NASA officials said yesterday that the costs of returning the grounded space shuttle to flight have risen as much as $900 million over original projections, raising the possibility that the agency may have to seek extra money from Congress next year or cut other space programs to fund the shortfall.

NASA Deputy Associate Administrator Michael Kostelnik said that "we'll be easily able to handle 2004," by searching within the agency for between $100 million and $200 million in extra money. But funding for 2005, with a projected shortfall between $400 million and $700 million, "is still an uncertainty," he added.

Nevertheless, Kostelnik emphasized that the cost projections may change and that even with a $700 million shortage, "we wouldn't need help in that regard until the fall of next year." He said NASA was unlikely to seek congressional help until 2005, and only if necessary.

"First we would look for resources with Space Operations and second within the agency," Kostelnik told reporters during a telephone news conference. "Then we would look to do something outside the agency later in the year."

NASA's announcement came 12 days after a key congressional committee passed a bill cutting the Bush administration's 2005 NASA budget proposal by more than $1 billion, dealing a sharp blow to the president's initiative to return humans to the moon and eventually send them to Mars. Bush has threatened to veto the bill.

One knowledgeable Republican source, who refused to be quoted by name because of office policy, acknowledged that Congress had heard about the shortfalls last month, and lawmakers "don't know what to think about it." While NASA is "acting responsibly" by voicing its fears early, the source said, the news "puts additional pressure on an already impossible budget -- and what are you going to take it from? And is this as high as [the shortfall] is going to get?"

The budget shortfalls were outlined in the newest version of NASA's "Implementation Plan for Space Shuttle Return to Flight and Beyond," which described the agency's ongoing efforts to ready the shuttle for a trip to the international space station in March. Kostelnik said preparations were "on track."

NASA grounded the orbiter after the Feb. 1, 2003, Columbia tragedy. The agency has been striving to implement 15 key recommendations by the blue-ribbon Columbia Accident Investigation Board, which analyzed the causes of the accident.

Some of these deal with vital systems, including changing the shuttle's external fuel tank so its foam insulation will not as easily break off and damage the wings during launch -- identified as the trigger of the Columbia disaster.

Others, however, deal with matters as seemingly mundane as forging an interagency agreement to ensure that the shuttle will be photographed while in orbit, and requiring NASA to simplify its definition of "debris."

In all, NASA has at least "conditionally closed" five of the recommendations and plans to comply with the rest by the end of this year: "We have a plan laid out," said John Casper, manager of the Space Shuttle Management Integration and Planning Office. "It's ambitious but doable."

Space Shuttle Program Deputy Manager Wayne Hale said NASA was also confident that planners would have a "safe haven" plan in place -- to enable a shuttle crew to stay aboard the space station until a backup shuttle could fly up to rescue them.

"This is an eventuality that we hope will never happen," Hale said. Preparing a backup mission would require converting a second shuttle into a rescue vehicle by loading it with special equipment, changing computer software, staffing it with a specially trained team of astronauts and getting it to the station before overloaded life support systems there were exhausted.

Current plans call for the backup to be ready to support the first two shuttle flights next year. "After that, we'll see where we go," said Space Shuttle Program Manager Bill Parsons. NASA could discontinue the plan, he added, because "there is a possibility that we'll have enough confidence in our system."