THE WOODLANDS, Texas—Planetary scientists, usually an affable lot, are plenty riled up at the moment.

The field is bristling at cutbacks, proposed last month by the Obama administration, to planetary science and especially to NASA's program of robotic Mars explorers. Researchers gathered here for the annual Lunar and Planetary Science Conference have taken turns railing against the cuts, in scientific lectures, in conference center hallways and in open meetings with NASA management. One NASA official, having briefed a group of scientists on one facet of the budgetary outlook, joked about needing a Kevlar flak jacket for his talk.

In the budget proposed by Obama for the 2013 fiscal year (pdf), NASA's planetary science division would lose more than $300 million compared with the previous year. The bulk of that would come from Mars exploration. If Congress agrees to the Obama plan for NASA, the Mars exploration program would lose roughly $225 million, a cut of more than 38 percent. (Some members of Congress have already protested the cuts; NASA Administrator Charles Bolden will appear before a House subcommittee this week to discuss the budget.)

The impacts of the proposed budget are already being felt—NASA has announced that it will withdraw from joint Mars missions in 2016 and 2018 with the European Space Agency (ESA). Jim Green, head of NASA's planetary science division, acknowledged during a NASA forum that the international partnership would need some repair. "We need to build back their confidence," he said of ESA. "We want to regain their trust."

The proposal carries an extra sting because NASA's Mars program has been among the agency's most productive enterprises of late, both in terms of science and public relations, thanks to missions such as the Mars Reconnaissance Orbiter, the Phoenix lander and the Spirit and Opportunity rovers. "Why indeed are we having a 38 percent cut in the Mars program?" asked James Head of Brown University during a lecture. "Have we been too successful?"

The latest mission, the behemoth Curiosity rover now en route to Mars, was beset by budgetary overruns and launch delays. But John Grunsfeld, head of NASA's Science Directorate, said during the NASA forum that the cuts were not punitive. Rather, he said, the agency was faced with a flat overall budget and increasing costs in certain other sectors that had to be offset elsewhere.

"The fact that the planetary budget—I think I'm not supposed to say 'got whacked'—the fact that the NASA planetary budget took such a big hit was really a result of those tough priority settings," Grunsfeld said. He added that it was not possible to link the planetary science cuts directly to gains elsewhere at NASA. "There really weren't trades from one division to another," he said.

But that is unlikely to satisfy some planetary scientists, who see an increasing share of NASA's science budget going to other projects, such as the James Webb Space Telescope, which is years behind schedule and already well over budget. Now it is clear that planetary science will have to pay, however indirectly, for pricy programs elsewhere at NASA.

"Instead of maintaining NASA's strengths by spreading the small amount of budget pain evenly among the agency's projects and programs, they decided to single out planetary science for a massively disproportionate budget cut," Jim Bell of Arizona State University in Tempe, president of the board of directors for the nonprofit Planetary Society, said during a question-and-answer session following Grunsfeld and Green's briefing. "And within planetary science, to single out one of our nation's most successful programs, the Mars program, for the lion's share of the cuts. I don't know, personally, what compels such hostility toward what's been NASA's singularly most successful endeavor in the past several decades."