Mission Specialists Steven A. Hawley left and Kathryn D. Sullivan and Loren J. Shriver prepare to enter the orbiter Discovery (.

STS-31 Mission Specialists Steven A. Hawley (left) and Kathryn D. Sullivan, and Commander Loren J. Shriver prepare to enter the orbiter Discovery from the 195-foot level at Launch Pad 39B during the culmination of the two-day Terminal Countdown Demonstration Test (TCDT). The TCDT, conducted March 19-20, is a dress rehearsal for launch, simulating final countdown from the T-24 hour mark up to T-5 seconds. Space shuttle mission STS-31 is set to lift off on April 12, carrying a five-member crew and the Hubble Space Telescope. Photo credit: NASA