Project Rover was a program initiated in 1953 by the US Atomic Energy Commission (AEC) and the US Air Force with a view to developing nuclear propulsion systems for long-range missiles. This research was begun as a backup to the chemically-powered rockets being developed for the Intercontinental Ballistic Missile (ICBM) program, since at the time researchers weren't sure they could build a chemically-powered rocket that would make it to the other side of the globe. When NASA replaced the Air Force's involvement several years later, the project was redirected toward nuclear thermal rocketry for application in spaceflight. By the early 1960s, Rover had been harnessed to the NERVA program: Rover focussing on research into small, powerful nuclear reactors, NERVA adapting those reactors to work inside a nuclear engine for propelling manned interplanetary spacecraft. Among the reactors built and tested in the Rover program were KIWI, Phoebus, Peewee-1, and Nuclear Furnace 1. In 1969 the United States abandoned its immediate plans for the human exploration of Mars, and Project Rover was canceled in 1973.