His four space flights include piloting the Discovery space shuttle mission in July 1995 to deploy the sixth and final Tracking and Data-Relay Satellite, and in February 2000 commanding Endeavour on the Shuttle Radar Topography Mission. He was then assigned to the Space Launch Initiative Project at the Johnson Space Center before leaving NASA. Kregel joined NASA in 1990 as an aerospace engineer and instructor pilot, and was selected as an astronaut in 1992. He resigned from NASA in December 2003 in order to pursue private interests.