Selected by NASA in January 1990, Harris became an astronaut in July 1991, and qualified for assignment as a mission specialist on future Space Shuttle flight crews. He served as the crew representative for Shuttle Software in the Astronaut Office Operations Development Branch. Harris was assigned as a mission specialist on STS-55, Spacelab D-2, in August 1991. He flew on board Columbia for ten days, (26 April 1993 – 6 May 1993); on the mission the Shuttle reached one year of accumulated flight time. Harris was part of the payload crew of Spacelab D-2, conducting a variety of research in physical and life sciences. During this flight, Harris logged over 239 hours and 4,164,183 miles in space.