WASHINGTON -- NASA will likely cull its herd of spacecraft slated for launch in the near term to accommodate a spike in mission costs, a top official said Thursday.

Ed Weiler, NASA's associate administrator for space science, said costs have risen on anywhere from 10 to 15 different projects, from a few percent to as much as 40 percent. To accommodate the increased costs, NASA must contemplate delaying or canceling missions.

"Having very cheap missions that fail is not a good thing," Weiler said during a Thursday conference call with reporters. "Having little more expensive missions that succeed is a good thing."

The sharp increases are due in part to risk-reduction measures instituted across the agency in the wake of the Mars Climate Orbiter and Polar Lander mishaps.

"I think what we're seeing is reality setting in. It's showing that people have learned some lessons from the Mars failures," Weiler said.

The fact that spacecraft projects are owning up to actual costs is a positive sign, Joseph Alexander, director of the Space Studies Board at the National Academy of Sciences, said in an interview last week.

"I think, overall, what we're seeing across the agency is a healthy effect," Alexander said.

But NASA must also face this prospect: the next-generation rockets the space agency is counting on to send many of those missions into space are taking longer to make it to the launch pad. And when the rockets eventually do, they will cost more than forecasted in earlier budgets.

"We also planned on certain commercial launch vehicles being available and tested at a certain price. And surprise, surprise -- some of those launch vehicles aren't going to be as cheap as some people promised," Weiler said. "So there's a lot of reasons why these budgets are going up."

Weiler cautioned any shakedown might not occur until late January or early February. He stressed that NASA has not cancelled a single mission, including plans for a first-ever visit to Pluto. Recent media reports, including a story by SPACE.com, have put Pluto-Kuiper Express at the top of the endangered mission list. (The probe may also lose its radioactive power source to a planned mission to Jupiter's moon Europa.)

"I have not cancelled the Pluto mission and I have not canceled any other mission, and other reports to that effect are false," Weiler said. "Have we considered canceling the Pluto mission? Yes, we have. Have we considered canceling the Space Interferometry Mission? Yes, we have."

That prospect has sown the seeds of panic among many in the space-science community, especially among backers of the Pluto shot. Tiny Pluto is the lone planet in our solar system never visited by a spacecraft.

"It's pretty clear this mission is in NASA's gun sights," Jonathan Lunine, a member of the original Pluto-Kuiper Express science-definition team, said earlier this week.

The Planetary Society hopes to rally its 100,000 members to forestall any such decision. The Pasadena-based space exploration advocacy group is goading its rank-and-file members to write Congress in an effort to forestall the cancellation of the mission to distant Pluto.

Louis Friedman, the Planetary Society's executive director, hopes doing so will drag the issue into the limelight. Already, he said, hundreds of the group's members have written, e-mailed and faxed members of Congress.

"It needs to be discussed rather than have a very quiet cancellation of a mission in the middle of summer when Congress is in recess," Friedman said.

Weiler said he would probably elect to cancel one or more missions to avoid budgetary woes from jeopardizing all of NASA's space exploration endeavors.

"Do you want to keep all the goodies that you've got in your fruit basket or do you want to throw one piece of fruit out and keep the other ones real healthy?" Weiler said. "And that's a tough choice because you'll take a lot of different advice on that subject."