From October 20 to November 5, 1995, Coleman served as a mission specialist aboard Space Shuttle Columbia on STS-73, the second United States Microgravity Laboratory mission. The mission focused on materials science, biotechnology, combustion science, the physics of fluids, and numerous scientific experiments housed in the pressurized Spacelab module. In completing her first space flight, Coleman orbited the Earth 256 times, traveled over 6 million miles, and logged a total of 15 days, 21 hours, 52 minutes and 21 seconds in space.
Carried USML-2 for microgravity experiments (attached to Columbia). Payloads: United States Microgravity Laboratory (USML) 2, Orbital Acceleration Research Experiment (OARE).
The launch of STS-83, the first Microgravity Science Laboratory (MSL-1) mission, was postponed for a day to replace some insulation around a water coolant line in Columbia's payload bay. Liftoff was further delayed 20 minutes due to anomalous oxygen readings in the orbiter's payload bay. STS-83 was cut short due to a problem with one of the three fuel cells that provide electricity and water to Columbia (flight rules required that all three must be operating). At 14:30 GMT on April 6 the crew were ordered to begin a Minimum Duration Flight (MDF). On April 8 the OMS engines ignited at 17:30 GMT for the deorbit burn, and Columbia landed on Runway 33 at Kennedy Space Center at 18:33 GMT.
With delays in International Space Station construction leaving ample room in the shuttle schedule, NASA made the unique decision to leave the equipment installed in Columbia and refly this mission with the same crew later in 1997 as STS-94.
STS-93 was first rolled out to pad 39B on June 7 1999. The Chandra/IUS-27 vehicle was placed in the payload canister on June 19. The first launch attempt was on July 20, but controllers aborted the launch at T-6 seconds, just before main engine ignition, due to a data spike in hydrogen pressure data. This was determined to be due to a faulty sensor and a second attempt was on July 22. A lightning storm prevented launch during the 46 minute window, and the launch was again scrubbed. Finally the vehicle lifted off the pad on July 23, but five seconds after launch a short in an electrical bus brought down two of the three main engine controllers. Backup controllers took over, but a further failure on the backup controller bus would have resulted in engine shutdown and the first ever attempt at an RTLS (Return To Launch Site) abort. To further complicate matters engine 3 (SSME 2019) had a hydrogen leak throughout the ascent, causing the engine to run hot. Controllers sweated as temperatures neared redline. The hot engine’s controller compensated as programmed by using additional liquid oxygen propellant. The final result was that the shuttle ran out of gas - main engine cut-off (MECO) was at 04:39 GMT, putting Columbia into a 78 km x 276 km x 28.5 degree transfer orbit. Columbia was 1,700 kg short of oxygen propellant and 5 meters/sec slower than planned. The OMS-2 engine burn at 05:12 GMT circularised the orbit 10 km lower than planned.
The orbiter payload bay contained only the Chandra spacecraft, the IUS, and the IUS tilt tableTthe following payloads were carried in the shuttle’s cabin: STL-B (Space Tissue Loss), CCM (Cell culture module), SAREX-II (Shuttle Amateur Radio Experiment), EarthKam, PGIM (Plant Growth Investigations in Microgravity), CGBA (Commercial Generic Bioprocessing Apparatus), MEMS (Micro-electric Mechanical System), and BRIC (Biological Research in Canisters) and SWUIS (the Southwest Ultraviolet Imaging System, an 0.18-m UV telescope to be used for airglow and planetary observations); GOSAMR (the Gelation of Sols: Applied Microgravity Research experiment) and LFSAH, the Lightweight Flexible Solar Array Hinge. MSX and SIMPLEX experiments were also to be carried out.
Chandra/IUS-27 was deployed from Columbia at 11:47 GMT July 23. Flight duration was limited; this was the heaviest shuttle (122,534 kg) and heaviest payload (19,736 kg) to that date. Columbia landed at 03:20 GMT on July 28 on runway 33 at Kennedy Space Center. Post-flight inspection found the presence of holes in the cooling lines on the nozzle of SSME 2019 (engine 3) which caused a hydrogen leak. A loose repair pin in the engine broke free and caused the failure. The cause of the short was found to be chaffed wiring inside the shuttle. The entire fleet was grounded for inspection and replacement of wiring as necessary.