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Shuttle Rollbacks 

"Rollback" is the term used when the Space Shuttle must be rolled back from the launch pad atop the Mobile Launcher Platform and Crawler-Transporter to the Vehicle Assembly Building (VAB). A variety of factors can necessitate a rollback, ranging from the threat of severe weather to the need for flight hardware repairs that cannot be performed at the launch pad. Shuttle rollbacks to date are listed in chronological order:
STS-9  Columbia October 19, 1983  
Columbia was rolled back to the VAB, de-stacked, and the orbiter returned to the Orbiter Processing Facility because of a suspect exhaust nozzle on the right solid rocket booster.
STS 41-D Discovery July 11, 1984  
Discovery was rolled back to the VAB following a pad abort on June 26. The vehicle was returned to the VAB, de-stacked, and the orbiter returned to the Orbiter Processing Facility to remove and replace the number three main engine which caused the abort.
STS 51-E/51-B Challenger March 5, 1985  
Challenger was rolled back due to a timing problem with the primary payload, the Tracking and Data Relay Satellite-B. The vehicle was de-stacked in VAB and the orbiter returned to Orbiter Processing Facility. This mission, 51-E, was cancelled and the orbiter re-manifested with 51-B payloads.
STS-35 Columbia June 12, 1990 1st rollback.
Columbia was rolled back twice during preparations for mission STS-35. The first time occurred after a hydrogen leak was detected in the external tank/orbiter 17-inch umbilical.
STS-38 Atlantis August 9, 1990  
Atlantis was rolled back to the VAB after tests confirmed a hydrogen fuel leak on the external tank side of the external tank/orbiter 17-inch quick disconnect umbilical. The vehicle was de-stacked.
STS-35 Columbia,  October 9, 1990 2nd rollback.
Columbia was rolled back a second time due to the threat of severe weather from Tropical Storm Klaus.
STS-39  Discovery March 7, 1991  
Discovery rolled back to the VAB after significant cracks were found on all four lug hinges on the two external tank umbilical door drive mechanisms. The vehicle was de-stacked.
STS-68 Endeavour August 24, 1994  
Endeavour was rolled back to the VAB after a pad abort due to an unacceptably high discharge temperature in the high-pressure oxidizer turbo pump on main engine number three. All three engines were replaced in the VAB.
STS-70 Discovery June 8, 1995  
Discovery was rolled back to the VAB  after Yellow Flicker Woodpeckers drilled about 195 holes on the external tank foam insulation, many of which were too high up on the tank to be accessed at the pad for repairs.
STS-69 Endeavour August 1, 1995  
Endeavour was rolled back to the VAB due to the threat of severe weather from Hurricane Erin which passed through Brevard County where the Kennedy Space Center is located.
STS-79 Atlantis July 10, 1996  
Atlantis was rolled back to the VAB  due to the threat of severe weather from Hurricane Bertha.
STS-79 Atlantis September 4, 1996  
Atlantis was rolled back to the VAB  due to the threat of severe weather from Hurricane Fran.
STS-96 Discovery May 16, 1999  
Discovery was rolled back to the VAB to repair hail damage to the external tank foam insulation.
STS-98 Atlantis January 2, 2001 1st rollback.
Atlantis began rollout to Launch Pad 39A, but an hour later stopped on the crawler path so engineers could troubleshoot a failed computer processor on the crawler transporter. Troubleshooting efforts were unsuccessful, so managers decided to roll Atlantis back into VAB high bay 3 using a secondary computer processor.
STS-98 Atlantis January 19, 2001 2nd rollback.
Atlantis was rolled back to the VAB due to uncertainty involving the integrity of the SRB cables.
 Source: Kennedy Space Center