By Adam Mann

A showdown over the course of Solar System exploration has ended with a qualified victory for Mars. NASA's planetary-science decadal survey, which sets mission priorities for 2013-22, firmly favors a mission to Mars over a rival one to Jupiter's icy moon Europa (see Nature 466, 168-169; 2010). But the decision marks the beginning of a much bigger battle: to secure the budget to lift the multibillion-dollar project off the survey's pages and into the heavens.

The decadal-survey committee's recommendations, released on 7 March at the Lunar and Planetary Science Conference in Houston, Texas, relied partly on President Barack Obama's 2011 budget request, which projected that NASA's annual planetary-science funding would grow from its current allocation of $1.36 billion to more than $1.6 billion by 2015. But Obama's 2012 budget foresees that funding dropping to $1.2 billion in 2016. On 3 March, planetary-sciences division director James Green told the NASA Advisory Council's science committee that this would create indefinite delays for both the Mars and Europa missions.

"This creates a big gap between what the decadal survey is planning on and what is available," agrees Fran Bagenal, a planetary scientist at the University of Colorado, Boulder, and former chairwoman of an external NASA planetary-science advisory committee, who was not involved in creating the report.

The details of the recommendation reflect the committee's attempts to navigate different budget scenarios and maintain a robust research agenda under cash-strapped conditions. "In prioritizing missions, the most important criterion was maximizing science bang per buck," says Steve Squyres, an astronomer at Cornell University in Ithaca, New York, and chairman of the decadal survey.

The top-ranked flagship mission, the Mars Astrobiology Explorer-Cacher (MAX-C), would use a rover to conduct in situ astrobiological experiments, and to collect and store samples for return to Earth. This mission would also deliver the ExoMars rover for the European Space Agency (ESA). "We are at the point in Solar System exploration where what we want to do is beyond the budget of a single nation," says Wendy Calvin, a geologist at the University of Nevada, Reno, and vice-chair of the decadal survey's Mars panel.

To allow room in the budget for other priorities, the report recommends that the mission should not fly in the next decade unless a billion dollars can be shaved off the estimated $3.5-billion cost to NASA.

The second-choice flagship mission will fly only if its costs can be cut, and if NASA gets a significant budget increase for planetary exploration. Indeed, it is excluded from the panel's 'cost-constrained' mission wish-list (see 'To boldly go ...').

The Jupiter Europa Orbiter (JEO) would map the Jovian moon to assess the extent of the ocean thought to lie beneath its icy surface -- a possible habitat for life. But the mission's estimated price tag -- $4.7 billion, adjusted to 2015 dollar values -- shocked panel members. "There were a lot of gasps when we saw the bottom line," says Stephen Mackwell, director of the Lunar and Planetary Institute in Houston, Texas, and vice-chair of the inner-planet panel. The high price was a key factor in tipping the decision towards a Mars mission and significantly lengthens the odds against the Europa mission getting off the ground this decade.

The decision to rank the JEO after MAX-C has not met with universal enthusiasm. "We've spent a little too much time on Mars," says Bagenal, adding that the red planet will be visited by the much-delayed and over-budget Mars Science Laboratory rover next year. She also finds it "hard to imagine" that MAX-C can stick to a $2.5-billion price tag, not least because it would rely on an as-yet-untested landing system.

The third flagship candidate is the Uranus Orbiter and Probe, expected to cost $2.7 billion. Like the other top recommendations, this project should be delayed or cancelled if costs increase significantly, the committee says. Indeed, these flagship missions should be the first to be put on hold if the planetary-science budget shrinks too much. This would protect the small Discovery missions and medium-sized New Frontiers projects, both of which are relatively low cost, narrowly focused and selected by competitive peer review. "One message we got from the community was not to let those missions get damaged from overruns on the flagships," says Squyres.

The survey does not prioritize competing Discovery missions -- the costs of which are capped at $500 million -- but advocates strongly that the program should continue to deliver 'economy probes' such as the Mercury surface, space environment, geochemistry and ranging (MESSENGER) mission, which is scheduled to arrive in orbit around the planet next week (see 'MESSENGER's arrival').

The report also suggests adding two projects over the next decade to the New Frontiers program. These projects cannot exceed $700 million and include the New Horizons mission, which launched in 2006 to study Pluto and the Kuiper belt. The additions would come from a list of seven candidates, including a sample-return mission to a comet, a Venus lander and an orbiter for Jupiter's moon Io. The committee also strongly endorsed the ESA-NASA Mars Trace Gas Orbiter. Expected to launch in 2016, the orbiter will study the behavior of trace gases in Mars's atmosphere -- particularly methane, which could indicate ongoing geological activity or even the presence of microorganisms.

Although funding helped to dictate the many trade-offs in the survey, Squyres says that the panel relied heavily on input from the planetary-science community and worked to build consensus before making decisions.

Despite some dissension, Squyres urges the planetary-science community to pull together behind the plan, speaking "with one clear and loud voice when talking to elected representatives in Congress to fight for funding".

"We have to be willing to implement some programs at a reduced cost," says Squyres, "and we have to be willing to fight."