WASHINGTON — After last year’s failed attempt to secure congressional backing for restarting domestic production of plutonium-238, NASA and the U.S. Department of Energy have redoubled their effort, sending lawmakers a report that warns continued inaction would jeopardize future science missions that depend on long-lasting spacecraft batteries powered by the critical material.

To meet the full scope of projected NASA and national security needs, the Department of Energy is proposing to re-establish the capacity to produce up to 2 kilograms of plutonium-238 per year — less than half the production rate the department was projecting last year.

The Department of Energy and NASA detailed the change of plans in a June report to Congress that estimates preparing national labs to produce plutonium-238 for the first time in more than two decades will take five to six years and cost as much as $90 million.

NASA has relied on plutonium-238 for decades to fuel long-duration spacecraft batteries known as radioisotope power systems that transform heat from the decaying plutonium into electricity. The Pluto-bound New Horizons probe launched in 2006 with 11 kilograms of the material onboard, and the Mars Science Laboratory rover will carry 3.5 kilograms into orbit when it launches in late 2011. But four subsequent missions planned between 2015 and the end of the decade could exhaust existing reserves, according to the document.

“We do have a small stockpile,” said Jim Adams, deputy director of NASA’s planetary division in the Science Mission Directorate in a June 28 interview, adding that three of the four missions in question could be powered using just 1.8 kilograms each of plutonium-238 to power a new, NASA-furnished radioisotope power system. These include Discovery 12 — a $425 million mission that would launch by 2016 — as well as a robotic lunar precursor planned under NASA’s marked-for-termination Constellation program and a joint U.S.-Europe Mars mission currently envisioned for 2018.

The fourth mission — a Jupiter Europa orbiter — would require a whopping 21.3 kilograms to power a traditionally reliable but less-efficient radioisotope thermoelectric generator that NASA has used for decades. “Beyond that, there is no supply,” Adams said.

NASA and the Department of Energy are asking for a combined $30 million in 2011 to complete initial design and development work on plutonium-238 production facilities and to begin purchasing equipment.

Ralph McNutt, a planetary scientist who co-chaired a National Research Council panel that examined the plutonium-238 issue in depth, said he expects the proposal to fare better this time around because it complies with 2010 appropriations language directing start-up costs to be split equally between the Department of Energy and NASA.

“Hopefully having this report out and having the budget structured the way it is, that will take care of the concerns that the appropriators had last year,” McNutt, a staff member at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., and the project scientist for NASA’s Messenger mission to Mercury, said in a June 30 interview.

The House Appropriations energy and water development subcommittee that oversees Department of Energy spending postponed a planned June 24 markup of 2011 spending legislation. No action has been taken in the Senate, which last year zeroed the White House’s plutonium-238 funding request.

The United States stopped producing plutonium-238 in the late 1980s when it shut down reactors at the Department of Energy’s Savannah River Site in South Carolina for safety reasons. U.S. nuclear laboratories remain able to process and package the material for use in radioisotope power systems, but they have been meeting NASA’s demand for the past two decades from a dwindling stockpile supplemented by periodic purchases from Russia.

The United States has taken delivery of 20 kilograms of plutonium-238 from Russia since the early 1990s. But after Congress rejected a 2009 proposal by U.S. President Barack Obama to restart plutonium-238 production domestically, Russia reneged on an agreement to deliver 10 kilograms of plutonium-238 to the United States in 2010 and 2011.

Since then, the Department of Energy has been renegotiating the agreement, a process that could delay delivery of the nuclear material by several years, according to an Energy Department official.

“The time to negotiate a new agreement and contract for and receive deliveries under a new agreement could take three to four years,” Department of Energy spokeswoman Jennifer Lee said June 30, adding that the department is “working with Russia to identify a path forward for completing planned purchases.”

In the meantime, bringing U.S. nuclear laboratories back on line is expected to cost less than the Department of Energy previously estimated, according to the document. In 2009, department officials told Congress at least $150 million would be needed to begin the painstaking process of restarting production over as many as seven years.

Now the department says NASA’s projected mission requirements can be met with less plutonium-238 than previously projected with an average production rate of just 1.5 kilograms per year, rather than the historic average demand for NASA and national security users of about 5 kilograms per year, the document states.

Adams said NASA attributes the reduced requirement in part to the agency’s planned use of Advanced Stirling Radioisotope Generator technology for future missions, a capability that could produce about four times more electrical power per kilogram of plutonium-238 than the traditional radioisotope thermal electric generators NASA has been using for decades.

“That efficiency directly correlates to the amount of plutonium that would be required,” Adams said.

Although Stirling technology is not new, the Stirling system has yet to be flight proven for deep space missions. Like radioisotope thermoelectric generators, the Stirling system converts heat from decaying plutonium into electricity. Where the two technologies differ is that the Stirling system has moving parts — vibrating pistons that make much more efficient use of plutonium-238 but could be prone to failure and cause interference problems for spacecraft instruments.

“These pistons have to move at roughly 100 times a second for up to 15 years without missing a heartbeat,” Adams said.

In addition, projected quantities of the plutonium-238 necessary for exploration missions are expected to be small compared with that needed for science missions, the document states, though Adams said it is unclear how much of the nuclear material may be needed for the robotic precursor missions outlined in NASA’s $4.2 billion exploration funding request for 2011.

NASA proposed canceling most of its Constellation program, a 5-year-old effort to build new rockets and spacecraft optimized for manned missions to the Moon. But the proposal has yet to be approved on Capitol Hill, where it has drawn fire from lawmakers whose states stand to lose jobs if the program is terminated.

“If, for some reason, either Constellation or whatever emerges out of the president’s new vision requires plutonium power, there’s no plutonium allocation for that, either,” Adams said, recalling that the president’s new direction for human spaceflight is based in part on sending precursor robotic missions to the Moon, asteroids and potentially Mars in advance of human explorers.

“There’s a risk that if they need to go places where the sun doesn’t shine, then they’re going to need plutonium-238,” Adams said.

Adams said that between the uncertainty surrounding Constellation and the potential for one or more of NASA’s next four science explorers to be powered by something other than nuclear energy, enough plutonium-238 remains to get the agency through to its next major outer planets mission — a Jupiter probe that would not launch until 2020 or later.

“We’ve set aside on the order of 20 or so kilograms of plutonium-238 for that,” he said. “After that, we run out of fuel.”