Coleman was selected by NASA in 1992 to join the NASA Astronaut Corps. In 1995, she was a member of the STS-73 crew on the scientific mission USML-2 with experiments including biotechnology, combustion science, and the physics of fluids. During the flight, she reported to Houston Mission Control that she had spotted an Unidentified flying object (UFO). She also trained for the mission STS-83 to be the backup for Donald A. Thomas; however, as he recovered on time, she did not fly that mission. STS-93 was Coleman's second space flight in 1999. She was mission specialist in charge of deploying the Chandra X-ray Observatory and its Inertial Upper Stage out of the shuttle's cargo bay.