NASA’s Voyager Space Probe’s Reserve Power, And The Intricacies Of RTG-Based Power Systems

Launched in 1977, the Voyager 1 and 2 space probes have been operating non-stop for over 45 years, making their way from Earth to our solar system’s outer planets and beyond. Courtesy of the radioisotope thermoelectric generators (RTGs) which provided 470 W at launch, they are able to function in the darkness of Deep Space as well as they did within the confines of our Sun-lit solar system. Yet as nothing in the Universe is really infinite, so too do these RTGs wear out over time, both from natural decay of their radioactive source and from the degradation of the thermocouples.

Despite this gradual drop in power, NASA recently announced that Voyager 2 has a hitherto seemingly unknown source of reserve power that will postpone the shutdown of more science instruments for a few more years. The change essentially bypasses a voltage regulator circuit and associated backup power system, freeing up the power consumed by this for the scientific instruments which would otherwise have begun to shut down years sooner.

While this is good news in itself, it’s also noteworthy because the Voyager’s 45+ year old Multi-Hundred Watt (MHW) RTGs are the predecessor to the RTGs that are still powering the New Horizons probe after 17 years, and the Mars Science Laboratory (Curiosity) for over 10 years, showing the value of RTGs in long-term exploration missions.

Although the basic principle behind an RTG is quite simple, their design has changed significantly since the US put a SNAP-3 RTG on the Transit 4B satellite in 1961.

Need For Power

Apollo astronaut photo of a SNAP-27 RTG on the Moon. (Credit NASA)
Apollo astronaut photo of a SNAP-27 RTG on the Moon. (Credit NASA)

Even on Earth it can be tough to find a reliable source of power that will last for years or even decades, which is why NASA’s Systems for Nuclear Auxiliary Power (SNAP) development program produced RTGs intended for both terrestrial and space-based use, with the SNAP-3 being the first to make it to space. This specific RTG produced a mere 2.5 W, and the satellites also had solar panels and NiCd batteries. But as a space-based RTG test bed, SNAP-3 laid the groundwork for successive NASA missions.

The SNAP-19 provided the power (~30 W per RTG) for the Viking 1 and 2 landers, as well as Pioneer 10 and 11. Five SNAP-27 units provided the power for the Apollo Lunar Surface Experiments Packages (ALSEP) that were left on the Moon by the Apollo 12, 14, 15, 16, and 17 astronauts. Each SNAP-27 unit provided approximately 75 W at 30 VDC of power from its 3.8 kg plutonium-238 fuel rod that was rated for 1,250 W thermally. After ten years, a SNAP-27 still produces over 90% of its rated electrical power, allowing each ALSEP to transmit data on moonquakes and other information recorded by its instruments for as long as the power budget allows.

By the time the Apollo project’s support operations were wound down in 1977, the ALSEPs were left with only their transmitters turned on. Apollo 13’s SNAP-27 unit (attached to the outside of the lunar module) made a re-entry to Earth, where it still lies – intact – at the bottom of the Tonga Trench in the Pacific Ocean.

The relative inefficiency of RTGs was readily apparent even back then, with the SNAP-10A experiment demonstrating a compact 500 W fission reactor in an ion-drive satellite that readily outperformed the SNAP RTGs. Although much more powerful per unit volume and nuclear fuel, thermocouple-based RTGs do have the advantage of absolutely zero moving parts and only passive cooling requirements. This allows for them to be literally stuck on a space probe, satellite or vehicle with thermal radiation and/or convection providing the cold side for the thermocouple.

These thermocouples employ the Seebeck effect, the Peltier effect in reverse, to turn the thermal gradient between two dissimilar electrically conductive materials into essentially a generator. Much of the challenge with thermocouple-based RTGs has been to find the most efficient and durable composition. Although Rankine-, Brayton- and Stirling-cycle RTGs have also been experimented with, these have the distinct disadvantage of moving mechanical parts, requiring seals and lubrication.

When considering the 45+ year lifespan of the Voyager MHW-RTGs with their relatively ancient silicon-germanium (SiGe) thermocouples, the disadvantages of adding mechanical components should be obvious. Especially when considering the MHW RTG two generations of successors so far.

Not Your 1970s RTG

While Voyager’s MHW-RTG was developed specifically for the mission by NASA, its successor, the creatively titled general-purpose heat source (GPHS) RTG, was designed by General Electric’s Space Division and subsequently used on the Ulysses (1990 – 2009), Galileo (1989 – 2003), Cassini-Huygens (1997 – 2017) and New Horizons (2006 – ) missions. Each GPHS-RTG produces about 300 W of electrical power from 4,400 W thermal, using still similar silicon-germanium thermocouples.

An interesting sidenote here is that even the solar-powered Mars rovers include a radioisotope unit, although in the form of a radioisotope heater unit (RHU), with the Sojourner Rover having three of these RHUs, and Spirit & Opportunity eight RHUs each. These RHUs provide a constant source of heat that allows scarce electricity from solar panels and batteries to be used for duties other than running heaters.

The GPHS module provides steady heat for a radioisotope power system. (Credit: NASA)
The GPHS module provides steady heat for a radioisotope power system. (Credit: NASA)

Meanwhile, the currently active Mars rovers, Curiosity and its twin Perseverance, get both electrical power and heat from a single multi-mission radioisotope thermoelectric generator (MMRTG) unit. These RTGs use PbTe/TAGS thermoelectric couples, meaning lead/tellurium alloy for one side and tellurium (Te), silver (Ag), germanium (Ge) and antimony (Sb) for the other side of the couple. The MMRTG is rated for a lifespan of up to 17 years, but is likely to outperform its design specifications by a considerable margin like the MHW-RTGs and others have. The Pu-238 fuel with an MMRTG is contained in General Purpose Heat Source (GPHS) modules, which serve to protect the fuel from damage.

The main failure mode of the SiGe thermocouples was migration of the germanium over time, which causes sublimation. This was prevented in later designs by coating the SiGe thermocouples with silicon nitride. The PbTe/TAGS thermocouples should provide further stability in this regard, and the MMRTGs in Curiosity and Perseverance have served as real-world duration tests.

A Matter Of Fuel

The Voyager 1 and 2 probes are well out of reach for a big service and maintenance session, so NASA had to get creative to optimize power usage. Although the backup power circuit was perhaps considered a necessity back in the 1970s in case there were power fluctuations from any of the three RTGs on each space probe, there is enough real-life monitoring data to support the suggestion that it may be superfluous, barring alien influences.

With the nearly 46 years of data from the Voyager RTGs, we can see now that thermocouple stability is essential to maintain a constant power output, with the decay of the plutonium-238 fuel source significantly easier to model and predict. Now that with the MMRTG units we should have addressed many of the issues that caused degradation of the thermocouples over time. The only missing ingredient is the Pu-238 fuel.

Most of the Pu-238 that the US had originally came from the Savannah River Site (SRS) before this facility and its special reactors was shutdown in 1988. After this the US would import Pu-238 from Russia before the latter’s stocks would also begin to run low, leading to the awkward position of the US running out of what is one of the best radioactive isotopes to use in RTGs for long-duration missions. With a short half-life of 87.7 years and only alpha decay, Pu-238 is both rather benign to surrounding materials, while providing significant amounts of thermal power.

With only enough Pu-238 left for the two MMRTGs in the current Mars rovers and two more after these, the US has now restarted Pu-238 production. Although Pu-238 can be created via a few different ways, the preferred way appears to be to use stockpiled neptunium-237 and expose it to neutrons in fission reactors or similar neutron sources, to generate Pu-238 via neutron capture. According to NASA, about 1.5 kg of Pu-238 per year should be enough to satisfy demand for future space missions.

A Tiny Spacecraft In The Dark

Voyager 1 is currently at a distance of 159.14 AU (23.807 billion km) from Earth, and Voyager 2 is only marginally closer at 133.03 AU from Earth. As a project that has its roots in the Space Race and has ended up outliving not only many of its creators, but also the geopolitics of the time, it is perhaps one of the few human-made constants with which we can all identify in some fashion.

As carriers of the golden discs that contain the essence of humanity, extending the life of these spacecraft goes beyond merely the science they can perform, out in the darkness of Deep Space. With every year extra we may learn a bit more and see a bit more of what awaits humanity beyond the reaches of this rather ordinary, out of the way solar system.

45 thoughts on “NASA’s Voyager Space Probe’s Reserve Power, And The Intricacies Of RTG-Based Power Systems

    1. ^ this, I was hoping for some sort of info on how they’ve done it, how the architecture of these things works (how do you software-reconfigure a power supply reliably?) that sort of thing.

    1. Most likely a few extra thermoelectric pairs that can be switched on if the system detects excess power draw from the instruments, otherwise unnecessary as excess voltage on the bus would stress the instruments’ own regulators more.

      1. I was referring to a typo which has since been changed.

        Still, I’d have thought they’d want all the thermoelectric units to be roughly equally sharing the job. Sounds like an interesting solution.

  1. From the description, it seems that the buffer was not intended to deal with the instability of the RTG, but to account for failures in the equipment which could drag the system voltage down, so it acts as a booster in case the computer detects a sagging voltage. Some extra thermoelectric pairs can be switched in and out as needed. This buys time to isolate the fault and shut the failing instrument down. If the extra power was on all the time, you would need an additional “regulator” device or devices to spend the excess power for the same point, which complicates matters and introduces another point of failure if it latches on.

    Of course this now leaves the risk that the aging equipment can malfunction and draw more power, which would then pull the whole thing down.

    1. That’s an explanation.
      I wonder if the extra power being unregulated does matter anymore.
      I mean, if the Voyagers are nowadays running “undervolted” anyway, the worst that could happen is that the extra therm. pairs increase the voltage levels to what used to be the normal voltage range. And since instruments are no longer being switched on/off all the time, the power draw should be quite constant. But I’m just a layman, of course.

  2. Probably spell check. Technical terms aren’t in it’s vocabulary. It’s one of those er/or words.
    There is no conserving the output for longer life from a RTG.
    If you start out with with a slowly decreasing voltage you would series regulate till the regulator voltage drop determines the useful life. Bypass the regulator and get that voltage drop back for more life. Maybe it’s a space hardened LM317 type. Even if it’s switched mode it still comes down to DC in DC out. Genius that there is a bypass mode which normally could fry things when new. Mid 70’s or older tech.

    1. If you draw too much power out of an RTG, it cools down and the efficiency goes down after a while, giving you less power than expected, so switching the “booster” at the source instead of the regulator can give you more power temporarily than if you just bypass a regulator.

      1. Besides, the linear regulator to spend the excess power also needs a cooling system with external radiators for any appreciable wattage, while you already got the cooling fins in the RTG to do the job. Every gram counts on a space craft.

  3. We´ll hit the wall well before we´ll have any chance of extending beyond the solar system. At the fastest speed of even tomorrow´s technology it would still take a veeeery long time.

    Just compare it to the speed we´re destroying our environment.

    We all live in a shrinking capsule and the time is counted. The only spaceship we got to survive for the future is precisely the one we are walking on. And we´re wasting it, trying to dominate nature in any way we can instead of understanding we´re part of it.

    We just don´t deserve this planet we have. A short-lived and short-sighted specie with the mind of a hunter-gatherer that will be just a fart on the geological scale, with a deep impact on the ecosystem.

    1. I suppose one of the problems is that human kind has no other species for comparison. Thus it must learn from its own failures. At the very least, human kind has good intentions, also. The majority doesn’t want to be bad or destroy this planet. It’s rather incompetence that causes this dilemma. But even in the most pessimistic outcome, nature will recover. Maybe after human kind has vanished, at worst. This planet had recovered many times in the past millions of years.

      1. incompetence is second, greed is first, by far. And third is numbness.

        I´m not worried for nature, there a many many ways it will recover. The change speed we´re imposing on the planet is very brutal, and we won´t make it, but it´s unlikely life will disappear from the planet anytime soon. The recovery won´t be as fast as the demise, of course.

        1. Hm. I suppose there’s some truth within.
          However, not all humans are alike. Natives and monks had been living in harmony with nature since ancient times, for example. If we’re lucky, some of them will remain and preserve things like music, art, fairy tales and mathematics. Those things deserve to be remembered.

          Last but nit least, humans had a positive influence on animals, too. They learnt things like compassion and gratefulness, among other things. Some wild animals do seem to find our pet dogs to be trustworthy, for example. And some wild animals even go so far to approach passengers if they need help. Just think of the many firefighters who save animals. There surely are good people, but they don’t stand out as much as they deserve it.

        2. “I´m not worried for nature, there a many many ways it will recover.”

          We ARE “nature.” What effects on the planet we have are just as valid as any other species that evolved here and those effects are absolutely minimal despite the doomsday hype and anecdotes. Do as I have done and calculate the percent by weight of the oh so terrible plastic pollution of the oceans. There are so many leading zeros to the right of the decimal point it’s hilarious and shows why scientific notation exists. There’s a great meme of every human on the planet ground into meat ball placed in Central Park. It’s just under ONE kilometer in diameter.

          The “we’re doomed” hype is just an attempt at more centralized CONTROL. Stop falling for it.

          “The whole aim of practical politics is to keep the populace alarmed (and hence clamorous to be led to safety) by an endless series of hobgoblins, most of them imaginary.” – H.L. Mencken (1880-1956)

          “It is simply no longer possible to believe much of the clinical research that is published, or to rely on the judgment of trusted physicians or authoritative medical guidelines. I take no pleasure in this conclusion, which I reached slowly and reluctantly over my two decades as editor of The New England Journal of Medicine.” – Marcia Angell (2009)

          “The case against science is straightforward: much of the scientific literature, perhaps half, may simply be untrue. Afflicted by studies with small sample sizes, tiny effects, invalid exploratory analyses, and flagrant conflicts of interest, together with an obsession for pursuing fashionable trends of dubious importance, science has taken a turn towards darkness.” – Richard Horton, editor of The Lancet (2015)

          Why Most Published Research Findings Are False
          John P. A. Ioannidis – 2005

          There is increasing concern that most current published research findings are false. The probability that a research claim is true may depend on study power and bias, the number of other studies on the same question, and, importantly, the ratio of true to no relationships among the relationships probed in each scientific field…Moreover, for many current scientific fields, claimed research findings may often be simply accurate measures of the prevailing bias. In this essay, I discuss the implications of these problems for the conduct and interpretation of research.

          1. “There’s a great meme of every human on the planet ground into meat ball placed in Central Park. It’s just under ONE kilometer in diameter.”

            [Reluctant Cannibal] does that make your mouth water?

          2. “The whole aim of practical politics is to keep the populace alarmed (and hence clamorous to be led to safety) by an endless series of hobgoblins, most of them imaginary.” – H.L. Mencken (1880-1956)

            I’d love to learn more about those non-imaginary hobgoblins….

          3. Winston, for endocrine disrupting chemicals (like many plastics) you don’t need high concentrations to drastically change biological systems. The whole POINT of hormones (the endocrine system) is that tiny amounts can have very very strong biological effects. So “lots of zeroes after the decimal point” isn’t itself any indication of safety. Not to mention, concentration gradients exist, especially in the ocean. So one area may have undetectable levels while another may be saturated. Add in some bioaccumulation and concentration up the food chain and ocean plastics are indeed bad news. No, the oceans won’t boil away or anything crazy like that, and I’m sure there’s a gross oversimplification that can be parroted to gloss over the problems. For example it’s easy to quote biological activity to claim that an area is teeming with life. If all that biological activity is toxic sludge it doesn’t really help the apex predators like you and me who rely on the food web.

    2. Eh, I’m sure we’ll figure it out. No point in fretting over it. I dumped over 500 gallons of perfectly drinkable water down the drain today while I was testing a configuration on this piece of equipment I’m working on, lol.

      1. Complacent optimism does not hold water in front of the scale of destruction we´re achieving down here. You pink-tainted glasses will start to crumble when your comfortable existence will be directly threatened. The world you´re living is the direct result of the frenetic exploitation of resources, and those shrink at fast speed. And everybody, no matter how they are comfortable now, will be impacted.

        1. Pessimism is much worse, though. It doesn’t add anything meaningful to the matter.

          It hurts moral, it puts people down, leads to resignation, stagnation, dead.

          Think of what Einstein said, ‘I’d rather be an optimist and a fool than a pessimist and right.’

          And that dude wasn’t just smart, but also wise and with lots of life experience. He enjoyed life as much as he could.

          In German, there’s an old saying, apparently by E. Kästner, “Es gibt nichts Gutes, außer man tut es!” (There’s nothing good, except you do it.)

          It means that it depends on us to make the world a good place, that everyone of us matters to do something good.
          And we can do it, if we really want to. It’s not too late until it’s too late. As long as people are around who care, there’s hope. We all must put aside our grief and pessimism and get going.

        2. >> You pink-tainted glasses will start to crumble when your comfortable existence will be directly threatened.

          When? Care to give a date? And, if that dates comes and goes and we’re still comfortable, then we’re agreed that you are wrong?

    3. O you are one of those people who think space exploration is a waste, we should spend the money fixing this planet, right? Well we wouldn’t even know what we are doing to this planet without the satellites in space to measure things for us. At minimum the technological advancement from space far outweighs its cost

        1. Complaining about humanity over and over alone doesn’t help, though.
          It just makes people give up and then they will mentally resign and nolonger care about environment.
          Which in turn makes you a part of the problem, rather than the solution.
          People who repeat those negative lines/point of views have a bad influence on others. Like a fire/virus that spreads.

          Constructive criticism would be better, thus, I think. Let’s use our imagination to find a solution, rather than being creative at pointing out why things must fail.
          That energy would be better spent on creativity.

      1. every time I take a walk outside I see what we have DONE to this planet. Every satellite or worse humans launching into space a little piece of Gaia dies.

        It hasn’t done ANYTHING for mankind.Like zilch…

        And to perpetuate the idea that space could safe mankind… we weren’t even able to save the bizon or the dodo…

        that’s how silly things are.

        1. I get yours and ono’s point and do agree in parts.

          However, optimism is what can save the day.
          Human kind has the power to heal the planet.

          In the pandemic, for example, when China made “a pause” and traffic/pollution was reduced, the air quality had quickly increased there.

          Human kind can do a lot to save the planet.

          a) It can turn salty water into drinking water by building desalination plants.
          b) It can bring extinct races back to life, researchers had collected seeds and DNA samples.
          c) Genetics can create plants that can live in though environments, like Mars. Or Earth, after the apocalypse. ;)

          d) Cities could be restructured to feature a home for plants and animal life.
          Trees on the roofs, ivy plants on the housewalls, “central parks” in the hearts of the cities. Convection cooling in new buildings. Human imagination finds a way.

          e) Cities under water. The seas are barely explored yet. Human kind might be able to use the vast resources without getting in the way with sea life. The thermal power down there is so vast, it should be sufficient for both sea life and humans.

          Unfortunately, the insight might not occur until we’re at the verge of destruction.
          On the bright side, the good people who are willing to change have a chance to make it.

          1. a) byproduct of desalination plants (brine) are rejected into seawater where it impacts water chemistry and ecosystems. It does not save the planet, and requires quite some energy to work. The benefit for humans it largely offset by the toll it take to underwater ecosystems. Oceans dead zones are already a problem
            b) seed banks can save some species, but many species of plants are in fact depends of other species (like one plant – one pollinator insect interaction). You´d get at most just one part of the puzzle.
            c) genes interactions are VERY complex. While it´s true one can alter, splice, edit genes, we´re way way far to make genetic editing that is beneficial to environment. Might look good at first, but most of the time it bring a lot of unexpected consequences

            e) The seas are barely explored yet, but much altered already, especially deep sea ecosystems that are very slow and very fragile are destructed as a massive scale. Just think how what proportion of the plastic waste that float (the visible part of the plasticberg) vs the plastic that sinks and destroys deep sea ecosystems. Those play a very important role, and we´re just starting to discover it. Cities under water are inept concept, that requires so so much resources and technologies to build, making these so much more wasteful than settlement on the surface.

        2. Advanced species have probably mastered interstellar travel. If they´re observing us, it´s likely that seeing how much death and destruction we leave behind, they would be concerned to see us being able to escape our neighborhood and would likely consider us as a threat.

          Same if we manage to build any superintelligence: it´s unlikely it would accept s a master: having an immature boss is at best no fun, but if you have no choice it´s pure hell.

          1. That’s the Steven Hawking nonsense, I suppose? I was quite disillusioned in his late years. Poor dude.

            No, I don’t think so. A sentient species so advanced wouldn’t harm humanity, since space faring species are rare in cosmic dimensions.
            They’d be above all unnecessary violence. Maybe they even faced the same troubles as us in their evolution.

            They would rather observe us, try to proserve the diversity on this planet. Maybe get into contact with us if it’s the right moment in time.

            Have a look at Star Trek, that’s more realistic than Hollywood’s invasion/instruction paranoia.
            Or let’s just think for ourselves instead of repeating nonse by some VIPs.

          2. What’s also to consider, humanity is very young.
            We’re just learning to walk.

            We’re certainly no threat in any way. We can barely leave earth orbit yet.

            And except earth, all the other planets are dead rocks or gas giants.

            With the exception of a few moons that might contain primitive bacteria and water ice.

            We certainly don’t pollute anything, that’s utter nonsense.

            It’s rather the contrary. Our probes might bring life into the universe, through the microbes that might travel with them. Seeds of life, so to say.

            That’s also the reason why NASA and other space agencies are very carefully and rather burn up their probes into the atmospheres if gas giants like Jupiter: They try not to unresponsibly endanger possible, already existing life on those moons.

            No idea where that “humanity is bad and pollutes everything” mentality comes from. Space is dead, no one cares out there.

            Humans are the only known species that cares and worries about these things. We’re alife, we *are* life. It’s us who can spread life into the universe.

            So please, stop being pessimistic. We have a problem here on earth, yes. And we must figure out a solution. Talking about guilt is no solution, it’s irrelevant, too. An open mind and optimism is tge key to find answers. Best regards.

        3. That’s nonsense. Satellites provide services that are essential for keeping our planet healthy: weather forecasting, and all kinds of land monitoring, for example. GPS makes travel more efficient. Communications satellites reduce the need to travel. I could go on.

          The negative impact of spaceflight on our planet is currently about a million times smaller than the negative impact of commercial air travel on our planet.

    4. The human condition is a simple to solve, neuter 50% of all newborns. Adjust % if needed. Overpopulation will be a thing of the past in 20-30 years. Promoting suicide as something noble and good will easily buy a few 100k less mouths to feed annually. Where there is a will, there is a way. I for one would happily see all the mexicants go, leaving the world to us mexicans.

  4. I was thinking it was a simple shunt regulator to stop surges when a load was switched off. Can anyone post a link to a proper technical description of the system?

  5. The Apollo 13’s SNAP-27 was not attached to the outside of the Lunar Module. A graphite cask containing the plutonium 238 fuel rod was. The fuel rod was removed from the cask and transferred to the generator during setup of the instruments on the lunar surface. The generator would not have survived re-entry, but the cask was designed to so that plutonium would not have been scattered in case of a launch failure.

  6. “The change essentially bypasses a voltage regulator circuit and associated backup power system,”

    The real life version of how Star Trek fixes problems. “I’ve rerouted the plasma conduits but it won’t withstand another hit like that last one.” “Do we have phasers Mr. Scott?” “Aye, one phaser bank, Captain.” “That’s all we need. Mr. Sulu…”

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.