WASHINGTON — NASA wants to put the outer planets within reach of budget-constrained space missions by making available a new type of radioisotope power system capable of cranking out abundant, long-lasting electrical power at a fraction of the cost of today’s plutonium-fueled spacecraft batteries.

To date, the high cost of building and fueling so-called radioisotope thermoelectric generators (RTGs) that transform heat from decaying plutonium-238 into electricity largely has limited their use to NASA’s pricier flagship-class missions, such as the $3.2 billion Cassini-Huygens probe that reached Saturn in 2004.

One notable exception is New Horizons, a Pluto flyby mission that lifted off early last year on a 15-year scientific voyage. New Horizons cost around $450 million, not including launch costs.

It is perhaps no coincidence then that more modestly priced missions are going to be given the nuclear option now that Alan Stern, New Horizon’s principal investigator, is running NASA’s Science Mission Directorate.

NASA in September issued a call for concept proposals outlining small planetary missions that require a nuclear power source, namely the Advanced Stirling Isotope Generator.

Jim Adams, deputy director of NASA’s planetary science division, said radioisotope power systems are essential if the agency wants to send probes into deep space without resorting to flying bigger and bigger solar arrays to provide electricity for power-hungry science instruments and subsystems.

“The further out you get from the sun, the less light you get on solar arrays, [and] the bigger your solar arrays have to be. Dawn is flying the largest solar arrays we’ve ever put into deep space,” Adams said, referring to the spacecraft NASA launched Sept. 28 on a nearly 5 billion kilometer journey to the asteroid belt. Dawn’s two solar arrays, which provide power for its ion engine, instrument and other subsystems, span 20 meters in length tip-to-tip. Adams said

Juno, the $750 million New Frontiers mission NASA aims to launch to Jupiter in 2011, likewise will depend on massive solar arrays, rather than nuclear batteries, for onboard power.

“Clearly, getting the power that we need in deep space, at some point the sun just is not going to cut it,” Adams said in an Oct. 22 interview. “So we need radioisotope power systems for deep space missions. The question is how can we do that affordably, reliably and safely? Stirling looks like it could be the answer.”

The piston-driven Stirling generator produces about four times more electrical power per kilogram of plutonium-238 than an RTG. Stirling has been in development in one form or another for more than 20 years but has yet to fly in space.

NASA considered using a Stirling generator for the 2009 Mars Science Laboratory, but decided in late 2006 to go with the better known RTG technology for the $1.5 billion rover mission.

Adam said the so-called Discovery and Scout Mission Capabilities Expansion concept studies the agency expects to fund this coming year are all about finding a good candidate for the first Stirling-powered mission the agency hopes to launch around 2013. NASA has set a Nov. 30 due date for proposals.

“Our notion is that we can expand medium-class missions into the outer planets by using this technology,” Adams said. “What we want now is for the community to tell us” what really great ideas are out there that can be done with a Stirling power system on a Discovery budget.

Discovery is a line of competitively selected planetary missions that are supposed to take no more than three years to develop and launch within a cost-cap that is generally between $300 million to $500 million. Scout, a similarly cost-constrained, competitive program, is focused exclusively on Mars missions, like the recently launched Mars Phoenix Lander.

Adams could not say what the cost caps would be for a Discovery mission launching in 2013 since the solicitation is still a ways off and NASA is currently wrestling with a number of budget variables, including what will happen to launch costs following the planned phase out toward the end of this decade of the Delta 2 rocket, the Discovery program’s workhorse launcher.

To encourage use of the Stirling system, Adams said NASA plans to treat it as government-furnished equipment that will not count toward the cost cap for the first mission to use it.

Adams would not say what the Stirling systems would cost to build, but he said the cost to get to the first flight unit is within $10 million or so of what NASA is spending to develop the Multi-Mission RTG for the Mars Science Laboratory. “That’s one of the things that convinced me that Stirling is now ready for prime time,” he said.

Once over the initial development hump, Stirling systems will cost about a fourth of what a comparable RTG would cost. “It turns out that the lion’s share of the cost is in the plutonium,” Adams said.

While NASA hopes to give the Stirling system its first in-space workout in 2013, the agency’s next flagship-class outer planets mission — currently penciled in for the 2015-2017 time frame — will be powered by RTGs.

Adams said NASA would like to see Stirling flight-proven on a medium-class mission before relying on it to power a multi billion-dollar flagship mission that NASA would expect to be well into development in 2013.

NASA’s latest embrace of nuclear power comes as the U.S. Department of Energy nears a decision on resuming domestic production of plutonium-238. The United States used to produce plutonium-238 at the Department of Energy’s Savannah River Site near Aiken, S. C., but stopped production in the 1980s and has been augmenting the U.S. inventory by buying the non-weapons grade material from Russia.

The Department of Energy announced in January 2001 that it intended to resume domestic production of plutonium-238 at the Oak Ridge National Laboratory in Tennessee, but those plans were altered following more stringent security requirements put in place following the Sept. 11 terrorist attacks. The Department of Energy currently is proposing to consolidate all radioisotope power system activity, including the resumed production of plutonium-238, at the Idaho National Laboratory near Idaho Falls. An environmental impact statement on the consolidation was issued by the department for public comment last year.

Department of Energy spokeswoman Angela Hill said Oct. 10 that the department expected to decide “in the next few months” whether to request 2009 funding for the consolidation effort. “Actual production would resume about five years after the start of the project,” Hill said.

Adams said NASA understands from talking to Energy Department officials that there is enough plutonium-238 available to meet the space agency’s forecasted demand through 2015.

Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...