Space nuclear technology has been used for power generation, propulsion, and heating. It was first used in 1961 with the launch of the U.S. Navy's Transit 4A navigation satellite powered by a radioisotope thermoelectric generator (RTG), and both the United States and Russia have since launched spacecraft with radioisotope thermoelectric generators and nuclear reactors to generate power. No nuclear rockets have been launched, but there was extensive design and testing of such rockets in the United States in the 1950s and 1960s.