There is little question some long-lived NASA planetary exploration missions will be at risk of cutbacks or cancellation later this year, but the agency's top planetary science official this week cautioned the science community not to presume which projects, if any, will see funding cuts.

Brilliant Saturn is suspended in the blackness of space in this view from Cassini. Photo credit: NASA/JPL/Space Science Institute
 
NASA's unparalleled fleet of planetary science missions are exploring Mars and Saturn, speeding toward Jupiter and Pluto, mapping the moon and Mercury, and dashing through the asteroid belt.

But the longevity of many of the missions will force difficult decisions for a review committee this spring tasked with deciding which projects NASA should keep funding.

Mission managers are finishing proposals to be submitted to NASA in April for consideration in a senior review, a process every two years in which an independent panel of respected scientists rank the value of continuing funding for each project.

The senior review board's recommendations will be announced in June, according to NASA. All of NASA's science divisions use a similar review to decide which missions most deserve continued funding.

Scarce funding, always a concern for NASA, is aggravated in this year's senior review cycle by the inclusion of the Curiosity Mars rover, which will complete its primary two-year mission this summer and must ask for approval for extended operations.

The Cassini mission in orbit around Saturn is also seeking money to keep flying. Running low on propellant, the spacecraft is on a trajectory to crash into Saturn in September 2017.

NASA is also trying to balance extending old missions going while developing new projects, such as another Mars rover scheduled for launch in 2020 and a robotic mission to Europa a few years later.

Scientists assembled this mosaic of images from the Curiosity rover last year, showing layers of sedimentary deposits in the Glenelg area of Gale crater. Photo credit: NASA/JPL-Caltech/MSSS
 
"This will be a very interesting competition," said Jim Green, director of NASA's planetary science division. "We have two very expensive flagship missions, Cassini and Curiosity, which are expected to operate even in an extended phase, along with a lot of our other missions, which are doing tremendous science at a lower cost."

Cassini and Curiosity each cost about $60 million per year to operate, but officials in charge of both missions say they are looking at ways to reduce costs.

Jim Erickson, Curiosity's project manager, said in an interview last fall that decreasing the mission's operating costs would require taking some creative steps.

"Once we got down to a planned steady state (after landing in August 2012), we haven't reduced the costs at all," Erickson said. "That's sort of the next step and takes some planning to do."

Erickson did not disclose specific cost reductions planned for Curiosity's extended mission, citing the competitive nature of the senior review.

Cassini's mission extension proposal includes an option for full funding by NASA and an alternate concept with 85 percent of the cost, according to a presentation in January by Cassini project science Linda Spilker.

The other projects under consideration in the senior review have smaller annual operating budgets.

Unless Congress boosts NASA's planetary science budget, or some missions are willing to live with severe cutbacks in funding and capability, there is a risk some spacecraft will be turned off.

Language in the Obama administration's fiscal year 2015 budget request sparked worries the Opportunity Mars rover and Lunar Reconnaissance Orbiter were destined for cancellation. Both missions receive no money in the White House's main budget request, but the projects are listed in an addendum dubbed the "Opportunity, Growth and Security Initiative," a separate $56 billion program not expected to gain traction in Congress.

But planetary science has strong supporters on Capitol Hill, including Rep. Adam Schiff, D-Calif., who sits on the House Appropriations Committee and represents the district containing NASA's Jet Propulsion Laboratory, which manages most of the missions up for review this year.

Imagery from the Lunar Reconnaissance Orbiter helped create a high-resolution interactive mosaic of the moon's north polar region. Photo credit: NASA/GSFC/Arizona State University
 
Speaking to a gathering researchers at the Lunar and Planetary Science Conference near Houston on Monday, Green said it was a "misunderstanding" to conclude Opportunity and LRO were singled out for cancellation.

"Congress has to make a decision," Green said. "Let's speculate and say Congress says no to the [Opportunity, Growth and Security Initiative] fund, what are we going to do? ... We go through a senior review. We'll order the missions, we'll make programmatic decisions, and we'll fund them. If LRO is on top or Opportunity is on top, they will be funded."

One scientist who questioned Green took umbrage with Opportunity and LRO's placement in the budget request's bonus fund and not the main document. "What mission would you like for me to put there?" Green asked. "Something other," the scientist replied.

Seven missions are on the planetary science division's senior review docket this year:

• The Curiosity rover, initially approved for a prime mission lasting one Martian year, is participating in the senior review for the first time this year after arriving on Mars in August 2012. Curiosity is driving toward Mount Sharp, a three-mile-high peak believed to harbor layered clay minerals containing clues about the red planet's ancient past.
  
  
• The Cassini mission is proposing an extension until late 2017, when the spacecraft will fly inside of Saturn's outermost rings before plunging into the gas giant's atmosphere. Cassini's mission has been extended twice since entering orbit in July 2004. Unlike other projects in the senior review, which are on the hook for two-year extensions, Cassini is seeking a three-year commitment from NASA to operate the spacecraft until the planned conclusion of its mission, supporting flybys of Saturn's moons Titan, Enceladus, Dione and Tethys.
  
  
• NASA's Mars Reconnaissance Orbiter, flying in orbit several hundred miles above the red planet, is seeking money for a fourth extended mission phase since it arrived at Mars in March 2006. MRO hosts a sharp-eyed high-resolution camera, a mineral mapping spectrometer and ground-penetrating radar. The orbiter also serves as a communications relay platform for the Curiosity and Opportunity rovers on the Martian surface.
  
  
• After a remarkable 24-mile journey across Mars, the Opportunity rover is exploring the rim of Endeavour crater, where scientists say they have found evidence of an ancient environmental that was capable of supporting microbial life. Opportunity landed in January 2004 at the start of a planned three-month mission, but the rover is still going and producing science results.
  
  
• Mars Odyssey is the longest-serving mission to ever visit Mars. The spacecraft entered orbit in late 2001 and still has fuel for nine or 10 years of operations, according to NASA. Odyssey is the primary communications link with NASA's rovers on Mars, and the probe is currently adjusting its orbit to fly over Mars during morning daylight, which scientists say could yield insight into ground composition, warm-season water flows found on steep slopes, and geysers spawned by dry ice during the spring thaw at the Martian poles.
  
  
• NASA is a junior partner on the European-led Mars Express mission, which has orbited Mars since December 2003. NASA supported development of the Mars Express subsurface radar and an instrument to monitor the interaction between the solar wind and Martian atmosphere to study what happened to the water that was once plentiful on Mars. NASA's limited involvement in Mars Express makes it the least costly mission in the agency's catalog of extended missions.
  
  
• The Lunar Reconnaissance Orbiter is up for a two-year extension. LRO would be NASA's only spacecraft exploring the moon over the next two years, with instruments to gather high-resolution images, search for ice deposits, map the moon's jagged terrain, and measure radiation in the lunar environment.

NASA's other planetary missions, such as New Horizons, Juno and Dawn, are still in their primary mission phases. And the MESSENGER spacecraft at Mercury was already granted an extension to March 2015, when engineers expect it to run out of propellant and impact Mercury.

"I would love for the community not to worry about where the money is, and how they're going to get it, because they need to write a proposal to get it," Green said. "They need to burn a hole in steel and write the best darn extended mission proposal they can such that our peer review process will allow us then to fund them."

Follow Stephen Clark on Twitter: @StephenClark1.