Global and Regional Data about the Moon

Despite the major achievements in studying the Moon during the last quarter-century, our global knowledge is still patchy and incomplete. Although the Moon is so close, access has always been limited. Half of the Moon remained completely unknown until spacecraft could be sent around it. Even with the Apollo program and the Soviet and U.S. robotic lunar explorations, humans have directly explored only 6 sites on the Moon and soft-landed robots at another 10 (not counting the Surveyor 3 site, which was also visited by the Apollo 12 mission; see Table 2.1 and Figs. 2.1 and 2.2). These sites are all on the lunar nearside, and most of them are near the lunar equator. Sample return missions (both human and robotic) have provided information on only nine sites; even with the addition of Antarctic lunar meteorites (whose exact sources are unknown), the lunar sampling grid is still minimal. To compensate for limited access, lunar scientists have two advantages. One advantage is technical. The instruments used for lunar exploration by both humans and robots, and the techniques used to analyze lunar samples back on Earth, were the most powerful and modern that could be applied. As a result, we have learned things about the Moon in a few years that took decades to learn about the Earth. A second advantage is the nature of the Moon itself. Compared to the Earth, the Moon is a simple world; it lacks the complications produced by plate tectonics, active volcanism, weather, water, and life. The geological history of the Moon, although complex and exciting, is potentially quite decipherable because it is simpler than Earth’s. This is why it has been possible, with only a few landings and sample returns, to establish the outlines of the Moon’s origin and development and to identify the new scientific questions to be answered by future missions. The next step in lunar studies is to acquire complete global information. It is paradoxical that the Apollo program, which provided so much information about a few parts of the Moon, did not also produce full global data about such key lunar characteristics as surface chemistry, surface mineral composition, gravity, magnetic fields, and topography—in short, a scientific database for the whole Moon as a single world. The need for more global lunar data is clear, and so is the method necessary to collect it—a polar-orbiting spacecraft, equipped with an array of scientific instruments and able to scan the entire Moon over the course of a year. Proposed at various times under various names, the mission is now being actively studied as the Lunar Observer (see section 11.4.6), and is regarded as a key step in the further exploration of the Moon by both robots and human beings.