The T-21 spacecraft is a full-scale model of the Surveyor spacecraft built by Hughes Aircraft Company under the management of NASA's Jet Propulsion Laboratory. The spacecraft was used as an engineering thermal control model for testing. The Jet Propulsion Laboratory refurbished the T-21 in 1968 to look like the Surveyor 7 spacecraft that was sent to the Moon. The T-21 is missing the solid retrorocket, which was located in the center of the craft. The retrorocket was used to slow the decent of the Surveyor spacecraft that landed on the lunar surface.
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 aluminum 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.