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I was watching a video on the engineering of Perseverance and it states that the rover is powered by 4.8kg of Plutonium Dioxide. I was wondering if that similar process of electricity generation can be applied here of earth (if it hasn't already). What makes Mars unique in that respect, and why did the team choose radioactive decay over solar panels?

If I had to think of an answer intuitively, I'd say it's because the process is hazardous to organics. Plus, the half life of the material on Mars might be under the time frame of when we actually settle and thrive on Mars (organics and all).

However, I still don't have a clear picture of whether or not we could use this tech for rovers here on earth, whether it's crawling the ocean depths or surveying places where life can't exist. Mars would seem to be more dangerous than say, a salt flat, or the ocean depths.

Kozuch
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ParthianShotgun
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    Related (solar vs. RTG): https://space.stackexchange.com/questions/1270/why-does-curiosity-use-an-rtg-rather-than-solar-as-spirit-and-opportunity-do-f – Eugene Styer Feb 23 '21 at 15:01
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    RTG fuel is very dangerous and incredibly expensive. Thermocouples and/or thermionic generators are getting increased attention. – ikrase Feb 23 '21 at 20:18
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    It's worth looking at the book "The Martian" by Andy Weir; the stranded astronaut uses as RTG in order to travel to a site on Mars. – No'am Newman Feb 24 '21 at 08:56
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    Why might it not, assuming you're Asking purely about technology and not efficiency? – Robbie Goodwin Feb 24 '21 at 23:12
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    Remember, Opportunity died in a dust storm that blacked out it's solar panels. Any solar power system on Mars would die when one of those mega dust storms hits, there is no defense. Curiosity and Perseverance don't rely on solar panels for survival, they can just wait it out. – Loren Pechtel Feb 25 '21 at 02:45
  • You might want to learn a thing or two more about plutonium. Your premise is incorrect. – Harper - Reinstate Monica Feb 25 '21 at 18:45
  • Can we? Yes. Should we? No. – Peter - Reinstate Monica Feb 26 '21 at 10:34
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    @Peter-ReinstateMonica We can't really. The cost of suitably fast decay isotopes is ginormous. No matter what scale of production was envisaged, and regardless of safety, the costs are many many many many orders of magnitude greater than alternatives. – Russell McMahon Feb 26 '21 at 22:35

8 Answers8

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RTG technology has been applied on Earth, many times, although not for transportation - they don't produce much power for their weight so any RTG powered vehicle would be very slow. Some pacemakers used to have plutonium batteries, and RTGs were used in remote sites to power sensors, lighthouses and the like in remote areas. It isn't used much anymore on Earth because the radioactive materials used can cause radiation sickness and burns, are toxic if ingested or inhaled, extremely costly to produce and there are better alternatives. There's also concerns that if stolen the materials could be used to make a "dirty bomb" - a radiological weapon which could kill or cause illness over a wide area.

The reason they are great for space applications is that they produce some waste heat which is useful for warming the spacecraft, and they deliver constant power day and night. Solar panels tend to get covered with dust on Mars, reducing their generating properties.

GdD
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    This article from the International Atomic Energy Agency gives some background on a) why and how the Soviet Union used to use RTGs in lighthouses and b) why the IAEA really wishes they hadn't. – Cadence Feb 23 '21 at 15:44
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    The BBC did an interesting 5-minute video on these lighthouses too. https://www.bbc.com/reel/video/p0931jtk/the-nuclear-lighthouses-built-by-the-soviets-in-the-arctic – Jason Feb 24 '21 at 01:13
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    The answer mentions nothing about (1) the RTG's puny power output, and (3) Mars rovers' puny power requirements because their travel distances are measured in the low 10s of KM per decade. – RonJohn Feb 24 '21 at 21:24
  • @RonJohn So why not add that in via an edit, like I just did? – Ian Kemp Feb 25 '21 at 15:30
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    I rejected that edit as it wasn't my intent. It sounds like you have your own answer. – GdD Feb 25 '21 at 15:34
  • @IanKemp StarfishPrime made the point in his answer. – RonJohn Feb 25 '21 at 17:32
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RTGs are expensive to produce, can be politically inconvenient to use, and in the form of a plutonium-bearing device, represent a potential nuclear proliferation hazard (though all RTGs might be used to construct a "dirty bomb").

To compound the issue, their power output simply isn't very big... Perseverance's generator cost about 75 million USD and provides a mere 110W fresh (source) which of course will decline a little bit every year.

Their use, therefore, is limited to places where solar and wind won't work, and recovery or access by humans for refuelling is likely to cost many millions of dollars each time. There can't be many places on Earth that fill those requirements, if any. And once you've identified such a place, you need a multi-year mission that only needs a few tens of watts, or a hundred watts at most. That's a pretty specialist piece of kit right there... I can't think of anything that would fit such criteria.

For deep ocean work, batteries and tethers seem to be the order of the day. Face it, you can get one of these awesome toys to go and visit the Titanic in person, and you'd still have plenty of change from 75 million USD.

Starfish Prime
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    If you mass-produce them like the Soviet Union did, the price per unit of RTGs goes way down. It also goes way down if your RTG merely needs to survive high-speed crashes, rather than atmospheric reentry. – Mark Feb 23 '21 at 23:19
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    @Mark the mass-produced RTGs the soviets made were things like the Beta-M which produce a tenth of the power of the Perseverance generator, don't last as long and (I think, though I'm not certain) are fairly hefty bits of kit, too. TANSTAAFL, etc etc. – Starfish Prime Feb 24 '21 at 08:11
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    Note Perseverance tops out at around 150 meters (~0.1 miles) per hour. (And it's actually the fastest Mars rover yet.) It may last a long time, but I doubt many people would be okay with a car that slow. – Darrel Hoffman Feb 24 '21 at 14:32
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    @DarrelHoffman the trick is to bring a skycrane with you, and start from somewhere high up... – Starfish Prime Feb 24 '21 at 14:36
  • Another example would be Antarctica: Not practically accessible during the winter months, solar only works in summer, wind power is apparently possible, but difficult, so it sounds like a great idea... but it turns out they mostly just use diesel generators, because they have to do annual resupply anyway (for food etc.). Maybe if you had a small, unmanned site with simple instruments, it might make sense. – Kevin Feb 24 '21 at 19:54
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    The second and third paragraphs are key. – RonJohn Feb 24 '21 at 21:19
  • @DarrelHoffman, if you're not doing off-road driving on uncertain terrain, a 100-watt motor can move you at a reasonable pace -- 100 watts is typical of low-end electric bicycles. – Mark Feb 24 '21 at 21:24
  • @Mark I was just going by what NASA claims is the max speed of whatever motor Perseverance is using - and that's on flat clear terrain, it's probably slower on average. (Presumably there are other limiting factors due to having to operate on Mars. Your electric bicycle probably won't do so well there...) – Darrel Hoffman Feb 24 '21 at 21:32
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    I'm an EE. A ~= 100 Watt continuous source is an immense resource. Per day that can be stored to around 2+ kWh / ~= 3 HP.hr. Certainly enough to live in luxury on Mars if you are a Mars lander. – Russell McMahon Feb 24 '21 at 22:26
  • @Kevin the Soviets and Americans used RTGs for long-life Antarctic stations, but they were all removed - the modern approach seems to be that solar+wind+battery is good enough for "it'll work most of the time, if it can be designed to reboot smoothly after an outage". – Andrew is gone Feb 25 '21 at 19:13
  • The cost is mostly because production of Pu-238 stopped decades ago, and a limited supply was made. If it were to actually be produced, the cost would go way down - it is very simple to produce. – IronEagle Feb 26 '21 at 16:42
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The answers claiming danger/toxicity are chasing something that's irrelevant. The real issue is that they don't produce all that much power*. If you want a car that can only drive a few hundred meters a day, an RTG will work just fine.

Solar panels aren't all that good on Mars. Not only is the sunlight weaker, and reduced even further by sun angle** and Martian dust storms), they tend to get covered with dust, reducing the power output even further. (Though I've always wondered why the engineers didn't include a brush that could be used by the robotic arm.)

*But they will do that dependably, and for many years without maintenance or refueling. Pioneer 10 & 11 use them, and they're stil working after nearly 50 years. The Voyagers are still running after 40+.

**Remember how the Spirit & Opportunity rovers had to find winter parking spots that would let them tilt towards the sun?

jamesqf
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    I hardly think that the toxic dangers of Plutonium are irrelevant on Earth regardless of its power output – Slarty Feb 24 '21 at 08:43
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    @Slarty petrol is highly toxic, and we still use it. My house is heated by gas: it’s not good if that escapes. I don’t see why a contained RTG is any worse. – Tim Feb 24 '21 at 09:46
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    @Tim Plutonium is a lot more toxic than petrol. – gerrit Feb 24 '21 at 10:11
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    @gerrit both will kill you. It hardly matters when they’re both in a sealed box. Plutonium has the advantage that you don’t need to open that box to add more each week a plutonium station. – Tim Feb 24 '21 at 10:13
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    Petrol is about 10.000x less toxic (in terms of the LD50oral dose) and degrades pretty quickly in nature. The pungent smell makes it almost impossible to ingest anyways. A couple of mg of 240-Pu will still be deadly after thousands of years and it's odorless. – couka Feb 24 '21 at 12:22
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    @couca: RTG devices use Pu-238, with a half-life of 88 years. Biologically toxic forever, of course, and not something we want dispersed in the environment. – CCTO Feb 24 '21 at 14:27
  • I've often wondered that about using a brush to clean solar panels. It wouldn't even need to be on a full robotic arm. A row of bristles on a rod that slides across the panel would do. – Jeanne Pindar Feb 24 '21 at 15:33
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    I think you underestimate how hard brushing (to actually get rid of dust/dirt and not just distribute it evenly and/or grind it into the glass) is. – Michael Feb 24 '21 at 17:04
  • Widespread use of RTG's would make it very easy to make dirty bombs and there is also widespread fear of nuclear anything. so yes, the danger (real or perceived) involved is very much relevant to why we wouldn't use them, even if they were very useful. See also: nuclear power in general. – eps Feb 24 '21 at 19:35
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    @Tim you are comparing a firecracker to TNT. – eps Feb 24 '21 at 19:36
  • @eps if you don’t put fire next to it, they’re both fine. Likewise if you don’t start eating fuel, you’re fine. – Tim Feb 24 '21 at 19:45
  • Potential for "repurposing" is about a co-equal factor for avoiding terrestrial use. The other is the immense cost of producing Plutonium for this purpose. – Russell McMahon Feb 24 '21 at 22:20
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    Nothing like scraping tiny rocks over your glass to scratch it up. There's a reason earthlings prefer to wash solar panels with water rather than sweeping them like a kitchen floor. – Lawnmower Man Feb 24 '21 at 22:24
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    @Slarty: You're missing my point. Even if RTGs were utterly, completely harmless, they simply wouldn't work for applications like cars. If they won't do the job, the toxicity or lack thereof is irrlevant. – jamesqf Feb 25 '21 at 03:23
  • @Lawnmower Man: You perhaps don't live in a desert. Brushing dust off glass surfaces is quite normal. Sure, water does a better job, if you have it handy, but that's even less likely on Mars than say out on the Black Rock. – jamesqf Feb 25 '21 at 03:26
  • @LawnmowerMan you could get better glass, of course. A nice thin layer of sapphire might do the job. – Starfish Prime Feb 26 '21 at 10:38
  • @jamesqf: On earth, wind-driven erosion produces nice, rounded sand grains. The effect is so bad that desert sand can't be used in cement. Plenty of Arab countries need to import sand! Obviously this is much less of an effect on Mars and the Moon, so you can expect much sharper grains. – MSalters Feb 26 '21 at 12:29
  • @MSalters: I'd expect wind erosion to have a much bigger total effect on Mars, since there's no water &c to compete with it. In any case, we're not talking about scratching glass over a period of many decades, but the occasional brushing of dust off the solar panels of your Martian rover. A few scratches might reduce the efficiency a bit, but I'd think not nearly as much as a coating of dust. – jamesqf Feb 27 '21 at 21:26
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Captain America: "How come [this shield is] not standard issue?"
Howard Stark: "That's the rarest metal on Earth. What you're holding there, that's all we've got."

RTG materials are special

In an RTG, you need some very special traits.

  • You need a half-life that is long enough to be useful. The power output will taper off, but you want a long enough half life that it tapers down only a little in the duration of the mission. Iodine-131 is plentiful, but it's gone in a month or two.
  • The half-life can't be too long. Decay events are what make the heat, and a long-half-life element like Uranium-235 will not make decay events often enough to make any useful heat.
  • The element must decay with the right kind of radiation - a kind trivially blocked. That is for human-safety reasons: a gamma emitter is Right Out. Alpha decay is ideal, as it's stopped by a few centimeters of ordinary air, or the skin - but must not be ingested.
  • The material must be anti-proliferation. Meaning that if it were captured by terrorists (or fell out of the sky right into their laps during a failed launch), they must not be able to make an atomic bomb. It cannot be fissile and cannot be a feedstock easily made fissile.
  • The material must be possible to separate from its origin and byproduct materials. For instance elements can be separated chemically, but adjacent isotopes of a heavy element are inseparable at an industrial scale. Enrichment of uranium is only possible because they are 3 apart: U-235 and U-238, and still requires state-level commitment (hence America's never-ending sparring match with Iran to prevent them from developing that ability).
  • The process that makes it can't quickly over-make it. For instance if you get the material by neutron bombardment, but that also converts the target material into a useless, inseparable material, that doesn't work at all.

Plutonium 238 ticks all the boxes... but...

Plutonium-238 has an 87 year half-life (just right) and is an alpha emitter. As an alpha emitter, it would make a poor 'dirty bomb' because it could only threaten humans as dust they breathed during the initial moments of the attack.

So let's make a bomb? Our buddy has a fission reactor, let's bombard it with neutrons for 30 days to make fissile Pu-239. *Great, we've done that, and now we have a slug of mostly Pu-238 with a little Pu-239. Let's do isotopic separation of -- Oh, snap. Even the United States can't do that.

Okay then, load it back in the reactor for a year and convert the kaboodle to Pu-239. That's working, except the Pu239 is also absorbing neutrons and becoming Pu240 and Pu241... which are too radioactive and will cause a bomb to misfire. So the fact that it already is plutonium actually helps the nonproliferation issue.

... but it's really hard to make.

As far as how to make Pu238, there are several processes to make it from spent reactor fuel. However, these are special processes, not just a "load it into a common power reactor with the normal fuel rods" situation*. Indeed, both the US and the Russians had shut down their Pu238 production, because their lines were manufacturing it using waste materials from bomb core production, which had ceased under the various START talks. Several countries had to start their own Pu238 manufacture operations, specifically for making RTGs.

Suffice it to say, Pu238 is rare, hard to make, and expensive.

* Canada's efforts to make it in a reactor is a special exception, because their reactors are. Unlike conventional BWR and PWR/VVER designs, the CANDU is specifically designed to allow fuel rod changes while the reactor is underway. This is a trait it shares with the Soviet RBMK and weapons reactors such as the Hanford "B", because hey, they were all after the same thing: the ability to achieve criticality on natural uranium, and remove rods in 30-90 days - perfect for breeding weapons grade plutonium. Canada wanted to have the ability in its back pocket, and Russia wanted to be able to scale up their existing ability quickly.

Harper - Reinstate Monica
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  • +1 Excellent overview of what is needed for an RTG material. One note: With regards to isotopic separation, it's much more the US won't do it, not can't. Legally, separation is restricted, scientifically, it can potentially be done in the back of a truck. – IronEagle Feb 26 '21 at 16:48
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Plutonium is exceedingly toxic both chemically and from a radiation stand point. It also has the potential to be converted into a nuclear weapon if sufficient can be gathered, so it is not the sort of thing that would be sensible to use for a vehicle on Earth. It is also not that plentiful and very expensive to manufacture.

It was used on Mars because it is a solid state power source with no moving parts so little to go wrong and on Mars repair is virtually impossible currently. It is also long lasting and the chemical and radiological issues are not so much of a problem on Mars. A big advantage over solar panels is the ability to operate during long sandstorms and not needing a battery for backup power at night.

Slarty
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    The plutonium isotope, 240-Pu, that is used in RTGs can not be used to make an atomic bomb. In fact, even a tiny amount of 240-Pu contamination can cause an atomic bomb to fizzle. Ironically, what poisons the atomic bomb is the fact that the stuff is so highly radioactive. But, terrorists don't need to make an atomic bomb. A "dirty" bomb would be plenty bad. – Solomon Slow Feb 23 '21 at 18:31
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    240-Pu can't be used to make a fission bomb true. A dirty bomb would sure be bad. – Slarty Feb 23 '21 at 18:44
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    Nuclear Pacemaker Fact Sheet and Plutonium Powered Pacemaker (1974) and The History of Nuclear Powered Pacemakers it's 238-Pu, not 240-Pu – uhoh Feb 24 '21 at 03:08
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    @SolomonSlow it's 238-Pu that's used in RTGs as well as in old pacemakers! – uhoh Feb 24 '21 at 03:10
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    @uhoh, Oops! You are right. I guess I copied the link from the wrong tab in my browser, and then I went with the link text. (I don't actually have anything to do with any plutonium isotope in my day-to-day life, so I don't normally have a keen awareness of which one is which.) – Solomon Slow Feb 24 '21 at 13:07
  • Making a "dirty bomb" sounds difficult though. Generally you need a whole fission reactor to burp to get people into a sufficient fright. Otherwise you just send in people in hazmat suitsm with radiation counters, hose everything down, and you are good to go. – David Tonhofer Feb 24 '21 at 18:59
  • @DavidTonhofer A 'dirty bomb' is traditionally used to mean a fission bomb aimed at maximising radiation - but for terrorist us, scattering whatever nasty material you can find over a suitable area using more comventional means would qualify. Using it to eg contaminate food products by mixing it into the source material (flour? sauce? ...?) could potentially cause more harm ("terror") from the panic caused than the actual material-caused damage. – Russell McMahon Feb 24 '21 at 22:23
  • Dirty bombs are far more a fear thing than a realistic threat. Pu-238 is an alpha emitter, completely harmless unless it gets inside your body--and note that if it gets in your digestive system it pretty much just passes through. It's only deadly if you inhale particles of it--they sit there irradiating one spot until it turns into cancer. However, it's quite a challenge to turn a block of metal into something that would stay in the air long enough to be inhaled by someone not there at the time of release. The threat of plutonium is greatly exaggerated. – Loren Pechtel Feb 25 '21 at 02:57
  • continued: Gamma emitters can kill without getting in your body--but if the bomb has enough to pose a hazard it simply kills the terrorists, they can't employ it. Beta emitters can't meaningfully harm you from outside the body. The only isotopes that actually pose an actual hazard are those that the body absorbs--and most of the stuff that poses a real risk has a short half life--it pretty much decays in the reactor cooling ponds. – Loren Pechtel Feb 25 '21 at 03:05
  • @Russell McMahon You make Plutonium sound quite harmless. – Slarty Feb 25 '21 at 10:30
  • @Slarty - I think that comment may have been directed towards Loren Pechtel :-) – Russell McMahon Feb 25 '21 at 12:21
  • " ... Plutonium-238 is given the highest relative hazard number (152) of all 256 radionuclides evaluated by Karl Z. Morgan et al. in 1963...." | Very interesting Wikpedia page. | My "suggestion" above re mixing it with food would be a rather effective use as a error weapon. Pu238 for RTG use produces about 0.6/ W/gram. A little goes a long way :-(

    https://wiki2.org/en/Plutonium-238

     – Russell McMahon Feb 25 '21 at 12:33
  • That plutonium isotope that may used for rtgs can not be used for bombs, it would be simply much too hot. For rtgs a short half live time is needed to get a high power density. For bombs a long half live time is needed to produce very little heat to stay cool. – Uwe Feb 25 '21 at 13:19
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In addition to other disadvantages, RTGs have pretty poor power densities, so there are much better solutions for most purposes like vehicles. They are practically restricted to niches where access for maintenance or refuelling is a problem. Such as Mars.

One of the biggest Earth users of RTGs was the Soviet Union, which used at least a thousand, to power unattended lighthouses in places like the high Arctic, where access, while easier than Mars, is still problematic.

But not problematic enough... after the fall of the Soviet Union, their remoteness left them vulnerable to vandalism and potentially worse...

And this has proved a pretty good reason for NOT using them on Earth. It has resulted in a major (and expensive) cleanup operation

Despite the issues, a new variant has been developed, uning a different isotope (Ni63) but it remains to be seen how well this pans out in practice.

user_1818839
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However, I still don't have a clear picture of whether or not we could use this tech for rovers here on earth, whether it's crawling the ocean depths or surveying places where life can't exist. Mars would seem to be more dangerous than say, a salt flat, or the ocean depths.

Not quite the same technique as an RTG, but using radioactive material as well (they use nuclear reactors), there are quite a few nuclear-powered vehicles, including:

A nuclear reactor is quite a lot bigger, heavier and more complex than an RTG, though, so they're not adapted for a small "rover". There was an attempt to use one on an aircraft but it was still a 16-ton thingy.

jcaron
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  • And then... There was project Pluto, a 600MW nuclear thermal ramjet that was actually statically tested for minutes of run time back in the late 1950s! Intended use was a supersonic low altitude cruse missile deploying multiple warheads. – Dan Mills Feb 24 '21 at 18:50
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To summarize the points from the other answers and keep things easy for future readers, here are the main points against the use of radioisotope thermoelectric generators on earth:

  • Not much power output: Power is on the order of a few hundred watts, not very useful for transportation
  • Extremely expensive: Millions of dollars per reactor, which makes sense when the budget is large and refueling is extraordinarily expensive, but those conditions rarely exist on earth
  • Nuclear proliferation hazard: Putting highly radioactive fission fuel where anyone can get ahold of it is a severe public safety risk, as it can be used to construct a dirty bomb. This is irrelevant on Mars
  • Large waste heat output: Radioactive things produce heat, and it never turns off. This is really useful on Mars, where keeping equipment warm in far-below-freezing temperatures is hard to begin with, but problematic in a warm environment where waste heat can damage the very devices it exists to power
  • Toxicity: Plutonium used isn't healthy stuff. It's very toxic, and radioactive as well, so a leak would require ridiculous decontamination procedures to prevent radiation poisoning and eventual cancers

Notably, all of these factors are either significantly lesser in impact or non-issues on Mars. Not much power is needed, as the device has a lifetime range on the order of a dozen kilometers/miles. Budgets are extraordinarily high just to get the thing to Mars, so the cost of the device itself matters far less. Nuclear proliferation isn't a problem unless you're concerned about the Martians, as the planet is about as close to inaccessible as you can get. Waste heat is not only not harmful on Mars, it's actually quite helpful, as it reduces the amount of engineering needed to keep the thing warm. Finally, toxicity isn't going to be a problem for quite a while, as I can't imagine many humans will be accidentally stumbling on the device.

For all of these reasons and others, RTG's make a lot of sense on Mars, but the list of situations on Earth where their benefits outweigh their tradeoffs is short to non-existent. Thanks to all the other answerers for the information in this summary, and please do feel free to look through the other answers for more in-depth discussion on any of the above reasons.

TheEnvironmentalist
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    Plutonium-238 is not a fission fuel. It can be bombarded with neutron to transmute it into Pu-239, which is fissile, but separating out the Pu-239 is impractical (i.e., much harder than separating U-235 from natural uranium). The bombardment also converts some of the Pu-239 to Pu-240, which is undesirable for weapons grade material. See & Harper's answer for more details. You could put Pu-238 in a conventional bomb to make a dirty bomb, but it'd have a very limited effect unless the plutonium dust were inhaled or otherwise ingested. – PM 2Ring Feb 26 '21 at 15:17
  • Plutonium's high toxicity is primarily due to its radioactivity. It doesn't appear to be more chemically toxic, per se, than other heavy metals. OTOH, if it does get into the body in a soluble form, it can be concentrated in the liver & bones, which is certainly bad. See https://chemistry.stackexchange.com/q/137129 – PM 2Ring Feb 26 '21 at 15:20