The SNAP-10A was the first and only nuclear reactor ever launched into space by the United States. Launched on April 3, 1965, aboard an Atlas-Agena D rocket, it was designed to test the feasibility of using a small nuclear reactor for long-duration space missions. Developed by the Atomic Energy Commission and NASA, the reactor provided 500 watts of electrical power for nearly 43 days before a voltage regulator failure in the spacecraft's control system caused the mission to end prematurely. Despite the failure, the reactor itself remained intact and posed no immediate risk.