The Value of the Moon

by Paul D. Spudis

  The accessibility issue also cuts against asteroidal resources. We cannot go to a given asteroid at will; launch windows open for very short periods and are closed most of the time. This affects not only our access to the asteroid but also shortens the periods when we may depart from the object to return our products to near-Earth space. In contrast, we can go to and from the Moon at any time, and its proximity means that nearly instantaneous remote control and response are possible. The difficulties of remote control for asteroid activities have led some to suggest that we devise a way to “tow” the body into Earth orbit, where it may be disaggregated and processed at our leisure. I shudder to think about being assigned to write the environmental impact (if you’ll pardon the expression) statement for that activity.

  So where does that leave us in relation to space resource access and utilization? Asteroid resource utilization has potential, but given today’s technology levels, it has uncertain prospects for success. Asteroids are hard to get to, have short visit times for round-trips, difficult work environments, and uncertain product yields. Asteroids do have low gravity going for them, which is both a blessing and a curse. In contrast, the Moon has the materials we want and in the form that we need. The Moon is close and easily accessible at any time and is amenable to remote operations controlled from Earth, in near-real time. We should go to the Moon first to learn the techniques, difficulties, and technology to conduct planetary resource utilization by manufacturing propellant from lunar water. Nearly every step of this activity, from prospecting and processing to harvesting, will teach us how to mine and process materials from future destinations, on both minor and planetary-sized bodies. Learning how to access and process resources on the Moon is a skill that transfers to any future space destination.

  The Moon: Our Next Destination in Space

  The Moon is the first extraterrestrial object after leaving Earth orbit and it is a highly desirable place to visit and utilize. Why would we not want to explore and use it? Yet, as we have seen, two presidential attempts to return to the Moon in the past twenty-five years have both ended in failure, stifled by bureaucratic process and the continuing siren call of Mars. Other nations clearly see the value of the Moon. Why can’t we?

  In part, America is the victim of its own early success on the Moon. The Apollo missions and the associated robotic missions that preceded them, were great technical and emotional triumphs. They produced sights and experiences that have yet to be surpassed, even by the technically more challenging (but also more prosaic) flights of the space shuttle and the construction of the ISS. It wet our appetites for more. Because of Apollo, there is a sense that we’ve been there, and overly anxious explorers don’t see a reason to return. This ignorance and quick dismissal about what the Moon has to offer is exploited by space advocates who have other agendas: to quest for life, to step onto new worlds, to build colonies and transform other planets. None of those motivations by themselves have had any better success in generating more—or even adequate—funding for the civil space program. In particular, the constant and recurring obsession with human missions to Mars has kept us from pursuing the more valuable and emphatically achievable near-term goal: a permanent return to the Moon.

  Simply put, most people are indifferent to space. This has been true since the beginning of spaceflight, even during the Apollo program.14 They are neither overly enthusiastic nor hostile to it; they are at best, mildly interested in space, occasionally becoming enthusiastic and patriotic in times of significant accomplishment. For years, space advocates have had the obsessive certainty that if they can impart to the public the same zeal that they feel for Mars or space colonies, or whatever their cause, that they will be showered with more money, forever. That hasn’t happened and it won’t. At best, there will be a modest level of ongoing federal funding—more or less what NASA has received since the end of the Apollo program.

  We must craft a program that will endure for decades, a program that makes steady, constant progress and returns tangible benefits with the levels of funding likely to be made available. Our challenge is to work with what we have. Yet, how can we craft a program that aims for big goals, like space settlement or planetary missions, under existing constrained budgets? I have spent the last few years exploring that question, and I believe there is a clear path forward.

  7

  How? Things We Should Have Been Doing

  Although several attempts to revitalize lunar exploration met with partial success, currently, the Moon is not a strategic destination for the United States. A general misunderstanding of the value of the Moon keeps stalling our plans to return. The Apollo program was a successful architecture for getting people to the Moon and that seminal experience still colors many viewpoints on how to approach a lunar return. The Apollo program was the product of historical circumstances born during a specific time and place. While that experience holds many lessons for us, we must resist using it as a guidebook to get back to the Moon.

  Questions on how to extend human reach beyond LEO have preoccupied the space community for years, with widely varying opinions on the appropriate steps to take, the order in which to take them, and how to implement the specific technical needs of each phase of human travel in deep space. Although many of these choices are a matter of personal preference, there is a common set of requirements that any trans-LEO architecture must satisfy. In what follows, I will outline some of the basic challenges of human spaceflight, the specific issues confronting travelers beyond LEO, and how these issues can be addressed.1

  Some Spaceflight Basics

  Rocket engines work through combustion. The chemical energy stored in propellant is released and expelled through a nozzle at high velocity. We have many choices—the type of fuel and oxidizer, the engine configuration, fuel flow rates and the geometry of the combustion chamber, as well as the mixing ratios and the nozzle diameter to vary the amounts of power a given rocket engine can generate. Regardless of how we might vary these parameters, we remain fundamentally limited in what we can put into space from the surface of the Earth.

  The principal limiting factors in spaceflight are the force of gravity and the amount of energy available for release in the chemical bonds of the propellant. We can do nothing about either of these two factors; they are dictated by nature. At best, we can be clever in our engineering by employing strategies like staging, and by varying the types of materials used to make structures. But varying these parameters work only at the margins, not at the fundamentals. Those fundamentals are described by something called the rocket equation, first formulated in 1903 by the Russian “Father of Astronautics,” Konstantin Eduardovich Tsiolkovsky. The rocket equation essentially says that for chemical fuels, a rocket must consist of about 80–99 percent propellant by mass. This depressing arithmetic informs us that the payload—the useful mass that we want to get into space—can be only a small fraction of the mass of the vehicle.2

  This simple fact of life, one that astronaut Don Pettit aptly terms “the tyranny of the rocket equation,” means that going into space is possible, but difficult and expensive.3 Typical commercial launch vehicles (CLV) are able to put 2–30 metric tons of payload into low Earth orbit, at a cost of between $30 million and $500 million per launch. These costs must include the necessary infrastructure costs, such as ground support, tracking, and insurance. All the air, water, food, and equipment the crew needs during the mission must be brought up by launch. This manifest is in addition to the mass of the launch vehicle, including its structure, tankage, and avionics.

  To achieve orbit, a payload must be launched along a carefully chosen trajectory with a rocket burn of precise magnitude and duration. It must be lifted above the atmosphere so that aerodynamic drag does not slow the payload down to roughly 100 kilometers above the Earth, a point called the Karman line, the boundary between air and space.4 It must be accelerated to a velocity of about 7.8 kilometers per second; at this speed, the distance traveled by the vehicle per unit time i
s greater than the magnitude of the curvature of the Earth. When this condition is achieved, the launched object will constantly circle the Earth—it is in orbit. At the altitudes of low Earth orbit (~200–300 km), traces of atmosphere occur, meaning that an orbit will eventually deteriorate over time. Because of atmospheric drag, a satellite in LEO eventually will reenter the atmosphere. To alleviate this problem, satellites carry small amounts of fuel that are burned in small rockets, thrusters fired in controlled bursts, to maintain its orbit.

  To go beyond LEO to high geosynchronous orbit (36,000 kilometers above Earth), an L-point, or to the Moon or planets (see figure 6.1), additional velocity (positive delta-v) must be imparted to the spacecraft by a rocket burn in the current direction of travel. An engine burn requires propellant, but existing launch vehicles reach LEO with empty fuel tanks. The only way around this problem is to include the fuel needed for trans-LEO travel as part of the payload, which further reduces the remaining fraction available for useful payload, or refuel the upper stage from a stored supply already in LEO. The first method requires the development of something called a heavy-lift launch vehicle (HLV). Such a rocket’s specific size is not rigorously defined, but typically, an HLV is able to put 50 to 100 metric tons or more into LEO. An example of an HLV is the Saturn V of the Apollo era, which could launch 116 metric tons into space. The Saturn V was the biggest launch vehicle the United States ever built and was sized specifically for the requirements of a human mission to the Moon, which included the Saturn IV-B upper stage, the Lunar and Command-Service Modules, and the liquid hydrogen-oxygen fuel needed to send the entire structure to the Moon.

  The alternative technique for travel beyond LEO is to store propellant at a depot in space, then refuel the departure stage from that source.5 The idea of propellant depots in low Earth orbit has drawn a lot of attention, especially from many armchair engineers who have never actually flown a mission beyond LEO. Although this sounds like a good idea—indeed, it is a spacefaring skill that we must eventually master—the hidden assumption of the depot concept is that we possess the capability to launch the propellant “cheaply” from Earth, usually via some magically inexpensive “commercial” source, and store it in orbit. This is a simplified account of the depot concept; many other complex variables must be considered such as propellant boil-off, transfer techniques, management of the arrivals and departures of the tankers, and manifesting the facilities and timings of each launch of propellant cargo. Propellant depots are something that we eventually will take advantage of, particularly when we are ready to export propellant from the lunar surface. For the moment, the use of depots is invoked primarily as a substitute for a heavy lift launch vehicle. In the future, once we begin to produce and export propellant from the Moon, depots will be essential for supplying the vehicles of cislunar and planetary spaceflight.

  A benefit Earth provides is that we can decelerate returning spacecraft using atmospheric friction dissipated as heat for braking, thus eliminating the need for propellant to slow down a returning spacecraft, thus making practical spaceflight possible. All returning human missions to date have used this technique, called aerothermal entry. A variant of this concept is aerobraking, in which a vehicle does not actually land, but uses the atmosphere to slow down enough to enter orbit around a planet. Although not used yet on human missions, this approach has been used on some robotic spacecraft sent to orbit Venus and Mars.6 As part of a system that can be reused and expanded, aerobraking is another skill that must be mastered in order to develop a permanent space transportation system.

  The rocket equation dictates that while travel to LEO is difficult, travel beyond it becomes increasingly more so. Although the actual numbers vary depending on the propulsion system and its fuel, putting a single kilogram in lunar orbit requires about five kilograms in LEO, while landing a single kilogram on the lunar surface requires about seven kilograms in LEO, most of which is propellant. A system that enables routine access to cislunar space—the volume of space between Earth and Moon, including the lunar surface—could be established by setting up staging areas where the intermediate travel segments to the varying levels of cislunar space might be launched. Examples of such staging areas include LEO—the ISS is one possibility; GEO, a useful location to access communications and weather satellites; the Lagrangian (libration) points, of which L-1 and L-2 are often mentioned; and lunar orbit, with a variety of possibilities. At these locations, different spacecraft and pieces may be assembled to travel to the next location; they would also be locales for the establishment of propellant depots. A network of transportation nodes will enable constant and routine flight throughout cislunar space.

  The tyranny of the rocket equation makes spaceflight difficult, and therefore expensive. It is possible to save some money by using clever engineering and some specialized tricks, but typically, such approaches only nibble around the margins and do not take big bites out of the core cost. This reality—the limiting arithmetic of spaceflight—cannot be addressed with finality as long as we haul everything we need up from the bottom of the deepest gravity well in the inner solar system. We will break loose from our tether once we learn how to create new capabilities by provisioning ourselves from what we find in space.

  Launch Vehicle Options

  After thirty years of service, the space shuttle was retired in 2011. Many observers regarded the shuttle as unsafe and inefficient, but although fourteen people died in two vehicle failures, 341 people safely made the trip to and from LEO, some taking multiple voyages. Moreover, the failure of two flights, Challenger and Columbia, out of a total of 135 flights, gives the space shuttle a 98.5 percent success rate, one of the best in the history of spaceflight. No one considers the loss of human life, even those who choose to challenge the limits, as anything but tragic, but each loss of vehicle and crew led to safer subsequent flights. At the end of the program, the shuttle was operating about as safely as any Earth-to-LEO transportation system could.

  An enduring problem with the shuttle system was the amount of time and effort needed to refurbish it after each flight. Of the shuttle stack, only the external tank was discarded; all other pieces were recovered and reused. The solid rocket motor segments were simply refilled with propellant. However, the orbiter required many man-hours of work to prepare for launch, especially the silica tiles used to protect the vehicle during the searing heat of reentry. Copious labor-intensive work on the thermal protection system vacuumed up money (during its years of operation, shuttle operations took up the major fraction of the human spaceflight budget). For this reason, some critics consider the shuttle a policy failure, in that it did not make spaceflight to and from LEO “cheap,” even though that was never one of its design goals.7 However, a better way to look at the shuttle is that its goal as a vehicle was to make spaceflight “routine”—and it did. Moreover, the size and design of the shuttle gave it some unique capabilities, some not available on any planned future American spacecraft.

  Now that the space shuttle is a historical relic, we are essentially in the beginning days of a new human spaceflight system. Currently under development are the remnants of Project Constellation: the Orion spacecraft and the Space Launch System (SLS).8 Orion can be configured to carry up to six passengers, four for cislunar flights, and has the capability to reside in space for about three weeks. This duration is adequate for almost all cislunar missions, but for missions beyond the Moon to, say, Mars or an asteroid, Orion will need additional modules for habitation, planetary landing, and other functions. In essence, Orion is only a single piece of a trans-LEO spacecraft system. Moreover, the design of the Orion command module is not conducive to satellite servicing, landing on a planetary object or extensive EVA; since it has no airlock, the entire spacecraft must be depressurized before crewmembers can egress.

  The new rocket under development is a heavy lift vehicle (HLV), the Space Launch System (SLS). The SLS is built with pieces derived from the retired shuttle system, including its engi
nes (modified shuttle main engines, burning LOX-hydrogen fuel), its solid rocket motors, and its central core tankage. In its basic form, the SLS can put about 70 metric tons into LEO; there are plans to increase that capacity, first to about 100 tons and ultimately to 130 tons. Depending upon the architecture, this core 70 ton payload capacity is adequate for most lunar missions. The largest variant of the SLS is scaled for human Mars missions staged completely from Earth. In such a case, eight to twelve separate launches are needed to assemble the 500+ ton Mars spacecraft in Earth orbit.

  The principal advantage of an HLV is that the number of launches needed to conduct a mission is minimized. Each launch has a finite probability for failure, which is multiplied by the total number of launches. An architecture that uses a smaller LV has greater total risk, even though the impact of the loss of a single vehicle is lessened. Moreover, because ground infrastructure tends to be limited for most launch vehicle systems, the management of resources such as personnel, timing, and processing streams becomes a significant factor in conducting trans-LEO missions, depending on how far the mission is to go and how difficult it might be, lunar missions being easiest and Mars missions being the most demanding. Cost is also a consideration; HLVs tend to have greater economies of scale in terms of dollars per kilogram delivered, but they carry higher initial development and operating costs.

  A return to the Moon can be accomplished using smaller launch vehicles and propellant depots. Several different architectures, with varying degrees of realism, have been developed to accomplish such a mission. In all cases, technical difficulties need to be solved before a viable transportation system is developed. The biggest unknowns are associated with the building and operation of propellant depots; delivery, storage, and transfer of propellant are technical issues that have yet to be demonstrated. These are particularly acute with cryogenic propellants (liquid oxygen and hydrogen) that have extremely low boiling points; some propellant will gradually be lost through sublimation regardless of how thermally well insulated the storage tanks are at the depot. One way to mitigate this loss is to store the propellant as water and crack it into its elemental form just prior to use. Such a strategy requires building a substantial infrastructure at the depot, including large solar arrays to generate high levels of electrical power to crack the water and processing facilities to capture and freeze the dissociated gases. This approach makes depots much more complicated facilities than simple storage tanks in orbit. It is possible to provision depots with storable propellants, that is, noncryogens that are much less susceptible to boil-off loss. But storables such as hydrazine and nitrogen tetroxide have much less specific impulse (total energy) when used, and the depots would not be configured to accept and use lunar-produced propellant in the future. The technical complexities associated with cryogenic oxygen-hydrogen depots make their development a protracted effort but after their establishment, one that provides the most extensibility, flexibility, and utility for spacefaring in the long run.