Plutonium-238 has been used as a fuel source by NASA for a number of missions, including the 2020 Mars missions with the Perseverance rover, the Apollo flights (1969–1972), the Viking 1 and 2 Mars landers (1976–1982), the 1977 launch of the Voyager 1 and 2 space probes, and the 2006 launch of the New Horizons mission to Pluto and planetary systems