The US planetary science community has laid out priorities for exploring the solar system, including guidelines for adjusting its recommended program should the budget grow or shrink. Vision and Voyages for Planetary Science in the Decade 2013–2022 was unveiled at the Lunar and Planetary Science Conference in Houston on 7 March.

The 400-plus-page survey was put together over two years with input from hundreds of planetary scientists in academia, industry, and NASA. It highlights recent discoveries across planetary science, spells out the most exciting scientific questions, and specifies which missions will help answer those questions. The survey was produced by the National Research Council at the behest of NASA and NSF and is intended to guide those agencies and Congress. It is the second decadal survey in planetary science; such community-led prioritization was initiated a half century ago by NASA’s astrophysics division and is now undertaken by all four of the agency’s science divisions.

Just a couple of weeks before the release of the planetary science decadal survey, President Obama’s proposed budget for fiscal year 2012 and the five-year horizon through 2016 was rolled out (see PHYSICS TODAY, April 2011, page 29). That horizon suggests that the field will have to tighten its belt even more than the survey committee had anticipated. James Green, director of NASA’s planetary science division, says that the FY 2011 budget request’s projection for FY 2013–16 falls about $800 million short of the projection from a year earlier (costs are in FY 2015 dollars). For context, the president’s request for planetary science in FY 2012 is about $1.5 billion.

Based on the survey’s guidelines, Green says that the Mars Astrobiology Explorer–Cacher (MAX–C), a multiyear, multicraft plan to gather and store samples on Mars with a later return to bring them to Earth, will have to be pared down. The goal of MAX–C is to collect samples from sedimentary rocks, which are not found in meteorites; such specimens would open the door to study water, potential prebiotic chemistry, and possibly remains of early Martian life. A joint program of NASA and the European Space Agency (ESA), MAX–C got the highest priority of three flagship missions, the most costly of the planetary science mission categories.

The second flagship mission is an orbiter around Jupiter’s icy moon Europa as a “first step in understanding the potential of the outer solar system as an abode for life . . . with the goal of confirming the presence of an interior ocean, characterizing the satellite’s ice shell, and understanding its geological history.” The third flagship mission is the Uranus Orbiter and Probe. Going to Uranus would break new ground because the ice giants—Uranus and Neptune—are the “only class of planet that has never been explored in detail.”

But here’s the rub: The planetary science survey says that the cost of flagship missions should be reined in, and they should be postponed or canceled outright rather than smaller, more frequent missions being sacrificed. Independent cost estimates by Aerospace Corp put NASA’s contribution to MAX–C at $3.5 billion, a “disproportionate share” of NASA’s planetary sciences budget, according to the survey. NASA’s portion of the MAX–C tab, the survey says, should be capped at $2.5 billion. Similarly, the tab for the Jupiter Europa Orbiter (JEO) would have to be slashed from an estimated $4.7 billion. But, whereas the survey committee sees ways to scale down MAX–C, it says flying JEO would require both downsizing and a bigger total planetary science budget.

“Scientists tend to have a love–hate relationship with the large flagship missions,” says Michael Drake, director of the University of Arizona’s Lunar and Planetary Laboratory (LPL). “They love the science, but hate that the flagships tend to eat their lunch.” Adds survey chair Steven Squyres of Cornell University, “If you did a flagship at all cost, you’d wind up with missions that have timescales of 10 to 15 years. You could go a decade, two decades, without getting any data. That leads to stagnation in the program. The community was emphatic that we maintain the faster-turnaround, lower-cost, highly productive New Frontiers and Discovery missions.”

New Frontiers missions are capped at about $1 billion each and Discovery missions at half that. Missions in those categories, unlike flagships, are initiated and competitively bid in the broader planetary science research community. The survey recommends that NASA select two New Frontiers missions from a specified set over the decade 2013–22 and says that Discovery missions should be selected and executed at the rapid pace of one every two years.

Among the survey’s “deferred high-priority missions” are trips to Ganymede, Mars, Titan, and Neptune. “My favorite target, Saturn’s moon Titan, ended up too low in the rankings,” says LPL’s Jonathan Lunine. Titan, he notes, “is in many ways the closest analogue to Earth. It is the only other solid body in the solar system that has an active hydrological cycle where there is rain”—liquid methane in the case of Titan. “We are suffering from too many good ideas, too many places to go, and insufficient resources,” Lunine says.

“We did mission prioritization primarily on the basis of bang for buck,” says Squyres, “where science return was judged by people on our panels and dollars were judged by an independent, conservative cost estimate.” The other criteria were balance across the program, technical readiness, and timing—or the availability of trajectory opportunities. Squyres notes that recommendations were made by “strong consensus, and most were unanimous. We had 16 members on the steering committee. Fifteen to one was okay. Fourteen to two was okay. Ten to six was not good enough.”

Other key recommendations in the survey are to protect and increase NASA’s “research and analysis” budget, which funds individual investigators, and to set apart about $100 million a year for technology development. “What always seems to happen,” says Squyres, “is that some mission gets into budget trouble, they need money from somewhere, and the technology program is a tempting target.” When that happens, he continues, a vicious circle is created: Technologies are not developed, missions with immature technologies are selected, and the new missions overrun their budgets. Examples of technologies specific to planetary exploration include aerocapture, in which a planet’s atmosphere is used to slow down a spacecraft and put it into orbit, and Advanced Stirling Radioisotope Generators, the survey’s top technology priority, a high-efficiency plutonium-based system to power spacecraft throughout the solar system. (See PHYSICS TODAY, January 2011, page 24.)

The survey notes that human spaceflight can “cannibalize space science programs” and urges that the human exploration program identify “objectives where human-tended science may advance our fundamental knowledge.” And it stresses the importance of international partnerships, even spelling out eight necessary ingredients for successful collaborations. Those include shared objectives, existing scientific communities, and clearly defined roles.

Green says that from his perspective, the decadal survey “is absolutely outstanding. We can’t do everything, but they give us decision-making rules for how to proceed. Even though this is an austere time, it is not all gloom and doom.”

“We need to fund the ongoing program,” Green says. He ticks off Cassini, which is expected to collect data about Saturn through 2017; Messenger, which went into orbit around Mercury in March; and missions scheduled to fly this year to asteroids, Jupiter, Mars, and the Moon. “This is spectacular stuff. If we do not fund these, we will have to turn things off,” he says. “And if you don’t start making selections, then you don’t have a future program.”

What the planetary science program might look like on a reduced budget, Green says, “could mean many things if we didn’t have the decadal survey. But since we do, it means going back to work with our international partners to try to create a different approach” to the flagship missions. Specifically, NASA and ESA are beginning discussions on how to downsize MAX–C while keeping the mission scientifically worthwhile.

Sedimentary rocks on Mars, known to contain hydrogen and oxygen, may be candidates for future visits to the planet as a means to study past environments that could have supported life. One such region is the mineral-rich Nili Fossae, photographed in 2007 by the HiRISE camera on the Mars Reconnaissance Orbiter mission.

Sedimentary rocks on Mars, known to contain hydrogen and oxygen, may be candidates for future visits to the planet as a means to study past environments that could have supported life. One such region is the mineral-rich Nili Fossae, photographed in 2007 by the HiRISE camera on the Mars Reconnaissance Orbiter mission.

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