Challenged by a volatile federal budget, a board of respected scientists recommends NASA should continue with small and medium-class planetary science missions over the next decade, even at the expense of potential flagship probes to collect samples from Mars and survey Jupiter's icy moon Europa.

Artist's concept of a Mars sample return mission, the No. 1 priority for NASA flagship missions as recommended by the decadal survey. Credit: NASA
 
A National Research Council committee highlighted the importance of a regular cadence of Discovery and New Frontiers missions, NASA's workhorse planetary science programs that launch fresh probes every few years.

"The problem with a program that's flagship-only, where you just have one or two big flagship missions and nothing else, is you get the science at very extended intervals. It's a long period of time with no science return," said Steve Squyres, chairman of the decadal survey panel.

The report released Monday will guide NASA's planetary science program from 2013 to 2022. The agency will seek to implement the recommendations beginning with next year's budget.

"The decadal report transcends Congress, transends changes in administration and is our guiding light that moves us forward year after year," said Jim Green, director of NASA's planetary science division.

The Discovery program should launch new robotic probes about every two years, according to the research panel. Larger New Frontiers missions should also continue, with NASA selecting two new probes in the next decade, the report said.

Both programs send robotic spacecraft to other planets, the moon, comets and asteroids.

Discovery missions should be cost-capped at about $500 million and New Frontiers probes at $1 billion. Both figures are in fiscal year 2015 dollars.

NASA is currently in the process of selecting new Discovery and New Frontiers missions for launch by 2017 and 2018, respectively.

The planetary science decadal survey was chaired by Steve Squyres, a Cornell University researcher who leads the science team for the Spirit and Opportunity rovers currently on Mars. Credit: NASA

"If we have a program in which there's no new science results in a couple of decades, that leads to stagnation, and that's not a good thing," Squyres told Spaceflight Now. "You want to have a program that's balanced among small, medium and large missions. It's important to have those small and medium missions because they're the ones that, for one thing, provide a steady stream of new discoveries. But they also are short turnaround missoins that are able to respond effectively to new discoveries."

The decadal report ranked more than two dozen missions and concluded the most promising planetary science experiments over the next 10 years involve research on the surface of Mars and at Jupiter's moon Europa.

But neither mission should take precedence over smaller NASA probes that launch more often, Squyres said in an interview Monday.

If there is enough money left over after funding the less ambitious missions, the space agency's top priority should be launching a rover to gather rock samples from the surface of Mars.

The Mars Astrobiology Explorer-Cacher, or MAX-C, mission could launch as soon as 2018. The project's current plan calls for a dual launch with the European Space Agency's ExoMars rover.

MAX-C is the first of at least three missions aimed at returning Mars surface samples to Earth. An orbiter and Earth return mission, plus a complicated and costly rocket to launch the samples from the surface of Mars, will have to be dispatched to get the specimens back to Earth.

The follow-on missions could be staggered, and the Mars samples may not make it back for decades.

An early design sketch of the MAX-C rover. Credit: NASA
 
In its current design, the MAX-C mission will cost $3.5 billion, according to an independent assessment by Aerospace Corp.

"This is too large a fraction of the planetary budget to be spending on Mars," Squyres told a crowd of scientists at the Lunar and Planetary Science Conference in Houston.

According to the decadal survey report, NASA should only pursue the MAX-C rover if the agency can reduce its own mission expenses to $2.5 billion. That will require significant "descoping" of the mission, meaning designers must remove some features of the rover to make it less costly.

Cutting the NASA's cost obligation on MAX-C to $2.5 billion will also require U.S. officials to re-open talks with ESA, which is a full partner in the Mars sample return effort.

"It is clear we must go back to the negotiating table and rework new agreements if we expect to maintain the partnership with our best foreign partner to date," Green said.

The next bilateral meeting between NASA and ESA to discuss Mars missions is scheduled for the end of March, according to Green.

The renewed negotiation could lead to a larger ESA contribution to the MAX-C mission, officials said. The fate of ESA's long-delayed ExoMars rover, Europe's most ambitious Mars mission to date, will depend on the upcoming bilateral talks.

"The cost of accommodating both of those (rovers) is what we believe drives the cost to $3.5 billion," Squyres said. "If you throw in an ESA contribution as well, we're quite confident that a good mission can be done for $2.5 billion. But we can't force all the bad news onto ESA, it's got to be a fair split between NASA and ESA."

The decadal survey also recommended NASA continue with its ongoing missions, including the 2016 launch of the Mars Trace Gas Orbiter, another joint project with Europe.

If NASA is unable to cut MAX-C's cost to an acceptable level, the agency's second-ranked flagship mission should be a satellite to orbit Europa, an icy moon of Jupiter believed to harbor a subsurface ocean.

But the cost curve for the Europa orbiter is even more steep than MAX-C. Aerospace Corp. concluded its probable cost, when accounting for known threats, is $4.7 billion. It would blast off from Earth in 2020.

Artist's concept of NASA's Europa orbiter mission. Credit: NASA
 
"That's a scary big number," Squyres said. "We just simply can't afford this mission at $4.7 billion."

ESA is working on a probe to Ganymede, another of Jupiter's four largest moons, to launch separately from the NASA mission. Both orbiters would arrive at Jupiter around the same time, conducting fruitful tandem observations.

The Europa orbiter will only be possible with a budget boost for the planetary science division and major changes to the mission itself that don't put other approved projects in jeopardy, according to Squyres.

The Europa mission should only be approved if NASA initiates a so-called "new start" with funding committed to the mission, the report said

According to Green, NASA doesn't expect such new starts, potentially putting a 2020 launch to Europa in question.

"Because we are in economically difficult times, we should not plan or expect new starts to occur," Green said.

MAX-C and the Europa mission were top two priorities from the last planetary science decadal survey in the early 2000s.

If the Mars and Jupiter missions aren't affordable, NASA should next consider a $2.7 billion satellite to orbit Uranus and study the ice giant's extensive moon system.

A $2.4 billion climate probe and lander to Venus and a $1.9 billion spacecraft to circle Saturn's icy moon Enceladus round out the decadal survey's list of recommended missions.