Discoverer 28, also known as Corona 9021, was an American optical reconnaissance satellite which was lost in a launch failure in 1961. It was the seventh of ten Corona KH-2 satellites, based on the Agena-B. The launch of Discoverer 28 occurred at 00:01 UTC on 4 August 1961. A Thor DM-21 Agena-B rocket was used, flying from at the Vandenberg Air Force Base. It failed to achieve orbit after the Agena's guidance and control system malfunctioned.