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Headshot of Emily Lakdawalla

Maybe, possibly, a nuclear-powered Discovery mission?

Posted by Emily Lakdawalla

08-01-2008 10:13 CST


One of the people who emailed me over the last few weeks to remind me to post about the selection of GRAIL was one Alan Stern, whose title is Associate Administrator for the Science Mission Directorate at NASA Headquarters (meaning he's head honcho at NASA for all the science missions, which include things like Cassini, the rovers, Dawn, Phoenix, MESSENGER, and GRAIL, but not Earth observing missions like Landsat or GRACE or human exploration, or, somewhat strangely, Lunar Reconnaissance Orbiter, which is controlled by the head of the division that controls human space exploration, NASA's Exploration Systems Mission Directorate). In his message to me, Alan pointed out that although GRAIL won't launch until 2011, because it's a lunar mission it has essentially no cruise phase, so it will begin returning science almost immediately. With both Stardust and Deep Impact having comet flybys at around the same time (Deep Impact on October 11 2010, Stardust on Valentine's Day 2011), it's going to be quite a time for the Discovery program.

Which is interesting, but didn't seem worth its own blog entry so I set aside Alan's email for a while. What I hadn't realized was that the third and final paragraph of his email to me contained some tantalizing news, which may have been out there already but which I had totally failed to clue in to. I'll try to be a good reporter and give the punch line at the beginning: NASA is considering opening up the option of nuclear power for the next Discovery mission, which could allow a huge variety of missions previously impossible under the relatively low Discovery budget; on top of that, the next Discovery mission may get started a year early.

Until now, Discovery missions have only been solar-powered. That limits Discovery to regions relatively close to the Sun -- Dawn, with targets in the asteroid belt, goes about as far as you can -- and, because of tradeoffs mostly involving mass, makes landers really hard to do. Having nuclear power as an option opens up a huge variety of new missions obtainable under a Discovery budget: things like a Mercury lander -- polar lunar rovers -- even missions to the outer solar system and Centaurs and what have you.

I chatted with Alan for a while on Friday exploring how this could work. To begin with, I need to explain what kind of nuclear power we're talking about: a Stirling generator, which is a technology that has not yet been used in space. I found a white paper dating to 2002 on what a Stirling generator is here; basically, it uses the heat from radioactive decay of plutonium-238 to run a piston-powered generator, converting 20-something percent of the energy from the plutonium into electrical power. Twenty percent doesn't sound great unless you compare it to the five- or six-percent efficiency of current radioisotope thermal generators (RTGs), including the "multi-mission RTG" or MMRTG that's going to fly for the first time on Mars Science Laboratory. The greater efficiency means that you need less plutonium to power a spacecraft; as Alan told me, an MMRTG requires 18 kilograms of plutonium, while a Stirling needs only two.

That's a really big deal. It's not just the cost of plutonium, which runs you several million dollars a kilogram. Even if you were willing to pay more, plutonium-238 is awfully hard to come by. I was introduced to the plutonium-238 supply problem at a May 2006 meeting of the Outer Planets Assessment Group. Almost all of the U.S. supply of plutonium-238 is spoken for. We're now buying it from Russia, which (as of May 2006) had delivered us 5 kilos of the precious stuff, with 5 more on order -- all that to supply the raw material necessary for the MMRTGs that will power Mars Science Laboratory. Beyond that, Russia has only 15 more kilograms of the stuff. Of that, as of May 2006, the U.S. Department of Energy had a contract to allow it to purchase 5 kilos, with an option to extend the contract to purchase the remaining 10. Nearly all of that is already earmarked for the next outer planets flagship mission, wherever it's headed. There isn't enough for any more MMRTGs after that. However, there will be enough plutonium for one Stirling.

Alan explained to me that the Stirling generator is almost ready for prime time. They have years of simulated time running the generator, and next month, they're going to fire up a flight model and run it for a year. Once it's run for a year without incident, he told me, he'd be quite comfortable seeing it on a Discovery mission, though not on an outer planets flagship mission. He said he wouldn't risk putting a never-flown power supply on the flagship mission, not without seeing it run successfully on a cheaper mission first.

You might wonder what principal investigator would be willing to take on this risk, but Alan told me there's a lot of appetite out there for a nuclear-powered Discovery mission because of all the possibilities it opens up. He said that Jim Green (director of the Planetary Science Division of NASA's Science Mission Directorate) put out a call for mission concept proposals, asking the science community what they'd do if the next Discovery mission was nuclear-equipped. He said they got more than forty proposals, of which they plan to select ten and fund them for a yearlong study. The proposals included all kinds of stuff previously inconceivable for Discovery: go look for ice at the lunar poles with a rover; go rendezvous with a Centaur, one of the small bodies like Chiron or Pholus that orbits in the outer solar system and may be an interloper from the Kuiper belt; send a probe into Saturn's atmosphere; go land on Mercury.

If they get one Stirling into space and it works, they'll presumably want to send more, but there's no more plutonium. The U. S. does have 300 kilograms of neptunium-237, the raw material necessary to produce plutonium-238. And we have the capability at Idaho National Laboratory to start production again in 2013, with five kilograms produced per year, of which half would be for NASA's use. However, Alan told me, there's a fight going on between the White House, NASA, and the Department of Energy over whether production is going to be restarted, and who's going to pay. The Department of Energy has always paid in the past, but they're now balking. Obviously Alan doesn't want to see money come out of NASA's budget to pay for it if he can do anything to prevent it. He didn't say this, but I assume that demonstrating that the Stirling works will create some pressure to restart production, regardless of who pays.

Alan wasn't 100% certain that the next Discovery announcement will include the option of using the Stirling power supply. Regardless of how it's powered, it looks likely that they'll be seeking proposals for new Discovery missions a year early, because there's enough money in the Discovery budget to do so. He said they wanted to select two Discovery missions this round but that they didn't quite have the money. The last straw, he said, was when the launch of Dawn slipped to September. That cost the Discovery program an additional $40 million, and ended hope of two Discovery missions being selected -- they only had enough money for one and a half. So the "and a half" will wind up meaning that the next Discovery mission, whatever it is, will probably get started a year early.

Finally, since I had him on the phone, I asked him about the status of the next outer planet flagship mission. Four mission concepts were being considered, one each to Titan, Enceladus, Europa, and the Jupiter system generally. It was announced late last year that of these four, three would be going ahead into the next round of studies; only Enceladus was eliminated, because, Alan said, the others were all equally strong. He said that they plan to downselect again to two mission concepts later this year, summer or fall, and to make the final selection about a year from now. He said that while they would like to make this process go as quickly as possible, but that they have to be sensitive to the fact that whatever outer flagship mission they select, they want to do it with international partners on board (either or both ESA and JAXA), and they have different timetables; ESA in particular has a big decision day coming later in 2008 on the next step in their cosmic vision, and NASA needs to cooperate with ESA on their selection calendar.

See other posts from January 2008


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