Scientists and space planners have long acknowledged that extended human residence on the Moon would be greatly aided by the use of local resources. This would avoid the high cost of lifting pay-loads against Earth's strong gravity. Certainly, lunar soil could be used for shielding habitats against the radiation environment. More advanced uses of lunar resources are clearly possible, but how advantageous they would be is presently unknown. For example, most lunar rocks are about 40 percent oxygen, and chemical and electrochemical methods for extracting it have been demonstrated in laboratories. Nevertheless, significant engineering advances would be needed before the cost and difficulty of operating an industrial-scale mining and oxygen-production facility on the Moon could be estimated and its advantages over transporting oxygen from Earth could be evaluated. In the long run, however, some form of extractive industry on the Moon is likely, in part because launching fleets of large rockets continuously from Earth would be too costly and too polluting of the atmosphere.
The solar wind has implanted hydrogen, helium, and other elements in the surfaces of fine grains of lunar soil. Though their amounts are small-they constitute about 100 parts per million in the soil-they may someday serve as a resource. They are easily released by moderate heating, but large volumes of soil would need to be processed to obtain useful amounts of the desired materials. Helium-3, a helium isotope that is rare on Earth and that has been deposited on the Moon by the solar wind, has been proposed as a fuel for nuclear fusion reactors in the future.
One natural resource uniquely available on the Moon is its polar environment. Because the Moon's axis is nearly perpendicular to the plane of the ecliptic, sunlight is always horizontal at the lunar poles, and certain areas, such as crater bottoms, exist in perpetual shadow. Under these conditions the surface may reach temperatures as low as 40 K (-388 °F, -233 °C). Some scientists have theorized that these cold traps may have collected volatile substances, including water ice, over geologic time, though others have expressed doubt that ice deposits could have survived there.
The Lunar Prospector spacecraft, which orbited the Moon for a year and a half, carried a neutron spectrometer to investigate the composition of the rego-lith within about a metre (three feet) of the surface. Neutrons originating underground owing to radioactivity and cosmic-ray bombardment interact with the nuclei of elements in the regolith en route to space, where they can be detected from orbit. A neutron loses more energy in an interaction with a light nucleus than with a heavy one, so the observed neutron spectrum can reveal whether light elements are present in the regolith. Lunar Prospector gave clear indications of light-element concentrations at both poles, interpreted as proof of excess hydrogen atoms. The observed hydrogen signature may represent the theoretically predicted deposits of water ice. However, the Kaguya spacecraft saw no evidence of water ice in a crater at the Moon's south pole where Clementine's radar had possibly detected frozen water.
A high priority for future lunar exploration is to send an autonomous robotic rover into a dark polar region to confirm the putative ice deposits, find out the form of the ice if it exists, and begin assessing its possible utility. If lunar ice can be mined economically, it can serve as a source of rocket propellants when split into its hydrogen and oxygen components. From a longer-term perspective, however, the ice would better be regarded as a limited, recyclable resource for life support (in the form of drinking water and perhaps breathable oxygen). Should this resource exist, an international policy for its conservation and management would be needed. Discovery of volatile substances anywhere on the Moon would be important both scientifically and for potential human habitation because all known lunar rocks are totally dry.
Even if no icy bonanza is discovered, the lunar polar regions still represent an important resource. Only there can be found not only continuous darkness but also continuous sunlight. A solar collector tracking the Sun from a high peak near a lunar pole could provide essentially uninterrupted heat and electric power. Also, the radiators required for eliminating waste heat could be positioned in areas of continuous darkness, where the heat could be dissipated into space.
The lunar poles also could serve as good sites for certain astronomical observations. To observe objects in the cosmos that radiate in the infrared and millimetre-wavelength regions of the spectrum, astronomers need telescopes and detectors that are cold enough to limit the interference generated by the instruments' own heat. To date, such telescopes launched into space have carried cryogenic coolants, which eventually run out. A telescope permanently sited in a lunar polar cold region and insulated from local heat sources might cool on its own to 40 K (-233 °C, or -388 °F) or lower. Although such an instrument could observe less than half the sky—ideally, one would be placed at each lunar pole—it would enable uninterrupted viewing of any object above its horizon.
One of the most beautiful and awe-inspiring phenomena that can be seen from our vantage point in the inner solar system is that of the eclipse. To see the Sun slowly disappear as if swallowed by some enormous beast or to see the Moon dim to a blood-red hue startled ancient peoples. Today, however, we know that we do not have to resort to myth to explain these events. Eclipses are the complete or partial obscuring of a celestial body by another, and in more general terms an eclipse occurs when three celestial objects become aligned.
From the perspective of a person on Earth, the Sun is eclipsed when the Moon comes between it and Earth, and the Moon is eclipsed when it moves into the shadow of Earth cast by the Sun. Eclipses of natural satellites (moons) or of spacecraft orbiting or flying past a planet occur as the bodies move into the planet's shadow. In the universe outside the solar system, the two component stars of an eclipsing binary star move around each other in such a way that their orbital plane passes through or very near Earth, and each star periodically eclipses the other as seen from Earth.
The phenomena of occulations and transits are related to eclipses. When the apparent size of the eclipsed body is much smaller than that of the eclipsing body, the phenomenon is known as an occultation. Examples are the disappearance of a star, nebula, or planet behind the Moon or the vanishing of a natural satellite or spacecraft behind some body of the solar system. A transit occurs when, as viewed from Earth or another point in space, a relatively small body passes across the disk of a larger body, usually the Sun or a planet, eclipsing only a very small area. Mercury and Venus, for example, periodically transit the Sun, and a natural satellite may transit its planet.
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