The Moon as a habitat

Because of its relative proximity, we begin our survey with Earth's single natural satellite, the Moon. Luna is the only solar-system world, other than the Earth, on which humans have walked. Many robotic probes have flown by, orbited, landed upon, or returned samples from the Moon. During 1968-1972, the crews of Apollos 8 and 10 orbited the Moon, Apollo 13 circled the Moon during its aborted mission, and the lunar modules of Apollos 11, 12, and 14-17 landed upon the lunar surface (Figure 2.1).

Following in the footsteps of the Apollo astronauts, future lunar pioneers will encounter an environment vastly different to any on Earth. The Moon orbits the Earth once every month, at an average distance of almost 400,000 kilometers. Contemporary rockets require about three days for a one-way lunar voyage.

Because the Earth has a mass 81 times that of the Moon, lunar gravity is much weaker than terrestrial gravity. Discounting the mass of the gear required to keep a person alive in the Moon's vacuum, a person weighs only one-sixth as much on the Moon's surface as on Earth.

Not only does the Moon not have an atmosphere, but water is either very rare or non-existent there. Two space probes—Clementine and Lunar Prospector—have returned preliminary data indicating that waterice from ancient comet impacts may exist in craters near the lunar poles that are shielded from sunlight. But until this is confirmed and we have a better idea regarding the magnitude of this resource, future lunar colonists must plan to recycle as much of their habitat's water as possible.

FIGURE 2.1 Apollo 15 astronaut David Irwin works on the lunar rover during an excursion nearMt Hadley. Hollow volcanic lava tubes in this region might someday house human settlers. (Courtesy NASA)

Because the Moon does not have an atmosphere, one might think that solar energy would be more effective in powering a lunar civilization than it is on Earth. However, because of tides produced by the much more massive Earth, the Moon's rotation rate around its axis is identical to its revolution rate around the Earth. This means that a lunar "day" consists of 14 Earth days of sunlight followed by 14 Earth days of darkness. Because of this factor and the lack of an atmosphere, lunar temperatures are quite variable, ranging from a low of-170 degrees Celsius to a high of 130 degrees Celsius.

Unless we restrict human settlement to its polar regions which will benefit from near-continuous sunlight, any permanent lunar residents will have to contend with these temperature variations. Also, some form of energy storage facility or auxiliary power source will be necessary to supplement direct solar energy during the long lunar nights.

Some consider the lack of a lunar atmosphere to be a great advantage for future astronomical facilities based on our Moon. It has been suggested that a radio observatory on the lunar farside would be shielded from terrestrial radio signals by the Moon's limb. However, much larger radio telescopes could be constructed in free space and shielded from terrestrial radio signals by bulk material or simply pointed away from the Earth.

FIGURE 2.1 Apollo 15 astronaut David Irwin works on the lunar rover during an excursion nearMt Hadley. Hollow volcanic lava tubes in this region might someday house human settlers. (Courtesy NASA)

Another possible function for a future human civilization—mining of material for use on the Earth or in space—also has issues. Some have suggested that copious reserves ofhelium-3, a form of helium ejected from the Sun and of possible significance to a future nuclear-fusion-based economy, exist in usable concentrations in the lunar soil. Although this is possible, concentrations are very low and the Apollo core samples only extended a meter or so into the lunar dust and regolith. We may have to go further—the atmospheres of the giant planets, or the solar wind—to tap into significant helium-3 reserves.

Advocates of lunar mining and science may come into conflict. No matter how clean a mining process, quantities of fugitive lunar dust will be raised above the surface. In the low lunar gravity, these dust grains may remain aloft for many months, partially obscuring the pristine lunar skies sought by some astronomers and contaminating sensitive optical instruments as the dust inevitably falls back to the surface.

Even though volatile material such as water is lacking or very rare on the Moon's surface, methods have been suggested that could be used to catapult mined lunar rock from there. This material might be of use in constructing solar-powered satellites to beam energy to Earth or as cosmic-ray shielding for large space habitats. However, near-Earth objects (NEOs)—a class of asteroids and comet nuclei that approach the Earth— might be a source of higher grade ore and may require less energy to access economically.

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