Twelve (12) human beings have walked on the moon, starting with Neil Armstrong and Buzz Aldrin on July 20, 1969. All twelve moonwalkers were American and participated in NASA’s United States Apollo program. Six crewed landings took place between July 20, 1969, and December 19, 1972. Two astronauts set foot on the moon during each of the six missions. A third crew member, the Command Module Pilot, stayed in orbit while the Lunar Module touched down on the moon. Armstrong and Aldrin were assigned to Apollo 11. Apollo 13 intended to land Jim Lovell and Fred Haise on the moon, but the mission was famously aborted due to an oxygen tank explosion, and the crew returned safely to Earth. NASA originally planned ten crewed missions to the moon, but the last three were canceled due to budget limitations, redirected priorities, and risk assessments. Two Saturn V rockets ended up being unused and are on display at the Johnson Space Center in Texas and the Kennedy Space Center in Florida.