The Moon as an enabling asset for spaceflight

Abstract

A consequence of the rocket equation is that significant quantities of mass can be lifted into orbit from the bottom of the deepest gravity well in the inner Solar System (Earth's surface) but only at enormous cost. After fifty years of experience and despite technical advances, launch costs continue to make up a major fraction of the cost of space exploration. One-off satellites e launched on expendable vehicles e are still the norm in today's spaceflight paradigm, the same template we have used for the past fifty years. We cannot make what we need in space because we lack both the infrastructure and resources to fabricate our needs at the various places in space where they are needed. The current lack of strategic direction in the American civil space program is at least partly a consequence of this dilemma. Because we must launch everything we need from the surface of the Earth, spaceflight e especially the human variety e is difficult and costly. This high cost makes justifying human missions into low Earth orbit and beyond difficult and thus, we engage in lengthy debates about appropriate destinations and activities. It is hard to come up with mission goals important enough to merit the spending of significant fractions of national wealth. Debates about mission objectives become mired exercises in scientific mythmaking (e.g., the quest for extraterrestrial life) or appeals to emotion involving “inspiration” or national prestige. Spaceflight is both useful and important for a wide variety of scientific, economic and national security needs. Satellites make up a critical part of the infrastructure of modern technical civilization. What is needed is the ability to move freely throughout space with a variety of capabilities, where various satellite assets reside, to upgrade, maintain and construct large distributed systems of greater potential and speed. As most satellites reside above low Earth orbit, in cislunar space (i.e., between Earth and Moon), the freedom to move throughout this region would enable us to not only access all of these space assets, but also to travel at will beyond low Earth orbit. However, the problem of the rocket equation remains e we arrive in LEO with empty fuel tanks and to travel farther, we must carry additional fuel and equipment with us. If we could instead provision ourselves once in space, virtually unlimited horizons would beckon. Fortunately, a source of materials and energy exists nearby that can be tapped to create new space faring capability. Unique among accessible space destinations beyond low Earth orbit, the Moon possesses material and energy resources in usable form to supply a space-based transportation infrastructure, thereby allowing us to establish a permanent presence in space. After nearly a decade of intensive robotic exploration,we nowhave a detailed understanding of the environment and deposits of the poles of the Moon. Because theMoon's spin axis obliquity is low (1.5! inclination), portions of the terrain near the poles are in either near-permanent sunlight or in permanent darkness. Areas in sunlight can support the generation of electrical power via the emplacement of solar photovoltaic arrays. Infrequent and brief periods of solar eclipse can be bridged through the use of rechargeable fuel cells, which generate electricity by combining hydrogen and oxygen into water. During the periods of solar illumination, solar arrays can then decompose the water back into its component gases, creating a completely reversible process. In addition to its benefits for power generation, illuminated zones near the lunar poles are characterized by constantly grazing incidence of sunlight, which keeps the surface at a nearly uniform temperature of about 220 K (50 !C). Thus, these polar areas of near-permanent illumination permit extended presence on the Moon, with both constant available power and a benign thermal environment. Surfaces within the permanently dark areas near the poles of the Moon have extremely low temperatures (25e40 K or 247! to 233 !C) and as a result, act as “cold traps” to collect and retain volatile materials, such as water. Although water is rare on the Moon, its surface is constantly bombarded by water-bearing objects, such as asteroids and comets. This water, vaporized on impact, is mostly lost to space, but it can be preserved if it gets into E-mail address: Spudis@lpi.usra.edu.