Thagard flew twice on space shuttle Challenger, STS-7 in 1983 and STS-51-B in 1985. Thagard used his medical skills to collect data on physiological changes associated with astronaut adaptation to weightlessness. He was a crew member on space shuttle Atlantis in 1989, for STS-30 where the crew deployed the Magellan Venus spacecraft to map that planet’s surface with radar instruments. Thagard’s final space shuttle flight was aboard Discovery in 1992 as payload commander.