Onizuka's first space mission took place on January 24, 1985, with the launch of mission STS 51-C on Space Shuttle "Discovery", the first Space Shuttle mission for the Department of Defense. He was accompanied by Commander Ken Mattingly, Pilot Loren Shriver, fellow Mission Specialist James Buchli, and Payload Specialist Gary E. Payton. During the mission, he was responsible for the activities of the primary payloads, which included the unfolding of the Inertial Upper Stage (IUS) surface. After 48 orbits around the Earth, "Discovery" landed at Kennedy Space Center on January 27, 1985. He had completed a total of 74 hours in space.