A mission specialist, his three space shuttle flights included the Discovery mission (September 1991) to deploy the Upper Atmosphere Research Satellite and the Columbia mission (March 1994) on which microgravity experiments were conducted. On 4 March 1994 Gemar flew as a mission specialist aboard STS-62 Columbia—the lowest space shuttle flight ever flown (altitude of 105 nautical miles); the mission lasted 13 days, 23 hours, and 16 minutes, making it the second-longest shuttle flight in history. In total, Gemar has logged more than 580 hours in space. Gemar was selected as an astronaut in 1985, and from 2006 until 2009 he worked as an on-air personality for HDNet, covering live space shuttle launches out of Kennedy Space Center, Florida.