The National Aeronautics and Space Administration suspended shuttle missions after the Columbia disaster in February 2003. It had hoped to stage a new flight in May this year with the shuttle Discovery.

But NASA said a new flight would not be held if a second shuttle could not be launched in time to rescue a crew on the International Space Station.

"In the unlikely event that all of our efforts to reduce risk and safely return the space shuttle to flight have failed, we have made plans to keep the space shuttle crew on the International Space Station and mount a rescue mission," NASA said in its latest report on preparations for shuttle flights.

"For the near term, we will not launch a space shuttle unless the second shuttle can be prepared and launched within the time the International Space Station can provide accommodation for the first shuttle's crew."

Atlantis is generally considered the reserve shuttle of the three remaining vessels. NASA said a rescue mission would only be carried out "in the most dire of circumstances and will not be used to justify flying unsafely."

Columbia broke up as it re-entered Earth's atmosphere on February 1, 2003, killing all seven crew. Following criticism of NASA procedures, the other three shuttles have been grounded since.

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