On 3 April 1966, the Soviet Union Luna 10 became the first to successfully orbit the moon. Launched on 31 March 1966 and arrived moon on 2 April 1966, Luna 10 became the first spacecraft to successfully go into orbit around another celestial body.
While in orbit, Luna 10 studied radiation levels, cosmic ray intensities, and took readings of the moon's weak magnetic field. The spacecraft successfully transmitted data for two months, circling the moon 460 times before its mission came to an end on May 30, 1966.
After a 92 hour voyage, the first successful United States Lunar Orbiter entered into orbit around the moon on 14 August 1966. The Lunar Orbiter program was designed to photograph possible sites for the Apollo landing missions and the far and near sides of the moon. Morever, the analysis of the motions of the spacecrafts helped determine the precise shape of the moon and any gravitational anomalies it might contain. It confirmed the moon is slightly pear-shaped, bulging about 1/4 mile (0.4 km) at its north pole.
Lunar Orbiter 1 sent back high-quality images (by television) of over two million square miles of lunar surface, including the first detailed images of potential Apollo landing sites. On its 26th orbit of the moon, the first pictures were taken, developed, and stored. Electronic scanning and transmission to earth commenced on 18 August.
After circling the Moon 527 times in 77 days, Lunar Orbiter 1 was deliberately crashed onto the moon's surface to prevent the possibility of its radio disturbing communications between earth and the Lunar Orbiter 2, whose launching was imminent.
While Lunar Orbiter 1 was sending photographs of the moon back to earth, the Soviets launched Luna 11 on 24 August 1966, which reached moon on the 28 August. Luna 11 was designed to test new technology, completed 277 orbits before its mission was terminated on 1 October 1966.
Luna 12 followed on 22 October, achieving lunar orbit and sent back a series of photographs of the moon surface, presumably including prime target landing sites.
Launched on 22 October 1966 and in lunar orbit on 25 October 1966, Luna 12's primary mission was to photograph the lunar surface and it took about 1,100 pictures including images of the Sea of Rains and the area surrounding the crater Aristarchus. The mission was terminated on January 19, 1967 after 602 orbits.
On 6 November 1966, Lunar Orbiter 2 headed for the moon and in lunar orbit on 10 November. Excellent pictures of potential Apollo landing sites in particular were returned.
Lunar Orbiter 2 took over 800 pictures during its mission, including an oblique view of the crater Copernicus that was voted one of the best images of the century by the press. It was deliberately sent crashing into the lunar surface on 11 October 1967, bringing to an end the successful mission.
Lunar Orbiter 3, boosted by its Atlas Agena D carrier was launched on 4 February 1967, and entered moon orbit on the 8th. Its mission was to photograph the prime site in eastern Mare Tranquillitatis, one of the potential landing site for Apollo.
The orbit of Lunar Orbiter 3 was altered several times during the mission to give controllers on Earth more experience with communications during lunar orbit. Lunar Orbiter 3 drew great attention when it photographed the Surveyor 1 spacecraft that landed on the moon in early June 1966. The mission ended on 9 October 1967 when it was deliberately crashed the spacecraft into the Moon.
Lunar Orbiter 4, unlike its predecessors, went into near polar orbit and sent back its first picture of the south polar region of the moon on 11 May 1967.
Lunar Orbiter 4 was the first to take pictures of the moon's south pole. It took images from orbit for eight months before it was crashed to the lunar surface.
The final in the Lunar Orbiter series, Lunar Orbiter 5, was launched on 1 August 1967, and after nearly 90 hours and travelled 255,000 miles (410,000 km) of flight, it entered into orbit. Its missions were to continue detailed photography of more than 40 preselected sites on the near side of the moon and to cover the unphotographed regions along the western equator on the far side.
Upon completion of the Lunar Orbiter 5 mission, over 99% of the moon's surface had been mapped (with data from all previous missions combined). The mission ended on January 31, 1968.
One spacecraft not so closely tied to the Apollo program was Explorer 35, launched on 10 July 1967, and sent 12 days later into a highly elliptical orbit around the moon. Explorer 35 was also known as a lunar-anchored interplanetary monitoring platform, or IMP. It found the moon had been volcanically active. No evidence of trapped radiation belts around the moon that is similar to the Van Allen belt surrounding the earth.
In April 1968, the Soviets placed their Luna 14 into moon orbit. It was instrumented to study the relationship between the masses of the earth and the moon, the lunar gravitational field, the interaction of the solar wind and the moon, and the characteristics of the motion of the moon in its orbit around the earth.