ARLINGTON, Va. — It is no longer certain when competition will begin for NASA’s next small- and medium-sized robotic solar system exploration missions, the head of the agency’s Planetary Science Division said here July 15.

“[If I were] to tell you exactly when the next Discovery call is, I would be making it up,” Jim Green, NASA Planetary Science Division director, told scientists gathered at a hotel here for a meeting of the NASA-chartered Outer Planets Assessment Group.

The same uncertainty applies to the New Frontiers series, Green said, adding that the start date for the next mission in both lines will not be decided until the White House releases its 2015 budget request in early 2014.

As recently as January, Green said competition for the next principal-investigator-led Discovery and New Frontiers missions would begin around 2015.

While that would have been par for the course for the New Frontiers program, which has had five-year gaps between launches so far, it already marked a major slowdown for the smaller Discovery line, which has launched 11 missions since 1996.

The most recent Discovery mission, the twin Gravity Recovery and Interior Laboratory probes, was launched in 2011. NASA’s next Discovery mission, a Mars lander called InSight that was selected last August, is scheduled to launch in 2017. The mission has a $425 million cost cap, which excludes launch costs.

New Frontiers missions are capped at $700 million, excluding launch costs. The first New Frontiers mission, the New Horizons probe now on its way to the Pluto system, launched in 2006. The Jupiter-bound Juno polar orbiter launched in 2011, and the Osiris-Rex asteroid-sampling mission, selected in 2011 as the third New Frontiers mission, is slated to launch in 2016.

Budget Battles

Green blamed ongoing federal budget woes for holding up the next Discovery and New Frontiers missions. But the Planetary Science Division had already been tapped to help cover the James Webb Space Telescope’s $8.8 billion price tag before across-the-board spending cuts known as sequestration threatened to put even more pressure on NASA’s budget.

The $17.7 billion NASA budget request the White House sent Congress early last year proposed cutting the Planetary Science Division’s $1.5 billion budget back to $1.2 billion in 2013 and plow about half of the savings into Webb, Earth Science and heliophysics.

The U.S. House of Representatives rejected the cut when it passed its 2013 NASA spending bill last summer, but the legislation stalled when a broader budget impasse prompted Congress in September to pass a six-month continuing resolution freezing spending at 2012 levels. But as Green informed the Outer Planets Assessment Group during its January meeting in Atlanta, NASA went ahead with its 2013 plan and cut planetary science back to $1.2 billion.

When Congress passed a second six-month continuing resolution at the end of March, lawmakers explicitly bumped Planetary Science back up to $1.5 billion.  NASA, however, informed its congressional oversight committees in a late May operating plan that it intends to spend just $1.2 billion on Planetary Science this year, the same as it is requesting for 2014.

Heidi Hammel, executive vice president for the Washington-based advocacy group, the Association of Universities for Research in Astronomy, said Congress has yet to accept NASA’s plan to keep planetary science funded at $1.2 billion for the rest of 2013.

“That’s bouncing back and forth right now between NASA, the [White House] Office of Management and Budget and Congress,” Hammel said during told the Outer Planets Assessment Group’s meeting here.

Curiosity vs. Cassini

With funds scarce in Planetary Science, NASA’s two active planetary flagship missions find themselves pitted against one another in an upcoming senior review where seasoned scientists will judge which ongoing missions are worthy of more funding.

The next senior review for planetary science missions is scheduled to begin in 2014 and conclude in 2015. Both the rover Curiosity, which landed on Mars last August, and the nearly 16-year-old Saturn-system orbiter Cassini are up for review, according to Green.

“Curiosity runs about the same as Cassini, it’s a $50 million to $60 million operation per year,” Green told the Outer Planets Assessment Group. “So we have an enormous problem on our hands in terms of trying to accommodate all of our operating missions, and yet move forward with our program.”

Cassini, which launched in 1997 and entered orbit around Saturn in 2004, may not be able to make a strong case. In the last meeting of the Outer Planets Assessment Group in January, Curt Niebur, lead program scientist for NASA’s New Frontiers line, said that the spacecraft’s Cassini Plasma Spectrometer instrument, shut down in June 2012 because of electrical problems stemming from suspected tin-whisker growths, was unlikely to be turned back on.

Pinching Plutonium

Elsewhere in the Planetary Science Division, budget concerns already have prompted NASA to abandon a concept it had been kicking around since 2012: a ground-based mock-mission known internally as M1 that would have tested a next-generation radioisotope power supply for use in future outer-solar-system expeditions.

“With the budget being what it is, it doesn’t like we’ll be moving forward on that,” Len Dudzinski, program executive for radioisotope power systems at NASA headquarters, said during the meeting.

The planned mock mission was conceived as a way to retire technical risks associated with the Advanced Stirling Radioisotope Generator, or ASRG, that Lockheed Martin Space Systems is building for NASA. At one point, the agency thought it could integrate an ASRG with one of the mission concepts competing with InSight to become the 12th Discovery-class mission. When selection officials chose the solar-powered Mars mission instead, the agency conceived M1, in which an ASRG would have been integrated with a mock spacecraft for long-duration ground testing.

Radioisotope power supplies use the heat from decaying atoms of Plutonium-238 — an isotope not currently produced in the United States — to produce electricity. NASA is in the middle of an effort, for which it is footing the bill, to resume domestic production of Plutonium-238, of which about 10 usable kilograms remain in the U.S. inventory. The space agency wants to be able to produce around 1.5 kilograms of Plutonium-238 a year, Dudzinski said July 15.

ASRGs will be lighter and more efficient than the current generation of radioisotope power supplies, one of which is now powering Curiosity on Mars. Lockheed Martin Space Systems — working under a $260 million U.S. Department of Energy contract it got in 2008 — is still expected to deliver two flight-ready ASRGs in 2016, Dudzinski said. The goal is to have both the plutonium and the power unit ready to support missions launching in 2020 or later.

“If we don’t get our high-efficiency radioisotope power system … we’re going to be limited in scope with the type of missions NASA as a whole can consider flying for the foreseeable future,” Dudzinski said.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.