The National Aeronautics and Space Administration (NASA ) is an independent agency of the U.S. federal government responsible for the United States' civil space program, as well as research in aeronautics and space. Headquartered in Washington, D.C., NASA operates ten field centers across the US and is organized into three mission directorates: Human Spaceflight, Research and Technology, and Scien Mirroring the Department of Defense's management concept of using redundant systems to build the first intercontinental ballistic missiles, NASA requested that the Air Force assign Major General Samuel C. Phillips to the space agency, where he served as the director of the Apollo program. Development of the Saturn V rocket was led by Wernher von Braun and his team at the Marshall Space Flight Center, deriving it from the Army Ballistic Missile Agency's original Saturn I. The Apollo spacecraft was designed and built by North American Aviation, while the Apollo Lunar Module was designed and built by Grumman. To develop the spaceflight skills and equipment required for a lunar mission, NASA initiated Project Gemini. Using a modified Air Force Titan II launch vehicle, the Gemini capsule could hold two astronauts for flights lasting over two weeks. Gemini pioneered the use of fuel cells instead of batteries, and conducted the first American spacewalks and orbital rendezvous operations. Wernher von Braun had advocated for NASA to develop a space station since the agency's creation. In 1973, following the end of the Apollo lunar missions, NASA launched its first space station, Skylab, aboard the final flight of the Saturn V rocket. Skylab heavily reused Apollo and Saturn hardware, with a repurposed Saturn V third stage serving as the primary orbital module. Damage sustained during its launch required the first crew to perform critical spacewalks to render the station habitable and operational. Skylab hosted nine missions and was decommissioned in 1974. It ultimately deorbited in 1979, two years prior to the inaugural flight of the Space Shuttle, making any attempt to boost its orbit impossible. In 1975, the Apollo–Soyuz test project represented the first international spaceflight and a major diplomatic accomplishment between the Cold War rivals, simultaneously marking the last flight of the Apollo capsule. During this mission, an American Apollo spacecraft docked in orbit with a Soviet Soyuz capsule.