His four missions include 141 days in space as a crew member aboard the Russian space station Mir, transported there on the Endeavour space shuttle in January 1998. He also flew on Discovery in March 2001 on the mission that took the second resident crew to the International Space Station, and undertook a space walk lasting more than six hours. His fourth mission was aboard Discovery in July–August 2005, the first space shuttle launch since the Columbia disaster of 2003. Thomas joined NASA's Jet Propulsion Laboratory in 1989 and was selected as an astronaut in 1992. He retired from NASA in 2014.