NASA deep-space mission plutonium-fueled thermoelectric generators with iridium containment

Tom's worked example of why iridium would be the perfect jet-engine material (3000°C melting point) but is impossible at scale (100 oz/year world production). Pivots into the actual deployed application: iridium spheres containing plutonium-238 for RTGs on deep-space probes, with the safety concern that a launch failure scatters nuclear-warhead material into the Atlantic.

If you wanted to make a really fancy engine, you could use iridium. The problem is they only mine about 100 ounces of iridium a year in the world. But iridium doesn't melt until over 3000° Centigrade. You could build a 4,000° engine out of iridium. No one's ever built a jet engine out of it — but we have built iridium engines, I take that back. For these deep-space flights, when NASA is going out to Pluto or Saturn or Uranus, anybody know how they get their energy?

Used as an example of an exotic welding application (iridium) — about once every 25 years there is a real need to weld iridium spheres around plutonium for deep-space spacecraft RTGs.

We can do iridium if you want. There is actually an application for welding of iridium about once every 25 years. They weld up little iridium spheres around plutonium. When NASA has a space shot that goes into deep space — passing by Uranus or Neptune or Pluto — there's not enough sunlight energy to power the spacecraft for 40 years. So they put a piece of plutonium in there which generates heat, and they have thermoelectric generators, which are very reliable electrical sources. The Voyager spacecraft that's been going for 30 or 40 years is powered by a piece of plutonium surrounded in iridium. Just in case it blows up when it's going up into orbit, rather than spreading plutonium all over the Atlantic Ocean, they end up with one little iridium sphere they can go look for. Don't tell anybody they're putting plutonium in spacecraft that could crash during the boost stage of the rocket, because the environmentalists would get all upset. NASA doesn't advertise that anymore.