WASHINGTON - With NASA's budget under pressure and the space shuttle set to retire, even the agency's most sacred cow - the 64-member astronaut corps - isn't safe from the possibility of cuts.

At the behest of the White House, the nation's top science advisers this month began a 10-month study of the appropriate "role and size" of the astronaut corps after the final shuttle mission next year.

The study, by the National Academies, reflects two realities: NASA's budget, squeezed by congressional budget hawks and its own cost overruns, needs every penny. And, more significantly, the United States may not need all these astronauts.

"Clearly, there won't be a lot of flying going on after the shuttle goes away," said Leroy Chiao, a former astronaut and a member of the 2009 presidential commission that examined NASA's future. "It's reasonable to say the astronaut office should be smaller. How much smaller depends on . . . what you want these guys to be doing."

At most, three shuttle missions remain on the calendar. Two require six-member crews; a third, a potential rescue mission, has a designated crew of four. An additional 11 astronauts are preparing for long-duration stays aboard the International Space Station - and are likely to get there aboard Russian Soyuz rockets.

That still leaves more than half of the 64 current astronauts - nine more are in training - without a mission.

That imbalance is a matter of timing, NASA said, as more than 30 additional station mission slots available through 2020 haven't yet been assigned. But critics say scientists and engineers could fill then without going through expensive astronaut training.

And astronauts won't be riding a new rocket beyond low Earth orbit for years.

Under a plan approved this fall by Congress and the White House, NASA's next spacecraft capable of carrying humans to the station won't be ready until 2017 at the earliest.

At the same time, the agency's manned-spaceflight plans are in flux. Under the Bush administration, the target was the moon by 2020. But President Obama asked NASA to aim for a first-ever trip to an asteroid by 2025.

Astronauts are expensive, noted Howard McCurdy, a space-policy expert and a professor at American University; keeping them on the government payroll isn't like "maintaining a couple of extra forest rangers." But before resizing the astronaut corps, he said, it's important to figure out its role.

"The big question is: Where we are going and when?" McCurdy said.

Joseph Rothenberg, a former NASA official and cochair of the National Academies study, framed the issue this way: "How do we maintain [astronaut] proficiency and make sure the capability we have doesn't erode due to idleness? At the same time, we want to make sure it [the astronaut office] is not overstaffed."

NASA officials said they base the size of the astronaut corps on a complicated series of factors that take into account everything from medical concerns to attrition rates and the skills needed for particular missions. The officials point out that the number of astronauts has dropped by more than half since a high-water mark of 149 in 2000.

Set against the rest of NASA's $18.7 billion budget, spending on the astronaut office is relatively small potatoes - in the tens of millions of dollars, though NASA wouldn't say just how much. The 47 civilian astronauts earn $65,000 to $101,000; the 17 military ones are paid by the Defense Department, which NASA reimburses.

The major cost is for training, including the corps' fleet of T-38 trainer aircraft; a "neutral buoyancy" water tank used to train spacewalkers; and other facilities at Houston's Johnson Space Center. NASA officials also did not provide an estimate for that cost in the 2010 budget.

The National Academies investigators have been asked specifically to examine the T-38 fleet. They also will look into the role and size of the astronaut corps and the facilities used to support them.

A key obstacle to cutting the astronaut corps, noted by McCurdy and others, is the immense symbolism tied to a team that once put men on the moon. And NASA tries to maximize that symbolism: Astronauts made 521 public appearances in 2009 - and have made 517 so far this year, NASA records show.

And concerns about NASA's future have not diminished the appeal of being an astronaut. More than 3,500 hopefuls applied for the 2009 class; the nine remaining members, expected to finish training next year, include two flight surgeons and a CIA officer.

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U.S. Rep. Pete Olson (R., Texas), whose district includes the Johnson Space Center, said the corps was a "national asset" that provides more than just expertise.

"They are the faces of human space exploration," he said. "There is no budgetary savings that can pay for the loss in stature."