Discoverer 21 was a USAF photographic surveillance satellite designed to assess how rapidly the Soviet Union was producing long-range bombers and ballistic missiles, and where they were being deployed. It was part of the Corona program which was also used to produce maps and charts for the Department of Defense and other US government mapping programs. Like Discoverer 19, it carried no film capsule. Its purpose was to test whether the rocket engine could be restarted in space.