The not-so-bad news in the president's 2013 budget proposal for NASA is that his request for $17.71 billion is only marginally less than what the agency got in 2012, including an overall science budget request of $4.92 billion that is not far below the 2012 level. The bad news—and fans of Mars should brace themselves—is that the budget, if approved, would officially signal the termination of NASA's participation in two European-led missions to the red planet in 2016 and 2018. However, NASA Administrator Charles Bolden says the agency isn't abandoning plans to explore Mars in the future—it is merely trying to come up with better ideas to do so without breaking the bank.
As reported on ScienceInsider last week , the president's request reduces NASA's planetary science budget from its current level of $1.5 billion to $1.2 billion in 2013. The bulk of that reduction comes from slashing the Mars exploration program from $587 million in 2012 to $360 million in the next fiscal year. The proposal also takes a bite out of the outer planets program, cutting it from $122 million to $84 million.
At a budget briefing at NASA headquarters this afternoon, Bolden did not offer a clear explanation about why the U.S. was pulling out of the 2016 and 2018 missions it had previously committed to  flying in partnership with the European Space Agency. "We are developing an integrated strategy to ensure that the next steps for the robotic Mars Exploration program will support long-term human exploration goals as well as science and meet the president's challenge to send humans to Mars in the mid-2030s," Bolden said.
The agency head said he had tasked a NASA panel to come up with recommendations over the coming months about how to restructure the Mars program beyond what's in the works right now, which includes Curiosity—the rover that is scheduled to land on Mars in August this year—and the Mars Atmosphere and Volatile Evolution Mission (MAVEN), which is due to launch in 2013.
"A problematic part of the ExoMars mission is that it was another multibillion dollar flagship mission," Bolden said. "Flagships are expensive. … We just could not afford to do another one." However, he refused to say whether the increased cost of a different flagship—the $8.8 billion James Webb Space Telescope (JWST)—was a factor in the agency's decision to terminate ExoMars. When reporters pressed Bolden on the issue, he on more than one occasion responded only by saying, "We're embarked on an ambitious program of space exploration," followed by, "We're in tough fiscal times."
However, Bolden did say that NASA wasn't ruling out alternate medium-class missions to Mars in 2018 or 2020, when the planet's location relative to Earth will provide a convenient opportunity to land a spacecraft on its surface. The agency was talking to the European Space Agency about the possibility of collaborating on such a mission, he said.
Briefing reporters later in the afternoon, John Grunsfeld, the new head of NASA's Science Mission Directorate, provided only vague clues about what an alternate Mars mission would look like. As part of the brainstorming to come up with the outlines of this future mission, he said, the agency was going to ask scientists: "What if you could go to Mars, what would you do, how would you do it?" Based on the ideas that are put forth, Grunsfeld said, NASA would go about planning a "basic mission" that "both answers scientific questions and supports future human exploration of Mars."