WASHINGTON — NASA announced plans Thursday to bring three crew members on the international space station home in a Russian vehicle by early May and replace them with a rotating bare-bones crew of two for up to two years while the space shuttle fleet remains grounded following the Columbia disaster.
But NASA Administrator Sean O'Keefe warned Congress that the space agency and its international partners are prepared to abandon the $100-billion orbiting station, at least temporarily, if adequate water and other critical supplies cannot be maintained.
NASA officials at Johnson Space Center in Houston said the construction of the space station, which is little more than half-built, has come to an abrupt halt. The space shuttles are a critical supplier of personnel, supplies and parts to the space station, so the station's scheduled completion date of 2006 will be pushed back, said NASA spokesman Rob Navias.
In his starkest evaluation yet of the peril facing the station since Columbia disintegrated on Feb. 1, O'Keefe said that if an unmanned resupply mission scheduled for June fails to reach the station, officials were likely to recommend that astronauts on it "dim the lights and come home."
While supply missions with Russian Progress craft have been considered relatively routine, O'Keefe repeatedly stressed the high stakes involved in his testimony. O'Keefe told lawmakers that the station probably could operate for six months to a year without a crew, "assuming no other unforeseen circumstances."
After that, NASA officials said Thursday, the station would not have enough propellant to keep it in its proper orbit path. The space station falls more than 600 feet each day because of gravity and often relies on the space shuttle's powerful thrusters to push it back up.
O'Keefe stressed that abandoning the station would be "the last possible alternative," adding: "There is a reticence, a deep reticence, to leave that unmanned for any period of time."
Shifting to two crew members could help space officials avoid that scenario by keeping supply needs -- particularly food and water -- to a minimum.
Even if two-person crews succeed in keeping the station maintained, the loss of the third crew member will have serious repercussions for scientific research in space. Many scientists have complained that three-member crews have little time as it is to conduct serious experiments on a station originally designed for seven-member crews.
Previously, NASA officials have estimated that upkeep of the station requires the full time of two astronauts and half the time of a third.
But O'Keefe revised that estimate Thursday. He said the U.S. astronaut and Russian cosmonaut now scheduled to replace Americans Kenneth Bowersox and Donald Pettit and Russian Nikolai Budarin would be able to continue research.
He conceded: "It's not optimum by any means." Navias later added that scientific work conducted on the station would slow down significantly.
The new crew, known as Expedition 7 and training at Star City in Russia, would fly to the station via a Russian Soyuz craft to be launched in late April or early May, O'Keefe said. A second two-person crew is expected to replace that one in October, Navias said.
Food, water and other supplies would be flown via unmanned Russian Progress vehicles in four missions this year and five missions in 2004 -- accelerating a previous schedule with one new mission each year.
Water has been an especially critical issue. Normally, the space shuttles that ferry crew members also refill the station's water supply.
The shuttle Atlantis had been scheduled to dock at the station in March to rotate crews before the Columbia disaster grounded the fleet indefinitely. O'Keefe said the 16 nations that support the station agreed on the plan Wednesday.
"Everyone's stepping up," O'Keefe said. "Everybody's doing what needs to be done to maintain this remarkable laboratory in space."
Lawmakers called the new plan prudent but wondered what would happen if the shuttle fleet is grounded for longer than the 18-month timetable O'Keefe outlined. After the Challenger explosion in 1986, shuttles were grounded for 2 1/2 years.
The NASA chief disclosed the plan in an appearance before the House Science Committee to outline the Bush administration's proposed $15.5-billion budget for the agency in the coming fiscal year.
Under sometimes sharp questioning, O'Keefe stoutly defended the agency's handling of an internal safety debate in the days before Columbia broke up over Texas while reentering the atmosphere.
That debate has come under increasing scrutiny as NASA has released e-mails that revealed serious concerns among engineers during Columbia's flight about the possibility that it had been fatally damaged by falling debris in its Jan. 16 launch.
Some lawmakers were incredulous at reports that word of the debate did not reach the highest levels of NASA until well after the disaster.