Even with its shuttles grounded, NASA can easily retrieve the astronauts aboard the international space station using Russian vehicles.
But if the space agency's remaining shuttles are out of service for an extended period in the wake of Saturday's catastrophe, as seems likely, it will prove difficult to maintain the station's operations.
If necessary, a Russian Soyuz vehicle attached to the space station could bring the three astronauts onboard back to Earth at a moment's notice.
But with Russia's ability to launch vehicles to the space station already compromised by budget problems, Saturday's accident could seriously jeopardize the continued operation of the outpost.
With no permanent crew aboard, the space station can operate in a ``dormant'' mode as long as occasional maintenance is performed by visiting astronauts. In fact, NASA had already been considering a ``demanning'' contingency for 2003 before Saturday's events.
But the longer the station went unoccupied, the greater the chances that it would deteriorate to an uninhabitable state. A dormant period would also cause a significant interruption in the station's continuing assembly and scientific research program.
Expedition Six, as the current crew is called, arrived at the station in November and is scheduled to return to in March. The crew consists of NASA astronauts Ken Bowersox and Don Pettit and Russian Soyuz commander Nikolai Budarin.
The shuttle mission that tragically ended Saturday over eastern Texas did not visit the space station. But the crews of the two spacecraft did speak by telephone on Jan. 28, the anniversary of the Challenger disaster that killed seven astronauts 17 years ago.
An unmanned supply vessel was to be launched Sunday from Russia's Baikonur Cosmodrome. It was scheduled to arrive at the orbiting station Tuesday.