It all comes down to Curiosity.

The big nuclear-powered Mars rover is NASA's last chance to drive scientific instruments around on the Martian surface in this decade. After that, it may be 2020 before the U.S. can afford to launch another rover to the red planet.

As recently as last fall, NASA had plans for launching a mission to bring back Mars samples to analyze on Earth as early as 2022. Now the expert team set up to develop new options for a downsized Mars program is working on the assumption that there will be surface samples delivered to orbit around Mars no later than 2033.

At that remove, it is difficult to predict whether they will be returned to Earth by an unpiloted vehicle or by human explorers, says Orlando Figueroa, a retired NASA “Mars czar” brought back from retirement to pick up the pieces left by President Barack Obama's fiscal 2013 NASA budget request.

Obama's space advisers have made their priorities clear. A May 7 “statement of administration policy” includes a threat to recommend that Obama veto the Republican-drafted House appropriations bill that includes NASA funding. The White House bureaucracy rejected proposed increases for Mars exploration and complained about a $330 million cut in seed money for the commercial vehicles NASA hopes will be able to deliver astronauts to the International Space Station later in the decade.

“This would increase the time the United States will be required to rely solely on foreign providers to transport American astronauts to and from the space station,” the Office of Management and Budget stated in policy language inserted by Deputy NASA Administrator Lori Garver. “While the administration appreciates the overall funding level provided to NASA, the bill provides some NASA programs with unnecessary increases at the expense of other important initiatives.”

House members passed the Republican-drafted fiscal 2013 appropriations bill that includes NASA on May 9. That bill would add $88 million to NASA's funding request for Mars exploration in fiscal 2013, on grounds that “[p]lanetary science has long been one of NASA's most successful programs, and the cuts proposed in the budget request will endanger this strong record,” says the report accompanying the legislation.

The Senate version of the bill would add more than $100 million to Mars exploration. But NASA's fiscal 2013 budget request called for a reduction of $226.2 million for Mars exploration, down from the $587 million the agency will spend on Mars this year. Most of the $360.8 million left over would go toward putting the Curiosity rover safely on Mars and operating it there (if it survives its untried “sky crane” landing), continuing development of the Mars Atmosphere and Volatile Evolution Mission (Maven) orbiter scheduled for launch next year, and operating the Mars exploration rover Opportunity and the two NASA orbiters above the planet.

Drafted by Democratic lawmakers, the Senate version of the NASA appropriations bill also would take a deep cut in NASA's commercial crew request of $830 million, trimming it to $525 million instead of the $500 million in the House version. Those and other differences will be ironed out by a conference committee after final Senate action and, in an election year, allowances must be made for a certain amount of partisan rhetoric. The White House policy statement, for example, charged that House Republican cuts in the bill “were made in the context of a budget that fails the test of balance, fairness and shared responsibility by giving millionaires and billionaires a tax cut and paying for it through deep cuts, including to discretionary programs.”

But the fact remains that “the train has left the station,” as NASA Planetary Science Director Jim Green said when asked if more funding would allow NASA to resume joint Mars exploration work with the European Space Agency. ESA has shifted to partnering with Russia for Mars exploration after NASA's bailout.

Figueroa, heading the Mars Program Planning Group (MPPG) as it drafts a downscoped, go-it-alone Mars exploration program, told planetary scientists on the NASA Advisory Council May 8 the $700-800 million that will be available for robotic Mars exploration by 2018 under the new NASA budget request will not support a rover. “A stationary lander may be possible in 2018,” Figueroa says. “A mobile lander, a rover, doesn't fit the budget we have available, so we need to jump one opportunity to generate enough funds to be able to do it.”

NASA already is “jumping” the 2016 planetary launch window—which comes around every 26 months—after pulling out of its plans to work with ESA on missions in 2016 and 2018 aimed at preparing for a Mars sample-return mission as early as 2022. Sample return remains the top priority among U.S. planetary scientists surveyed in the National Research Council's decadal survey in the field, and Congress strongly supports that goal. Figueroa says a rover would be more useful in meeting it than a stationary lander.

The MPPG was established to present options for a new U.S. Mars program that both retains the sample-return goal, and draws on the funds and expertise of NASA's Human Exploration and Operations (HEO) Mission Directorate, the Office of the Chief Technologist (OCT) and the agency's chief scientist, as well as the planetary science organization within NASA's Science Mission Directorate. The MPPG is working with those NASA organizations, and the larger U.S. and international planetary science communities, to develop options that will fit within the diminished U.S. Mars-exploration budget while contributing as much as possible to human knowledge of the planet.

As an alternative to a stationary lander, the funds expected to be available in 2018 probably also would support a new orbiter that could help pinpoint potential landing sites for future rovers that could characterize and cache samples for eventual return to Earth. An overarching objective of the MPPG is to develop “pathways” that would ensure samples from Mars's surface would be in orbit above it no later than 2033. That year would be particularly advantageous for a Mars-Earth transit, Figueroa says, and was not necessarily picked for other reasons.

Another factor in play as the MPPG develops options for NASA mangers is how well they “infuse” new technology into exploration plans. One area Figueroa says OCT and HEO are examining closely is laser communications, which would provide broadband links for robotic and human exploration missions.

The MPPG plans to make its recommendations in August, to give NASA managers and the White House time to incorporate their decisions on a future pathway into the agency's fiscal 2014 budget request. If election-year jousting leads to a flat continuing resolution (CR) instead of a new funding bill for NASA, Green says there is no guarantee the Science Mission Directorate would continue to fund Mars exploration at fiscal 2012 levels. Instead, he warns, managers could even decide to cut it under a CR below the amount requested for fiscal 2013.

That leaves Curiosity as the only near-term alternative to the aging solar-powered Opportunity rover, which is just returning to work again after wintering on a Sun-facing slope near the rim of the Endeavour crater.