The NASA approach for technology development for missions is to (1) wait for a mission need, and (2) upgrade the technology available at that time, however inadequate.
This is illustrated with two important NASA technologies: in situ resource utilization (ISRU) and recycling wastewater. It also serves as a review with 49 references provided.
NASA funding for ISRU has been sporadic and minimal, probably because no mission was being implemented that used ISRU. The state of the technology remains underdeveloped. For example, CO2 in the Mars atmosphere supplies carbon and oxygen. However, we still do not have a viable system to acquire CO2 and compress it with acceptable power requirements and adequate lifetime.
NASA technology for recycling wastewater was developed for the International Space Station. It requires frequent attention with replenishment and replacement of subsystems. This system appears to be inadequate for Mars missions and there is no evidence that NASA has a viable plan to fix that.
This is an editorial perspective on two important NASA technologies: the use of in situ extraterrestrial resources for the production of propellants (ISPP), and recycling water as a major part of life support for long missions. It is also a commentary on NASA's approach to technology, which is to develop technology within missions, rather than independently to enable missions.
NASA decides on its major initiatives, and NASA employees provide loyal support, while corporations, big and small, seek to contribute to the enterprise while getting "a piece of the action". Follow the money. The current NASA plan to return human crews to the Moon has been greeted with widespread enthusiasm, particularly regarding ISPP. Hardly anyone seems to be concerned with recycling water. Aside from the specifics of these two technologies, there doesn't seem to be a mechanism for judging whether the whole lunar enterprise makes sense. NASA system engineers appear to view their role as finding support for whatever NASA management decides. The new lunar initiative provides funds, and neither NASA system engineers nor businesses can afford to take issue with the oncoming avalanche.
Recycling of water in space is a very challenging technology. The present state of the technology is a patchwork quilt that gets by in LEO with frequent servicing and replacement but does not suffice for long missions to Mars. NASA doesn't appear to have either the budget or the will to develop reliable water cycling technology. There does not seem to be a plan, a roadmap, or a vision of a new, reliable water cycling system. Instead, NASA has taken a defensive position for a system that repeatedly requires servicing, repair, and replacement, and NASA suggests that might be adaptable to long-term missions.
Who then can provide an independent review and assessment of these ventures if NASA system engineers view their role as justifying management decisions? The people best equipped to provide an independent view of the big picture of these enterprises are those who are not stakeholders, nor dependent on NASA for support, yet possess lengthy experience in relevant technologies and systems. Retirees with long experience, who are not beholden to the establishment for support might fit this need.
The concept of using indigenous resources on Mars to produce propellants for ascent from Mars originated in an important paper by Ash, Dowler, and Varsi [11Ash RL, Dowler WL, Varsi G. Feasibility of rocket propellant production on Mars. Acta Astronaut. 1978;5:705-724.]. The widely used term “ISPP” (In Situ Propellant Production) originated in that paper. This was later generalized to “In situ Resource Utilization” (ISRU) as engineers devised concepts for using resources for other purposes than propellants [22Sanders G, Kleinhenz J. In situ resource utilization (ISRU) strategy—scope, plans, and priorities. In: Proceedings of the NASA Advisory Council (NAC) Technology Innovation and Engineering Committee; 2023; Washington, DC, USA.]. Jerry Sanders and Diane Linne have been and remain leading NASA advocates for ISRU, and they continue to make presentations in favor of ISRU on the Moon and Mars [22Sanders G, Kleinhenz J. In situ resource utilization (ISRU) strategy—scope, plans, and priorities. In: Proceedings of the NASA Advisory Council (NAC) Technology Innovation and Engineering Committee; 2023; Washington, DC, USA.,33Sanders G, Kleinhenz J, Linne D. NASA plans for in situ resource utilization (ISRU) development, demonstration, and implementation. Presentation to COSPAR; 2022. Available from: https://ntrs.nasa.gov/api/citations/20220008799/downloads/NASA%20ISRU%20Plans_Sanders_COSPAR-Final.pdf]. I was also an advocate for ISRU in my books [44Rapp D. Human missions to Mars. 3rd ed. Heidelberg, Germany: Springer-Praxis Books; 2023.,55Rapp D. Use of extraterrestrial resources for human space missions to Moon or Mars. 2nd ed. Heidelberg, Germany: Springer-Praxis Books; 2018.]. Despite the change in reference from ISPP to ISRU, the reality is that propellant production offers the most practical near-term advance in ISRU with potential near-term benefits, and most of the technical accomplishments in ISRU since the original paper by Ash, Dowler, and Varsi were aimed at propellant production on Mars and Moon.
Starting in the 1990s, advocates for ISRU within the NASA community were able to obtain funds to support occasional small technology tasks in ISRU. For example, previously as the manager of a Mars Technology Program administered by JPL, funds were provided for an experimental study of a Mars ISRU system utilizing the Sabatier process – reported in 1994 [66Zubrin R, Price S, Mason L, Clark L. Report on the construction and operation of a Mars in-situ propellant production unit. AIAA-94-2844. Available from: https://marspapers.org/paper/Zubrin_1994.pdf] and 1997 [77Clark DL. In-situ propellant production on Mars: a Sabatier/electrolysis demonstration plant. In: Proceedings of the 33rd Joint Propulsion Conference and Exhibit; 1997 Jul 6–9; Seattle, WA, USA. AIAA 97-2764.]. Sanders and Linne also were able to find support for several ISRU technology tasks via SBIRs and other sources of NASA funding. For example, Clark and Payne [88Clark DL, Payne K. CO2 collection and purification system for Mars. AIAA 2001-4660. Las Vegas, NV, USA; 2001.] were funded through advocacy by Sanders [88Clark DL, Payne K. CO2 collection and purification system for Mars. AIAA 2001-4660. Las Vegas, NV, USA; 2001.].
ISRU technology continued to advance in laboratories in the early 21st century. For example, see Zubrin, Muscatello, and Berggren [99Zubrin RM, Muscatello AC, Berggren M. Integrated Mars in situ propellant production system. J Aerosp Eng. 2013;26:43–56.].
In the Griffin era (2005-2007) when NASA contemplated a major campaign to return humans to the Moon, the ISRU focus shifted to lunar concepts. Many lunar ISPP concepts were put forth [1010Abbud-Madrid A. Space Resources Roundtable. Available from: https://www.lpi.usra.edu/publications/absearch/?keywords_all=roundtable+vii&num=100]. After the NASA lunar initiative was cancelled by then-President Obama, the state of NASA ISRU technology returned to sporadic, intermittent funding of small technology tasks at low Technology Readiness Level (TRL).
The rise and fall of MOXIE
Scientists and engineers are people. People are individuals. Events transpire. Opportunities spring up. Actual costs are always higher than allocated, particularly for unprecedented programs. Pressures build up. Individuals respond in their own ways, sometimes irrationally. The real world is run by people and sometimes luck plays a role.
In August 2013, Dr. Subbaro Surampudi of JPL found out somehow that an Announcement of Opportunity (AO) was soon to be released by NASA calling for a validation demonstration of ISRU on Mars. Rao and I began informal planning and campaigning for writing a proposal – along with a few others at JPL. It was difficult to convince a reluctant JPL managementto proceed. One prominent Assistant Laboratory Director declared "JPL is not in the ISRU business".
On September 24, 2013, seemingly out of nowhere, the anticipated AO was released by NASA calling for a demonstration of ISRU on Mars with an allocated budget of $30 million. Our JPL team continued to advocate to JPL management to propose, but JPL hesitated. Finally, JPL assented and a team from JPL, MIT and a small company (OxEon Energy LLC) worked on the proposal for the "MOXIE" project. Michael Hecht (of MIT) was chosen as Principal Investigator. However, JPL could not confirm a cost estimate nor obtain high-level signatures for approval in time for the December 2013 deadline date for proposal submission. Then, by good fortune, a government shutdown took place and the due date for proposals was postponed several weeks into January 2014. That provided JPL with enough time to submit the MOXIE proposal on the last day, and as it turned out, the MOXIE team won the competition.
It is important to emphasize that this AO was fundamentally not a call for technology development, but rather a technology demonstration. Never mind that the technology was not already developed. The AO was not signed by its creator. It was just issued by NASA. Its origin and history will probably never be told.
It was not a call for a lab demonstration, but an in situ demonstration on Mars. Why demonstrate on Mars? Did we not know enough about the composition of the Mars atmosphere? Was there some fear that some trace contaminant might interfere? That seems very farfetched.
It seems most probable that the AO called for demonstration on Mars because (1) NASA prefers demonstrations rather than development, and (2) NASA prefers flight projects over lab projects.
Since the technology for MOXIE had to be developed as part of the implementation, the first few years were mainly devoted to developing the electrolysis stack, the compressor, and the dust filter. Adapting these technologies to flight required a very great amount of engineering. The final runout cost overran to about $55 million, and I roughly estimate that perhaps $43 million could be attributed to engineering. Had the same system never been tested on Mars and only been tested in the lab, I estimate the total cost would have been $12 million. Since a duplicate system was run in the lab prior to every run carried out on Mars, the data from Mars could be viewed as superfluous.
The point here is that NASA would never have provided double-digit funding for a lab ISRU project. NASA is willing to develop technology within a flight project but not so much in preparation for flight projects.
The JPL implementation team developed the MOXIE hardware and software and OxEon Energy LLC provided the electrolysis stacks. In the early stages of development, several crises arose regarding requirements, costs, and design. It soon became apparent that the original cost estimate was far too low and the performance requirements from the AO were too stringent. Balancing budget against effort in an unprecedented technology is difficult. The greatest engineering challenge was cramming all the subsystems into the allocated one cubic foot container. In this atmosphere of continual stress, the manager of the implementation team was not receptive to inputs from the science team. As a result, the final design, though workable, lacked several capabilities of importance to the science team. For example, there were no leads from the ends of the electrolysis stack, so we had no direct measurement of the voltage across the stack. During the course of operations on Mars, we spent what seemed to be endless efforts to estimate that critical quantity, and cope with other limitations of the hardware.
Why mention all this? It is because workers in space systems must understand that mission cost is usually impossible to estimate accurately, that all cost estimates are low, and have to be low to win a contract, that implementation teams and science teams have different priorities, and good interchange between them enhances the end product, that managers, engineers, and scientists are people, and people react to stress in various ways, and dedicated long hours of hard work produce a good end result, even if some of the players are irrational sometimes along the way.
Nevertheless, the system built by the implementation team and operated by the science team culminated in a successful demonstration of a small proof-of-concept system on Mars over a two-year period in 2022-2023 [1111Hoffman JA, Hecht MH, Rapp D, Hartvigsen JJ, SooHoo JG, Aboobaker AM, et al. Mars Oxygen ISRU Experiment (MOXIE)-Preparing for human Mars exploration. Sci Adv. 2022 Sep 2;8(35):eabp8636. doi: 10.1126/sciadv.abp8636. Epub 2022 Aug 31. PMID: 36044563; PMCID: PMC9432831.-1313Rapp D, Hoffman J, Meyen F, Hecht M. The Mars oxygen ISRU experiment (MOXIE) on the Mars 2020 Rover. Paper presented at: AIAA Space2015 Conference and Exhibition; 2015 Aug 31; Pasadena, CA.] The system employed solid-oxide electrolysis of CO2 from the Martian atmosphere to produce pure oxygen. I roughly estimate the final cost of the project to be about $55 million, which dwarfs the total of NASA expenditures on all other ISRU projects over three decades.
In the process of implementing MOXIE, a world-class laboratory for solid oxide electrolysis was set up to support Mars operations and add lab data that could not be obtained on Mars. Solid oxide electrolysis is a rapidly growing field in terrestrial climate control and NASA could potentially leverage much larger investments in this technology by the Department of Energy (DOE) and other agencies than NASA could afford itself. However, NASA has shown no interest in funding research in that lab. Furthermore, NASA seems to have forgotten about MOXIE in its planning documents. MOXIE is rarely (if ever) mentioned in publications and announcements. It seems to have slipped into oblivion at NASA. A recent article entitled " In Situ Resource Utilization (ISRU) Strategy—Scope, Plans, and Priorities" fails to even mention MOXIE [22Sanders G, Kleinhenz J. In situ resource utilization (ISRU) strategy—scope, plans, and priorities. In: Proceedings of the NASA Advisory Council (NAC) Technology Innovation and Engineering Committee; 2023; Washington, DC, USA.]!
Mars atmospheric CO2 as a feedstock
The Mars atmosphere provides an essentially unlimited source of carbon and oxygen.
Most ISPP schemes that have been proposed or studied require a large inlet flow rate of CO2 from the Mars atmosphere. For example, Rapp and Hinterman [1414Rapp D, Hinterman E. Adapting a Mars ISRU system to the changing Mars environment. Space: Science and Technology. 2023;3: 0041. DOI: 10.34133/space.0041.] analyzed a full-scale version of MOXIE technology to produce 3 kg/h of oxygen, and this required 14 kg/h input of Martian CO2. [1414Rapp D, Hinterman E. Adapting a Mars ISRU system to the changing Mars environment. Space: Science and Technology. 2023;3: 0041. DOI: 10.34133/space.0041.] At Mars pressure, this amounts to a huge volumetric flow rate (900 cubic meters per hour). If you read almost any article on Mars ISRU, you find one box in the system diagram that shows "acquisition and compression of Martian CO2". One might think a priori that the first thing to attack Mars ISRU is obtaining a suitable flow of compressed CO2. The problem is that 46 years after the Ash, Dowler, and Varsi paper, NASA lacks any practical means of providing such flows of CO2 into reactors.
Three approaches for providing compressed CO2 have been proposed: mechanical compressor, cryogenic freezing, and sorption cycling.
Considering the problems inherent in cryogenic freezing and sorption cycling, I guess that a mechanical compressor is most likely to turn out to be the most practical approach after further study and development. However, the only experience we have is the mechanical compressor used on MOXIE [1111Hoffman JA, Hecht MH, Rapp D, Hartvigsen JJ, SooHoo JG, Aboobaker AM, et al. Mars Oxygen ISRU Experiment (MOXIE)-Preparing for human Mars exploration. Sci Adv. 2022 Sep 2;8(35):eabp8636. doi: 10.1126/sciadv.abp8636. Epub 2022 Aug 31. PMID: 36044563; PMCID: PMC9432831.-1313Rapp D, Hoffman J, Meyen F, Hecht M. The Mars oxygen ISRU experiment (MOXIE) on the Mars 2020 Rover. Paper presented at: AIAA Space2015 Conference and Exhibition; 2015 Aug 31; Pasadena, CA.]. This compressor was designed for a lifetime of at least 100 hours, and it required considerably more power than the basic thermodynamic requirement, due to several design aspects that dissipated additional power. Hinterman (in discussions with the manufacturer) estimated that more advanced designs could greatly reduce the power requirement, but that remains to be demonstrated, and the lifetime remains uncertain, especially compared to the requirement for 10,000 hours of continuous operation [1414Rapp D, Hinterman E. Adapting a Mars ISRU system to the changing Mars environment. Space: Science and Technology. 2023;3: 0041. DOI: 10.34133/space.0041.].
The cryogenic approach of alternately freezing and sublimating CO2 has been proposed and was briefly studied in the lab [1515Clark DL, Payne KS, Trevathan JR. Carbon dioxide collection and purification system for Mars. AIAA 2001-4660. AIAA Space 2001 Conference and Exposition; 2001; Albuquerque, NM.-1717Shah M. CO2 freezer testing. Available from: https://tfaws.nasa.gov/wp-content/uploads/3_ISRU-CO2-Freezer-for-TFAWS-2018.pdf]. A scaled-up version was tested briefly [1818Meier AJ, Grashik MD, Shah MG, Sass J, Bayliss J, Hintze P. et al. Full-scale CO2 freezer project developments for Mars atmospheric acquisition. In: AIAA Space Forum; 2018 Sep 17–19; Orlando, FL.]. It is remarkable that power requirements were hardly considered, if at all, in these references, although Meier, et al. [1818Meier AJ, Grashik MD, Shah MG, Sass J, Bayliss J, Hintze P. et al. Full-scale CO2 freezer project developments for Mars atmospheric acquisition. In: AIAA Space Forum; 2018 Sep 17–19; Orlando, FL.] did mention the cooler energy requirement was roughly 5 kJ per g of CO2 frozen.
We can do a short "gedanken experiment" regarding power for this system. Aside from the various other factors that affect power requirements for accumulating pressurized CO2, there is a basic energy requirement to freeze CO2 and sublimate CO2. For each 100 g of CO2 that is frozen or sublimated, the process requires 59.3 kJ, so one round trip of freezing and sublimation will require first removing 59.3 kJ and then adding 59.3 kJ. The cryocooler that removes CO2 for freezing will require very roughly about ten times that amount of energy as power input, for a power requirement of about 590 kJ. Therefore, to carry 100 g of CO2 through the freeze/ sublimation cycle, the minimum energy requirement is about 650 kJ. Assuming each step required about 1.5 hours, as indicated in lab studies, the power for freezing 100 g of CO2 would be:
(590) (1,000)/{(1.5)(3,600)} = 109 W
and the power to sublimate 100 g of CO2 would be
(59) (1,000)/{(1.5)(3,600)} = 11 W
The net flow rate of CO2 is 100/3 = 33.3 g/h.
Rapp and Hinterman (2023) showed that a full-scale solid-oxide electrolysis ISPP system to produce 3 kg/h of O2 would require an input of about 14 kg/h of CO2, about 420 times as large as the hypothetical 33.3 g/h system we reviewed [1414Rapp D, Hinterman E. Adapting a Mars ISRU system to the changing Mars environment. Space: Science and Technology. 2023;3: 0041. DOI: 10.34133/space.0041.]. The minimum power requirement (neglecting losses) for the freezing cycle would be 420 x 109 = 45,800 W. The CO2 freezing approach requires an impractical power requirement. This approach has been advocated by some at NASA who never bothered to make this simple calculation.
In addition to the power requirement, the sheer complexity of the system as shown in Figure 1 is challenging, especially compared to a mechanical compressor which would be just one rectangle on a flow diagram.
1818Meier AJ, Grashik MD, Shah MG, Sass J, Bayliss J, Hintze P. et al. Full-scale CO2 freezer project developments for Mars atmospheric acquisition. In: AIAA Space Forum; 2018 Sep 17–19; Orlando, FL.]" /> Figure 1: System diagram for testing cryogenic compression of CO2. [1818Meier AJ, Grashik MD, Shah MG, Sass J, Bayliss J, Hintze P. et al. Full-scale CO2 freezer project developments for Mars atmospheric acquisition. In: AIAA Space Forum; 2018 Sep 17–19; Orlando, FL.]
Another approach is a sorption compressor. A sorption compressor contains virtually no moving parts and achieves its compression by alternately cooling and heating a sorbent material that absorbs low-pressure gas at low temperatures and drives off high-pressure gas at higher temperatures. By exposing the sorption compressor to the cold night environment of Mars (roughly 6 Torr and 200 K at moderate latitudes), CO2 is preferentially adsorbed from the Martian atmosphere by the sorbent material while a good part of the 4.5% of other gases in the atmosphere is vented. During the day, when solar electrical power is available, the adsorbent is heated in a closed volume, thereby releasing relatively pure CO2 at significantly higher pressures for use in a CO2 conversion reactor.
There are several significant challenges in such a sorption compressor, particularly delays in executing the heating and cooling cycles, and the large amount of sorbent mass involved. If the requirement is 15 kg of CO2 per hour, and the round trip storage of CO2 on sorbent yields about 12% CO2, then the amount of sorbent required is (15)(t)/(0.12) where "t" is the duration of the warm/cold cycle (in hours). If it were say, 12 hours, then 1,500 kg of sorbent would be required. The physical arrangement of such a system would be challenging.
Brooks, et al. [1919Brooks KP, Rassat SD, TeGrotenhuis WE. Development of a microchannel ISPP system. PNNL Report, PNNL-15456; 2005.] developed an approach using microchannel structures to support thin layers of sorbent [1919Brooks KP, Rassat SD, TeGrotenhuis WE. Development of a microchannel ISPP system. PNNL Report, PNNL-15456; 2005.]. This significantly reduced the cycle time for heating and cooling. However, the capacity of a single cell is low and this necessitates a complex array of a very large number of interconnected cells to produce a significant flow rate of compressed CO2. Merrell, et al. [2020Merrell RC. Microchannel ISPP as an enabling technology for Mars architecture concepts. AIAA 2007-6055; 2007.] advanced this approach [2020Merrell RC. Microchannel ISPP as an enabling technology for Mars architecture concepts. AIAA 2007-6055; 2007.]. Linne, et al. [2121Linne DL, Gaier JR, Zoeckler JG, Kolacz JS. Demonstration of critical systems for propellant production on Mars for science and exploration missions. In: AIAA 2013-0587; 2013.] reported further details on this approach [2121Linne DL, Gaier JR, Zoeckler JG, Kolacz JS. Demonstration of critical systems for propellant production on Mars for science and exploration missions. In: AIAA 2013-0587; 2013.]. However, they said:
"Although the device was fabricated and preliminarily tested ..., the project was prematurely shut down because of changing priorities within NASA programs and inadequate data was obtained in order to fully understand the operational characteristics of the system, especially under Mars atmospheric pressure conditions" [2121Linne DL, Gaier JR, Zoeckler JG, Kolacz JS. Demonstration of critical systems for propellant production on Mars for science and exploration missions. In: AIAA 2013-0587; 2013.].
None of these studies appear to have actually tested such a device within a chamber containing simulated Mars atmosphere during diurnal cycles. The technology might work, but the challenges include heavy mass, high power requirements, and inherent complexity leading to uncertain reliability over thousands of cycles. It is difficult at this stage to evaluate the merits of this approach. "Changing priorities within NASA programs" might mean giving up on Mars ISRU and focusing on the Moon.
ISPP on the moon
Starting around 2018, NASA decided to embark on a major new program to return humans to the Moon. Like weathervanes in the prevailing wind, almost all ISRU research became refocused on lunar ISPP, and NASA lost interest in Mars ISRU, as evidenced by the fact that typical NASA presentations on ISRU in 2023-2024 either barely mention MOXIE, or more likely omit it altogether – even though MOXIE was by far, the most significant (and most costly) ISRU project ever implemented [33Sanders G, Kleinhenz J, Linne D. NASA plans for in situ resource utilization (ISRU) development, demonstration, and implementation. Presentation to COSPAR; 2022. Available from: https://ntrs.nasa.gov/api/citations/20220008799/downloads/NASA%20ISRU%20Plans_Sanders_COSPAR-Final.pdf].
In the years from 2019 to 2024, NASA system engineers and affiliated personnel in space industries published several proposals, plans, and analyses of approaches for lunar ISRU. The most likely possibility is to locate the system at a crater near the lunar south pole to retrieve putative water ice stored in permanently shadowed regions of the crater. Power within the crater might be supplied by beaming from a solar mirror located on a ridge of the crater [2222Elliott J, Austin A, Colaprete T. ISRU in support of an architecture for a self-sustained lunar base. Paper presented at: 2019 70th International Astronautical Congress (IAC); 2019 Oct 21–25; Washington, DC, USA.,2323Kleinhenz JE, Paz A. Case studies for lunar ISRU systems utilizing polar water. ASCEND; 2020 Nov 16–19. Available from: https://ntrs.nasa.gov/api/citations/20205007966/downloads/PolarWaterISRUstudy_Kleinhen]. Rapp (2024) analyzed these plans and concluded they were very complex and difficult to implement, involved significant risk, and had no return on investment compared to bringing resources from Earth [2424Rapp D. Near term NASA Mars and lunar in situ propellant production (ISPP): complexity vs. simplicity. Space Sci Technol. 2024.,2525Rapp D. The value of utilization of extraterrestrial resources for propellant production for space exploration—a perspective. Acad J Engrg Studies. 2024;3(4).]. As he put it: "NASA produces propellants so it can go to the Moon. NASA goes to the Moon to produce propellants".
More generally, the case for (or against) implementing ISRU on Mars (and by inference the Moon as well) was reviewed by Rapp (2024) in the context of the greatly reduced launch costs now being introduced by Space X. He concluded that use of ISRU is more expensive and riskier but launching repeated large loads introduces logistic challenges even if it is affordable [2424Rapp D. Near term NASA Mars and lunar in situ propellant production (ISPP): complexity vs. simplicity. Space Sci Technol. 2024.,2525Rapp D. The value of utilization of extraterrestrial resources for propellant production for space exploration—a perspective. Acad J Engrg Studies. 2024;3(4).].
None of the above is ironclad. Rapp's negative conclusion should give one pause, but the problem is that the relative cost and risk of mission alternatives are difficult to estimate at this early stage, and these are the parameters that are critical in evaluating the merits of alternative mission designs. There are too many uncertainties.
What seems to be missing most of all regarding proposals for ISPP processes, is a full end-to-end description, analysis, and cost estimate of all the many steps involved in developing any mission option through stages of development, prospecting, validating in situ, implementing, installing and initiating, and operating autonomously for any specific ISPP process on the Moon or Mars. This would be compared to a baseline of bringing propellants from Earth. In all the literature on ISPP or ISRU, there seems never to be a comparison to bringing propellants from Earth.
Every ISPP process is power-hungry because ISPP inevitably involves breaking strong chemical bonds (either O-H in H2O or C-O in CO2). A full-scale extrapolation of MOXIE technology that produces 3 kg of oxygen per hour for fourteen months (30 tons total) would require about 25 kW of continuous electrical power [1414Rapp D, Hinterman E. Adapting a Mars ISRU system to the changing Mars environment. Space: Science and Technology. 2023;3: 0041. DOI: 10.34133/space.0041.]. But MOXIE is the simplest ISPP process. Processes that involve digging and transporting regolith, obtaining water, purifying water, electrolyzing water, and producing both CH4 and O2 are far more complex and would require additional power. Published plans for lunar ISRU involve a gradual increase in power levels to hundreds of MW, although such high levels of power on the Moon appear to be science fiction rather than science [2626Thomas G, Granger M, Csank J, Gardner B. Establishing a lunar surface power grid. In: 2022 Conference on Advanced Power Systems for Deep Space Exploration (APS4DS); 2022 Aug 30.].
ISRU processes that require prospecting to locate the best deposits of water-bearing regolith on the Moon or Mars will entail a series of gradually more specifically local observations beginning remotely, then in situ over distance, and ending in in situ at precise locations, to validate autonomous mining of water-bearing regolith, derive water from the regolith, determine water purity, and transport raw and spent regolith between sites to reactors, hundreds or thousands of times. On Mars, where favorable launch conditions recur every 26 months, it will take about four sequences of missions (possibly more), spread 26 months apart, to carry out prospecting. The cost seems likely to be several billion dollars. Furthermore, the requirements for landing site location might not coincide with the best location for ISRU. On the Moon, prospecting will require operating in the permanently shadowed regions of south polar craters with power supply a major challenge. The cost there is also likely to exceed several billion dollars.
The most attractive ISPP process is producing oxygen from the CO2 in the Martian atmosphere, using MOXIE electrolysis technology [1414Rapp D, Hinterman E. Adapting a Mars ISRU system to the changing Mars environment. Space: Science and Technology. 2023;3: 0041. DOI: 10.34133/space.0041.]. This ISPP system can be placed almost anywhere on the Mars surface, and if a power system is connected, the ISPP system can be turned on and allowed to run continuously for months, with no mining and moving regolith, and essentially no moving parts except the likely mechanical compressor to draw in Mars atmosphere. No prospecting is involved, and power levels are probably the lowest for any ISPP system. However, NASA seems more interested in utilizing Martian water, which requires prospecting, mining, and transporting regolith [2727Kleinhenz J, Collins J, Barmatz M, Voecks G, Hoffman S. ISRU technology development for extraction of water from the Mars surface. NASA presentation; 2022. Available from: https://ntrs.nasa.gov/api/citations/20180005542/downloads/20180005542.pdf].
It is noteworthy that Kleinhenz, et al. [2727Kleinhenz J, Collins J, Barmatz M, Voecks G, Hoffman S. ISRU technology development for extraction of water from the Mars surface. NASA presentation; 2022. Available from: https://ntrs.nasa.gov/api/citations/20180005542/downloads/20180005542.pdf], like many other such publications regarding ISPP and ISRU, present a detailed roadmap, year by year of a campaign that never was going to happen, and likely never will happen [2727Kleinhenz J, Collins J, Barmatz M, Voecks G, Hoffman S. ISRU technology development for extraction of water from the Mars surface. NASA presentation; 2022. Available from: https://ntrs.nasa.gov/api/citations/20180005542/downloads/20180005542.pdf]. Indeed, NASA has employed middle managers to plan ISRU roadmaps over the last two decades, and NASA mostly ignored them. I plead guilty to having been a participant in the early exercises [2828Sanders G, Duke M. In-situ resource utilization (ISRU) capability roadmap progress review; 2005. Available from: https://ntrs.nasa.gov/api/citations/20050205045/downloads/20050205045.pdf,2929Sanders G. Results from the NASA capability roadmap team for in-situ resource utilization (ISRU); 2005. Available from: http://www.marsjournal.org/contents/2006/0005/files/SandersDuke2005.pdf].
In the year 2000, G. Sanders published an update on ISRU for NASA in which he made a very reasonable set of recommendations, including establishing a strategic plan and a NASA office of ISRU, which NASA did not do. NASA did "encourage other government agencies to support the program with technologies and funding while encouraging industry and other organizations to invest their resources into this area" with their SBIR program. After the year 2000, Sanders continued to point out the virtues of ISRU to NASA every year, however, with limited response from NASA.
Most recently, NASA has been examining a short-stay Mars mission that does not utilize ISRU [3030Rucker M. NASA’s Strategic Analysis Cycle 2021 (SAC21) Human Mars Architecture. NASA Report; 2021. Available from: https://ntrs.nasa.gov/citations/20210026448,3131Rucker M. NASA’s Strategic Analysis Cycle 2021 (SAC21) Human Mars Architecture. NASA ESDMD Mars Architecture Team; 2022 Mar 7; IEEE Aerospace Conference; Big Sky, MT.]. The return on investment is minimal. This ill-advised and ill-considered mission concept has many faults, as pointed out by Rapp [44Rapp D. Human missions to Mars. 3rd ed. Heidelberg, Germany: Springer-Praxis Books; 2023.]. One example is:
"The mission includes 4 crewmembers, arranged as two pairs. One pair would land on the Mars surface, while the second pair would remain in a 5-sol Mars orbit. It is not clear that the crew in orbit serves any purpose or performs any useful duties? To return, the surface pair would ascend and rendezvous with the pair in orbit, and the foursome would return to Earth in the ERV. The only impact on the mission from adding two crewmembers to remain in orbit seems to be to double the requirements for the transfers to and from Mars, and to expose the crewmembers in orbit to incredible boredom" [44Rapp D. Human missions to Mars. 3rd ed. Heidelberg, Germany: Springer-Praxis Books; 2023.].
The ISPP concept was introduced in 1978. Starting in the 1990s, quite a number of small technology development tasks at low technology readiness numbers (TRL) were funded as initial efforts to probe possibilities for ISPP, and later ISRU. The SBIR Program was very beneficial in this regard. Most of this work was devoted to proof of concept of processes. Lunar ISRU made some progress in 2005-2007 in the Griffin era but this died out when the lunar initiative was cancelled. The MOXIE Project was an anomalous one-and-done large effort from 2014 to 2023 that demonstrated ISPP on Mars. The NASA mission study DRA-5 claimed significant benefits to ISRU in Mars missions [3232Drake BG. Mars design reference architecture 5.0 study - executive summary. Available from: https://www.nasa.gov/wp-content/uploads/2015/09/373669main_2008-12-04_mars_dra5_executive_summary-presentation.pdf?emrc=db5841].
However, NASA's response to activity in ISRU technology was minimal. Low-level funding was provided intermittently and sporadically, and NASA seems to have forgotten the accomplishments of the MOXIE project. Sanders's year 2000 plea for an office dedicated to ISRU was never fulfilled. Until about 2023, ISRU was on NASA's back burner. NASA appeared to regard MOXIE as an interesting concept worthy of keeping an eye on.
In 2020, NASA announced its new initiative for sustained lunar exploration and development [3333NASA’s plan for sustained lunar exploration and development. NASA web publication; 2020. Available from: https://www.nasa.gov/wpcontent/uploads/2020/08/a_sustained_lunar_presence_nspc_report4220final.pdf?emrc=5aa8ef]. However, it is noteworthy that NASA listed six "zero-level" goals, and ISRU was not mentioned.
An important part of the plan was the establishment of a "Gateway" to "offer astronauts easier crew returns, a safe haven in the event of an emergency, the ability to navigate to different orbits around the Moon and later, an advancement in human life support systems". However, Zubrin [3434Zubrin R. Lunar Gateway or Moon Direct? Space News; 2019. Available from: https://spacenews.com/op-ed-lunar-gateway-or-moondirect/] claimed that the Gateway was not only unnecessary but was counterproductive because it added significant costs and long delays in implementation. With new much lower-cost launch vehicles, direct transfer from LEO to the lunar surface is not only possible but preferable [3434Zubrin R. Lunar Gateway or Moon Direct? Space News; 2019. Available from: https://spacenews.com/op-ed-lunar-gateway-or-moondirect/].
The NASA announcement referred to lunar ISRU obliquely:
"Other technologies are in early development for significant long-term benefits. For example, ISRU will enable the production of fuel, water, and/or oxygen from local materials, enabling sustainable surface operations with decreasing supply needs from Earth" [3535Using space-based resources for deep space exploration. NASA announcement; 2023. Available from: https://www.nasa.gov/overview-in-situ-resource-utilization/].
Later, lunar ISRU was included as a major part of the current lunar initiative to return humans to the Moon – at least in press releases. For example, the NASA announcement said:
"NASA’s Lunar Surface Innovation Initiative will develop and demonstrate technologies to use the Moon’s resources to produce water, fuel, and other supplies as well as capabilities to excavate and construct structures on the Moon" [3535Using space-based resources for deep space exploration. NASA announcement; 2023. Available from: https://www.nasa.gov/overview-in-situ-resource-utilization/].
Of greater significance was a NASA industry forum to introduce a forthcoming NASA Request for Information from industrial partners regarding ISRU technology [3636NASA invites stakeholders to STMD’s LIFT-1 industry forum. NASA announcement; 2023. Available from: https://www.nasa.gov/general/stmd-lift-1-industry-day/].
In addition to official NASA announcements regarding lunar ISRU, several mid-level managers at NASA published papers with enthusiastic support for large-scale NASA investments in lunar ISRU [3737Sanders G. NASA Lunar ISRU strategy. Presented at: What Next for Space Resource Utilization? Workshop; 2019 Oct 10; Luxembourg.,3838Araghi K. NASA Lunar ISRU overview. Presentation to Korean Institute of Geoscience and Mineral Resources (KIGAM) ISRU Workshop; 2022 May 3.] Others in the NASA community have proposed very large-scale lunar ISRU enterprises [ 22 22Elliott J, Austin A, Colaprete T. ISRU in support of an architecture for a self-sustained lunar base. Paper presented at: 2019 70th International Astronautical Congress (IAC); 2019 Oct 21–25; Washington, DC, USA.].
At this point, it remains unclear to what extent those at NASA who control the purse strings plan to invest in lunar ISRU. At one end of the scale is testing various approaches for ISRU as an adjunct to the main mission. At the other end of the scale is building the whole mission around large-scale ISRU. Either way, the return on investment is likely to be nil compared to bringing resources from Earth [2424Rapp D. Near term NASA Mars and lunar in situ propellant production (ISPP): complexity vs. simplicity. Space Sci Technol. 2024.,2525Rapp D. The value of utilization of extraterrestrial resources for propellant production for space exploration—a perspective. Acad J Engrg Studies. 2024;3(4).].
Summary of ISPP
NASA is willing to develop technology as part of implementing a space mission, but NASA is reluctant to invest significant funds into generic technology development that would be useful in future missions. The state of ISRU technology, forty years after Ash, Dowler, and Varsi remain primitive. Even collection and pressurization of Mars CO2 remains at an early emergent state. The one time that NASA did invest significant funding into ISRU via MOXIE, it was part of a NASA mission and the majority of funds were used for engineering rather than technology development. It is ironic that even MOXIE now seems to be forgotten by NASA.
Recycling water
Providing a crew with water for life support on long missions in space is a major challenge. It is widely believed that recycling wastewater is essential to supply water for long, distant missions.
In regard to the water supply to a distant human crew, there is a serious lack of coverage by NASA of the most basic factor of all: crewmember water requirements. How does the quality of life for a crewmember vary with the amount of water provided per day in various space missions? The water requirements (kg per crew member per day) remain uncertain [44Rapp D. Human missions to Mars. 3rd ed. Heidelberg, Germany: Springer-Praxis Books; 2023.]. These requirements are likely to be greater for longer trips. How can NASA design reliable water supply systems for long missions if even the basic requirements are not understood?
Rapp [3939Rapp D. Mars ascent propellants and life support resources—take it or make it? IgMin Res. 2024 Jul 29;2(7):673-82. IgMin ID: igmin232; DOI: 10.61927/igmin232. Available from: igmin.link/p232] reviewed previous estimates of water requirements for crewmembers on long missions and encountered a severe lack of detailed studies and data [3939Rapp D. Mars ascent propellants and life support resources—take it or make it? IgMin Res. 2024 Jul 29;2(7):673-82. IgMin ID: igmin232; DOI: 10.61927/igmin232. Available from: igmin.link/p232]. Typically, a few papers from a couple of decades ago published a number in a table with no backup and no explanation of the source or the connotations. Bagdiagian, et al. [4040Bagdigian RM, Dake J, Gentry G, Gault M. International Space Station environmental control and life support system mass and crew time utilization in comparison to a long duration human space exploration mission. In: 45th International Conference on Environmental Systems; 2015; Seattle, WA. Paper ICES-2015-094.] said the ISS produced “an average daily potable water production rate [of] approximately 12.7 kg/day, corresponding roughly to a 3.4-person water processing rate” [4040Bagdigian RM, Dake J, Gentry G, Gault M. International Space Station environmental control and life support system mass and crew time utilization in comparison to a long duration human space exploration mission. In: 45th International Conference on Environmental Systems; 2015; Seattle, WA. Paper ICES-2015-094.]. That would imply a crewmember would only require 3.8 kg of water per day, which seems unreasonably skimpy even for very short-term visits to the ISS, but this level is unthinkable for long missions. The quality of life on a long trip to Mars resulting from various levels of water supply remains a mystery. Rapp [3939Rapp D. Mars ascent propellants and life support resources—take it or make it? IgMin Res. 2024 Jul 29;2(7):673-82. IgMin ID: igmin232; DOI: 10.61927/igmin232. Available from: igmin.link/p232] attempted to piece together an overview from the fragmented background, and he suggested that a minimal survival level might be about 7 kg/crewmember/day while perhaps about 17 kg/crewmember/day would provide Earth-like accommodations. These remain guesses.
The present state of technology for recycling water on space missions is the system that was developed for and used on the International Space Station (ISS). The system was designed as an interconnected set of subsystems so that each subsystem could be independently serviced, replenished, repaired, or replaced as needed.
Bagdigian, et al. [4040Bagdigian RM, Dake J, Gentry G, Gault M. International Space Station environmental control and life support system mass and crew time utilization in comparison to a long duration human space exploration mission. In: 45th International Conference on Environmental Systems; 2015; Seattle, WA. Paper ICES-2015-094.] provided a review of service and failure data for the Water Recovery System (WRS) on the ISS. The WRS consists of two main assemblies, The Urine Processor Assembly (UPA) and the Water Processor Assembly (WPA). These subassemblies each encompass several subsystems [4040Bagdigian RM, Dake J, Gentry G, Gault M. International Space Station environmental control and life support system mass and crew time utilization in comparison to a long duration human space exploration mission. In: 45th International Conference on Environmental Systems; 2015; Seattle, WA. Paper ICES-2015-094.].
Bagdigian, et al. [4040Bagdigian RM, Dake J, Gentry G, Gault M. International Space Station environmental control and life support system mass and crew time utilization in comparison to a long duration human space exploration mission. In: 45th International Conference on Environmental Systems; 2015; Seattle, WA. Paper ICES-2015-094.] stated the key issue very concisely:
"Over the last two-and-a-half decades, the International Space Station’s (ISS) Environmental Control and Life Support System (ECLSS) has grown and evolved in size, complexity, and capability. The functions that it performs today are many of those that will need to be performed in the future aboard spacecraft and habitats that will enable long-duration human exploration missions to destinations beyond low Earth orbit. Regardless of the particular deep space destination, it is widely accepted that highly reliable ECLSS systems that depend minimally on expendable equipment will be required. An important question, particularly in today’s fiscally constrained environment, is how well suited is the ISS ECLSS suite of technologies to meeting the needs of future missions? To help begin answering this question, the maintenance history of the ISS Water Recovery and Oxygen Generation Systems has been surveyed." (underline added) [4040Bagdigian RM, Dake J, Gentry G, Gault M. International Space Station environmental control and life support system mass and crew time utilization in comparison to a long duration human space exploration mission. In: 45th International Conference on Environmental Systems; 2015; Seattle, WA. Paper ICES-2015-094.].
Bagdigian, et al. [4040Bagdigian RM, Dake J, Gentry G, Gault M. International Space Station environmental control and life support system mass and crew time utilization in comparison to a long duration human space exploration mission. In: 45th International Conference on Environmental Systems; 2015; Seattle, WA. Paper ICES-2015-094.] provided data and information regarding this question, but their results are enigmatic, and they did not seem to reach a clear answer to the above question. The answer depends upon the criteria used for evaluation. Figure 4 of Bagdigian, et al. [4040Bagdigian RM, Dake J, Gentry G, Gault M. International Space Station environmental control and life support system mass and crew time utilization in comparison to a long duration human space exploration mission. In: 45th International Conference on Environmental Systems; 2015; Seattle, WA. Paper ICES-2015-094.] work shows that over a 6.5-year period, there were several hundred "repair and replacement" events. There are so many data points clustered in the graph that an exact count of the number of such events is difficult. My rough guess is about 300 events, averaging about an hour each, or one event every ~8 days. The original ECLSS installation had a mass of 1,383 kg. Replacements due to failure required 941 kg, and replacements due to consumption of planned expendables required 1,284 kg. This system produced on average 12.7 kg/day of water. Reviews were also provided on specific subsystems as to their design lifetime and experience on the ISS [4040Bagdigian RM, Dake J, Gentry G, Gault M. International Space Station environmental control and life support system mass and crew time utilization in comparison to a long duration human space exploration mission. In: 45th International Conference on Environmental Systems; 2015; Seattle, WA. Paper ICES-2015-094.].
One might conjecture a water cycling system for Mars based on the ISS system, in which the basic system is backed up by redundant units, multiple replacement units, and repair capabilities, and the crew is expected to service the system roughly every eight days. Slightly different systems would be needed for crew transfer in zero-g (about 200 days each way) and in Mars gravity (about 500 days). Since 2010, Jones has repeatedly made the point that the current NASA life support system is inadequate for long-term missions, and he analyzed various approaches for accommodating the unreliability of recycling systems using redundancy and spares [4141Jones HW. Life support with failures and variable supply. In: 40th International Conference on Environmental Systems; 2010; Barcelona, Spain.].
Jones [4242Jones HW. Developing reliable life support for Mars. In: 47th International Conference on Environmental Systems; 2017 Jul 16–20; Charleston, SC. Paper ICES-2017-84.] carried out an analysis of the question:
"Can a life support system be kept operating on the way to Mars using only redundant systems and spare parts?" [4242Jones HW. Developing reliable life support for Mars. In: 47th International Conference on Environmental Systems; 2017 Jul 16–20; Charleston, SC. Paper ICES-2017-84.].
He concluded that the failure history of the ISS suggests that common-cause failures are likely to occur and will probably require design changes rather than being reparable with spare parts.
If we imagine what a scaled-up cycling system might look like for a crew of six, and assume that the 12.7 kg/crewmember/day production rate suffices for one crewmember for a level somewhere between survival and maximal, and we assume that the 6.5-year history on ISS could be applied here, the masses of a system per crewmember would be about 3.7 tons. For a crew of six, each of the three cycling systems for crew transfer and Mars would have a mass of 22 tons. There would need to be a 22-ton ECLSS on transport to Mars, on Mars, and on return from Mars.
A major question would be how the "Orbital Replacement Units" ORUs would be stored, and how practical it might be for the crew to carry out a servicing every eight days.
While NASA made some improvements in this ECLSS system over the past fifteen years, the system remains problematic for long missions [4343Broyan JL, et al. NASA environmental control and life support technology development for exploration: 2020 to 2021 overview. In: 50th International Conference on Environmental Systems; 2021. Paper ICES-2021-384.]. More recently, Jones pointed out the impact of reduced launch costs and suggested that bringing water from Earth might now be almost competitive with recycling [4444Jones HW. The recent large reduction in space launch cost. In: 48th International Conference on Environmental Systems; 2018 Jul 8–12; Albuquerque, NM. Paper ICES-2018-81.,4545Jones HW. Take material to space or make it there? In: 2023 ASCEND Conference; 2023; Las Vegas, NV.]. Rapp further analyzed the trade between "take it or make it" and concluded that a system where survival levels of water brought from Earth, and recycling to provide better living conditions, would be optimum [3939Rapp D. Mars ascent propellants and life support resources—take it or make it? IgMin Res. 2024 Jul 29;2(7):673-82. IgMin ID: igmin232; DOI: 10.61927/igmin232. Available from: igmin.link/p232]. Some at NASA have engaged in complex mathematical analyses for the use of spares [4646Owens AC, Jones CA, Cirillo W, Klovstad J, Judd E, et al. Integrated trajectory, habitat, and logistics analysis and trade study for human Mars missions. ASCEND 2020; 2020; Virtual.], or for test programs [4747Owens AC, Cirillo WM, Piontek N, Stromgren C, Cho J. Analysis and optimization of test plans for advanced exploration systems reliability and supportability. In: 50th International Conference on Environmental Systems; 2021. Paper ICES-2020-199.,4848Maxwell AJ, Wilhite A, Ho K. Spare strategy analysis for life support systems for human space exploration. J Spacecraft Rockets. 2021;58(5):1-12. DOI: 10.2514/1.A34849.], but NASA needs better hardware rather than more mathematics.
The main thesis of this paper is that NASA is reluctant to make significant investments to develop generic technology for future missions, and the lack of progress in developing ISRU and ECLSS is offered as evidence toward that conclusion.
NASA has many ardent supporters for ISRU within its ranks, but NASA only invested significant funds into one ISRU project (MOXIE) because it was a space mission to Mars, and the preponderance of funding was for engineering rather than technology development. Ironically, NASA seems to have forgotten MOXIE as it now thinks exclusively about the Moon. The state of ISRU technology remains undeveloped and will remain so until a mission compels development within the mission. That is likely to occur in the years ahead as NASA progresses in its lunar initiative. But this will be a very challenging effort and likely to be less rewarding than bringing propellants from Earth.
While NASA has ardent supporters for advances in ISRU within its ranks, the NASA programs in life support technology seem invisible, and published papers seem defensive or abstruse. The life support establishment does not seem to have absorbed and responded to the many papers published by Harry Jones (NASA systems engineer) from 2010 to 2024. NASA's report on progress in life support technology is underwhelming. In fact, even for the most basic quantity: the requirements for water per crewmember are vague and not well understood.
NASA developed an ECLSS within the ISS mission that has served the ISS well despite the need for frequent replacement and repair services, which could be accommodated with so-called "Orbital Replacement Units". NASA does not appear to have the will to develop an advanced ECLSS for distant, long-duration missions. Perhaps, someday, when a human mission to Mars is finally decided upon, the ECLSS of the future will be developed within the mission. Meanwhile, NASA will continue to rely on the ISS ECLSS.
In addition, it is shown that there is a severe lack of internal checks and balances within NASA. NASA system engineers line up like weather vanes to the prevailing wind from on high, and they see their role as finding ways to justify management decisions, rather than review them independently and neutrally. As a result, NASA's plans for the Moon and Mars include some poor choices. The establishment of a "Gateway" to the Moon is counter-productive and the plan to process putative ice in polar craters is likely to have no return on investment compared to bringing propellants from Earth. The latest plan for Mars is a short-stay mission ("plant the flag and run") cannot be justified on any logical grounds.
Recommendation
The following is recommended:
1) NASA system engineers should be allowed and encouraged to make independent appraisals and analyses of missions proposed by administrators without feeling pressure to justify poor choices.
2) Proposed new missions should be subject to Non-Advocate Reviews where open discussion allows non-advocates to challenge mainline thinking in a positive and constructive way.
3) All proposed approaches to technologies for remote application in missions, such as ISRU or ECLSS, should always be compared to a "brute force" baseline such as bringing resources from Earth, before investing in them.
4) As step 1 for ISRU, NASA needs to develop a long-life, energy-efficient means of acquiring and compressing Mars CO2.
5) As step 1 for ECLSS, NASA must for the first time, analyze water requirements for crewmembers on long missions.
As step 2 for ECLSS, NASA needs a more thorough analysis of the limitations of the ISS ECLSS, and begin early system definition for the next generation.
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Address Correspondence: Donald Rapp, 1445 Indiana Ave., South Pasadena, CA 91030, USA, Email: drdrapp@earthlink.net
How to cite this article: Rapp D. Use of Extraterrestrial Resources and Recycling Water: Curb Your Enthusiasm. September 27, 2024; 2(9): 775-784. IgMin ID: igmin247; DOI: 10.61927/igmin247; Available at: igmin.link/p247
Figure 1: System diagram for testing cryogenic compression o...
Ash RL, Dowler WL, Varsi G. Feasibility of rocket propellant production on Mars. Acta Astronaut. 1978;5:705-724.
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![System diagram for testing cryogenic compression of CO2. [18]](https://www.igminresearch.jp/articles/figures/igmin247/igmin247.g001.png)
