteroids can be processed to produce both bulk 

and precious metals. Likewise, Martian rock and soil could also 

be processed by the techniques discussed here. 

4.1 Desirable Products From Lunar Regolith

Other than O2; and metals, glass, and related ceramics and Si

are valuable as structural and power materials. Each subsection 

gives examples of practical applications of these materials for 

an eventual lunar colony.

4.1.1 Metals 

Metals are a ubiquitous structural material, and will certainly 

be used in lunar manufacturing. Structural metals include Fe

and steel, aluminum (Al), and titanium (Ti); each valuable for 
NASA/TM—2015-218867 11

different uses, and all available as elemental components of 

lunar material. Metals are also used as wires conductors. From 

its elemental abundance in lunar soil and high electrical conductivity, Al is the clear choice for wires. A second possible 

choice is calcium, not used on Earth because of high reactivity 

with O2, but a possible conductor for vacuum applications.

4.1.2 Glass and Ceramics 

Transparent glass is a required material in forming solar arrays (Ref. 35). Ceramics are also useful as insulators. Glass or 

ceramic fibers are useful for structural composites. The primary 

glass-forming material, silicon oxide, is abundant on the Moon, 

in the form of silicates. Transparency requires refining, most 

particularly to remove trace amounts of Fe and other transitionmetal oxides, which produce color centers. Silicon dioxide is 

not only a simple formula for glass but an engineered material 

with many additives selected to produce required properties. 

Several of the oxides which are used to adjust the properties of 

glass are not abundant on the Moon. Sodium oxide (Na2O), the 

main component of “soda lime glass,” is typically used to reduce the melting point, allowing easier working. Boron oxide 

(B2O3), to produce borosilicate glass, is typically used to adjust 

the thermal expansion coefficient. New glass compositions will 

have to be invented to reduce, or eliminate, the amounts of these 

materials that are rare in lunar soil.

4.1.3 Silicon

One of the most important issues for settlement is production 

of power. Many different semiconductors can be used to produce photovoltaic cells, but from the standpoint of lunar abundance of materials, the clear choice for locally-manufactured 

cells is Si solar cells (Ref. 39). Silicon suitable for semiconductor applications is a highly purified product; parts per billion of 

some impurities is sufficient to degrade the properties. Thus, a 

processing sequence for making solar-cell grade Si must include purification steps.

4.1.4 Civil-Engineering Materials

In addition to structural materials, lunar settlements will 

undoubtedly require less highly processed material. Although 

habitation structures on the Moon will not be made of ordinary 

bricks (since habitats must hold pressure, and hence will be tension structures), there will still be the need for the equivalent of 

concrete, asphalt, and bricks. Many possibilities for such bulk 

material exist, including sintered or melted regolith bricks, material produced from slag from other processes, or composite 

materials comprising aggregate fill cemented with a ceramic 

matrix.

4.2 Processing Methods

A processing sequence can be broken into three main steps:

(1) Acquisition and beneficiation (if required) of feedstock; 

(2) Reduction; and (3) Refining of the desired raw materials and 

purification to the required level. What follows is a brief overview followed by a focus primarily on three different 

approaches to the second step, reduction.

4.2.1 Processing Overview

The acquisition portion of the processing is a sequence of 

prospecting (if required), materials acquisition and mining, 

grinding or otherwise preparing the material for processing, and 

(for some sequences) beneficiation of the input material to 

increase the concentration of the desired mineral. Preferably, 

the sequence selected could be fed from regolith that is available at any lunar location, minimizing and possibly eliminating 

the need for prospecting and for beneficiation. 

The reduction step comprises stripping the O2 away from 

oxides. This step produces the main product, O2. Lunar regolith 

is primarily silicates, in which the oxides are in the form of O2

bridging between Si atoms, chemically bonded to metal cations 

in a strongly-bound net. The reduction process therefore requires breaking the Si-O2 bonds.

After the O2 is produced, the byproduct is reduced (or partiallyreduced) metals. The resultant product may be a mixture of metals. To turn this into useable raw material, the desired materials 

must be separated and purified to the levels needed. Many 

sequences for O2 production have been previously reviewed

(Refs. 1 to 3). Sequences of interest here are those that reduce the 

main components of lunar regolith, to produce metals.

4.2.2 Magma Electrolysis

Magma electrolysis is conceptually the simplest method of 

refining regolith into reduced metals and O2. It consists of heating lunar soil to the melting point, then running electrical current through the melt to electrolyze the anions (primarily O2) at 

one electrode, and the cations (metals and Si) at the other

(Ref. 36). However, the details are complicated. A significant 

difficulty is the extremely high temperatures needed, from 1300 

to 1450 °C, which result in significant practical difficulties.

4.2.3 Calcium Process

A two-step calcium process is feasible as a method of reducing temperatures needed (Ref. 37). It requires considerably 

lower temperatures than direct magma electrolysis, and produces O2 with considerably higher efficiency than H2 or carbothermal reduction methods. The first step consists of 

reducing the regolith by calciothermic reduction. This is done 
NASA/TM—2015-218867 12

by heating of regolith in the presence of metallic calcium, to 

convert the silicates into metals plus calcium oxide. Regolith is 

heated with metallic calcium at a temperature greater than the 

melting point of calcium, 845 °C. The reaction rate is increased

by use of finely-ground reactants as well as excess calcium;

Equation (13) shows a typical reaction:

MgFeSiO4 + 4Ca → 4 CaO + MgFeSi (13)

The second step (14) regenerates the metallic reactant, by electrolyzing the calcium oxide in a molten salt at 825 to 900 °C, to 

produce metallic calcium and O2:

CaO → Ca (metal) + 1⁄2 O2 (gas) (14)

Since calcium and calcium oxide are soluble in calcium chloride (CaCl2), the chloride or a CaO/CaCl2 eutectic mix can be 

used as a flux to accomplish the reactions in a liquid solution. 

Further separation and purification steps can be taken from this 

point to produce refined product for other processing.

4.2.4 Fluorine Process

A basic reduction process requires heating regolith in the 

presence of fluorine. The fluorine displaces the O2 from the 

rock (collected as a useful product), producing fluorides. The 

fluorides are displaced by potassium to produce reduced metals 

and potassium fluoride (KF), which is electrolyzed (Refs. 35

and 38).

Silicon and Ti produce volatile fluorides, SiF4 and TiF4, both 

gaseous at processing temperatures. The tetrafluorosilane can 

be easily purified to semiconductor-grade by distillation. The 

remaining metals are produced in the form of fluoride salts, 

which must be reduced to the metals. The Fe and Al fluorine 

salts are directly reduced with potassium:

FeF3 + 3K → 3KF + Fe (15)

AlF3 + 3K → 3KF + Al (16)

Calcium and magnesium fluorides are not reduced by potassium, and are returned to oxide form by potassium substitution:

CaF2 + K2O → 2KF + CaO (17)

MgF2 + K2O → 2KF + MgO (18)

The oxides are then available for glassmaking. The reactants, 

fluorine and potassium, are returned in the form of KF, and are 

then recovered by electrolysis in a KF/NaF/CaF2 eutectic at 

676 °C.

4.3 Impact of Geographical Location

On the lunar surface there are two main types of soils available. These consist of the older, brighter soils at high topographies, termed highlands, and the darker basaltic lava flows of 

the mare located at lower topographies (Ref. 4). A display of 

the topography on the Moon is shown in Figure 8 (Ref. 40). The 

regolith at the highlands and mare have different compositions 

and therefore different O2 production processes are applicable 

to each of these areas. The H2 reduction process for producing 

O2 can yield 1 to 5 percent of O2 per unit volume of ilmenite

(Ref. 24). For comparison the carbothermal O2 production process requires high enough temperatures to melt the regolith 

(~1800 K), but can be used with most of the metal oxide compounds found in the regolith. The O2 yield of the process is approximately 15 percent by weight (Ref. 4). 

Figure 8.—Topographic map of the Moon determined by the Clementine mission of 1999.
NASA/TM—2015-218867 13

Figure 9.—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.

For locations within the mare regions, either H2 reduction or 

carbothermal reduction can be used as an O2 production process. Since ilmenite is not found in usable amounts in the highland regions (Ref. 41), the carbothermal process would be 

utilized within these regions. This means higher temperatures 

and more precise concentrator optics are required.

Though both the mare and the highlands have different regolith characteristics, they both have one significant problem; for 

approximately 14 Earth days, most of the mare and highland 

topographies are enshrouded in darkness (Ref. 42). Because the 

Moon’s axis is angled 1.5° from the ecliptic, the South Pole is 

almost constantly bathed in sunlight; during lunar summer, the 

entire South Pole is visible, 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. 42).

Figure 9 shows the percentage of time regions near the South 

Pole geographies are illuminated during lunar winter. Such 

areas would be an ideal place for a constantly operating solar 

concentrator that could generate approximately twice the 

amount of O2 than an ISRU system anywhere else on the Moon. 

However, there are minimal deposits of Fe and Ti located at the 

poles (Ref. 41). Less than 1 percent of ilmenite is present which 

means H2 reduction would not be the preferred O2 production 

method; carbothermal reduction or other non-mineral-specific 

processes must be pursued.

5.0 Beyond the Moon: Future Human 

Exploration Options

From the 1950s through the 1980s ambitious robotic and human missions were planned for outer space—targeting places 

from Mercury to the outermost reaches of the solar system (Refs. 

43 to 47). Although robotic missions continue to be endorsed today, any new human exploration of the solar system still awaits 

promotion.. While investments in robotic missions have continued, human exploration of the solar system has awaited new invigorating steps. Beyond the Moon, Mars represents an obvious 

destination for human exploration, and also a rich destination for 

use of in-situ resources. Mars resource utilization is incorporated 

into reference plans for exploration of Mars (Ref. 48). The literature on Mars resource extraction and utilization is sufficiently 

extensive that it will not be covered in this paper. Future human 

missions to other destinations such as Mercury and Saturn will 

also require long-term investments. Currently, Mercury and Saturn have robotic missions returning invaluable data on those 

planets and their environs. These data have provided insights that 

will ensure the success of future missions. With its proximity to 

the Sun, Mercury has extremely high temperatures and requires 

special high heat flux considerations for long-term human visits 

or bases. In contrast, temperatures at Saturn and its moons require 

designs for cryogenic environments. The possibilities for ISRU

may allow more effective robotic missions and human visits to 

these planetary targets.

5.1 Mercury

Mercury is the closest planet to the Sun; ranging from a perihelion of 46,000,000 km to an aphelion of nearly 

70,000,000 km. The high temperature, high heat flux environment at Mercury and the tenuous surface emanations of several 

major chemical species (sodium, etc.) will likely pose challenges to long term human visits. Permanently shadowed craters offer a valuable niche for longer-term human visits and 

planetary bases. Such craters offer cryogenic temperatures 

while the Sun-facing surface is at a temperature of 590 to 

725 K. The north Polar Regions of Mercury have been identified as a likely location for such permanently shadowed craters

(Refs. 49 to 51). Water ice is also likely to be in these craters, 

further aiding and assisting any human exploration(s). Short exploratory missions can be accomplished with hopping ascentdescent vehicles from the base at the shadowed crater.

Figure 10 shows locations of the shadowed craters (Ref. 50). 

Figure 11 depicts the temperatures that would exist in and near 

the craters (Ref. 51). The crater could accommodate a small 

base or at least an initial landing site. The lander’s temperature 

could stay within the nominal operating temperatures of traditional spacecraft. The temperature distribution in the crater 
NASA/TM—2015-218867 14

Figure 10.—Permanently shadowed craters in Mercury’s north 

polar region (from Ref. 50).

Figure 11.—Temperature ranges outside and inside 

permanently shadowed craters (from Ref. 51).

would allow construction of the base at the warmer side of the 

crater and then the frozen volatiles would be extracted with cryogenic mining machines.

5.2 Mercury Missions

A human round trip mission to Mercury was assessed. The 

mission ΔV values for the round trip Mercury missions were 

derived from the literature (Refs. 52 to 55). The highest ΔV case 

was selected from this data: an Earth departure ΔV of 5.2 km/s, 

a Mercury arrival ΔV of 10.9 km/s and a Mercury departure ΔV 

Figure 12.—LEO masses of human round trip missions to 

Mercury.

of 8.7 km/s (Ref. 52). At Earth a capsule enters the atmosphere

to return the crew directly to Earth. The capsule’s mass is

4,350 kg; the round trip time is 585 days with a 40-day stay time 

at Mercury (Ref. 52). In this case, the vehicle does not land on 

Mercury. The LEO masses of both chemical propulsion and 

nuclear thermal propulsion (NTP) vehicles were estimated. 

Figure 12 compares the LEO masses for two types of chemical 

propulsion systems and two NTP systems. The interplanetary 

chemical propulsion systems used tankage dry mass coefficients of 3 and 5 percent of the total propellant mass in the tankage. In many cases, these dry masses may be deemed to be 

optimistically low; however, they allow some relative comparison of the chemical propulsion and the nuclear mission cases.

The NTP vehicles dry mass was 15 percent of the propellant 

mass. In current NTP designs, an Isp of 900 s is nominally used

(Refs. 39 and 56). Somewhat lower Isp values were used for 

these missions: 800 and 850 s, respectively. These lower Isp values were assumed given the high heat flux environment of 

Mercury and the degraded Isp values would reflect the added 

propellant used for propellant cooling and/or refrigeration. The 

chemical propulsion systems required between 17,146 and 

31,228 MT to accomplish the mission. The NTP vehicles 

required approximately an order of magnitude less mass in 

LEO: 1,667 to 2,300 MT. Based on our prior analysis, the stage 

and lander mass was estimated with a mass scaling Equation (19) (Ref. 57).

Mdry,stage (kg) = Mdry,coefficient• Mp (kg) (19)

A Mercury landing vehicle mass was also estimated; the oneway ∆V for the lander was 3.5 km/s (Ref. 58). The ascent ∆V 

was also 3.5 km/s. These ∆V values accommodate approximately 19 percent for gravity losses for each maneuver; this 
NASA/TM—2015-218867 15

gravity loss ∆V is added to the orbital velocity for a 100 km 

orbit which is 2.945 km/s. The lander Isp was 480 s. The higher 

Isp was chosen for the lander as the engine used a higher engine 

expansion ratio that the interplanetary transfer vehicle

(Ref. 57). The smaller engine size would allow a higher expansion ratio, given the typical volume constraints for space vehicles. The dry mass coefficient was 20 percent of the total 

propellant load. While the Mercury missions will likely require 

more aggressive thermal control (propellant shielding, cooling, 

etc.), that thermal control system mass is accommodated in the 

payload mass of the vehicle. The payload delivered to the surface was 10 MT. Figure 13 compares the mass in LEO of a oneway lander and a round trip lander. The masses were 140 MT 

for the round trip lander and 27 MT for the one-way lander. 

Thus, using ISRU on the surface of Mercury to replenish the 

lander’s propellant would allow a savings of 113 MT on this 

mission. Additional analyses are needed to investigate the mass 

reductions for the interplanetary transfer vehicle to carry the 

lander to Mercury. Another option would be to carry five

landers to Mercury rather than carry simply one lander; many 

more permanently shadowed craters could then be visited on 

one mission. The interplanetary vehicle carrying the five

landers could be sent on a lower energy trajectory than the 

human flights, thus saving additional mass launched into LEO 

in the overall Mercury architecture.

Figure 13.—LEO masses of landers for missions to Mercury.

TABLE VI.—SPACE VEHICLE DRY MASS COEFFICIENT 

AND ROCKET ENGINE SPECIFIC IMPULSE (Isp)

Technology Isp,

sec

Mdry,coefficient,

kg/kg M,p

Chemical-1 450 0.03

Chemical-2 450 0.05

Chemical lander 450 0.20

NTP-1 800 0.15

NTP-2 850 0.15

Using Mercury resources to augment the human missions 

was investigated. An ISRU system’s effect on reducing the 

LEO mass (see Table VI for details) was analyzed. For the 

NTP-1 and -2 systems, cases were computed where the 

Mercury departure ∆V propellant was supplied at Mercury. The 

Mercury departure stage is brought from Earth with no propellant. Hydrogen would be produced from the water at the northern polar craters, and transported to orbit. For the NTP-1 case, 

94 MT of H2 would be transported to orbit. With NTP-2, the 

propellant mass required in Mercury orbit is 82 MT. With the 

in-situ H2 production, the LEO mass of the NTP-1 case is reduced from 2300 to 760 MT, as shown in Figure 12. Similar 

large LEO mass reductions are enabled for the NTP-2 system; 

using ISRU, the 1667 MT LEO mass is reduced to 588 MT.

Additional summary data on mission design is summarized

in a review. Figure 14 provides a map of the one-way ∆V and 

trip time for a wide range of planetary targets (Ref. 55). Fast 

missions to Jupiter and Mercury are possible with ∆V values of 

80 to 100 km/s. Nuclear propulsion systems may someday 

allow such ambitious missions and if augmented by ISRU, such 

mission will be within our technological reach.

5.3 Venus

The next planet out from the Sun, Venus, is one of the most 

hostile surface environments in the solar system, with a surface 

temperature of 450 °C at an atmospheric pressure of 92 bars. 

Nevertheless, Venus, as a rocky planet with a thick atmosphere, 

is the planet in the solar system that is in many ways most similar to the Earth, and hence scientific study of Venus is of significant interest. The atmosphere, consisting primarily of CO2

and N2, with clouds of sulfuric acid, represents a resource that 

may be of interest in long-term human settlement.

As with Earth, the temperature in the atmosphere decreases 

due to adiabatic lapse as the altitude increases, and at an altitude 

of about 56 km above the surface, the temperature reaches 

Earthlike values of about 20 °C, at a pressure of about half a 

bar. If human habitats were to be emplaced at Venus, this altitude would be an Earthlike location (Ref. 59). 

Terraforming, the process of altering a planetary environment to make it hospitable to life, has been proposed for both 

Venus and Mars (Ref. 60). Terraforming Venus is a process 

which would require long periods of time and large expenditures of energy, but which may not be beyond the reach of future engineering capabilities (Ref. 61).

5.4 Jupiter and Its Moons

Jupiter, the largest planet in the solar system, is a challenging 

target for exploration. Because of the high gravity and deep 

gravitational well, Jupiter itself is probably beyond the reach of 
NASA/TM—2015-218867 16

Figure 14.—One-way interplanetary mission ∆V versus trip time for various targets (from Ref. 54, used with permission1).

human exploration, but the extensive system of Jovian moons, 

including the four large Galilean moons and a number of 

smaller moons. The innermost moon, Io, has a volcanic, sulfurrich surface; Europa shows an icy surface, while Callisto and 

Ganymede show evidence of regolith on the surface covering 

ice. Europa and Ganymede, and very likely also Callisto, also 

shows evidence of a subsurface ocean. Of these, only Callisto 

lies outside the Jovian radiation belts, and hence is a reasonable 

target for human exploration (Ref. 62). 

Beyond the large Galilean moons, the smaller moons of Jupiter are asteroid-like bodies, which very likely are partly or 

mostly ice in composition. The Trojan asteroids, leading and 

trailing Jupiter’s orbit around the Sun, also represent sources of 

ice. Thus, the moons of Jupiter may well be a source of volatile 

elements, primarily water, which may represent useful 

resources for production of fuel and life-support consumables.

5.5 Saturn and Its Moons1

Saturn is one of the outer planets. Its orbit has a perihelion of 

1,352,600,000 km and an aphelion of 1,514,500,000 km. The 

Cassini spacecraft has conducted an extensive series of flybys 

of the Saturnian moons. During these flybys, cameras and instruments capture and data on the moons’ composition, atmosphere and cloud cover (on the moon Titan), volcanoes, plumes, 

rotation, and gravity.

Titan is the largest moon of Saturn. It is the second largest 

moon in the solar system, the only moon with an atmosphere, and 

the only body in the solar system (other than Earth) that shows 

bodies of liquid on its surface. It is thus one of the most 

scientifically interesting targets for exploration in the solar system. Its intriguing nature includes a N2 and CH4 atmosphere and 

a subsurface ocean, as shown in Figure 15 (Ref. 63). Recent 

 1 This figure was originally presented at the American Astronautical 

Society (AAS) 16th Annual Meeting, Anaheim, CA, and was originally 

published in the AAS publication “Space Shuttles and Interplanetary 

Flight,” ed. L. Larmore and R.L. Gervais, Vol. 28, Advances in Astronautical Sciences, 1970, p. 364 (Copyright © 1970 by American Astronautical Society Publications Office, Web Site http://www.univelt.com)
NASA/TM—2015-218867 17

Figure 15.—Possible present day cross-section of Titan (from Ref. 63, Tobie, G., Lunine, 

J.I., Monteux, J., Mousis, O., and Nimmo, F., “The Origin and Evolution of Titan,” in 

Titan: Interior, Surface, Atmosphere and Space Environment, Müller-Wodarg, I., Griffith, C.A., Lellouch E., and Cravens, T.E., eds., Cambridge Univ. Press, Cambridge, 

U.K., 2012, pp. 24-50.) Used with permission. 

flybys of the Cassini spacecraft have shown direct visual evidence of the northern lakes which are likely composed of CH4. 

Based on measurements and theories of the evolution of Titan, a 

large ocean of water and ammonia may exist below the icy 

surface. Large lakes in the northern Polar Regions have been seen 

on Titan’s surface, and they are likely composed of CH4.

Figure 15 shows the possible nature of Titan’s interior, surface, 

and atmosphere. While CH4 can be used as an effective rocket 

propellant, its N2 could be used in cold gas propulsion or electric 

propulsion (resistojet, arcjet or magneto-plasma-dynamic 

thrusters).

5.6 Atmospheric Mining in the Outer Solar 

System

Atmospheric mining in the outer solar system (AMOSS) can 

be a powerful ISRU tool in extracting fuels from the outer planets and allow fast human and robotic exploration of the solar 

system. Preliminary designs of aerospacecraft with gas core 

rocket nuclear engines for mining the outer planets were developed (Ref. 39 and 56). Helium-3 (3

He), a possible fuel for advanced nuclear fusion reactor cycles based on the D3

He 

reaction, would be extracted from the atmosphere and stored for 

final delivery to orbital assets. 

The outer planets Uranus and Neptune are the most reasonable targets for such atmospheric extraction of 3

He, since their 

atmospheres, although primarily H2, contain significant 

amounts of He, and the gravitational potential is much less steep 

than that of Jupiter or Saturn.

Analyses showed that gas core nuclear rocket (GCNR) 

engines can reduce the mass of such aerospacecraft mining 

vehicles very significantly: from 72 to 80 percent reduction 

over NTP solid core powered aerospacecraft mining vehicles. 

While this mass reduction is important in reducing the mass of 

the overall mining system, the complexity of a fissioning 

plasma gas core rocket is much higher than the more traditional 

solid-core NTP engines. Additional analyses were conducted to

calculate the capture rates of 3He, H2, and 4He during the mining 

process. Very large masses of H2 and 4

He are produced every 

day during the often-lengthy process (multi-day) of 3

He capture 

and gas separation.

Figure 16 shows the mass of H2 needed for the gas core rocket 

and the potentially excess H2 captured every day (Ref. 56). 

Typically, these very large additional fuel masses can dwarf the 

requirements needed for H2 captured for ascent to orbit. Thus, 

the potential for fueling small and large fleets of additional exploration and exploitation vehicles exists. Aerial vehicle
NASA/TM—2015-218867 18

Figure 16.—Helium-3 mining time and H2 capture (mass per 

day) versus atmospheric gas capture rate for Neptune 

AMOSS.

designs can take on many configurations. Additional aerospacecraft or other uninhabited aerial vehicles, or balloons, rockets, 

etc., could fly through the outer planet atmospheres, for global 

weather observations, localized storm or other disturbance investigations, wind speed measurements, polar observations, etc. 

Deep-diving aircraft (built with the strength to withstand many 

atmospheres of pressure) powered by the excess H2 or 4

He may 

be designed to probe the higher density regions of the gas giants.

Based on these past analyses, there will likely be several possible future avenues for effective use the gases of the outer planets for exciting and scientifically important atmospheric 

exploration missions. The analyses focused on Uranus and 

Neptune, as these planets offer vast reservoirs of fuels that are 

more readily accessible than those from Jupiter and Saturn (as 

these latter planets require lower energies needed to attain orbit 

and present less danger from powerful atmospheric lightning) 

and, with the advent of nuclear fusion propulsion, may offer us 

the best option for fast interplanetary travel and the first practical interstellar flight.

6.0 Conclusions

As amply demonstrated in this retrospective survey, “living 

off the land” imposes constraints for utilization of power, propellants, and other expendables. The normal rigors (mass, space 

limitations and environmental challenges) imposed by space 

travel are magnified by a resource-limited situation. On the 

other hand, enhanced energy efficiency and minimal launch 

mass will simplify missions (and increase successful outcomes) 

by resultant limitations to planned activities. Thus the technical 

hurdles and challenges alluded to above will stimulate development of technology solutions for space exploration that can be 

spun off to solve terrestrial problems for defense, and dual-use

applications such as and commercial transportation, power 

generation, and efficient resource utilization. Another important aspect of the GRC effort that should certainly not be 

overlooked is the variety of individual collaborations fostered, 

organizational partnerships formed, and mentored students educated and into the aerospace community as a result of the technical work.

In-flight utilization of waste and trash to produce essential materials such as water, fuel(s), and O2 is an anthropogenic-waste 

application of an ISRU approach for sustainability. For prudent 

and efficient utilization of terrestrial raw materials and energy for 

transportation fuel production, re-use or recovery of hydrocarbons and waste is becoming increasingly critical in overall mission designs. In an effort to provide fresh water through 

desalination in a clean, economical, and environmentally friendly 

way, a solar powered water desalination facility located in Yuma, 

Arizona, was considered. Such a facility powered by solar concentrators would be able to fund itself during operation, assuming 

public financing covered costs for the plant; unless the capital 

equipment costs of solar concentrators decrease, the facility 

would not be able to recoup the initial startup cost. Nevertheless, 

the system would still save money in the long run.

Production of O2 from local materials can fuel a transportation system that supports the lunar base, as well as serving as a 

possible product for use in propulsion systems either commercially or to support exploration of other locations. Replacement 

O2 is also a necessary product for life support, since even 

closed-ecological life support systems will not have perfect efficiency of recycling. Because the Moon’s axis is angled 1.5°

from the ecliptic, the South Pole is almost constantly bathed in 

sunlight; during lunar summer, the entire South Pole is visible, 

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. Such areas would be an ideal place for a 

constantly operating solar concentrator that could generate approximately twice the amount of O2 than an ISRU system anywhere else on the Moon.

Fast missions to Jupiter and Mercury are possible with ∆V 

values of 80 to 100 km/s. Nuclear propulsion systems may 

someday allow such ambitious missions and if augmented by 

ISRU, such mission will be within our technological reach. 

With its proximity to the Sun, Mercury has extremely high temperatures and requires special high heat flux considerations for 

long-term human visits or bases. There will likely be several 

possible future avenues for effective use the gases of the outer 

planets for exciting and scientifically important atmospheric exploration missions. The analyses focused on Uranus and Neptune, as these planets offer vast reservoirs of fuels that are 

relatively readily accessible; with the advent of nuclear fusion 

propulsion, these most distant gas giants may offer us the best 

option for fast interplanetary travel and the first practical interstellar flight.
NASA/TM—2015-218867 19

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