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NASA's cancellation of Advanced Sterling Radioisotope Generator casts doubt on future deep-space missions


November 24, 2013

The Mars rover is powered by radioactive thermoelectric generator (dark cylinder on rear) (Photo: NASA)

The Mars rover is powered by radioactive thermoelectric generator (dark cylinder on rear) (Photo: NASA)

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NASA has announced the cancellation of the decade-old program to develop a Sterling Radioisotope Generator for deep-space missions. This program was a response to the critical shortage in radioactive isotopes in general, and plutonium-238 in particular, in the US and worldwide. NASA will now be depending on rebuilding a Pu-238 production system, an option that is not without its drawbacks and challenges.

Radioisotope thermoelectric generators work by converting the heat generated by a radioactive isotope to electricity using an array of thermocouples. In particular, Pu-238 fuel, which generates about half a kilowatt of heat per kilogram of isotope, has been used to power space missions and remote military installations since the early 1960s. RTGs have been carried to the Moon, and power most deep-space probes from Voyager through New Horizons, as well as the Curiosity Mars Rover.

The problem is that no one on Earth has made Pu-238 for RTGs for a couple of decades, and the supply has just about run dry.

Why should we care? According to one NASA official, if NASA doesn't find a new source of Pu-238 for its RTGs by 2022, "then we won't go beyond Mars anymore. We won't be exploring the solar system beyond Mars and the asteroid belt." Without Pu-238, a decades-long hiatus in deep-space space missions is to be expected.

Pu-238 has nearly ideal properties for powering space missions whose duration is years to a few decades. It is, unfortunately, quite difficult to produce in pure form (artificial isotopes are like that). The historic cost of Pu-238 has never been clear, as production for civilian uses was largely a side effect of weapons production and military satellite uses.

In 1992, the US arranged to buy 30 kg of Pu-238 from Russia for US$6M ($200K/kg). Even counting the effects of inflation, this was a real bargain, although in the end only about 20 kg was delivered.

Cost estimates for Pu-238 production following a $150M restart cost were $6M per kg this year, an estimate probably several times too low when considering that a $20-30M yearly program is expected to produce only 1-2 kg/yr.

If the price does settle into the range of $10-15M/kg, then the cost of fuel for a typical deep-space probe (about 8 kg or 18 lb) would be in the neighborhood of $100M, significantly increasing mission cost, and with it, mission viability.

NASA's Planetary Science Division (PSD) has been investing in development of a radioisotope-fueled Sterling engine generator (SRG) to take over the role of RTGs for future deep-space probes. While the particular program that was just cancelled began in 2009, the overall program has been active for more than a decade.

The reason to develop Sterling radioisotope generators is that an RTG only converts about 6 percent of the heat energy from decay of the radioisotope into electricity. The 8 kg of Pu-238 that powers the latest generation of deep-space probes generates about 4.4 kw of decay heat, but only 300 watts of electric power.

Instead, the combination of a Stirling engine that is powered by decay heat and an efficient electric generator offers an overall conversion efficiency into electric power of about 25 percent, meaning that an SRG could supply the 300 watts of electricity to a deep-space probe using only about 2 kg of Pu-238, representing a savings of roughly $75M, but also the ability to launch more missions. With current RTG designs, the deep-space program will be facing very long intervals waiting for Pu-238 supplies between launches.

Instead, we will stick with fuel-inefficient RTGs and a very limited Pu-238 source. Moreover, while the NASA PSD is now expected to pay the Department of Energy for both the Pu-238 restart and production costs, the DOE budget for this program has not been transferred to PSD, as is usually done in such cases. NASA is receiving about $10M per year at present to carry out a $150M restart project in the next seven years.

It appears to this occasionally paranoid writer that the only purpose of this plan, if indeed it was conceived with a purpose in mind, is to largely shut down NASA's Planetary Science Division by preventing it from carrying out a sustainable launch program. It does seem clear that, intended or not, this is likely to be the effect.

Source: Lunar and Planetary Institute

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer. All articles by Brian Dodson

Errr..what? This smells of political sabotage...mad!

Simon Sammut

Sad. Interestingly, in the sixties when the U.S. had top tax rates sitting at 70 to 77% for the top earners, things got done, I have been told there was even some flying to the moon going on.

But I guess jobs created by government agencies having people earn money from working in research and space travel can't be counted as jobs, right? We're all better off saving pennies and putting them into mutual funds that invest in factories making crap abroad.


Looks like Los Alamos small nuclear reactor is the answer. The weight might be a problem though.

Dave G

Would this device be the same as the Stirling Radioisotope Generator based on the work of Reverend Dr Robert Stirling (25 October 1790 – 6 June 1878) the inventor of the Stirling engine?

Michael Crumpton

@ BeWalt When the tax rate was reduced the tax revenue increased. The overwhelming evidence is that raising tax rates that are over over 20% will result in lower revenue. The only reason for higher tax rates is to punish the successful for their hard work. Reducing the money spent on welfare to the same percentage of the federal budget as it was in the 1960 would balance the budget.


@ Michaelc Yes. Stirling cycle are remarkably efficient but but the time that material engineering had starting producing materials to make Stirling cycle engines reliable in high energy applications the physical and intellectual infrastructure for ICE which throttles better was almost universal.


How is it even possible Pu238 is in short supply in the USA?! It's a real head scratcher that this is the case after over a half century of massive nuclear weapons and power programs in this country. It must be that this isotope cannot be made in a reactor and that it has to be found in nature.


Nostromo47 , it's exactly the opposite. The rapid decay of Pu238 - what causes it to generate so much heat - means it does not exist in nature. We have to make it in a nuclear reactor. There was a pile of it made as a byproduct of the buildout of the US nuclear arsenal. But like so much else in the US our glory days of building nukes (if you dare call it that) are in our past. And since no one wants to invest in anything even remotely like infrastructure anymore, it's going to take years - maybe forever - to get the capability back. Hey, maybe China will sell us some?

Mark Lewus

Sterling engines move. Anything that moves isn't going to last long enough for deep space missions, and is probably going to mess up telemetry as well.


@ christopher

All the moving parts are inside the pressure container and I imagine they picked gas that does not migrate through the material the pressure container is made of.

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