Skeptical senators grilled NASA administrator Sean O'Keefe (search) on Wednesday on whether President Bush's vision of returning astronauts to the Moon and exploring Mars is feasible in light of strained federal budgets.

In the past, Congress has seen NASA project budgets balloon well beyond what they were ever projected to cost, said Sen. Byron Dorgan (search), D-N.D.

"Space flight is costly," said Dorgan. "I don't want to be a wise guy, but we've been promised the Moon before."

Earlier this month, the president sought to chart a new course for the space agency, focusing on a return to the Moon by 2020 in preparation for manned missions to Mars and beyond. To pay for the project, Bush plans to request a 5-year, $1 billion increase in NASA's budget with an additional $11 billion diverted from other NASA projects.

O'Keefe said details of the budget, due to be released Feb. 2, will provide a clearer picture of how NASA will achieve its lofty goals. It will be followed by a sweeping reorganization of the agency to focus efforts on the new goals, he said.

"I would hope members would examine that budget before making judgments on its adequacy or efficacy," O'Keefe told the Senate Commerce, Science and Transportation Committee (search).

The administration is proposing mothballing the space shuttle at the end of the decade, which would save NASA about $4 billion a year, and quickly ending its obligations to the International Space Station, which costs about $1.7 billion annually.

O'Keefe said NASA can fund the president's program and meet Bush's goal of cutting the deficit in half within five years. He said the budget strategy is flexible, but it will not force future Congresses to cover the costs.

"Space flight: You can't do it on the cheap and I just don't think a billion dollars increase over five years — that's $200 million a year — is going to do it," said Sen. Bill Nelson, D-Fla., the only current member of Congress who has been in orbit. He was a crew member on a 1986 space shuttle flight.

Committee chairman John McCain, R-Ariz., cited reports that the new space proposal could cost between $170 billion and $600 billion. A more modest proposal laid out by the first President Bush had projected costs of $400 billion to $500 billion and never got off the ground.

McCain said the public is justifiably apprehensive about the initiative.

"When we look back on the past cost of programs at NASA, there has been one constant and that's been that the costs have exceeded the initial estimates," said McCain. "What's different?"

O'Keefe said the president's space agenda is not an "aggressive schedule" and the there is room built into the budget to allow for unforeseen costs.