Luna 13 landed on the lunar surface between the craters Selencus and Craft in the Oceans of Storms on 24 December 1966. It transmitted photographs of the lunar surface, which were shown in Moscow on Christmas day. The mission ended on December 30, 1966, when the spacecraft's supplies were depleted.
In addition to camera equipment, Luna 13 carried a gamma radiation measuring device and a rod that was poked into the ground to determine its bearing strength. Experiments conducted on soil beneath the spacecraft showed that the mechanical properties of the moon's surface are similar to those common for ground on earth, though its surface density is much. Other instruments onboard measured dispersed radiation from a gamma source and recorded cosmic corpuscular radiation on the lunar surface.
Although Surveyor 2 failed to soft land on 20 September 1966, the Surveyor program continued during 1967, with alternating launches from the Lunar Orbiter program. The first of several Surveyor spacecrafts launched during 1967 was Surveyor 3, touched down in Oceanus Procellarum on 20 April a few miles off target and about 380 miles (610 km) east of Surveyor 1.
As Surveyor 3 came in for a soft landing on the moon, one of its thrusters didn't turn off at the right time and the spacecraft bounced a couple of times before it came to rest in the Ocean of Storms. About 6,300 lunar surface photographs were transmitted back to earth, in addition sent invaluable data from its digging and scraping mechanism that determine the mechanical properties of the lunar soil. The soil sample analyzed confirmed the moon could bear the weight of an Apollo manned spacecraft. Unfortunately, the images were severely compromised by the presence of dust on the mirror.
Surveyor 5, launched on 8 September, carried an alpha-ray scattering experimental package to conduct chemical tests on the soil composition.
Despite a serious helium leak that occurred during its trip to the Moon, Surveyor 5 touchdown successfully on 11 September 1967. Once on the ground, experiment work began to test analyzed the soil beneath it. The experiment indicated the existence of basalt and silicon. Surveyor 5 survived a lunar night and commenced operations again on October 15. The final transmission from the spacecraft was received on 17 December 1967.
However Surveyor 6 had a more difficult mission because of the rough terrain on which it had to land in the Sinus Medii, just south of the equator. Its mission was to photograph an area that was supposed to be covered by its predecessors - Surveyor 2, 4 missions that had failed to soft land on the moon.
In addition to taking thousands of photographs in Sinus Medii, Surveyor 6 made soil measurement with its onboard analyzer. Then on 17 November 1967, Surveyor 6 was commanded to make a "hop" on the moon's surface - its engines were fired to lift itself off the lunar surface 10 feet (3 meters), and then setting down a few feet away from the original landing site. The spacecraft then took pictures of its former landing site, looking for any evidence of a crater created by the rocket's exhaust. No crater was found, indicating that the moon's surface was solid. Last contact with the spacecraft was 14 December 1967.
With the successful conclusion of Surveyor 6 operation, scientists were convinced the surface in the mare regions of the lunar equatorial zone was suitable for the forthcoming manned Apollo landings. The Surveyor program had thus completed its investigations for Apollo, while the last spacecraft in the series, Surveyor 7, could be used for other scientific research, like exploring highland area around the crater Tycho. It was reasoned that the site would provide scientists with the opportunity to examine, both geologically and chemically, the material thrown out of the volcano at the time of the formation of Tycho.
Surveyor 7 landed in the lunar highlands near the crater Tycho. Devices were carried onboard to analyze chemically the material scooped out. Scientists used the scoop on the spacecraft to "weigh" lunar rocks, based on how much current was needed to lift each rock. Images sent back indicated some of the lunar rocks had been molten at some time in their history. The mission was successfully completed on 21 February 1968.
With Surveyor 7, the US unmanned lunar exploration program came to an end, temporarily at least, as all its resources were focused on manned operation within the Apollo lunar landing progam.
