He logged more than 1 040 hours during his four space shuttle flights, which included the 15-day Columbia mission in July 1994 that conducted microgravity experiments. He was assigned to the sixth resident crew for the International Space Station in November 2002, but was held back because of medical issues involved in the long-duration mission. Thomas worked at the Lockheed Engineering and Sciences Company, where he reviewed space shuttle payloads, before joining NASA's Johnson Space Center as a materials engineer. He was selected as an astronaut in 1990 and retired from NASA in 2007.