After the successful launching of the first artificial satellites, the Soviet Union’s Sputnik 7 in October 1957 and the United States’ Explorer 1 in early 1958, these two countries began launching probes to the moon. Initially, the Soviet Union was the more successful in its lunar space program, but the most spectacular achievement was made by the United States in July 1969, when Neil Armstrong and Edwin Aldrin became the first men to set foot on the lunar surface.
The Soviet Luna missions
The Soviet Union began its lunar program with the launch of Luna 7 on January 2,1959. Weighing about 790 pounds (360 kilograms), it carried instruments to measure magnetic fields and solar and cosmic radiation and passed within 3,700 miles (6,000 kilometers) of the moon. Nine months later, the next Soviet lunar probe, Luna 2, hard-landed on the moon’s surface, thereby becoming the first probe to reach another world. And shortly afterward, in October 1959, Luna 3 made the first circumlunar flight and sent back the first photographs of the far side of the moon—during a period in which the United States was experiencing a series of failures in its lunar probe program.
Following the success of the first three Luna missions, the Soviet Union devoted much of its space effort to Mars and Venus probes. In 1963, however, it launched Luna 4, which attempted a soft landing on the moon. But the probe by-passed the moon on April 6, 1963 and went into orbit around the sun. After four further attempts, the Soviet Union achieved the first successful soft landing with Luna 9, which landed on the moon on February 4,1966 and began transmitting close-up pictures of the lunar surface.
Three years later Luna 15 was launched— on July 12,1969 (3| days before Apollo 77)— with the intention of soft landing on the moon, taking a soil sample, and returning to earth. The Soviet probe was successfully placed in lunar orbit—where it remained during the Apollo flight—but crashed on the moon during its descent to the surface. Luna 16, however, was successful. Launched on September 12,1970, it soft-landed on the moon, where its automatic drilling rig bored down to a depth of just over a foot and extracted some 0.2 pound (100 grams) of rock and soil. This sample was then placed in a sealed container and launched back to earth, where it was recovered and its contents analyzed.
The first robot lunar explorer
Luna 17, launched on November 10,1970, was a great technical success for the Soviet space program. The unmanned craft landed safely on the moon and, after checks by its television cameras, one of its two ramps was lowered on command from ground control on earth. Off the top of the craft rolled Lunokhod 7—an eight-wheeled robot vehicle which, under guidance from earth, was capable of moving about on the moon’s surface. Warmed by the circulation of heated gas, it withstood the extremely cold lunar nights—during which the temperature typically falls to below —200° F. ( — 130° Cl—and remained functional for more than 10 months. In this period, it traveled nearly 7 miles (11 kilometers), testing the soil at points on its path and sending back many television pictures of the surface. In January 1973, the Soviet Union repeated this success with the Luna 21 mission, which carried Lunokhod2. This second “moonwalker” traveled about 3| times the distance of its predecessor and also sent back television pictures of the surface.
The Soviet Union, by this time surpassed by the United States in the field of lunar exploration, thereafter devoted much of its space effort to sending probes to the planets, such as Venus.
The first American lunar probes
Shortly after the success of its first earth satellite, Explorer 1, the United States launched three small moon probes—Pioneer 0, Pioneer 1, and Pioneer2. Each weighed 84 pounds (38 kilograms) and was equipped with a camera, a micrometeorite detector, and a magnetometer, and they were intended to go into orbit around the moon. All three failed in their objective but valuable experience was gained for future missions. Moreover, before falling back into the southern Pacific Ocean after a 43-hour flight, Pioneer 1 recorded the extent of the Van Allen radiation belts in the earth’s atmosphere, previously discovered by Explorer 1.
The next two American lunar probes, Pioneers (launched in late 1958) and Pioneer 4 (launched in early 1959), were even smaller, weighing only 13 pounds (6 kilograms). The aim of their missions was to fly past the moon at close range. Again, neither probe was completely successful, the closest approach being achieved by Pioneer 4, which passed within about 37,000 miles (60,000 kilometers) of the moon but failed to transmit pictures back to earth.
With the development of the powerful Atlas launching rockets, a new series of Pioneer probes was planned. These probes were more sophisticated than the previous Pioneers, each being a 39.5-inch (100-centimeter) sphere with four solar paddles and hydrazine propulsion units. Four probes were launched between September 1959 and December 1960, but all failed disastrously because of problems with the rocket booster. Meanwhile, Soviet Luna probes had photographed 70 per cent of the moon’s far side.
The Ranger spacecraft
The American moon program began to achieve a measure of success with the introduction of the Ranger spacecraft. They were stabilized in all three axes, had a high-gain antenna pointing at the earth, and an array of instruments directed toward the moon. Despite the effort put into the project, all of the first six missions failed to perform perfectly.
Ranger i and Ranger 2, intended as test vehicles, achieved only a low orbit above the earth. Ranger 3, Ranger 4, and Ranger 5 were prepared for attempts to hard-land a capsule on the moon’s surface and then transmit seismic and meteorite data back to earth. The plan was to release a small capsule 13 miles (21 kilometers) above the moon, allow it to fall toward the surface then, by firing retrorockets, slow down the capsule to zero velocity at a height of about 1,100 feet (335 meters), from where it would free-fall, and finally hit the surface at about 120 miles (190 kilometers) per hour. On impact, the capsule was designed to right itself so that its aerial pointed toward the earth. But this ingenious device was never put to the test because all three missions malfunctioned. However, Ranger 4 did reach the moon, becoming the first American probe to land on the lunar surface (it crash-landed on the far side in April 1962).
At about this time there was a marked change in emphasis of the American lunar program, stimulated by President John Kennedy’s declaration in 1961 that the United States’ space effort should be directed toward landing a man on the moon.
The first step was to replace the early seismic-capsule Rangers with a new series of Ranger probes equipped with an extensive array of cameras. Ranger 6, the first of the new models, was launched in early 1964, en route for a hard landing on the moon. But although it landed on target, its cameras failed to transmit pictures.
The next (and final) three Ranger missions were successful, however. Launched between July 1964 and March 1965, Ranger 7, Ranger 8, and Ranger9 relayed more than 17,000 high-resolution pictures back to earth. Surface features, 1,000 times smaller than any visible from earth, could be clearly seen in some of the photographs; Ranger9s final series of pictures, for example, were taken from about 5,000 feet (1,500 meters) above the lunar surface and showed details as small as one foot across.
The Surveyor probes
The next logical step for the United States was to attempt a soft landing on the moon, which was achieved in May 1966 with the Surveyor 1 mission (four months after Luna 9s touchdown on the surface). Launched by an Atlas-Centaur rocket, Surveyor 1 landed successfully near the lunar equator after a 63-hour flight. As it approached the lunar surface, retrorockets decelerated the craft until it was nearly stationary at a height of about 12 feet (4 meters), from which it free-fell to the surface. Shock-absorbers on the legs softened the impact. Designed to withstand the extremes of lunar temperature, Surveyor 1 was equipped with a television camera capable of rotating through a complete circle. Over a six-week period, this camera relayed more than 11,500 photographs of the lunar surface.
The next three successful missions, Surveyors, Surveyors, and Surveyors, also landed near the equator, whereas Surveyor 7, the final one in this series, landed outside the rim of the crater Tycho in the Southern Hemisphere. Each of these probes carried a mechanical scoop to carry out investigations of the lunar soil. Surveyor 3 dug 7 inches (18 centimeters) into the lunar soil and sent back photographs of the results. Surveyors carried special equipment to analyze the chemical composition of the lunar soil—the first time this had been done. The analysis showed that the moon’s soil is reasonably similar to terrestrial basaltic soil. More importantly perhaps, the Surveyor probes proved that the technology existed for a manned landing on the lunar surface.
The Orbiter probes
Having established the feasibility of landing a man on the moon, the United States launched another series of unmanned probes with the object of assessing potential landing sites for the future manned Apollo missions, in the year following August 1966, five Orbiter probes were put into low orbits around the moon. They photographed more than 3 million square miles (5 million square kilometers) of potential Apollo landing areas. They also made the first complete photographic survey of the far side of the moon.
An interesting feature of the Orbiter spacecraft was the way in which they recorded photographic images. Unlike the Ranger and Surveyor craft, which used conventional television cameras, the Orbiters took photographs on film. The photographs were developed on the craft and the resultant images electronically scanned, the information then being relayed to earth. The scanner examined each photograph in small strips, and for this reason the Orbiter photographs have a faintly striped appearance.
Manned landings on the moon
The successes of the unmanned Ranger, Surveyor, and Orbiter programs—and of the early manned Apollo missions—prepared the way for landing men on the moon. The United States achieved this goal at the first attempt, the Apollo 11 mission (launched on July 16, 1969). After going into lunar orbit, Neil Armstrong and Edwin Aldrin transferred to the Lunar Module (the landing craft), leaving Michael Collins in the Command Module, which continued to orbit the moon. On July 20, the Lunar Module landed safely on the moon. The two astronauts spent more than 21 hours on the surface (including about two hours outside the Lunar Module), after which they returned to their module, took off, and rejoined the orbiting Command Module. On July 24, the three astronauts landed back on earth, splashing down in the Pacific Ocean.
In November 1969, Apollo 12 made another successful manned moon landing, but the next attempt, Apollo 13 (launched on April 11, 1970), was a failure. During the outward journey, an explosion caused a total loss of the main power supplies. The planned lunar landing was canceled, and only by using the Lunar Module’s air and power was a disaster averted. The three astronauts returned safely to earth on April 17.
There were four further Apollo missions to the moon, all successful; Apollo 17, launched on December 7,1972, was the last in the series. After the Apollo missions, the United States concentrated on developing the reusable Space Shuttle.