STS-400: 'Just in case'

When the space shuttle Atlantis blasts off on NASA's final mission to service the Hubble Space Telescope, the shuttle Endeavour and a four-man crew will be standing by for launch on a mission space agency managers hope will never be needed: an emergency rescue flight to bring the Atlantis astronauts back to Earth if heat shield damage or some other problem prevents a safe re-entry. Unlike flights to the International Space Station, the Atlantis astronauts, working in a very different orbit, cannot seek "safe haven" aboard the lab complex if Columbia-class damage is incurred during the climb to space or later, due to impact with space debris. In either case, Endeavour could mean the difference between life and death for Atlantis' crew. More

(Source: SpaceFlightNow.com - May 7)