DTR operations were terminated on Voyager 2 in 2007, and Voyager 1 in 2015, but I don't know if this was related to mechanical problems with the tape recorders or just due to the lack of need given the data collected by the remaining instruments.

Actually, when I mentioned thermoelectric conversion I wasn't referring to an RTG (which is just nuclear decay) but rather to the true nuclear reactors that the Soviets put in space:

https://en.wikipedia.org/wiki/Romashka_reactor https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor

They used thermoelectric conversion rather than a Sterling engine with moving parts (although I guess TOPAZ is technically "thermionc coversion").

Yes, it means exactly that - though soon you'll start hitting into issues with solar panel efficiency and thermal management. It's very hard to keep things cool in space.

The space electricity tanker idea is theoretically possible, but might not make any economical sense. For it to be worth it, it would have to store a lot of energy, so that it could ship back more than the cost of moving it around, while also being competitive with simply building more bigger collectors further away (and closer to the industry). But maybe a highly eccentric orbit, with a very low perihelion, would work.

Not totally sure if the math works out on this, but I could see moving the energy-intensive industry closer to the Sun, and having it use beamed energy to move resources and products to itself and back.

Take a look at the solar panels on the Parker Solar Probe: http://parkersolarprobe.jhuapl.edu/index.php#spacecraft

It did: https://www.space.com/1929-nasa-pluto-mission-draws-dozen-pr...

(Not many, but more than zero.)

Stirling engines have a pair of seals that are extremely hard to make durable, and to my knowledge only Whisper Systems has cracked this problem to the point where you won't lose working gas (or seals) in a timespan much shorter than those 10 years. It's a stupidly hard engineering issue, without it there likely would have been far more adoption of the Stirling cycle for production machinery.

That's the usual way things are done. If you buy for instance a simple 5:1 gearbox for industrial use, it will quite likely be a sealed casing 'greased for life'.

Not that I'm disagreeing with you really - mechanical devices have high failure rates. 10 years maintenance-free is a bit of a dream. As a mechanical engineer I'm interested to see whether the Stirling engines involved have radically different scaling to optimise for reliability, or NASA just plan to do the engineering really well.

Of course manned maintenance may be possible - they are touting this technology for Mars bases. Plus robotic maintenance is going to become more of a thing over time.

There is a Stirling engine design which only has a piston as the single moving part, called a "lamina flow Stirling engine".

Note that while implementations usually show a crank and rod with a flywheel, it could just as easily use a magnet and coil to generate electricity.

That get's you down to a single part.

Then you have this:

https://en.wikipedia.org/wiki/Thermoacoustic_heat_engine

...that gets you down to something that can generate sound from a heat differential, and you could couple that sound to some kind of transducer to generate electricity. Still a moving part, though.

You probably can't get zero moving parts and yet have it do useful work, but you can get really close I think.

There's some work being done in superfluid discovery which would create the sort of conditions you're looking for but physics discoveries move incredibly slowly and some of the materials are rare or difficult to create. However, that sort of "lubricate, seal, and forget" kinetic system is essential for a lot of surface work in space. Increasingly important on Earth as well.

As a heuristic, the less mechanical parts in any system, the more efficient it is over time.

Ideally, everything that's built is engineered like a spacecraft, because to some extent it is, and is onboard one.

Sounds like there is some redundancy. These are intended for manned missions so occasionally replacing parts might be part of maintenance.

And the bigger chance of failure like Rosetta could not send anymore, because the solar panels were blocked ...

If you want to use the RTG to power thrusters, then I'd think that doing the "detour" via a stirling engine to create electrical current to turn into thrust might be unnecessary.

There must be some way to directly turn radioactivity (or heat) into thrust with sufficiently high exhaust velocity!?

Cool, thanks! It's really surprising to me that inserting a mechanical intermediary has such a large efficiency advantage.

Hmmmm a normal boiler for a 4-person family home is about 8-10kW so again, probably not. At least not for heating. But for normal power consumption should be ok, especially if paired with some storage device to account of periods of higher consumption(running a 3kW kettle for couple minutes for example).

They're very low power, make a lot of waste low-grade heat, and use an inefficient electricity generation approach.

"over 20 years, a three-megawatt wind turbine can deliver 80 times more energy than is used in its production and maintenance." [0]

Beyond that, Steel is 100% recyclable^, and that accounts for over 65% of US steel production. Recycled steel can be done in arc furnaces, requiring no coal coke. [1]

Info like this is at the tip of your fingers.

[0] https://www.worldsteel.org/en/dam/jcr:f07b864c-908e-4229-9f9...

[1] https://seekingalpha.com/article/3785906-metallurgical-coal-...

^ This rate is never achieved. Global average is ~90%, as some countries aren't efficient: http://www.steel.org/sustainability/steel-recycling.aspx

But on deep space missions, aren't you aiming for 50+ years, or ideally potentially unlimited if you want to hit alpha centauri?

Voyager 1 is going for 50 years ...

The half-life of fuel for these reactors is very long - over 100,000 years. So for all practical biological effects they are basically stable. If it exploded before it was activated (far into space), it would be about the same danger to people and the environment as the rest of the exploding metal the rocket was made of.

Pretty much any uncontrolled explosion propels highly toxic material over a large area (rocket fuel is not lickable). I don't see how alpha radiation is inherently worse than hydrazine.

For that case there are launch escape systems typically used for crewed missions.

The alternatives still seem worse.

Lasers are still subject to the diffraction limit, regardless of the quality of focus.

We can examine this limit in the context of delivering energy to a remote object. This analysis will be simple - we assume that the optics are perfectly aligned (dubious - pointing is difficult); we assume nothing about the absorption properties of the object, which will necessarily need to be very high efficiency. The angular size T of a laser beam of wavelength L emitted by an aperture of diameter D_a is roughly L/D_a. Similarly, using the small angle approximation, this angular size T at the object itself will be the width of the beam, W, divided by the distance between the object and the aperture, D_o: T = W/D_o. Ultimately, this gives us the width: W = L*(D_o/D_a).

What does this tell us about the practicality of such a system? Visible light, wavelength roughly 500 nm, is perhaps a solid guess for a real system. Realistically the aperture size is probably limited to about 10 meters, but we can go even further and assume a synthesized aperture of a realistic system being 100 meters. You would want to get all of your beam for power transmission, so lets assume an upper limit for the beam width at the object to be 100 meters as well - probably unrealistic, but maybe solar sail/ultralight absorbers could get there. Throwing these numbers in gives a maximum range of... 2x10^10 meters. This is roughly a tenth of an AU. In comparison, this is about 50 Earth-Moon distances... and only a quarter of the distance between Earth and Mars at their closest approach. Coincidentally, this is also about one light-minute.

Any real power delivery system, using current tech and without assuming convenient fictions, will have a much more limited range. In short, lasers are not really perfect rays, even though they are approximately so over scales we typically encounter; at astronomical scale, diffraction always wins. This is why it is usually way better to bring the power with you - especially as you lose solar irradiance as you get further from the Sun. And for bringing power with you, nothing beats nuclear for energy density.

Saw a documentary recently and they're doing just this.

I don't think people respond well to this kind of objective reasoning, it just doesn't work very well as an argument. At worse it can sound like you are calling them dumb. We need to explain the benefits, and why they are worth a tiny amount of risk. And accept that sceticism and concern are exactly what an intelligent layman should be exhibiting. Wondering if the thing is going to explode and cause cancer is a perfectly legitimate question to ask. To pursuade people you need to engage with them on an emotional level, not just bash them dismissively.

In your zeal to defend nuclear energy, you're come around full circle, defending something indefensible, with arguments far removed from actual science.

Nuclear energy may be safe, if done under near-perfect condition, with extremely large budgets to understand risks, and plans to mitigate incidents. There is truth to the argument that nuclear power saves lives compared to coal power.

But nuclear power isn't inherently safe. It is, by its very nature, extremely dangerous. Actual nuclear scientists understand that. See, for example, Feynman's work on nuclear safety during the Manhattan project.

Strapping a nuclear reactor to a rocket is almost by definition a dirty bomb. Depending on the height and mode of an eventual launch failure, the result could be anything from Tchernobyl to a less-dramatic yet more deadly dispersal of nuclear material in the upper atmosphere.

The dose-response relationship of radiation exposure is largely linear, meaning the latter event might just increase your risk of brain cancer by 0.005%. Yes, you may consider it negligible. But statistically, it would kill half a million people.

To blithely state that "nuclear is safe, hoho, why shouldn't we strap Plutonium to a rocket, you environmental nincompoops" has nothing to do with science, and gives science a bad name. Maybe NASA could come up with a way to protect a reactor during a missile launch. But it wouldn't be easy, and saying "why wouldn't we?" on TV would not inspire confidence in their abilities, but doubts in their sanity. The right thing to say on TV is "We're sending a reactor into space, and here are the mechanisms we've come up with to make it safe..."

An RH-72B[1] cask is a container for shipping trans-uranic nuclear waste, probably to the WIPP site.

[1] http://www.wipp.energy.gov/WIPPCommunityRelations/images/pho...

To some extent we are radioactive material on the surface streets of the major cities, because we have consumed soil!! Even our steel tools can be somewhat radioactive, refinement concentrates point sources.

However, containment vessels like that are usually for refined ores, which end up in reactors, smoke detectors, medical devices, radiotherapy machines, you name it. Too expensive and possibly hazardous to airlift.

As far as I know, production of atomic weapons has stopped in this country and we're not even sure if they still work (half-lives--they rot). Could've been materiel for a stewardship program at the Hanford Site, or waste being evacuated from the Hanford Site.

Generally speaking, the remaining atoms transported and transmuted are for peace. That's why Russia brokers Uranium to the States, and why people in Kazakhstan and elsewhere continue to mine fissile ore.

If you were to shut down the remaining fission plants of Planet Earth, civilization as we know it, would end almost immediately, such is our need for these fuels. It would not be pretty--consider the amount of electric heat, the number of electric stoves with 50+ year duty lifetimes. That's why Japan recycles spent fuel at the French reactor.

Could be related to all the Navy nuclear stuff around town like the shipyard at Bremerton or the Bangor sub base

But then your Carnot efficiency goes way down.

Yeah, but this subthread is about protest reasons :) The reactor in the article (and the half-life of it's fuel) can't be the reason people are protesting.