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Plutonium-238 and the Outer System

Powering up a spacecraft is a lot easier to manage in the Sun-rich environment inside the orbit of Mars than it is out past the orbit of Jupiter. Solar panels provide plenty of power for a satellite in near-Earth orbit, for example, but moving into the outer system invokes the need for RTGs — radioisotope thermoelectric generators — powered by radioactive decay. If you read through the specs on the FOCAL mission design presented here last Friday, you saw that this attempt to reach the Sun’s gravitational lens would demand 20 RTGs, and thus requires resumed production of plutonium-238.

What’s happened is that US production of 238Pu was halted as far back as 1988, leaving us with stockpiles that should be sufficient for missions in the pipeline through the end of this decade. That’s the view of Leonard Dudzinski, NASA’s Radioisotope Thermoelectric Generator program executive, who was speaking at the opening session of the 43rd Lunar and Planetary Science and the Nuclear and Emerging Technologies for Space 2012 conferences. The two conferences were held in conjunction with each other in Texas from March 19 to 23.

There have been negotiations between the US and Russia to buy plutonium from Moscow, but according to Dudzinski. quoted in a release from Aviation Week Aerospace Daily & Defense Report, the talks have stalled and, in any case, Russia’s own supplies of plutonium are rapidly dwindling.

Thus it’s good to hear that NASA and the Department of Energy are poised to fund and restart the production of plutonium-238 within six to seven years — NASA won congressional approval to spend $10 million during 2012 on the restart and is seeking similar amounts in 2013-14. Meanwhile, the development of the Advanced Stirling Radioisotope Generator (ASRG) continues, the latter possibly to be used on Discovery-class missions like the Titan Mare Explorer (TiME) and Comet Hopper, both of which are being reviewed by NASA for a 2016 launch.

Image: Artist’s impression of the Titan Mare Explorer (TiME) capsule. TiME would perform the first direct inspection of an ocean environment beyond Earth by landing in, and floating on, a methane-ethane sea on Saturn’s largest moon. Credit: Johns Hopkins University Applied Physics Laboratory/Lockheed Martin.

From the report:

DOE is close to completing a restart assessment that attempts to make the best use of current production facilities to generate 1.5-2 kg (3.3-4.4 lb.) of plutonium-238 annually… In all, NASA counts 28 of the radioactively fueled missions over a half century, requiring a 3-kg average annual production. The Voyager 1 and 2 missions, launched in 1977, remain active as they exit the Solar System.

Of more modern vintage are the Mars Science Laboratory (MSL) and Curiosity rover, along with the New Horizons mission, which is scheduled for a flyby of Pluto/Charon in 2015. The Advanced Stirling Radioisotope Generator (ASRG) has never flown, but it would be useful indeed, increasing the heat-to-power-generation efficiency of plutonium to 30% from the 5-7% allowed by the solid-state RTGs in use on MSL and New Horizons. As the various agencies move to restart plutonium-238 production, a parallel step will be to learn whether ASRG is flight-worthy.

It’s worth considering the context in which all this occurs:

The provocative backdrop includes the proposed sharp drop in proposed spending on planetary science in NASA’s 2013 budget; an ambitious decadal road map from the National Research Council (NRC) that includes missions to assess the habitability of Jupiter’s moon Europa and Saturn’s moons Enceladus and Titan; and a three-year-old NRC report that warns U.S. space leadership is imperiled if it does not resume plutonium-238 production.

Missions to the outer system demand RTG power sources, without which the exploration of high-value targets like Titan and Enceladus cannot proceed. We should learn this year whether the TiME mission, which would attempt to land a probe in one of the lakes of Titan, or the Comet Hopper mission to explore a comet from its surface are selected. In any case, a wide range of potential missions remain in the queue, including the fascinating AVIATR, an ambitious concept to fly a heavily instrumented airplane through the thick atmosphere of Titan. I’ll be talking more about AVIATR tomorrow as we continue to ponder power generation in deep space.


Comments on this entry are closed.

  • philw1776 April 4, 2012, 10:53

    Don’t let Helen Caldicott’s disciple physicist Dr Michio Kaku read this. He tried to stop the Cassini mission because of plutonium.

    Hey! Maybe now that Pluto is no longer a regular planet, plutonium may be less dangerous.

  • Mike Lockmoore April 4, 2012, 12:21

    Kirk Sorensen has been promoting Thorium as a better nuclear fuel for Earth-based power, but one that would could produce some nice side-products like plutonium-238, even in the initial R&D reactor regime. Here’s a Google talk on YouTube: http://www.youtube.com/watch?v=tdusXIvyLFQ.

  • Adam April 4, 2012, 16:27

    The ASRGs will be very useful for would be missions to deep space if their power-to-mass ratio is better than solid-state RTGs. Might hit 1,000 AU in under a century.

  • Enzo April 4, 2012, 16:42

    And we just sent an RTG to Mar, a place where solar panel have been shown to work for years, especially if a cleaning mechanism is included.
    That shows the low priority represented by outer planet exploration, even when Mars looks less habitable by the day.

  • ljk April 4, 2012, 18:06

    I well remember the hubbub about the RTGs on Cassini in 1997, much of it made by people who responded like sheep to the word nuclear without having much a clue in regards to physics and science. Or Saturn.

    I did not hear or see anything about Curiousity’s RTG, but that may be due to people having other more immediate priorities these days.

  • Mike April 4, 2012, 20:46

    Not exactly on topic but I’ve just learned Kepler has been approved for a 4 year mission extension. WOO-HOO! The Kepler and NASA websites have the details regarding this great news. Now we just hope Kepler stays healthy
    and working nominaly for the next 4 years at least if not longer.

  • ljk April 4, 2012, 22:59

    Plutonium to Pluto: Russian nuclear space travel breakthrough

    Moscow, Russia (RIA Novosti) Apr 05, 2012 – A ground-breaking Russian nuclear space-travel propulsion system will be ready by 2017 and will power a ship capable of long-haul interplanetary missions by 2025, giving Russia a head start in the outer-space race. The megawatt-class nuclear drive will function for up to three years and produce 100-150 kilowatts of energy at normal capacity.

    The new project proposes the use of an electric ion propulsion system. The engines exhaust thrust will be generated by an ion flow, which is further accelerated by an electric field.

    The nuclear reactor will therefore “supply” the necessary amount of electric power without unwanted radioactive contamination of the environment.

    Xenon will serve as propellant for the engines.

    It is under development at Skolkovo, Russia’s technology innovation hub, whose nuclear cluster head Denis Kovalevich confirmed the breakthrough to Interfax. “At present we are testing several types of fuel and later we will start drafting the design,” he said.

    Full article here:


  • Siderite April 5, 2012, 1:34

    Better still would be if one could find and process uranium or plutonium in space. I don’t think there have been enough investigations on nuclear material found in situ.

  • Joy April 5, 2012, 3:36

    This real world issue concerning the difficulty and cost of scraping together a few kilos of Pu 238 for outer planets probes in the early 21st century (harder than in 1972 when the Pioneers 10/11 were launched) makes a good postscript to the discussion about using bulk antimatter to fuel interstellar craft. The gap between concept and capability is so many orders of magnitude — the head spins.

  • Eniac April 5, 2012, 5:30

    Missions to the outer system demand RTG power sources, without which the exploration of high-value targets like Titan and Enceladus cannot proceed.

    Actually, reactors would be much more preferable as a power source. More power, throttleable, and comparably ubiquitous U-235 as fuel. I want to see a high temperature, radiation cooled solid state reactor with thermionic conversion. That should be the ultimate in reliability and longevity, if practical.

    AFAIK, serious space reactor development was stopped many decades ago. Some of those old ones were pretty good. Imagine what we could do with a modern effort.

  • jkittle April 5, 2012, 16:11

    ljk – The Russian work sounds great. I remain skeptical about their ability to follow though, they have been talking about nuclear reactors in space for years, ( the US has too, for that matter) . I am not sure what the problem is other than they are really not needed or desirable in low earth orbit, and present missions to the outer solar system are scattered efforts mostly launching one or two probes before moving on to other approaches.
    So.. can we get a nuclear power system with enough energy output to power manned missions? If not we can focus more energy on high value probes and “robotic” missions.
    By the way, Kepler data will be what drives the imagination for interstellar travel for a while, the victory in getting the funding continued there should be celebrated. for statistical reasons it will be harder to find the planets like earth around stars like our sun and in the habitable zone, but we only need to find a few to get a measure of how many there are. i am guessing there are a lot, which we can prove by detecting a few.

  • kzb April 10, 2012, 7:38

    I looked at the article linked to on the Russian nuclear engine.
    I’ve failed to spot what exactly is “the breakthrough”, and also I can’t think why a 100-150kW device is in the “megawatt class”?

  • ljk April 25, 2012, 22:31

    Plutonium Production Restart Prepared

    April 24, 2012 18:04

    Merryl Azriel

    NASA has powered deep space missions with Plutonoum-238 (Pu-238) for 50 years. However, the United States stopped production of the isotope during a nuclear decommissioning phase in the 1980′s. Since then, the supply has been dwindling, with experts warning that if production doesn’t restart soon it will be impossible to conduct missions beyond Mars after 2020. It looks like that restart may now be in the works.

    “We have turned the spade in starting the project for renewed plutonium production,” said Wade Carroll, DOE’s deputy director of space and defense power systems in March according to a Space.com report, “It’ll take probably five or six years before the next new plutonium is available.”

    Full article here: