The U.S. space agency might try to launch space shuttle Atlantis on May 11, a day earlier than scheduled, administrators announced Thursday in Houston.
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This will be the last time that a second shuttle -- in this case the Endeavour -- will sit on a launch pad ready to rescue a mission's crew in case of an emergency. On missions to the international space station, the station itself serves as the emergency backup. A mission to Hubble will put the shuttle in a different orbit, preventing access to the space station.

A rescue shuttle has been prepared for such mission since wing damage at liftoff led to the loss of the Columbia and its crew on February 1, 2003.

Administrators said Endeavour could lift off in seven days if it is needed.