Exploration by Spacecraft First Robotic Missions

Following the launch in 1957 of the U.S.S.R.'s satellite Sputnik, the first spacecraft to orbit Earth, it became obvious that the next major goal of both the Soviet and the U.S. space programs would be the Moon. The United States quickly prepared and launched a few robotic lunar probes, most of which failed and none of which reached the Moon. The Soviet Union had more success, achieving in 1959 the first escape from Earth's gravity with Luna 1, the first impact on the lunar surface with Luna 2, and the first photographic survey of the Moon's far side with Luna 3. After the National Aeronautics and Space Administration (NASA) was founded in 1958, the U.S. program became more ambitious technically and more scientifically oriented. Initial spacecraft investigations were geared toward studying the Moon's fundamental character as a planetary body by means of seismic observation, gamma-ray spectrometry, and close-up imaging. Scientists believed that even limited seismic data would give clues toward resolving the question as to whether the Moon was a primitive, undifferentiated body or one that had been heated and modified by physical and chemical processes such as those on Earth. Gamma-ray measurements would complement the seismic results by showing whether the Moon's interior had

One of the first recorded views of the Moon's far side, part of a 29-photograph sequence taken by the Soviet Luna 3 spacecraft on October 7,1959. Mare Smythii, which lies on the boundary between the near and far sides, is the circular dark patch below and left of centre, and Mare Moscoviense is the dark circle at upper right. At lower right, appearing as a dark spot with an inner white dot, is the crater Tsiolkovskiy with its central peak. NASA/Goddard Space Flight Center sufficient radioactivity to serve as an active heat engine, and they would also give some information on the chemical composition of the lunar surface. Imaging would reveal features too small to be seen from Earth, perhaps providing information on lunar surface processes and also arousing public interest.

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 Yury 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 manned 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

Planet Earth rising above the lunar horizon, an unprecedented view captured in December 1968 by Apollo 8 astronauts as their orbit carried them clear of the far side of the Moon. NASA

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. The Luna samples that were returned to Earth were valuable because they were collected from eastern equatorial areas far from the Apollo sites.

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 astronautsFrank 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 manned 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 Ni, comparable to the Apollo program's Saturn V. After several launch failures of the Ni, the program was canceled in 1974.

After the Apollo missions, lunar scientists continued to conduct multi-spectral 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 geochemi-cal properties by an automated spacecraft in polar orbit above the Moon and

Apollo 15 astronaut James B. Irwin standing in back of the lunar roving vehicle; the lunar module (LM) is at left with the modular equipment storage assembly (MESA) in front of it. Apollo 15 was launched July 26,1971. NASA

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.

The Clementine and Lunar Prospector spacecraft, operating in lunar polar orbits, used complementary suites of remote-sensing instruments to map the entire Moon, measuring its surface composition, geo-morphology, topography, and gravitational and magnetic anomalies. The topographic data highlighted the huge South Pole-Aitken Basin, which, like the other basins on the far side, is devoid of lava filling. Measuring roughly 2,500 km (1,550 miles) in diameter and 13 km (8 miles) deep, it is the largest impact feature on the Moon and the largest known in the solar system; because of its location, its existence was not confirmed until the Lunar Orbiter missions in the 1960s. The gravity data collected by the spacecraft, combined with topography, confirmed the existence of a thick, rigid crust, giving yet more

Multispectral image of the Moon's Mare Tranquillitatis and Mare Serenitatis regions (upper left and lower right, respectively) derived from observations by the Galileo spacecraft in December 1992 during its second lunar flyby. The image was constructed from multiple exposures made at different wavelengths. The photo indicates both low and high concentrations of titanium on the planet. NASA/Goddard Space Flight Center evidence that the Moon's heat source has expired. Both spacecraft missions hinted at the long-considered possibility that water ice exists in permanently shadowed polar craters. The most persuasive evidence came from the neutron spectrometer of Lunar Prospector.

Toward the end of the first decade of the 21st century, there was a revival of lunar exploration on the part of several spacefaring nations. In 2007 Japan

The Moon's south polar region in a mosaic of images made by the U.S. Clementine spacecraft from lunar orbit in 1994. The mosaic, which is centred on the south pole and combines the illumination received over more than two of the Moon's solar days (each about 29 Earth days), reveals the existence of appreciable permanently shadowed areas where water ice could exist. Ice deposits, if they could be mined economically, would constitute an important resource for a future manned lunar outpost. NASA/Goddard Space Flight Center launched the Kaguya satellite, which mapped the Moon's gravity field. That same year, China launched its first lunar probe, Chang'e 1, and the U.S. search engine company Google announced a $30 million prize to the first privately funded team to land a robotic rover on the Moon. In 2008 Chandrayaan-1, the first Indian lunar probe, studied the Moon's mineralogy.

Future planned unmanned missions include other satellites in the Kaguya, Chang'e, and Chandrayaan series. Russia and the U.K. have announced probes, Luna-Glob and MoonLITE, respectively, scheduled for launch in 2014, that will study the Moon's seismic activity. The U.S. has planned three lunar probes: the Lunar Reconnaissance Orbiter (LRO, 2009), which will map the Moon; the Lunar Atmosphere and Dust Environment Explorer (LADEE, 2011), which will study the Moon's atmosphere, and the Gravity Recovery and Interior Laboratory (GRAIL, 2011), which will accurately map the Moon's gravity field. These three missions will pave the way for the return of astronauts to the Moon in 2020 as part of the Constellation program, which will carry four astronauts in Orion, the main spacecraft, and Altair, the lunar lander.

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