WASHINGTON — Having lost a chance to send a next-generation nuclear power source to space in 2017 as part of a small planetary exploration mission, NASA is preparing to test its Advanced Stirling Radioisotope Generator (ASRG) as part of an Earth-bound mock mission.

The purpose of the mock mission, known internally as Mission 1, or M1, is to retire technical risks associated with the unproven ASRG before trusting it to power a big-ticket, must-succeed planetary flagship mission, said Len Dudzinski, program executive for radioisotope power systems at NASA headquarters here.

NASA thought it could buy down ASRG risk by flying the plutonium-powered unit on the next Discovery-class planetary science mission, which is launching in 2017. However, NASA decided last August to pass over two ASRG-powered mission proposals in favor of a solar-powered Mars lander called InSight.

“For the purposes of developing an ASRG for future missions, most notably the flagship missions, it was a lost opportunity to really retire the risk for integrating and fueling and flying the system and proving it out in space,” Dudzinski said in a Feb. 20 interview. “It had been hoped that had we flown on Discovery, that that would have retired the risk for putting the ASRG on the Europa flagship mission to fly some time after 2020.”

Members of the NASA-chartered Outer Planetary Advisory Group have identified a $2 billion concept called Europa Clipper as their preferred outer solar system flagship. However, there is currently no money for such a mission in NASA’s cash-strapped Planetary Science Division, which is operating under a $1.19 billion budget that is projected to decline further through 2015 before rebounding to about $1.2 billion in 2017.

But there is money for the ASRG program. NASA is on track to spend about $36 million in 2013 on ASRG-related research, Dudzinski said, while Lockheed Martin Space Systems — working under a $260 million U.S. Department of Energy contract it got in 2008 — is expected to deliver two flight-ready ASRGs by October 2016.

That schedule, Dudzinski said, leaves time to get an ASRG aboard the next Discovery mission after InSight. Competition for what would be NASA’s 13th Discovery mission since starting the program of cost-capped, scientist-led planetary science missions in 1992, is set to begin in 2015. Dudzinski estimates that the Discovery 13 mission would launch around 2020, by which time the M1 project should be able to accomplish most of its risk-retirement work.

What exactly the mock mission will entail Dudzinski was not ready to say.

“In the absence of having that real mission, what the program is doing is … defining a mission that would be Earth-bound but it would, as much as possible, answer a lot of unanswered questions about using an ASRG,” Dudzinski said.

ASRGs create electricity by converting heat produced by decaying atoms of plutonium-238. This radioactive isotope has powered every deep-space mission since Voyager 1, which launched in 1977. An ASRG would be about four times more efficient, and 7.5 kilograms lighter, than the Multi-Mission Radioisotope Thermoelectric Generator powering the $2.5 billion Curiosity rover now exploring Mars and slated for use on the Curiosity clone NASA plans to send to the red planet in 2020.

The cost of the M1 project is not yet known, Dudzinski said, because the team that will run M1 at the Glenn Research Center near Cleveland is still fine tuning project requirements. M1 would not begin until at least October, the beginning of the U.S. government’s 2014 budget year.

In the meantime, Glenn-based M1 project manager Diane Malarik is now reaching out to the two teams whose outer-planets proposals lost to InSight back in August: the Titan Maritime Explorer proposed by Proxemy Research of Gaithersburg, Md., and Comet Hopper, proposed by a team at the University of Maryland in College Park.

Dudzinski said the two teams are being asked to provide input on how the M1 project can be used “to retire the risks to their satisfaction.”

“It may very well be that they would like to see an ASRG fueled and tested while it’s operating under plutonium heat and perhaps even integrated on a spacecraft bus, or simulated spacecraft bus, while it’s in that condition,” he said. “So that’s all [to be determined]. I’m promised that I will have a project plan and a request for a budget this spring when we go through our budget process.”

The Titan Maritime Explorer team wanted to send a boat-like probe to explore the hydrocarbon seas of Saturn’s largest moon. Ellen Stofan, vice president of Proxemy Research and principal investigator for the mission, told SpaceNews that NASA’s Radioisotope Power Supply office had reached out informally to her team for feedback on integrating ASRGs into future missions. Stofan’s team plans to give NASA a written response “sometime this spring,” she said.

Comet Hopper principal investigator Jessica Sunshine, a University of Maryland astronomy professor, did not reply to requests for comment.

With NASA signaling that tightening budgets have lowered its tolerance for risk even in its traditionally innovative Discovery line, Stofan is worried that dependence on radioisotope power could put future plutonium-dependent proposals at a competitive disadvantage.

Stofan said she intends to point out in her team’s written report to NASA that even government-furnished ASRGs are not exactly free.

In its announcement of opportunity for the Discovery 12 competition, NASA said that any missions proposing an ASRG had to set aside $20 million to cover, among other things, environmental compliance costs associated with handling the power pack’s nuclear fuel.

“I would argue this would exceed what one would normally spend on a power system,” said Stofan, a former researcher at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “That’s a disincentive to use the technology.”

For that reason, “missions with radioisotope power technology are likely to be riskier,” Stofan said. “This difference in risk must be accounted for in selections between these missions and non-radioisotope power supply missions that are in competition.

“Ideally, you would not mix the two.”

Dan Leone is the NASA reporter for SpaceNews, where he also covers other civilian-run U.S. government space programs and a growing number of entrepreneurial space companies. He joined SpaceNews in 2011.Dan earned a bachelor's degree in public communications...