The recent Time magazine has an article about the New Horizons probe that is going to study the planet Pluto (or whatever it is). It mentions that the craft will be powered with a tiny amount of plutonium.
So my simple-minded question is, how does energy from plutonium get converted into useable energy for a spacecraft?
All you ever wanted to know about (warning, PDF!) radioisotope heater units More info here.
It’s a certain isotope of plutonium, plutonium-238. It’s not the same isotope used to build weapons.
Pu-238 releases a lot of energy as it decays and as such it generates heat. A ball of Pu-238 the size of a baseball could boil water.
A radio thermal generator is just a thermo-electric converter. Basically it turns heat directly into electricity, without having to use steam or anything as a go-between.
Cool! It’s like a nuclear battery. And it looks small enough to fit in my camera.
(You don’t suppose…?)
No, I don’t. It would be heavy, expensive, and utterly unworkable in PR terms alone. After all, everyone knows that all nuclear material is a bomb waiting to go off and give you cancer and make all your children’s hair fall out even as it renders you sterile as a mule.
That doesn’t even make sense, but it’s what everyone knows.
OK, can someone explain how this heat is converted directly into electricity?
And is this also somehow used for propulsion as well as for the electrical equipment on the craft?
I wasn’t too serious about the consumer electronics angle, but seeing the tiny size and the thought that I’d never have a camera die on me, regardless of the weather extremes (after all, this thing runs in outer space!) was too attractive not to consider. But then I guess it would cost a bit more than the thirteen bucks I pay now for my rechargables. And just how heavy is that sucker?
And just how is heat directly converted to electricity?
The Pluto mission just uses it to power onboard electronics, radio transmitter, gyros, etc. It’s a one-way trip so there isn’t much need for propulsion along the way - just a big kick at the start, and coast all the way. It has conventional chemical thrusters for orbital corrections and pointing.
In theory you can use RTGs to power ion thrusters, but they are very power-hungry and I don’t think anybody has done it. Ion thrusters are usually combined with big solar panels (aka solar-electric propulsion).
I imagine it won’t work very well in hot weather. Thermocouples generate electricity from a temperature difference. (As do all generators, for that matter.) The isotope is a source of high temperature, but you also need a low-temperature heat sink (radiator, cooling water, cool air, etc).
Anway, the one in the photo is not a generator, but a heater designed keep critical components warm. A generator is made up of an isotope, thermocouple and a radiator, and is usually quite large.
The Pluto mission just uses it to power onboard electronics, radio transmitter, gyros, etc. It’s a one-way trip so there isn’t much need for propulsion along the way - just a big kick at the start, and coast all the way. It has conventional chemical thrusters for orbital corrections and pointing.
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A very slight nitpick: New Horizons is going to do a gravity assist at Jupiter.
Every spacecraft that has gone to the outer Solar System has used RTGs for power (that would be Pioneer 10 & 11, Voyager 1 & 2, Galileo and Cassini), because solar power just isn’t practical when you get that far from the Sun. (IIRC, Cassini would have required solar panels the size of a tennis court to power its systems!) The Viking Landers also used RTGs.
I am also given to understand that they are very common in classified military space applications; you don’t want honkin’ solar panels on your spy satellites, after all.
Yes, thanks.
I don’t think there are stealth satellites, if that’s what you’re implying. Everyone knows where everyone else’s spy satellite is, and there’s not much point in making them less visible. I know some early military satellites used RTGs, but I can’t think of any reason to use one now. (Nuclear reactors are another matter, they can produce much more power. Reactors have been used in space to power high-power radar systems.)
By the way, RTGs were also used to power instruments left on the Moon by Apollo missions. One of them didn’t make it there (Apollo 13) and is lying in the bottom of the Tonga Trench. (Just once, in the 70s…)
With regards to military spacecraft, I’m merely repeat what I hear from “sources close to NASA” 
about how many RTGs are available for civilian use, vs. how many actually exist, and “reallocation” of plutonium from NASA to the military for “national security purposes.” This is much less of a big deal now that the DOE has made arrangments to buy some Russian plutonium, and also plans to begin another phase of US plutonium production.
So, if I can get some clarification, in “Back to the Future,” when Marty proclaims,
“Are you telling me that this sucker is nuclear?” and Doc Brown goes:
“No, no, no. This sucker’s electrical.”
He basically meant he was using this method? Even though from what it seems, this method really can’t generate 1.21 Giga watts?
Probably not. RTGs are used for long-duration (many decades) low-power applications. Doc Brown’s nuclear generator somehow produces a short burst of power, using up all the plutonium in the process. It’s probably more closely related to a nuclear bomb than anything else.
[Doc Brown voice] Great Scott!