"The exploration vision for solar system exploration is affordable in both the short-term and the long-term," Sean O'Keefe told skeptical senators on the Commerce, Science and Transportation Committee.

Bush called two weeks ago for development of a new spacecraft capable of traveling to the moon as early as 2015, then possibly on to Mars.

He would give the National Aeronautics and Space Administration an additional billion dollars over five years, in addition to its annual budget of 15.4 billion dollars.

The Congressional Budget Office this week said the US budget deficit will explode to a record of nearly half a trillion dollars in 2004, prompting concern about the costs of the new space program.

O'Keefe insisted Bush's request "will be fiscally responsible and consistent with the administrations goal of cutting the budget deficit in half within the next five years."

"I would hope members would examine that budget before making judgments on its adequacy or efficacy," he told the committee.

But senators from both parties were critical.

"Space flight is costly," said Democrat Byron Dorgan from North Dakota. "I don't want to be a wise guy, but we've been promised the moon before."

Republican John McCain from Arizona pointed to studies that estimated Bush's space plans would cost 170 to 600 billion dollars.

"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," he said.

SPACE.WIRE