A large deficit in NASA's troubled shuttle program threatens to seriously delay and possibly cripple President Bush's space exploration initiative unless the number of planned flights is cut virtually in half or the White House agrees to add billions of dollars to the human spaceflight budget.

Sources familiar with ongoing negotiations between NASA and the White House say the administration has no intention of spending extra money to deal with a shortfall that some space experts say could exceed $6 billion from 2006 to 2010, when NASA plans to retire the shuttle for good.

The source of the deficit is the travail that has plagued the shuttle program since the Columbia disaster in 2003. After a single flight by Discovery this summer, the orbiters -- grounded for 21/2 years after Columbia -- are out of action again until at least May while engineers work to make them safer.

One option being considered to close the shortfall is to limit the number of flights to two per year -- 10 in all -- and cut the workforce. But shuttle program manager Wayne Hale said in a televised news conference yesterday that "frankly it doesn't save you very much money. . . . From my point of view, that's a non-starter."

NASA Administrator Michael D. Griffin has said that terminating the shuttle program would be just as expensive as keeping it going. The shuttle routinely consumes more than 30 percent of NASA's budget.

The impasse has put the future of Bush's "Vision for Space Exploration" in doubt less than two years after it was announced. Without extra money, experts say, NASA could have trouble developing a new "crew exploration vehicle" by 2014, as originally planned, let alone fulfilling Griffin's wish to fly it by 2012. The dilemma is also fueling an odd confrontation between the administration and Congress, where once-wary lawmakers now appear willing to provide the extra funding even as the White House backs away from its own initiative.

"The decisions made over the next few weeks will determine whether the Bush White House is serious about supporting the vision," said John Logsdon, director of George Washington University's Space Policy Institute. "We've reached a watershed."

The cornerstones of the Bush initiative, announced in a speech on Jan. 14, 2004, are to use the shuttle to finish the international space station by 2010, develop the crew exploration vehicle by 2014, return humans to the moon by 2020 and eventually move on to Mars.

Bush called the plan "a journey, not a race," to be completed without appreciable increases in NASA's budget.

Initially, Congress expressed suspicion that the initiative was either a grandiose but empty gesture or a risky project that would cannibalize established NASA programs to raise the needed funding. Last year, it took an eleventh-hour arm-twist by then-House Majority Leader Tom DeLay (R-Tex.) to win passage of NASA's $16.1 billion budget, but this year lawmakers easily passed the 2006 budget -- for the full $16.5 billion the White House requested.

The difference was that Griffin, confirmed in April of this year, earned congressional trust by reorganizing NASA and segregating the shuttle and exploration vehicle programs from the rest of NASA's portfolio.

Where he has not fared so well, however, is in allaying lawmakers' misgivings about the "gap" in human space travel between the end of the shuttle program in 2010 and the first manned flights of the new exploration vehicle in 2014.

Griffin said earlier this year that NASA now projects that the new spaceship would fly by 2012, with a return to the moon by 2018, but he was unable to satisfy those who want to close the gap completely.

Sen. Kay Bailey Hutchison (R-Tex.), chairman of the science and space subcommittee, said on Sept. 19 that she "will do everything possible to keep the shuttle and crew exploration vehicle programs on course," and her words have come to encapsulate the dilemma now facing NASA.

Under the budgets projected for the next five years, experts outside and within the Bush administration agree, it will be impossible -- by several billion dollars -- to complete the planned shuttle missions and finish the new spacecraft by 2012, or maybe even by 2014.

NASA and the White House budget office said they could not comment on the shortfall before the official 2007 budget rollout in February, but several expert sources described ongoing negotiations to find a solution. They spoke on the condition of anonymity because they did not want to become part of the public debate or were not authorized to speak for their employers.

"The budget associated with the Vision's announcement in 2004 [anticipated] that the shuttle's demands would significantly decrease in 2008 and 2009," said one longtime NASA watcher. "That was a myth."

Griffin acknowledged as much at a Nov. 3 House Science Committee hearing, saying the plan to finish the space station and retire the shuttle in 2010 faces a "$3 billion to $5 billion" funding shortfall.

A committee document placed the deficit at "nearly $6 billion," and some sources said even that figure could be low. NASA's budget difficulties have also been complicated by having to pay for about $400 million in special projects inserted, mostly by senators, into the agency's 2006 funding.

The sources said the White House is juggling several proposals to close the deficit, but one industry source said, "None of the choices are good -- NASA's in a box."

But the White House, struggling with the costs of the Iraq war and Hurricane Katrina, is unlikely to find billions more for space travel. "There's really no place Griffin can go," said one source familiar with the negotiations. "The White House gave him the best deal he could expect. He can go back to the well, but it's not going to happen."

Several sources confirmed that the budget office in the early negotiations proposed stopping shuttle flights altogether. "It sucks money out of the budget, and it's a dead-end program," one source said.

But "that argument's over," another source said. "The political side of the White House said, 'We're keeping it.' If you kill the shuttle right now, it will be heavy lifting for your foreign policy because of the international obligations" around the space station.

A proposal under consideration would keep the full complement of shuttle flights -- 18 to finish the space station and one to service the Hubble Space Telescope -- and let completion of the crew exploration vehicle slip to 2014, if necessary, or even beyond.

"The president said originally there would be a four-year gap, and that's realistic," one source said. "My personal view, though, is whatever date you set . . . it will slip."

Some negotiators believe they could salvage the 2012 delivery date if NASA goes to "serial processing," using only one team of engineers to prepare shuttle flights one at a time. The sources said this would drop the total number of flights to about 10.

"But what kind of a space station do you get out of that?" one source asked. "And while you can reduce the workforce and maybe save some money, you don't know how much, and you're not making anyone happy. This is nobody's first choice."

The fourth possibility, the one probably favored by Congress, is to fully fund both the shuttle and the new spacecraft, thus eliminating the entire four-year gap and ensuring a seamless transition to a new era in human space travel.

"The message to the White House is to pony up," one congressional source said. "The green-eyeshade approach undermines the president's legacy. You've got to have a space station worthy of the name, and you've got to close the gap."