Non-radioactive RTG alternatives

[jwl]

Shortage of Pu-238, for use as a power supply on radioisotope thermoelectric generators, might be a problem for future space probe missions, since the US and Russia haven't made very much of it since the cold war and it doesn't occur naturally. But all alternatives I've heard for this are also radioactive, and one of the reasons they aren't used now is that there is a higher radiation risk.

So consider the scenario where an experimental, less safe alternative to Pu-238 goes wrong somehow, and the resulting political overreaction means no radioactive stuff is allowed into space whatsoever. What kind of non-radioactive alternatives could be proposed to replace them, in probes to the ice giants or further?

[Lord Revan]

well problems faced here is that you need a power source that's a) reliable b) last for a long time without refuel c)is not dependent on stellar proximity-

for example solar panels 2 A&B pretty well but are highly dependent on solar proximity and thus cannot be used much futher then 1AU from the sun as the power you get from the sun starts to become insufficient.

[Napoleon the Clown]

Chemical sources of energy just don't supply enough energy for a long enough period of time.

Solar panels become rapidly less effective as you get further away. When you're in a shadow, you're not going to be getting power.

No fission power in space means no power in space at ice giant distances or beyond.

[Eternal_Freedom]

Yeah I really don't think you can do this kind of mission without RTGs in a small enough probe. I mean, sure, if we ever crack fusion power we could use that on a larger spacecraft to travel out there but thats the best you'll manage.

[Sea Skimmer]

Somewhere between Jupiter and Saturn's orbits solar power becomes functionally useless. So little energy will be collected that the spacecraft won't even be able to keep its electronics warm enough to operate, let alone actually power them. That's one of the useful features of an RTG for long range missions, it not only produces electrical energy but the much more prolific waste heat warms the thing up. Solar power spacecraft have avionics heaters. The solar powered mars landers and rovers had to shutdown in the winter largely because they couldn't keep themselves warm. Battery power lets them warm back up when the sun is shining again.

RTGs have appeal for certain kinds of missions closer then Jupiter too, but one could brute force past that by simply building bigger and thus much more expensive to launch spacecraft. Chemical energy indeed has no utility at all here. We have more chemical options for efficient low amperage power then we used too but it just does not scale to years.

The Pu-238 issue is being dealt with any in case as I have brought up probably five times before when people start mentioning it as the doom end of space exploration. The US congress allocated money to solve the issue for the last several years and Oak Ridge National Laboratory has advanced to the pilot production phase. Just a few days ago they announced the first small batch for tests had been extracted. It is hardly an earth shattering thing to do, it just wasn't being done because low and behold, we had enough Pu-238 to meet needs until recently, which is why they stopped making it in the first place. It doesn't store without power loss so no reason to have a large stockpile.

[biostem]

Is there some method of launching material that's not radioactive, then somehow making it radioactive after it's traveled a safe distance from the Earth? I suppose you'd need some sort of particle accelerator to add on more protons or something, which would be large and cumbersome to include in the probe...

[Batman]

What would be the point?

[biostem]

What would be the point?

I was trying to think of a way of launching non-radioactive material into space, then making it radioactive to they could get all the advantages, but not have any of the risks if there should be a failed launch/detonation before it left orbit. Perhaps they could construct such a facility in orbit, then refine the material there, have the probe dock and take on the material, then leave Earth orbit...

[Batman]

All of which would mean a fuckton more of potential-nay, likely-damage to the environment than the Plutonium on the probe ever would.

[Guardsman Bass]

If you used a small nuclear reactor, it wouldn't be any more radioactive than the fuel naturally is until you activated it in space after launch IIRC. You probably wouldn't be able to do the typical "swing around the inner solar system planets picking up speed" thing, though, if it mean it had to swing by Earth at high speed with an active reactor.

[Adam Reynolds]

Long term a potential replacement is cheap fusion power on much larger probes with much more powerful electric engines, but in any short term, nothing can possible beat RTGs for long endurance space probes. It's the same way conventional rockets are simply superior for now.

If anything were more effective, it would already have been used. Never bet on engineers not having thought of something you came up while reading an article on the internet. Not to say that it is impossible that there is something you came up with that is unique, but it is highly likely you are simply attempting to reinvent the wheel.

If you used a small nuclear reactor, it wouldn't be any more radioactive than the fuel naturally is until you activated it in space after launch IIRC. You probably wouldn't be able to do the typical "swing around the inner solar system planets picking up speed" thing, though, if it mean it had to swing by Earth at high speed with an active reactor.

The problem with a full nuclear reactor is one of scale, which means ever larger rockets or massively reduced range when dealing with unmanned probes. Though it would likely be a good idea for something like a manned mission to Mars(either as a means to create fuel for the return rocket as was described in Mars Direct, or as a power source for a reusable spacecraft as used in Mars Semi-Direct and The Martian).

[Napoleon the Clown]

Is there some method of launching material that's not radioactive, then somehow making it radioactive after it's traveled a safe distance from the Earth? I suppose you'd need some sort of particle accelerator to add on more protons or something, which would be large and cumbersome to include in the probe...

Well, it could pick up some rather lively particles if you parked it in the Van Allen belt for a long time, but it would take so long to accumulate much of anything that it's pointless. For something to be radioactive its nucleus has to be unhappy about existing in its current state. It takes a lot of energy to get it into this unhappy state, because it doesn't want to become unhappy.

Think of it as sort of being like a spring. Not a great analogy, but stick with me here. This spring requires truly tremendous amounts of force to compress, but once you do oh baby does it jump. In nature, this "spring" is "compressed" when enormous stars go supernova. The amounts produced this way are vanishingly small, so we have to make it ourselves using particle accelerators. Not an easy process, and we start with radioactive substances. I guess you could do a similar thing to lighter elements to produce radioactive isotopes, but you still run into energy problems. We're talking smashing atoms into each other at 99% the speed of light. Nothing that we can get in orbit will produce that amount of energy.


So in a word: No.

[Jub]

Well, it could pick up some rather lively particles if you parked it in the Van Allen belt for a long time, but it would take so long to accumulate much of anything that it's pointless. For something to be radioactive its nucleus has to be unhappy about existing in its current state. It takes a lot of energy to get it into this unhappy state, because it doesn't want to become unhappy.

Think of it as sort of being like a spring. Not a great analogy, but stick with me here. This spring requires truly tremendous amounts of force to compress, but once you do oh baby does it jump. In nature, this "spring" is "compressed" when enormous stars go supernova. The amounts produced this way are vanishingly small, so we have to make it ourselves using particle accelerators. Not an easy process, and we start with radioactive substances. I guess you could do a similar thing to lighter elements to produce radioactive isotopes, but you still run into energy problems. We're talking smashing atoms into each other at 99% the speed of light. Nothing that we can get in orbit will produce that amount of energy.

So in a word: No.

Thinking along that route we could look at transmuting other elements into gold, it's not exactly like turning something into an isotope, but it gives a ballpark figure. Scientists working at the GSI Helmholtz Centre for Heavy Ion Research could, theoretically, create two million gold atoms per second. At that rate, it would take 50 million years to make a gram of gold. I don't even want to think of the energy used over that same span...

[jwl]

All of which would mean a fuckton more of potential-nay, likely-damage to the environment than the Plutonium on the probe ever would.

I'm talking about a hypothetical political overreaction here, not an actual technological problem. If it doesn't involve launching radioactive materials from earth, that would be fine.

[Napoleon the Clown]

No radio-isotopes into space means no long-range probes. Period. Solar can't provide the power, chemical doesn't have the longevity.


I will give an example: You know the sun? That thing that accounts for 99.9% of the mass of our solar system? If it relied strictly on chemical reactions for energy release it would have burned out in the span of 10k years. So... yeah.

Pu-238 has a half-life of ~80 years. Name a chemical reaction that can go on for eighty years and fuel smaller than an orange. And you'll still have Pu-238 after those 80 years. There simply isn't one. Chemical reactions are horrifically inefficient at releasing energy from a given amount of mass. Literally the only way we could have non-radioactive fuel and still have probes is if we got good at storing anti-matter and could make enough of it for cheap enough to not bankrupt an economic superpower. Of course, if you lose containment on anti-matter of that sort of quantity an accident with a radio-isotope will look just adorable.

I outright ignore using fusion in probes because the experiments claiming to be able to get more energy out of it than is put in always turn out to be bullshit. Unless there's a way to get fusion at temperatures under ten million kelvin, it ain't gonna happen.

[GrandMasterTerwynn]

Shortage of Pu-238, for use as a power supply on radioisotope thermoelectric generators, might be a problem for future space probe missions, since the US and Russia haven't made very much of it since the cold war and it doesn't occur naturally. But all alternatives I've heard for this are also radioactive, and one of the reasons they aren't used now is that there is a higher radiation risk.

So consider the scenario where an experimental, less safe alternative to Pu-238 goes wrong somehow, and the resulting political overreaction means no radioactive stuff is allowed into space whatsoever. What kind of non-radioactive alternatives could be proposed to replace them, in probes to the ice giants or further?

Abandoning exploration of the solar system beyond Jupiter. Since anything that produces radioactivity is out, that obviously rules out RTGs, actual fission reactors, several fusion reactor designs (as they induce radioactivity in the reactor structure,) and antimatter (as antimatter reactions produce gamma rays, and could possibly generate radioactive materials if the right kinds of atoms eat an antiproton.)

That, or colonize Mars and dig for uranium there, since I saw your follow-up post on not launching radioactive materials from Earth (the Moon might work too, but might be too close for comfort for the NIMBYs in this scenario.)

[Guardsman Bass]

rockets or massively reduced range when dealing with unmanned probes. Though it would likely be a good idea for something like a manned mission to Mars(either as a means to create fuel for the return rocket as was described in Mars Direct, or as a power source for a reusable spacecraft as used in Mars Semi-Direct and The Martian).

I'm not talking about a big, megawatt reactor. Think more along the lines of a multi-kilowatt reactor, like SAFE or the SP-100. The latter was aiming for a mass of about 4 metric tons - that's heavier than an RTG system, but it also gives it way more power.

[jwl]

Actually thinking about it, if unenriched uranium was banned for space takeoffs because it wouldn't take long for someone to point out that depleted uranium is deliberately exploded at ground level in tank shells with no consequence. So I suppose it might be politically okay to send up stuff with ~the radioactivity of depleted uranium or less. Would it be possible to enrich uranium in space?

[Grumman]

Actually thinking about it, if unenriched uranium was banned for space takeoffs because it wouldn't take long for someone to point out that depleted uranium is deliberately exploded at ground level in tank shells with no consequence. So I suppose it might be politically okay to send up stuff with ~the radioactivity of depleted uranium or less. Would it be possible to enrich uranium in space?

You could, but it would be stupid to. Depleted uranium has less than half as much U-235 in it compared to natural uranium, and less than one tenth as much compared to reactor-grade uranium. This means that if you want one ton of reactor-grade uranium processed on site, you'd need to fly ten tons of depleted uranium up to orbit.

[Sky Captain]

Russians at some point during Cold War had small few kilowatt class nuclear reactors in orbit to power some spy sattellites. Similar small scale reactors could be used on long range space probes. Activate reactor only after reaching Earth escape velocity and there would no radiation risk at all.