The U.S. lunar exploration program got off to a rocky start. The first six attempts in the Ranger series, between 1961 and 1964, failed to accomplish their objective of just hitting the Moon. The last three were successful, however. Ranger 7 collided with the lunar surface (as intended) on June 28, 1964. Five U.S. Lunar Orbiter spacecraft, launched in 1966 and 1967, were successfully placed in orbit around the Moon, and they relayed back to Earth high-resolution images of much of the lunar surface. Between 1966 and 1968, seven Surveyor missions soft-landed on the Moon and performed detailed analyses of the surface.

Many of these unmanned U.S. missions were performed in support of the manned Apollo program. On May 25, 1961, at a time when the U.S. space program was in great disarray, President John F. Kennedy declared that the United States would "send a man to the Moon and return him safely to Earth" before the end of the decade, and the Apollo program was born. On July 20, 1969, less than 12 years after Sputnik and only 8 years after the statement of the program's goal, Apollo 11 commander Neil Armstrong became the first human to set foot on the Moon, in Mare Tranquilitatis (Sea of Tranquility). Three and a half years later, on December 14, 1972, scientist-astronaut Harrison Schmitt, of Apollo 17, was the last.

The astronauts who traveled in pairs to the lunar surface in each lunar lander (see the accompanying photograph) performed numerous geological and other scientific studies on the surface. The later landers brought with them a "lunar rover—a small golf cart—sized vehicle that greatly expanded the area the astronauts could cover. Probably the most important single aspect of the Apollo program was the collection of samples of surface rock from various locations on the Moon. In all, some 382 kg of material was returned to Earth. Chemical analysis and radioactive dating of these samples revolutionized our understanding of the Moon's surface history—no amount of Earth-based observations could have achieved the same results.

Each Apollo lander also left behind a nuclear-powered package of scientific instruments called ALSEP (Apollo Lunar Surface Experiments Package) to monitor the solar wind, measure heat flow in the Moon's interior, and, perhaps mostimportant, record lunar seismic activity. With several ALSEPs on the surface, scientists can

By any standards, the Apollo program was a spectacular success. It represents a towering achievement of the human race. The project goals were met on schedule and within budget, and our knowledge of the Moon, Earth, and the solar system increased enormously. But the "Age of Apollo" was short-lived. Public interest quickly waned. Over half a billion people breathlessly watched on television as Neil Armstrong set foot on the Moon, yet barely 3 years later, when the program was abruptly canceled for largely political (rather than scientific, technological, or economic) reasons, the landings had become so routine that they no longer excited the interest of the American public. Unmanned space science moved away from the Moon and toward the other planets, and the manned space program foundered. Perhaps one of the most amazing—and saddest—aspects of Apollo is that today, nearly three decades later, no nation on Earth (including the United States) has the desire, the capability, or the money to repeat the feat.

In 1994, the small U.S. military satellite Clementine (see Interlude 8-2) was placed in lunar orbit, and performed a detailed survey of the lunar surface. In January 1998, NASA returned to the Moon for the first time in a quarter century with the launch of Lunar Prospector, another small satellite on a 1-year mission to study the Moon's structure and origins. Prospector's first task was to survey craters near the lunar poles, to search for water ice hinted at by Clementine's detectors a few years earlier. Since the Sun never rises more than a few degrees above the horizon, as seen from the Moon's polar regions, temperatures on the permanently shaded floors of craters near the poles never exceed about 50 K. Consequently, scientists had theorized, ice there could have remained permanently frozen since the very early days of the solar system, never melting or vaporizing and hence never escaping into space. In early March, 1998, Prospector mission scientists announced that sensitive equipment on board the spacecraft had indeed detected large amounts of water ice—perhaps hundreds of millions of tons, in the form of tiny crystals mixed with the lunar regolith.

Plans do exist to establish permanent human colonies on the Moon, either for commercial ventures, such as mining, or for scientific research. Proposals have also been made to site large optical, radio, and other telescopes on the lunar surface. Such instruments, which could be constructed larger than Earth-based devices, would enjoy perfect seeing and no light pollution. None of these projects is scheduled to become reality in the near future, although the discovery of water on the lunar surface alleviates at least one major logistical problem associated with such an undertaking. After a brief encounter with humankind, the Moon is once again a lifeless, unchanging world.