The Value of the Moon

by Paul D. Spudis

  We do not know how to send people to Mars at this time. The difficulties with a human mission to Mars fall into several categories: technical, programmatic, and fiscal. The manned space program has conducted long-duration spaceflight, built a heavy-lift launch vehicle, and conducted landings on the Moon. But for a variety of reasons, getting humans to Mars is much more difficult. Mars is much farther away from Earth, varying between 140 to 1,000 times (55 to 400 million km) the distance of Earth to the Moon (400,000 km). No known trajectory can shorten the months of transit; most robotic missions take nine months. Although issues of crew deconditioning caused by microgravity appear to be mostly resolved from flight experience on the ISS, months of exposure to hard cosmic radiation and the occasional possible solar particle event, requires some type of shielding. People need to breathe, eat, and drink, so those consumables must be carried with them. Mars is bigger than the Moon (its gravity is about 3/8 that of the Earth, compared to the 1/6 g of the Moon); thus, it requires more energy to descend and land on the surface of Mars. This applies to the return trip as well. The larger gravity well of Mars means that bigger landers and more fuel are needed. Although there is an atmosphere on Mars, it is more than one hundred times thinner than Earth’s, so we cannot rely solely on aerothermal entry to slow down the spacecraft; a significant propulsive maneuver is required. This issue, the EDL (entry, descent, landing) problem,10 is one for which we have no solution at present.

  The composition of the martian atmosphere is virtually pure carbon dioxide (CO2) and thus, not breathable; the thinness of the atmosphere requires people to wear pressure suits. The surface is not completely shielded from cosmic rays and solar UV radiation; Mars does not have a magnetosphere like the Earth, which means that it is a hard radiation environment, limiting the permissible time for surface exploration. The soil on Mars is very fine, probably consisting of clay minerals, and owing to the presence of perchlorates and other highly oxidizing substances, it appears to be highly reactive chemically. If inhaled—and some dust inevitably will be brought into the crew cabin—dust could result in caustic chemical reactions in the bronchia of the crew’s lungs. In addition, no one knows how well humans will cope with the reduced gravity of the martian surface after spending multiple months in microgravity.

  The biggest problem with a human Mars mission comes right at the beginning. In order to carry the fuel, consumables, equipment, and vehicles that the crew will need for this trip, it will require the launch of between 500 and 1,000 metric tons into low Earth orbit.11 Getting this mass into orbit will require eight to twelve launches of a heavy lift rocket, each carrying about 130 metric tons to space. The vast bulk of this mass is the fuel required for the trip, most of which is burned at the beginning of the voyage to insert the vehicle into its planetary trajectory. But that’s not the end of the issue. The fuel will probably be cryogenic hydrogen and oxygen; if we use storable propellant, multiply the mass needed by factors of two to three. After it is delivered to orbit to await the eventual mission to Mars, the super cold cryogenic fuel will quickly boil off from solar heating. It would be a race against the clock to get enough fuel in one place in orbit at the right time. For now, we have no solution to the problem of fuel boiling off in the landers, both for descent and ascent, during the cruise phase of a manned mission to Mars.

  Other problems arise in terms of the scheduling of the multiple HLV launches and coordinating their payload manifests. Only two HLV launch pads (Launch Complex 39) exist at Cape Canaveral. One is currently unavailable, leased to a private company. Thus, we would need to launch all of these vehicles from a single pad. To get the pieces of the Mars mission in one place and ready to go, we must deal with an enormous scheduling and manifest problem, as well as the logistics of multiple HLV deliveries. After that, the next hurdle would be assembling and fueling the Mars vehicle in space.

  The cost of a Mars mission conducted in this manner is estimated at several tens of billions of dollars per trip. Is such a cost politically viable? Regardless of the propaganda spun by a hopeful New Space community, there are no magic bullets to lower this enormous cost. We still need the same mass in LEO, and the “lowering” of launch costs, which in any event is only on the order of factors of two or three at best, might turn a $500 billion mission into a $450 billion mission. For context, we currently spend about $18 billion per year on our civil space program, of which about $8 billion is designated for human spaceflight.

  Faced with these realities, it should be evident that Mars is very far from Earth, technically and fiscally. But the hardwired dreams of living on Mars have left space advocates of all persuasions chasing their tails, locked in a 50-year exercise by the promises of politicians or administrators who tell us, “Yes, we will be embarking on a new program to send humans to Mars.” What follows, as night follows day, is that people get spun up and start conducting feasibility studies; new vehicles are designed, and lovely color artwork showing people rappelling down the walls of one of the canyons of Valles Marineris is produced. And then, yet again, the dismal mathematics of a Mars mission becomes evident. But, we are told, not to worry: The mission is at least a couple of decades into the future. Somehow, the money and the political support for more money still will magically appear at the right time. Certainly, if we can assemble a Mars advocacy group, one that shows we have clout and that strikes fear in the hearts of our elected officials, we will get more money. To date, these methods and declarations have accomplished nothing. But, our leaders tell us, this will change as soon as we find a way to get the public excited about space—that “excitement” causes money to flow into the space program. After 50 years, is it not time to admit that this approach isn’t working?

  An article of faith among the true believers is that interest in the Moon and planning for lunar bases has kept them from achieving their lifelong dream of strolling across the red plains of Mars. The reality is exactly the reverse: It is the fixation with sending people to Mars that has kept us from doing any human missions beyond LEO. Looking over the history of post-Apollo planning, from Nixon’s Space Task Group in 1969 to the Vision for Space Exploration in 2004, all efforts to get people into trans-LEO space have run aground on the realities of the enormous technical and cost difficulties of human Mars missions.12 During the VSE, NASA was more concerned with devising a lunar “exit strategy” than it was with getting people back to the Moon in the first place.13 The dirty little secret is that most politicians love human Mars missions not because they have any desire or interest in doing them but because it is an excellent and proven way to keep the space community pacified by selecting a goal that is so far into the future that no one will be held accountable for its continuing non-achievement. What a remarkable accomplishment for America’s efforts in space: once we had a real space program that some thought was faked, and now we have a fake space program that many believe is real.

  The only way we will ever get people to Mars is through the construction of a transportation system that enables the routine movement of cargo and people throughout space. An Apollo-style crash program to send humans to Mars is highly unlikely to ever materialize. We need to acquire and learn certain spacefaring skills and technologies, including reusable space-based vehicles, staging nodes in deep space, in situ resource utilization, and the manufacture of propellant from water. If we possessed these capabilities, a human mission to Mars, while still challenging, would become more feasible. We can learn those skills and acquire those technologies on the Moon.

  Why not Mars? Because it’s too far, too difficult, and too expensive.

  Why Not Asteroids?

  At first glance, it might seem that asteroids, specifically the near-Earth objects (NEO), answer the requirements for future human destinations. NEOs are beyond low Earth orbit, they require long transit times and so simulate the duration of future Mars missions, and we have never visited one with people. However, detailed consideration indicates that NEOs are not the best choice as our next destination in
space.

  Most asteroids reside not near the Earth but in the asteroid belt, a zone between the orbits of Mars and Jupiter. The very strong gravity field of Jupiter will sometimes perturb the orbits of these rocky bodies and hurl them into the inner solar system, where they usually hit the Sun or one of the inner planets. Between those two events, they orbit the Sun, sometimes coming close to the Earth. NEOs can be any of a variety of different types of asteroids, but are usually small, on the order of tens of meters to a few kilometers in size. As such, they do not have significant gravity fields of their own, so missions to them do not “land” on an alien world, but rather rendezvous and station-keep with them in deep space.

  The moniker “near Earth” is a relative descriptor. These objects orbit the Sun just as the Earth does, and depending upon the time of year, vary in distance to the Earth from a few million kilometers to hundreds of millions of kilometers. Getting to one NEO has nothing to do with getting to another, so visiting multiple NEOs during one trip is both difficult and unlikely. Because the distance to a NEO varies widely, we cannot just go to one whenever we choose: Launch windows open at certain times of the year, and because the NEO is in its own orbit, these windows occur infrequently and are of very short duration, usually a few days. Moreover, due to the distances between Earth and the NEO, radio communications will not be instantaneous, with varying time lags of tens of seconds to several minutes between transmission and reception.

  Although there are several thousand NEOs, few of them are potential destinations for human missions. This is a consequence of two factors. Because space is very big, even several thousand rocks spread out over several billion cubic kilometers of empty space results in a very low density of objects. Second, many of these objects are unreachable, requiring too much velocity change from an Earth departure stage; this can be the result of either too high of an orbital inclination (out of the plane of the Earth’s orbit) or an orbit that is too eccentric (to varying degrees, all orbits are elliptical). These factors result in reducing the field of possible destinations from thousands to a dozen or so, at best.

  There are few asteroid targets and it takes months to reach one. Long transit time is sold as a benefit by advocates of asteroid missions: Because a trip to Mars will take months, a NEO mission will allow us to test out the systems for Mars missions. But such systems do not yet exist. On a human mission to a NEO, the crew is beyond help from Earth, except for radioed instructions and sympathy. A human NEO mission will have to be self-sufficient to a degree not present on existing spacecraft. Crew exposure to the radiation environment of interplanetary space is another consequence of long flight times. This hazard comes in two varieties: solar flares and galactic cosmic rays. Solar flares are massive eruptions of high-energy particles from the Sun, occurring at irregular, unpredictable intervals. We must carry some type of high-mass shielding to protect the crew from this deadly radiation, and this “storm shelter” must be carried wherever we go. Because Apollo missions were only a few days long, the crew simply accepted the risk of possible death from a solar flare. Cosmic rays are much less intense, but constant. The normal ones are relatively harmless, but high-energy versions (heavy nuclei expelled from ancient supernovae) can cause serious tissue damage. Although the crew can be partly shielded from this hazard, they are never totally protected from it.

  When the crew finally arrives at their destination, more difficulties await. Many NEOs spin very rapidly, with rotation periods on the order of a few hours at most. This means that the object is approachable only near its polar area. Because these rocks are irregularly shaped, rotation is not the smooth, regular spin of a planet, but is more like that of a wobbling toy top. If material is disturbed on the surface, the rapid spin of the asteroid will launch this debris into space, creating a possible collision hazard to the human vehicle and crew. The lack of gravity means that “walking” on the surface of the asteroid is not possible; crew will “float” above the surface of the object, and just as occurs in Earth orbit, each touch of the asteroid surface (action) will result in a propulsive maneuver away from the surface (reaction).

  We would need to work quickly at the asteroid because we would not have much time there; loiter times near the asteroid for most opportunities are a few days. Why so short? Because the crew wants to come home. The NEO and Earth continue to orbit the Sun, and we need to make sure that the Earth is in the right place when we arrive back at its orbital position. In effect, we will spend months traveling there in a vehicle with the habitable volume of a large walk-in closet, have a short time at the destination, and then spend months on the trip home.

  In general terms, we already know what asteroids are made of, how they are put together, and what processes operate upon their surfaces. Most NEOs will be ordinary chondrites. We know this because ordinary chondrites make up about 85 percent of all observed meteorite falls. This class of meteorite is remarkable not for its diversity, but for its uniformity. Chondrites are used as a chemical standard in the analysis of planetary rocks and soils to measure the amounts of differentiation or chemical change during geological processing. One chondrite is pretty much like all the others.

  Questions that could be addressed by human visitors to asteroids concern their internal makeup and structure. Some appear to be rubble piles, while others are nearly solid. Why such different fates in different asteroids? By using active seismometry (acoustic sounding), a human crew could lay out instruments and sensors to decipher the density profile of an asteroid. Understanding the internal structure of an asteroid is important for learning the internal strength of such objects; this is an important factor in devising strategies to divert a NEO away from a collision course with Earth.

  An alleged benefit of travel to an asteroid is that they have resource potential. I agree, putting the accent on the word “potential.” Our best guide to the nature of these resources comes from the study of meteorites—NEOs that have already collided with the Earth. The resource potential of asteroids lies not in the chondrites, but in the minority of asteroids that have more exotic compositions. Metal asteroids make up about 7 percent of the population and are composed of nearly pure iron-nickel metal, with some inclusions of rocklike material as a minor component. Other siderophile (iron-loving) elements, including platinum and gold, make up trace portions of these bodies. A metal asteroid is an extremely high-grade ore deposit, potentially worth billions of dollars, if we were able to get these metals back to Earth.

  However, from the spaceflight perspective, water has the most value. A relatively rare asteroid type contains carbon and organic compounds, as well as clays and other hydrated minerals. These bodies contain significant amounts of water (up to 20 weight percent). Finding a water-rich NEO would create a logistics depot of immense potential value.

  A key advantage of asteroids as a resource is a drawback as an operational environment: They have extremely low surface gravity. Getting into and out of the Moon’s gravity well requires a change in velocity of about 2,380 meters per second each way; to do the same for a typical asteroid requires only a few meters per second. This means that a payload launched from an asteroid rather than the Moon saves almost 5 kilometers per second in delta-v, a substantial amount of energy. From the perspective of energy accessibility, the asteroids beat the Moon as a source of materials.

  Yet there remains the challenge of working in very low gravity, as well as other difficulties that exist in mining and using asteroidal, as opposed to lunar, resources. First is the nature of the feedstock or “ore.” Water at the poles of the Moon is not only present in enormous quantity, tens of billions of tons, but is also in a form that can be easily used: ice. Ice can be converted into a liquid for further processing at minimal energy cost; if the icy regolith from the poles is heated to above 0°C, the ice will melt and water can be collected and stored. The water in carbonaceous asteroids is chemically bound in mineral structures. Significant amounts of energy are required to break these chemical bonds to free th
e water, at least two or three orders of magnitude more energy than to melt ice, depending on the specific mineral phase being processed. So extracting water from an asteroid (present in quantities of a few percent to maybe a couple of tens of percent) requires significant energy; water ice at the poles of the Moon is present in greater abundance (up to 100 percent in certain polar craters) and is already in a form that is easy to process and use.

  The processing of natural materials to extract water has many steps, from the acquisition of the feedstock, to moving the material through the processing stream, to the collection and storage of the derived product. At each stage, we typically separate one component from another; gravity serves this purpose in most industrial processing. A challenge to asteroid resource processing is to devise techniques that do not require gravity, including related phenomena, such as thermal convection, or to create an artificial gravity field to ensure that things move in the right directions. Either approach significantly complicates the resource extraction process.

  The great distance from the Earth and poor accessibility of asteroids compared to the Moon works against resource extraction and processing. Human visits to NEOs will be of short duration, and because radio time lags to asteroids are on the order of minutes, direct remote control of processing will not be possible. Robotic systems for asteroid mining must be designed to have a large degree of autonomy. This may become possible but presently we do not have enough information on the nature of asteroidal feedstock to design, or even envision, the use of such robotic equipment. Moreover, even if we did fully understand the nature of the deposit, mining and processing are highly interactive activities on Earth and will be so in space. The slightest anomaly or miscalculation can cause the entire processing stream to break down, and in remote operations, it will be difficult to diagnose and correct the problem and restart it.