The drawing outlines essential steps in a Lunar Orbiter mission following launch and transit to the Moon's vicinity. In step 1 the spacecraft fired its velocity control rocket to make a course correction. In step 2 the rocket fired again to deboost the spacecraft into its initial orbit of the Moon. Here its orbit was adjusted, and the first pictures were made (3) before the Orbiter changed orbital parameters (4) to assume an elliptical orbit that brought it closer to the lunar surface for further photographic coverage (5).

In the early years of the space race, the United States engaged in a series of key programs designed to fly past, orbit, impact, and soft-land on the Moon. The implementation of projects such as Ranger, Lunar Orbiter, and Surveyor was critical to the eventual success of the Apollo program, NASA's major program to land astronauts on the surface of the Moon before the end of the 1960s.

The United States took an early interest in the Moon. Less than a year after the launch of the Soviet Sputnik satellite that set off the “space race,” the U.S. Air Force Ballistic Missile Division assembled a series of small probes designed to enter orbit around the Moon. These spacecraft, called Able (and later, Pioneer), carried rudimentary infrared TV scanners to take pictures of the lunar surface for about two weeks while in orbit around the Moon. Unfortunately, none of the three Air Force Able probes ever made it to the Moon. After a failure in August 1958, the second, launched in October 1958, reached about one-quarter of the way to the Moon and returned valuable data on the Van Allen radiation belts that encircle the Earth. The third Able, launched in November 1958, failed to reach beyond 1,500 kilometres of the Earth.

 
Ranger 7 took this image, the first picture of the Moon by a U.S. spacecraft, on 31 July 1964 at 13:09 UT (9:09 AM EDT) about 17 minutes before impacting the lunar surface

Air Force attempts were followed by two Army launches in 1958 and 1959, carrying probes designed for more simple flybys of the Moon. They were also to test an experimental imaging system. The first, Pioneer 3, reached about one-fourth of the way to the Moon. The second, Pioneer 4, had the distinction of being the first U.S. spacecraft to reach “escape velocity,” i.e., speed sufficient to leave Earth orbit. Due to an engine malfunction, however, the probe passed by the Moon at a range of about 59,545 kilometres, much farther than originally planned. Pioneer 4 eventually became the first U.S. probe to go into solar orbit.

The final group of early lunar probes was built by Space Technology Laboratories for NASA, and all were designed to reach lunar orbit. They were launched by the Atlas Able booster. All three (of a planned four), launched between November 1959 and December 1960, failed to reach the Moon.

 
Surveyor 7 mosaic of the rim area of Tycho from the highland region north of the crater on the Moon. Surveyor 7 landed 10 January 1969 and took about 21,000 photos over a month, some of which were used to make up this mosaic.

With these failures behind it, NASA embarked on several new projects designed specifically to support the Apollo program. The first of these was Ranger, managed by NASA's Jet Propulsion Laboratory (JPL). Originally, the Ranger spacecraft, launched by the Atlas Agena B, were designed to land simple instrumented capsules made of balsa wood on the lunar surface. The first two experimental spacecraft failed in their test missions. Ranger 3, the first fully operational spacecraft, missed the Moon and flew into solar orbit. Ranger 4 lost power during the three-day-long outbound flight. Because of an accurate initial trajectory, however, it hit the surface of the Moon on April 26, 1962, becoming the first U.S. spacecraft to reach the Moon, three years after the Soviets achieved a similar success with Luna 2. Ranger 5, launched in October 1962, also missed the Moon.

After these failures, NASA introduced a new Ranger design that had more modest missions goals. Instead of landing a probe on the lunar surface, these “Block III” Rangers were designed to take high-resolution photos prior to impact using a set of six cameras. Ranger 6, launched in January 1964, successfully impacted on the Moon, but the camera system did not function. Success finally came with Ranger 7, launched in July 1964, which took more than 4,000 impressive photos of the Moon as it careened to the northern rim of the Sea of Clouds. The photos were, in many cases, one thousand times better than photos of the Moon taken from the Earth. Ranger 8 was another resounding success. It took thousands of photos prior to impact on the Sea of Tranquillity in February 1965, an area that was the target for the first Apollo landing in 1969. Ranger 9 continued the spate of successes, taking nearly 6,000 spectacular photos before crashing into the Alphonsus crater in the lunar highlands in March 1965.

The next NASA program, also managed by JPL was the much more ambitious Surveyor project. NASA originally conceived the Surveyor program in 1963 as a lander/orbiter combination project, but later scaled it down to only soft-landing. Each lander comprised a three-legged triangular aluminium structure with a large solid propellant retro-rocket engine at the base. The lander was equipped with an advanced imaging system. After three tests of the Atlas Centaur booster in 1965-1966, NASA launched Surveyor 1 in May 1966. The mission was a resounding success. The spacecraft landed successfully in the Ocean of Storms on June 2, 1966 and took more than 11,000 photos of the surface over a month-long period. Although Surveyor 2 failed, Surveyor 3 successfully landed on the Moon in April 1967. In addition to an imaging system, the lander also included a remote scooper arm to determine the density of lunar soil. Experiments showed that the lunar soil had the consistency of wet sand. More than two years later, in November 1969, Apollo 12 astronauts Charles Conrad, Jr. and Alan Bean landed their Intrepid Lunar Module about 180 meters from Surveyor 3 and recovered some its parts to evaluate the environmental effects of a long period on the Moon's surface. Surveyor 4 was a failure, but Surveyors 5, 6, and 7 successfully landed on the Moon in 1967 and 1968, returning vast amounts of photographs and data on the Moon that were critical to designing experiments for the Apollo missions. In total, the five successful Surveyors returned more than 87,000 photos of the Moon and showed that it was feasible to soft-land a large probe on the Earth's only natural satellite.

NASA conducted the Surveyor program in parallel with the Lunar Orbiter project, which was designed to accurately map the lunar surface (down to one-meter resolution). The program's primary goal was to allow mission planners to select the safest, but most scientifically interesting landing sites for the Apollo missions. The project was managed by NASA's Langley Research Centre. Each Lunar Orbiter weighed about 380 kilograms and was launched by the Atlas Agena D rocket. Each carried an imaging system that could develop exposed film, scan the photos, and broadcast them back to Earth. Five Lunar Orbiters were launched between August 1966 and August 1967. All were highly successful, and in total, mapped roughly 99 percent of the lunar surface. Lunar Orbiter 1 became the first spacecraft to take a picture of the Earth from the Moon. While the first three orbiters mapped potential Apollo landing sites, the last two conducted more general scientific surveys and also mapped the lunar gravitational field.

During two of the three final piloted Apollo missions to the Moon in 1971 and 1972, the crews of Apollo 16 and 17 each released a single small satellite known as the Particles and Fields Satellite to conduct scientific experiments. Each of these probes weighed about 80 pounds (36 kilograms) and operated for a few months, successfully returning data on magnetic fields and other interplanetary phenomena.

Besides the Pioneer, Ranger, Surveyor, and Lunar Orbiter spacecraft, NASA also attempted to send three Explorer spacecraft to the Moon. Two of them, launched in 1967-1968, were International Monitoring Platform (IMP) vehicles. After an initial failure, the second IMP, Explorer 35, successfully studied the solar wind, the interplanetary magnetic field, the lunar gravitational field, and other interplanetary phenomena from lunar orbit. The third vehicle, Explorer 49, reached lunar orbit in June 1973 and studied low-frequency radio emissions in the solar system. It was the last U.S. mission to the Moon for 21 years. With the end of the Apollo program in 1972, NASA redirected its deep space program to other targets.

The United States finally resumed robotic exploration of the Moon in 1994 with the launch of Clementine, jointly managed by the Ballistic Missile Defence Organization (BMDO) and NASA. Clementine, carrying 25 technology and scientific instruments on board, successfully entered orbit around the Moon in February 1994. During its first two months in lunar orbit, it took about 1.6 million digital images of the Moon. In December 1996, Clementine's scientific team caused a sensation when they announced that the spacecraft's instruments had detected ice at the bottom of a lunar crater near the Moon's south pole.

The most recent NASA mission to the Moon was that of Lunar Prospector, a part of NASA's Discovery program of low-cost missions. Lunar Prospector successfully entered orbit around the Moon in January 1998. During its year-long primary mission, the probe discovered an estimated six billion tons of water ice trapped in the shadows of the Moon's polar areas, thus bolstering the value of Clementine's earlier findings. The mission was managed by NASA's Ames Research Centre.