Early studies

Investigations of the Moon and some understanding of lunar phenomena can be traced back to a few centuries BCE. In ancient China the Moon’s motion was carefully recorded as part of a grand structure of astrological thought. In both China and the Middle East, observations became accurate enough to enable the prediction of eclipses, and the recording of eclipses left data of great value for later scientists interested in tracing the history of the Earth-Moon system. (See eclipse: Uses of eclipses for astronomical purposes.) Several early Greek philosophers saw reason to believe that the Moon was inhabited, although they did not base their conclusion on scientific principles. The Greek astronomer and mathematician Hipparchus, on the other hand, took an experimental approach: observing Earth’s round shadow creeping across the Moon during a lunar eclipse, he concluded that Earth must be spherical and that the Moon was an independent world, and he correctly explained the Moon’s phases and accurately estimated the distance between the two bodies. Later, Mayan calendars were constructed that reflected the results of careful observation and long-range prediction.

For centuries, knowledge about the Moon accumulated slowly, driven by astrological and navigational needs, until an outburst of progress began in the Renaissance. In the early 1600s the German astronomer Johannes Kepler used observations made by Tycho Brahe of Denmark to find empirically the laws governing planetary motion. Kepler wrote a remarkable work of science fiction, Somnium (“The Dream”), that describes the life of imagined inhabitants of the Moon and correctly portrays such facts as the high temperature of the Moon’s sunlit side. In 1609–10 Galileo began his telescopic observations that forever changed human understanding of the Moon. Most effort hitherto had been devoted to understanding the movements of the Moon through space, but now astronomers began to focus their attention on the character of the Moon itself.

Among nine U.S. Ranger missions launched between 1961 and 1965, Ranger 4 (1962) became the first U.S. spacecraft to strike the Moon. Only the last three craft, however, avoided the plaguing malfunctions that limited or prematurely ended the missions of their predecessors. Ranger 7 (1964) returned thousands of excellent television images before impacting as designed, and Rangers 8 and 9 (both 1965) followed successfully. The impact locale of Ranger 7 was named Mare Cognitum for the new knowledge gained, a major example of which was the discovery that even small lunar features have been mostly subdued from incessant meteorite impacts.

After a number of failures in the mid-1960s, the Soviet Union scored several notable achievements: the first successful lunar soft landing by Luna 9 and the first lunar orbit by Luna 10, both in 1966. Pictures from Luna 9 revealed the soft, rubbly nature of the regolith and, because the landing capsule did not sink out of sight, confirmed its approximate bearing strength. Gamma-ray data from Luna 10 hinted at a basaltic composition for near-side regions. In 1965 the Soviet flyby mission designated Zond 3 returned good pictures of the Moon’s far side.

In the mid-1960s the United States carried out its own soft-landing and orbital missions. In 1966 Surveyor 1 touched down on the Moon and returned panoramic television images. Six more Surveyors followed between 1966 and 1968, with two failures; they provided not only detailed television views of lunar scenery but also the first chemical data on lunar soil and the first soil-mechanics information showing mechanical properties of the top few centimetres of the regolith. Also, during 1966–67 five U.S. Lunar Orbiters made photographic surveys of most of the lunar surface, providing the mapping essential for planning the Apollo missions.

Apollo to the present

After the Soviet cosmonaut Yuri Gagarin pioneered human Earth-orbital flight in April 1961, U.S. President John F. Kennedy established the national objective of landing a man on the Moon and returning him safely by the end of the decade. Apollo was the result of that effort.

Within a few years the Soviet Union and the United States were heavily engaged in a political and technological race to launch crewed flights to the Moon. At the time, the Soviets did not publicly acknowledge the full extent of their program, but they did launch a number of human-precursor circumlunar missions between 1968 and 1970 under the generic name Zond, using spacecraft derived from their piloted Soyuz design. Some of the Zond flights brought back colour photographs of the Moon’s far side and safely carried live tortoises and other organisms around the Moon and back to Earth. In parallel with these developments, Soviet scientists began launching a series of robotic Luna spacecraft designed to go into lunar orbit and then land with heavy payloads. This series, continuing to 1976, eventually returned drill-core samples of regolith to Earth and also landed two wheeled rovers, Lunokhod 1 and 2 (1970 and 1973), that pioneered robotic mobile exploration of the Moon.

In December 1968, acting partly out of concern that the Soviet Union might be first in getting people to the Moon’s vicinity, the United States employed the Apollo 8 mission to take three astronauts—Frank Borman, James Lovell, and William Anders—into lunar orbit. After circling the Moon three times, the crew returned home safely with hundreds of photographs. The Apollo 9 and 10 missions completed the remaining tests of the systems needed for landing on and ascending from the Moon. On July 20, 1969, Apollo 11 astronauts Neil Armstrong and Edwin (“Buzz”) Aldrin set foot on the Moon while Michael Collins orbited above them. Five more successful crewed landing missions followed, ending with Apollo 17 in 1972; at the completion of the program, a total of 12 astronauts had set foot on the Moon.

Twenty years later the Soviet Union admitted that it had indeed been aiming at the same goal as Apollo, not only with a set of spacecraft modules for landing on and returning from the Moon but also with the development of a huge launch vehicle, called the N1, comparable to the Apollo program’s Saturn V. After several launch failures of the N1, the program was canceled in 1974.

fter the Apollo missions, lunar scientists continued to conduct multispectral remote-sensing observations from Earth and perfected instrumental and data-analysis techniques. During Galileo’s flybys of Earth and the Moon in December 1990 and 1992 en route to Jupiter, the spacecraft demonstrated the potential for spaceborne multispectral observations—i.e., imaging the Moon in several discrete wavelength ranges—to gather geochemical data. As a next logical step, scientists generally agreed on a global survey of physical and geochemical properties by an automated spacecraft in polar orbit above the Moon and employing techniques evolved from those used during the Apollo missions. Finally, after a long hiatus, orbital mapping of the Moon resumed with the flights of the Clementine and Lunar Prospector spacecraft, launched in 1994 and 1998, respectively.