WASHINGTON — Criticized for cutting millions of dollars worth of science grants and delaying or canceling a number of smaller, researcher-proposed missions, NASA’s science chief said she was willing to reconsider what she puts on the chopping block but that ultimately her budget ax must fall somewhere.

Mary Cleave, NASA’s associate administrator for science, was back in the hot seat March 6 defending her budget cuts before an ad hoc committee of the National Research Council’s Space Studies Board chartered to assess the agency’s science plans.

Several days earlier, Cleave made her first appearance before the House Science Committee as NASA’s science chief, sharing the spotlight with a slate of influential scientists who did not like her budget priorities and told lawmakers that small missions and research grants are so important that it is worth considering scaling back NASA’s big, expensive flagship science missions to find extra money for them.

Appearing alone before the Space Studies Board here, Cleave had a chance to explain the tough choices she faced in preparing a 2007 budget request that has been roundly criticized by a science community that had been counting on a bigger increase than the 1.5-percent raise NASA requested for 2007 .

Cleave described for the committee a $5.3 billion science budget that was being eroded from within by cost growth on programs both big and small even before NASA cut $3.1 billion from the Science Mission Directorate’s five-year plan to cover a severe shortfall in the space shuttle budget.

To live within a budget not projected to keep pace with inflation through the end of the decade, Cleave canceled several missions still early in their development, reduced the amount of funding that would be made available to scientists to analyze data streaming back from spacecraft already in operation , reduced the number of spacecraft that will be sent to Mars in the decade ahead, pushed out competitions to select a new slate of small missions and postponed getting started on bigger missions that have been on NASA’s agenda for years.

Space Studies Board Chairman Len Fisk told Cleave that he did not see a clear, priority-driven strategy behind what was cut and what was not. “In some ways this is a survival budget,” Fisk said. “You got a top-line number and you tried to do the minimum amount of damage.”

Cleave indicated she is willing to make changes. “We worked hard on it, but it doesn’t mean we can’t improve it,” she said. “We did our best. It was very uncomfortable. It’s obvious some people don’t agree with our decisions.”

Cleave said she was open to putting more money into research grants if that is what the science community wants, but warned that another mission would be canceled as a result.

“We will take down another mission to restore [research and analysis],” she said.

“There aren’t many left to take down,” one scientist quipped.

“That’s my point exactly,” Cleave replied.

The Space Studies Board’s Committee on An Assessment of Balance in NASA’s Science Program is due to deliver its report this spring.

Meanwhile, some programs that have found themselves on NASA’s chopping block are trying their best to get off.

Proponents of the Stratospheric Observatory for Infrared Astronomy (SOFIA), a telescope-equipped jetliner, have been busy drumming up support for their embattled program in Congress. SOFIA’s first science flight, according to NASA, has slipped at least three years to 2008, and the estimated cost of completing the flying observatory and operating it for 20 years have risen substantially. NASA has not made a decision to cancel the mission, but included no money for the program in its 2007 request. Cleave has told lawmakers that if NASA goes ahead with SOFIA, it will have to cancel an Explorer-class mission to get the money for it.

Although Cleave did not specify which Explorer mission would be cut, sources tracking NASA’s budget decisions said it would probably be the Wide-field Infrared Survey Explorer, or WISE, a small space telescope mission managed by the Jet Propulsion Laboratory, Pasadena, Calif. In what some are reading as a preparatory move for restoring SOFIA, NASA informed the WISE team March 2 that its 2006 budget was being cut in half, and that the start of the project’s detailed design and development phase was being postponed until at least October pending availability of funds.

Also on March 2, Dawn, a Discovery-class mission to explore two asteroids, got official word that it was canceled due to major cost and schedule issues that first came to light last year. Cleave said Dawn’s price tag had grown from $370 million to about $450 million. Even though NASA already has spent $280 million on the mission to date, she said the Dawn team’s proposal for getting the project back on track was not credible.

But within a couple days of Cleave telling the Space Studies Board why she canceled the mission, NASA announced that it would take another two weeks to review the decision before making a final determination about Dawn’s fate. NASA released a statement March 9 saying that NASA Associate Administrator Rex Geveden would lead the review.

Comments: bberger@space.com

Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...