The Deep Space Energy Crisis Could Soon Be Over

On the face of it, powering most spacecraft would appear to be a straightforward engineering problem. After all, with no clouds to obscure the sun, adorning a satellite with enough solar panels to supply its electrical needs seems like a no-brainer. Finding a way to support photovoltaic (PV) arrays of the proper size and making sure they’re properly oriented to maximize the amount of power harvested can be tricky, but having essentially unlimited energy streaming out from the sun greatly simplifies the overall problem.

Unfortunately, this really only holds for spacecraft operating relatively close to the sun. The tyranny of the inverse square law can’t be escaped, and out much beyond the orbit of Mars, the size that a PV array needs to be to capture useful amounts of the sun’s energy starts to make them prohibitive. That’s where radioisotope thermoelectric generators (RTGs) begin to make sense.

RTGs use the heat of decaying radioisotopes to generate electricity with thermocouples, and have powered spacecraft on missions to deep space for decades. Plutonium-238 has long been the fuel of choice for RTGs, but in the early 1990s, the Cold War-era stockpile of fuel was being depleted faster than it could be replenished. The lack of Pu-238 severely limited the number of deep space and planetary missions that NASA was able to support. Thankfully, recent developments at the Oak Ridge National Laboratory (ORNL) appear to have broken the bottleneck that had limited Pu-238 production. If it pays off, the deep space energy crisis may finally be over, and science far in the dark recesses of the solar system and beyond may be back on the table.

Hot and Ready

Hot out of the oven – not. Pu-238 oxide RTG fuel slug glows red hot due to its radioactivity after removal from a graphite insulating blanket. Each slug can produce 62 watts of heat. Source: Los Alamos National Lab

The development and use of RTGs for space missions closely parallels the build-up of space programs in the middle of the previous century. The first RTG was invented in 1954 in the “Atoms for Peace” era of efforts to find non-destructive ways to harness the power of the atom. The promise was great; essentially unlimited power with no moving parts, that could be scaled up or down to fit a huge range of applications, from powering remote terrestrial applications like lighthouses and remote weather stations to running implantable pacemakers with a power source that would outlive the patient.

It would not be until 1961 that the first RTG would go to space, aboard a Navy navigation satellite. The first of the deep-space missions to sport RTG power were the Pioneer missions in the early 1970s, which paved the way for the ultimate test of the RTG: Voyager 1 and Voyager 2. Each of those spacecraft uses three RTGs containing 4.5 kg of Pu-238, producing 480 watts of total power per vehicle at launch. More than forty years later, the RTGs are still working, their output greatly diminished by the passage of a fair fraction of the 87.7-year half-life of the fuel pellets and general degradation of the electrical system. But they still work, and probably will for at least another year or two.

Cutaway of a General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG). The Pu-238 fuel pellets are encased in the stack of GPHS blocks in the center. Source: NASA Radioisotope Power Systems

The design of RTGs for space is fairly simple. Radioactive fuel is pressed into pellets that are covered with protective materials. The fuel is nestled into a container called the heat source, whose only job is to get hot. The heat source is slipped into another container, this one lined with thermocouples. The inside surface, in direct contact with the hot canister of decaying fuel, has the hot junctions of the thermocouple, while the cold junctions face out into the vacuum of space. The temperature difference is the key to creating electric power via the Seebeck effect, which is the same idea behind Peltier chips.

The Voyager mission’s RTGs were the “multi-hundred watt RTG” (MHW-RTG) that used silicon-germanium thermocouples, 312 per RTG. Later missions like Cassini and Galileo used a different design, the GPHS-RTG, or “general-purpose heat source RTG.” These RTGs were very similar to the MHW-RTG, with similar electrical design but a better, more efficient fuel package. The most recent RTGs are the “multi-mission RTGs” (MMRTGs) which have advanced thermocouples using lead telluride and an alloy called TAGS (tellurium, silver, germanium, and antimony). The Mars Curiosity rover has MMRTGs, as does the Mars 2020 Rover.

Robots to the Rescue

The nuclear alchemy used to produce Pu-238 and other radioisotopes is complex and extremely dangerous to conduct. Pu-238 was originally produced by bombarding uranium-238 with deuterium nuclei in a reactor, but later it was discovered that the production of Pu-239 for bomb cores yielded byproducts that could be more easily converted into Pu-238. Starting in the mid-1960s, all the Pu-238 for US civilian and military use was produced by neutron irradiation of neptunium-237 followed by chemical separation.

In 1988, the nuclear reactors at the Savannah River Site in South Carolina were turned off, and America’s Pu-238 spigot dried up. Even when the reactors were operating, production was a slow and hazardous process, resulting in only a few kilograms a year. Since 1993, NASA has sourced its Pu-238 from Russia, but they only managed to supply 16.5 kg of the stuff before they too shut down production.

In December of 2015, Oak Ridge National Laboratory in Tennessee produced the first Pu-238 in the US in nearly 30 years – 50 grams worth. The production process was laborious, with the bulk of the work going into making neptunium-237 pellets. The pellets were made by hand by adding Np-237 and aluminum powder and squeezing it into pellets suitable for neutron bombardment.

The best the lab could manage by hand was about 80 neptunium pellets a week, far short of the goal of 275 pellets a week. To achieve that level of production and ramp up from 50 grams of Pu-238 a year to 400 grams, ORNL has recently introduced an automated method of producing neptunium pellets. Details are hard to come by – plutonium production methods tend to be somewhat closely guarded for national security reasons – but if the neptunium pellets turn out to perform well in the reactors, ORNL could well be on the way to rebuilding the Pu-238 stockpile.

True, even if this automation advance proves itself, the total production capacity of Pu-238 in the US will still be under half a kilogram per year. But if the technology works, it can be replicated at both Los Alamos National Laboratory and at the Idaho National Lab, tripling the nation’s output and approaching NASA’s goal of 1.5 kilograms on plutonium-238 a year by 2025.

NASA appears optimistic that the deep-space energy crunch is nearing an end thanks to the new technology, but it’s still a long way from over. Nearly all of the 35 kilograms total inventory of Pu-238 that was available in 2015 has either been slated for future missions or is unsuitable for use in RTGs for deep-space.

41 thoughts on “The Deep Space Energy Crisis Could Soon Be Over

  1. “…implantable pacemakers with a power source that would outlive the patient.”

    Doctor: The good news is we’ve installed a pacemaker which will last longer than you will. The bad news is it doesn’t have enough shielding, so you’re also getting radiation therapy for the cancer which you’ll undoubtedly develop. :P

    Has anyone ever tried to power a submarine with an RTG? It seems like it would make for a very very quiet submarine.

      1. Source for this? When I search for information I get either sensationalist media reports or very sober articles like these:
        Quotes: “As of 2003, there has yet to be a single human death officially attributed to plutonium exposure”, “so far, no human is known to have died because of inhaling or ingesting plutonium and many people have measurable amounts of plutonium in their bodies”.
        Bernard L. Cohen also writes in his book (and quotes in the above article) ‘The Nuclear Energy Option’ ( of how calculations show plutonium workers should have a 99.5% chance of lung cancer from their work, yet none of them have any such indication. Granted, that was in 1977, so they may have developed cancer since.

        Don’t get me wrong, I’d love to be proven wrong on this, but it seems to me its toxicity is exaggerated.

    1. I believe most of the noise made by a sub’s propulsion these days is cavitation by the propeller. Think I read somewhere that the water in contact with the blades is basically boiling, and is quite noisy.

      1. You would be mistaken. At flank or emergency speed, it’s a possibility, however, as normal operational speed the props are designed to not cavitate, and have been for a very long time(the original design program was on state of the art paper tape). The main source of noise is the support equipment for the reactor and the life support systems. The reactor is the worst one, as it can’t be shut down when you’re trying to hide.

      2. Matthew Trey – With the American subs screw cavitation is greatly reduced with the Mitsubishi milling machine that enhances the screw’s blades.The real noisemaker that really challenge rigging for good quiet is the compressor for the nuke.Today our subs are amazingly quiet. Magnetic bearings help too. Like a hole in the water.

    2. External combustion has been however with other sources of heat, i.e. diesel.

      I’m really not comprehending the limitation to implement an external combustion thermonuclear system other than they’re still classified. Maybe I’m being optimistic.

      This is an interesting article showing a trajectory I haven’t observed before on page 4:

      I’ve always thought once I learned about the RTG’s and the NASA external combustion systems (I want to say Ruby lined cylinders & Higher Pressure Operations in space that I’m not finding the source at the moment… that I thought would be perfect underwater and especially like at a thermal vent position) the external combustion method might be a way to preserve water more… since most the vapor just goes up into the atmosphere somewhere. Seems there is also potential for improvements in materials science to allow for desalination in the turbine plants if not external combustion.

      Back to RTG’s… it’s interesting now wikipedia has some of the information I wasn’t able to find online regarding the use of RTG’s throughout the Soviet Union:

      Scary actually in regards to the security and oversight potential in those systems. I mean… I thought the medical communities issues were bad enough. Especially, with government sourced raw materials going to systems operations that in some cases make more than 1 million dollars a day. Wondering why there are issues with radio-logical oversight in general??? Seems I read somewhere that there was a population control agenda to cause nuclear disasters instead of nuclear bombs. Makes sense why there were and are concerns over “dirty bombs”… not just from wireless technology hacking electrophysiological functions.

  2. Glad to see this issue getting more attention, the Pu-238 shortage has been a big deal for basically any mission looking to go farther than the Moon. With more Plutonium in the pipeline, would be interesting to see NASA make some progress on their Stirling RTG.

  3. This simply will not be allowed under the New Green Deal. And no launches until NASA can make a solar powered launch vehicle and mitigate any carbon debt created during R&D and production.

  4. “The nuclear alchemy used to produce Pu-238 and other radioisotopes is complex and extremely dangerous to conduct.”

    The first successful chemical isolation (and discovery) of element 94 (produced by a cyclotron bombarding U-238 with deuterons to produce Np-238, which then decays to Pu-238) was carried out during February 23-25, 1941, in Gilman Hall Lab Room 307 at the University of California, Berkeley, by Glenn Seaborg’s graduate student, Arthur C. Wahl.

  5. The Germans have made some great strides in submarine technology with fuel cell technology. No dangerous radiation involved. RTG’s are ok but they do introduce danger to potential lifeforms. I mean TITAN is suspected to harbor life according to NASA and ESA. They are planning on sending a submarine probe to TITAN to explore its ocean(s). It will undoubtedly be RTG powered. They already left a RTG powered Huygens probe on the surface.

    If some clever inventor could figure out how to recycle hydrogen from a fuel cell, one might figure out how to power a spacecraft out past Neptune sans RTG.Or figure out how to feed a fuel cell with methane from Titan and other methane producing moons Instead of RTG’s we could have a next level BLOOM box. Many large corporations power their buildings today with Bloom Energy servers and methane (natural gas) – byproduct = H²0.

    1. “If some clever inventor could figure out how to recycle hydrogen from a fuel cell, one might figure out how to power a spacecraft out past Neptune sans RTG.Or figure out how to feed a fuel cell with methane from Titan and other methane producing moons Instead of RTG’s we could have a next level BLOOM box. ”

      Might not be the most efficient processes… raw materials seem present and have to study the cycles classical methods and determine areas of opportunity for continuous improvement to increase efficiency and reduce systems mass/volume requirements:

    2. sonofthunderboanerges says: “They already left a RTG powered Huygens probe on the surface.”

      Uh, no. Whether due to wishful thinking or plain ignorance, that’s not correct. Huygens does (did) not have an RTG. Cassini, its delivery vehicle (among other spectacular things), had three RTGs, but Huygens was battery powered.

      Come on: it only had a lifetime of less than four hours. No RTG required for that.

      To recycle hydrogen from fuel cell exhaust (water) you could carry along an electrolyzer, and power it from an RTG. Oh, wait…

      1. @Paul – OK I got the letters wrong. Huygens had 35 RHU’s. Batteries? Yeah I guess you could call them NUCLEAR batteries. RHU’s have plutonium too.

        On recycling hydrogen from a HFC (i.e. Bloom box)? I think you forgot what HFC do. They can make electricity and lots of it.

    3. Here on Earth we have a notion of “fuel” only because we are surrounded by oxygen, but outside Earth, somewhere where atmosphere is methane, oxygen is the fuel, and it is in short supply. And regarding recycling the hydrogen from fuel cell, fuel cell is using up oxygen and producing water. To recycle water (in sense to get back oxygen and hydrogen) you need to add energy, so basically it is back to square one: where is the needed energy going to come from?

    4. Hydrogen is used (oxidized) in a fuel cell. To “recycle” it you have to reduce it, that means invest more than the energy you get out of the cell first. If you are able to do this, you have another energy source and you would not need the fuel cell firsthand.
      So “recycling the hydrogen from a fuel cell” is just bullshit, there is no free energy.
      These bloom boxes probably need air or oxygen. Both not generally available in space. You had to take fuel AND oxidizer with you for chemical energy sources.

  6. You forgot to mention that some laboratories are studying how to use Americium-241, which not only has a MUCH longer half life (something like a thousand years), but is also available in larger quantities. The main downside to Am241 as an RTG is that you need a lot more of the stuff — it doesn’t produce as much power-per-watt as Pu-248 — and it also needs more shielding, since it emits alpha AND some gamma radiation (RTGs only really want Alpha rads).

    Source: I’m actually in the process of researching a DIY RTG for a space probe project I’m working on.

  7. Wow – awesome awareness stuff on powering systems, but this wont be available for the general public due to bomb-making, it would provide a decent amount of power for someone using say, like an EXOSKELETON for mobility, thats one avenue i would have researched but no point as its never going to be for the general public unless some-how the plutonium or very close alternative can be sold to the general public that is unable to be used as a bomb, now that would be so damn awesome !!

    So solar, mini engine for powering/recharging battery packs systems installed in such exoskeleton’s so people that are bed-bound can get out the house/room and have mobility again, plug in charging for the batteries if power cannot be installed on the exoskeleton for saving weight on the overall exoskeleton, Nuclear Power would be even better but shielding again would increase the weight of suc a system, but would last for many many years – this would be super-awesome if possible in the future

  8. if we use solar energy to drive an induction heater base on tesla power transformer we can generate great heat to produce steam and drive a turbine-alternator to get great energy from the sun.

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