A number of significant technological hurdles have to be overcome before humans can successfully explore Mars. (credit: NASA)

The challenges of manned Mars exploration

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Gravity

In the 1988 OExP studies it was recommended that “An artificial gravity research program must be initiated in parallel with the zero-gravity counter-measure program if the U.S. is to maintain its ability to begin exploration in the first decade of the next century.” However, for the short traverses required for going to the Moon, artificial gravity is not required. Since ESAS has its hands full planning for the Moon, artificial gravity for transits to Mars seems to have been ignored—at least for the next decade or so. A search reveals that the above quote from the 1988 report is the only place that the term “artificial gravity” appears in the ESAS Report.

The Mars Society DRM report said:

An artificial gravity system was deemed necessary for the MSM’s outbound hab flight to (1) minimize bone loss and other effects of freefall; (2) reduce the shock of deceleration during Mars aerobraking, and (3) have optimal crew capabilities immediately upon Mars landing. Experience with astronauts and cosmonauts who spent many months on MIR suggests that if the crew is not provided with artificial gravity on the way to Mars, they will arrive on another planet physically weak. This is obviously not desirable. Unless a set of countermeasures that can reduce physiological degradation in microgravity to acceptable levels is developed, the only real alternatives to a vehicle that spins for artificial gravity are futuristic spacecraft that can accelerate (and then decelerate) fast enough to reach Mars in weeks, not months. To save on mass, the MSM uses an artificial gravity system with the habitat counterbalanced by a burned-out [launch vehicle stage], as in Mars Direct.

A recent paper provides a review of low gravity effects on physiology. ESAS does not seem to have addressed this subject yet.

Living off the land

In situ resource utilization (ISRU) is a technology to reduce the mass that must be brought from Earth by using indigenous resources. The proper figure of merit for evaluating the investment in ISRU is the cost of missions with ISRU vs. the cost of missions without ISRU. Since cost estimates are notoriously difficult to make, a typical surrogate is to compare the IMLEO with and without ISRU. Another important figure is the mass of the largest vehicle that must be placed into LEO for a related sequence of missions (with and without ISRU) that determines size of required launch vehicle.

Since the merit of ISRU can only be evaluated in the context of a DRM, and most DRMs are either ill-defined or based on very optimistic assumptions, evaluation of ISRU benefits remains quite subjective. Nevertheless, ISRU advocates continue to chant mantras in favor of ISRU and these have swayed ESAS thinking.

Visionaries and optimists have proposed various lunar ISRU schemes for extracting oxygen from regolith. However, all of these seem to require an autonomous reactor that operates at very high temperatures (typically in excess of 1500°C) with solids entering from above and spent slag exiting the bottom, and gaseous oxygen moving counter-flow to the regolith. This is a chemical engineering nightmare on Earth; on the Moon it is unimaginable. Visionaries and optimists have also proposed recovery of volatiles embedded in the regolith from solar wind. It is likely that one would have to excavate and process at least 20,000 tons of regolith to acquire one ton of volatiles—and this appears likely to be very impractical.

The only form of lunar ISRU that seems to have any chance of making practical sense is acquisition of water-ice bearing regolith from permanently-shaded polar areas. Whether this turns out to be feasible or not depends on the existence, distribution, accessibility, and requirements for extraction of water. We have evidence of existence of such ice deposits from orbit observations, but it needs in situ corroboration. We have no data on distribution, accessibility and requirements for extraction. That being the case, technical work on lunar ISRU needs to concentrate on three things: prospecting for water ice, mission analysis for ice recovery, and excavation techniques. There is no need for developing complex chemical processes. Water ice is the only potentially useful resource, and we know how to melt ice and electrolyze water. It remains unclear whether the cost and material needed to gather regolith, extract the water, and power the whole process is greater than the value of the water extracted (if it can be extracted).

There are further anomalies in the ESAS approach to lunar ISRU. These include the following:

• There is no indication that ESAS will make the necessary effort (i.e. send several in situ precursor missions with rover and subsurface coring capability to locations indicated by remote sensing) to locate near-surface ice and validate its accessibility. Early plans seem inadequate.
• Although NASA plans to spend $80 million on lunar ISRU technology over the next six years, a significant part of this is for oxygen extraction from regolith that does not need to be done in polar areas, and there is an apparent disconnect between this technology program and the precursor missions to do prospecting.
• If ISRU is really doing its job, the benefit ought to be in terms of a reduction in LEO mass, whereas the ESAS plan seems to add 121 mT to LEO mass because of ISRU. (An entire launch of a heavy lift launch vehicle is devoted to transporting ISRU equipment to the Moon and the return on investment is unknown).
• All of the above not withstanding, the only place where near-surface ice can exist on the Moon is in deeply shadowed craters near the poles. There is no solar energy in these areas. That means that nuclear energy will be needed for ISRU. But there probably aren’t enough isotopes around to power such systems with RTGs, and NASA has only developed reactors in PowerPoint. So we have a major mismatch between the use of solar energy and the desire to process water ice in dark areas near the poles.
• By locating the outpost on such a site, it would seem to eliminate any hope of using the outpost to excavate near-surface ice from shadowed areas, considering the distances and terrain that heavy vehicles would have to traverse. This, in turn, would imply that ISRU will involve oxygen from regolith silicates and not ground ice as the prime feedstock. Yet oxygen from regolith is likely to be an impractical approach.

Mars ISRU has been widely discussed. For example, the notion of risk in ISRU is discussed in this document. Other relevant topics include transport and storage of hydrogen and occurrence of water on Mars.

ISRU on Mars is a very different situation than it is on the Moon. The main differences are:

• Because of the required long stay on Mars, the requirement for life support consumables is well over 100 mT, which translates into a much higher mass in LEO. Although this figure can likely be reduced by a recycling life support system, that system would have to operate for 500–600 days on the surface without failure.
• The requirement for propellants for ascent from Mars is typically several tens of mT, depending on the ascent process, and as in the case of consumables, this translates into a much higher mass in LEO.
• The combined required IMLEO for life support consumables and ascent propellants may be several hundred mT, which translates into several launches with a 125-mT launch vehicle. Use of ISRU could potentially reduce or eliminate the need for these extra launches.
• Unlike the Moon, Mars has a ready supply of carbon and oxygen in the easily acquired atmosphere.
• Unlike the Moon, Mars has significant near-surface deposits of water (mainly in the form of ground ice) widespread across much of the planet.
• The combination of atmospheric CO₂ and water from regolith provides feedstocks on Mars that enable proven, relatively simple Sabatier-electrolysis processing for propellants, and water for life support.
• In conclusion, Mars ISRU is far more easily implemented and has far more mission impact than lunar ISRU.

The only unknowns regarding Mars ISRU are:

• What are the requirements for excavating water-bearing near-surface regolith and extracting water?
• In the case of equatorial water-bearing near-surface regolith, is the water in the form of ground ice or mineral hydrates?

Unfortunately, neither ESAS nor the Mars Exploration (Science) Program has any specific plans to investigate these questions.

Power

All of the previous Mars DRMs from the 1990s used nuclear reactors for power on the surface of Mars except for the Dual Landers DRM, which used solar power. However, the Dual Landers DRM was never documented and now seems to have been discarded by ESAS. In all cases, a significant amount of autonomous deployment and integration is assumed on the surface of Mars.

The ESAS Report (pp. 201–204) discusses alternative power systems for lunar missions and manages to conclude that “all of the power systems considered will need considerable development,” without ever facing up to the huge advantages of a nuclear reactor, although it is implied that a nuclear reactor may be required for Mars.

It is inconceivable that ISRU on Mars will be feasible without a nuclear reactor because of the high power requirements and the difficulties involved in starting and stopping processing 500 to 600 times. Even without ISRU, it taxes one’s credulity to imagine a human mission dependent completely on solar power. The requirement to locate the reactor some great distance from the outpost imposes additional requirements for autonomy, mobility, and cabling. Power may be a major stumbling block for Mars human missions. NASA does not seem disposed to invest in the needed technology, at least in the short run. The required IMLEO for the power system is likely to require the majority of one 125-mT to LEO launch.

Schedules

Prior to about 2020, almost all ESAS funding will go into lunar programs, and Mars will be on the far back burner. After 2020, the ESAS Plan calls for parallel funding of the lunar outpost(s) and “Mars development.” The probability that this plan will be implemented through 2020 is not great. That is why ESAS is so desperately pushing their plan forward at breakneck speed. However, there has never been a plan of this sort that did not encounter delays, overruns, and other difficulties. Therefore, if ESAS says they will be ready to begin work on Mars in 2020, we must expect that it might be closer to 2025 or 2030. The requirements for lunar outposts will likely usurp most Mars funds between 2020 and 2025, and maybe out to 2030. Nevertheless, if we adopt an optimistic posture we might assume that they are ready to do serious Mars development around 2020. We must take stock of the starting point in 2020.

It seems likely that by 2020, ESAS will have:

• Very little progress on Mars aerocapture and direct entry for large (greater than 30 mT) vehicles.
• No progress on nuclear thermal propulsion.
• No in situ experiments to validate existence and requirements for excavation of near-surface water resources.
• No progress on Mars ISRU technology.
• Uncertain lifetimes and reliability of long-term life support systems.
• No nuclear reactor.
• No progress on artificial gravity.
• No clue how to deal with radiation threat.

On the positive side, ESAS should have in place:

• Lunar habitat designs that can be modified for use on Mars.
• Life support systems that function reliably for several months.
• Regolith moving equipment that has been field-tested on the Moon.
• Earth departure systems based on hydrogen/oxygen propulsion.
• Rendezvous and assembly capabilities in LEO and lunar orbit.

If ESAS embarks on programs to develop needed technologies, it will be hampered by high costs, and long delays between in situ demonstrations (26 months between launch opportunities).

No one knows how much the required technology development program will cost nor how many decades it will take to carry it through. NASA has been able to design lunar missions based mainly on existing technology, but Mars will be a different story. It may require many launches with a 125 mT to LEO launch vehicle to implement one human mission to Mars. The ESAS plan for Mars depicting six cargo launches seems to be based on wishful thinking.

Does NASA have the stomach to spend many billions on Mars technology development and demonstration over more than a decade after 2020 (in parallel with lunar outpost buildup)? Dealing with fail-safe delivery of life support consumables and zero-g effects will present major challenges. Development of large-scale aero-entry systems and demonstration at Mars will require multiple large-scale precursor flights. That would be unprecedented. And regardless of anything else, is there any solution to the radiation problem? I think not.

In parallel with this ESAS enterprise, there is a Mars Exploration Program managed by JPL, primarily addressing Mars science via robotic missions. Most notable here is the Mars Exploration Rovers (MER) mission that sent two rovers roaming around equatorial Mars for over two years in a program that was a major success.

No good deed goes unpunished, and the reward for this success has been severe cutbacks in the program's future mission expectations to the point where people at JPL are asking once again “What can we do with orbiters?” because funding for the more ambitious rover missions may be difficult to obtain (despite the fact that these costs are miniscule compared to human mission costs). In addition, JPL has been systematically isolated from the ESAS enterprise, given no lead roles and only minor secondary roles. The Robotic Lunar Exploration Program (RLEP) was not assigned to JPL despite its obvious superiority for this role, because of other internal political pressures within NASA.

Yet even within the JPL-led Mars Exploration Program (MEP), there are aspects that cause wonderment. The principal goal, motivation ,and emphasis of the MEP is the search for life (past or present) on Mars. The adoption of this goal, with its innately very low probability of occurrence, introduces several anomalies in the logic of it all. Life requires liquid water, and liquid water can only exist well below the surface. Similarly there are no organics on Mars—at least not to one part in a billion. Yet we continue to hear about the MEP searching for organics and near-surface liquid water, and developing life-detection instruments.

The MEP does not seem to have any interest in prospecting for accessible near-surface water ice and mineral hydrates, both for its value as a resource that could provide a source of propellants and power (via solar-powered electrolysis) to enable long-term, long-range exploration of the surface of Mars robotically, as well as for its science implications regarding the evolution and history of water distribution on Mars.

There does not seem to be much commonality between ESAS and the MEP.

By 2020, the full impact of the pending world oil shortage should be apparent, and coupled with the location of major deposits in unstable parts of the world, we might find that coping with oil shortages and energy problems in general will take center stage and reduce the imperative to send humans to Mars.

The earliest possible date that NASA could (under the best of circumstances, using the most optimistic assumptions) send humans to Mars would be 2040. By then, however, the chaos produced by a burgeoning world population, each bent on using up energy as fast as the Americans, is bound to lead to major dislocations. The future of the space program is difficult to predict under such circumstances.

Dr. Donald Rapp is a former senior research scientist and division technologist at JPL. He served as manager of the Mars Exploration Technology Program there, studying in situ propellant production, solar power on Mars, and other Mars-related technologies.

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