What was originally conceived as a mission of preventive maintenance turned more urgent on November 13, 1999, when the fourth of six gyros failed and Hubble temporarily closed its eyes on the universe. Unable to conduct science without three working gyros, Hubble entered a state of dormancy called safe mode. Essentially, Hubble "went to sleep" while it waited for help.
NASA decided to split the Third Servicing Mission (SM3) into two parts, SM3A and SM3B, after the third of Hubble's six gyroscopes failed. In accordance with NASA's flight rules, a "call-up" mission was quickly approved and developed and executed in a record 7 months!
The crew of STS-103 including astronauts Curtis L. Brown Jr., Scott J. Kelly, John M. Grunsfeld, Jean-François Clervoy, Michael Foale, Steven L. Smith, and Claude Nicollier ensured that the Hubble Space Telescope continued its mission into the 21st century.