Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2004 NAE Symposium on Frontiers of Engineering (2005)

B. KENT JOOSTEN

NASA Lyndon B. Johnson Space Center

Houston, Texas

The National Vision for Space Exploration calls for an American return to the Moon in preparation for the human exploration of Mars and other destinations. The surface environment of the Moon presents many challenges for human operations, but recent findings from robotic and Earth-based studies indicate that the lunar polar regions may offer advantages in terms of thermal conditions, availability of solar energy, and access to local resources. Although accessing these regions represents challenges in terms of orbital dynamics and propulsive performance, methods of accessing the polar regions are being actively investigated. Robotic missions are also being planned to gather environmental data on the poles and other areas of the Moon.

The National Vision for Space Exploration (NASA, 2004) laid out by President Bush in January 2004, calls for a return to the Moon in preparation for the human exploration of Mars and other destinations. According to this plan, the Moon will provide an operational environment for demonstrating human exploration technologies and capabilities within relatively safe reach of Earth. These capabilities included sustainable exploration techniques, such as the utilization of space resources, and human-scale exploration systems, such as power generation, surface mobility, and habitation and life support systems. In addition, lunar

missions will pursue scientific investigations, such as gathering geological records of the early solar system.

Because the Moon’s rotation rate is tidally locked to its revolution about the Earth, the Moon is a “slow rotator.” The lunar diurnal period is 29.5 days. The long lunar night, combined with the absence of a lunar atmosphere, leads to large variations in surface temperature, from 117°C (243°F) to –170°C (-272°F) near the equator (Heiken et al., 1991). In addition to the severe cold during the lunar night, the lack of solar illumination for almost 15 days would mean an extended human exploration mission would have to rely upon nonphotovoltaic (i.e., nuclear) power sources.

During the Apollo Program, missions landed early in the lunar day and departed before the thermal conditions of lunar noon had set in. The polar regions of the Moon were essentially unexplored either by the Apollo Program or the Lunar Orbiter robotic missions of the 1960s. The polar regions were never considered as targets for the Apollo landings because of the constraints of orbital mechanics (i.e., high propulsive requirements and the absence of “free-return” abort capabilities). However, recent robotic and Earth-based observations of the Moon have revealed exciting new information about these previously unexplored regions.

Both the 1994 Clementine spacecraft and the Goldstone Solar System Radar have indicated the existence of permanently shadowed and nearly permanently lit areas in the rough surface topography in the vicinity of the lunar poles (Bussey et al., 1999; Margot et al., 1999). This is possible because of the very low inclination (only 1.5 degrees) of the lunar equator with respect to the ecliptic plane, which results in very shallow solar incidence. Illuminated areas of the poles are thought to have surface temperatures of –53°C (–63°F); the permanently shadowed regions would be only a few degrees above absolute zero (perhaps around –233°C or –387°F).

Both the shadowed and lit terrain, which appear to be in close proximity, are of interest for human lunar exploration. The nearly permanently illuminated areas offer moderate thermal conditions and potentially abundant solar power. The permanently shadowed regions have long been hypothesized to harbor “cold traps” that might have preserved volatiles, such as water ice deposited by millennia of cometary impacts. In fact, in 1998, the Lunar Prospector spacecraft detected trapped hydrogen, possibly in the form of water ice, in these regions (Feldman et al., 1998). If water were accessible in sufficient quantities, it would be a key contributor to sustained human presence on the Moon. Robotic lunar missions are planned to refine the topographic information and assess the true availability of resources.

The Apollo missions did not attempt to use lunar polar regions as landing sites for several reasons. First, free-return trajectories, which allow the outbound trip to the Moon to be aborted and the spacecraft returned to Earth with little additional propulsion requirement, are not compatible with injection into lunar polar orbit. Polar orbits slowly drift into alignments that are not compatible with a return trajectory. Second, by choosing near-equatorial parking orbits and landing sites, crews could begin the return journey to Earth at nearly any moment with little propulsion performance penalty. Finally, there were strict limits on sun angles on the lunar sites during landing to ensure that the crew had good visibility of terrain relief.

Analyses are under way to meet these challenges. Future missions will address propulsion system failures by having additional levels of system redundancy and reliability rather than by free-return aborts. The orbital alignment issues can be addressed in several ways: (1) additional maneuvering capabilities can be built into the mission profile (with the associated propellant weight penalties); (2) an alternative to the Apollo “lunar orbit rendezvous” technique can be used, perhaps using Earth-Moon Lagrange points as a staging location; or (3) “safe havens” can be established on the lunar surface, thus negating the need for return to Earth at any given moment and making it possible to wait for more optimal return trajectories. We also expect that automated descent, landing, and hazard-avoidance technologies will obviate the need for strict lighting conditions.

Lunar polar locations may have environmental and resource advantages over equatorial sites and may enable future lunar mission to meet the goals laid out in the National Vision for Space Exploration. The challenges associated with human access to the polar regions are understood. With data from future robotic missions and advanced human mission capabilities and technologies, we should be able to address and overcome these challenges.

Bussey, D.B.J, P.D. Spudis, and M.S. Robinson. 1999. Illumination conditions at the lunar south pole. Geophysical Research Letters 26(9): 1187–1190.

Feldman, W.C., S. Maurice, A.B. Binder, B.L. Barraclough, R.C. Elphic, and D.J. Lawrence. 1998. Fluxes of fast and epithermal neutrons from Lunar Prospector: evidence of water ice at the lunar poles. Science 281(5382): 1496–1500.

Heiken, G.H., D.T. Vaniman, and B.M. French. 1991. Lunar Sourcebook. Cambridge U.K.: Cambridge University Press.

Margot, J.L., D.B. Campbell, R.F. Jurgens, and M.A. Slade. 1999. Topography of the lunar poles from radar interferometry: a survey of cold trap locations. Science 284(5420): 1658–1660.

NASA (National Aeronautics and Space Administration). 2004. The Vision for Space Exploration. NP-2004-01-334-HQ. Washington, D.C.: NASA Headquarters. Available online at: <http://www.nasa.gov/pdf/55583main_vision_space_exploration2.pdf>.