WASHINGTON (AFP) – Space shuttle Atlantis astronauts aboard the International Space Station delayed their second spacewalk Saturday after being jolted awake by a false alarm, NASA said.

Mission specialists Mike Foreman and Randy Bresnik had just two hours of sleep before a depressurization alarm sounded around 0253 GMT, forcing them to exit the Quest airlock, where astronauts "camp out" before spacewalks to purge nitrogen from their bloodstream to prevent decompression sickness.

Mission control in Houston, Texas confirmed the original alarm and subsequent alerts set off around the orbiting outpost were false.

The astronauts were allowed 30 extra minutes to snooze before their six-hour spacewalk, which will be 30 minutes shorter than planned.

Saturday's spacewalk, the second exterior work effort of the shuttle's 11-day mission, is intended to install a new system for storing spare parts on the space station's exterior.