The Apollo program was a United States human spaceflight program carried out from 1961 to 1972 by the National Aeronautics and Space Administration (NASA), which landed the first astronauts on the Moon.[1] The program used the Saturn IB and Saturn V launch vehicles to lift the Command/Service Module (CSM) and Lunar Module (LM) spacecraft into space, and the Little Joe II rocket to test a launch escape system which was expected to carry the astronauts to safety in the event of a Saturn failure.[2] Uncrewed test flights beginning in 1966 demonstrated the safety of the launch vehicles and spacecraft to carry astronauts, and four crewed flights beginning in October 1968 demonstrated the ability of the spacecraft to carry out a lunar landing mission.
Apollo achieved the first crewed lunar landing on the Apollo 11 mission, when Neil Armstrong and Buzz Aldrin landed their LM Eagle in the Sea of Tranquility and walked on the lunar surface, while Michael Collins remained in lunar orbit in the CSM Columbia, and all three landed safely on Earth on July 24, 1969.[3] Five subsequent missions landed astronauts on various lunar sites, ending in December 1972 with twelve men having walked on the Moon[4] and 842 pounds (382 kg) of lunar rocks and soil samples returned to Earth, greatly contributing to the understanding of the Moon's composition and geological history.[5]
Two Apollo missions were failures: a 1967 cabin fire killed the entire Apollo 1 crew during a ground test in preparation for what was to be the first crewed flight;[6] and the third landing attempt on Apollo 13 was aborted by an oxygen tank explosion en route to the Moon, which disabled the CSM Odyssey's electrical power and life support systems, and made the propulsion system unsafe to use. The crew circled the Moon and were returned safely to Earth using the LM Aquarius as a "lifeboat" for these functions.[7]