Voyager spacecraft iridium thermoelectric generator

Plutonium in an iridium sphere powers Voyager's thermoelectric generator. Eagar saw "about 50% of the world's supply of iridium" in a file cabinet at Oak Ridge during a job interview.

Ruthenium — the only application I've ever heard. Hexagonal close-packed. A guy came to me once who wanted to make ruthenium BBs, basically, for ballpoint pen tips. It's hexagonal close-packed, so I had to show him how to melt it — surface tension would make it spherical. Osmium — I know of no application of osmium except among the biologists. They use osmium tetroxide to stain cells. When you get to osmium and iridium, these things are produced in hundreds of ounces per year in the world. They're not exactly something you're going to choose for structural material. Although iridium is used for structural material. The Voyager spacecraft, that's heading out of the solar system now — officially headed out of the solar system after, what, 30 or 35 years — is powered by plutonium in a thermoelectric generator. There's not enough solar radiation, when you get that far away from the sun, to provide the energy to go beaming back the signals. So the Voyager spacecraft was plutonium in an iridium sphere. Iridium melts at a high temperature. If you add a little tungsten, some other things to it, you'll have ductility. When I was graduating, I was interviewed by Oak Ridge, and they offered me a job. But I saw a file cabinet down in Oak Ridge, Tennessee, that had about 50% of the world's supply of iridium, because they were making these little spheres for NASA, for the Voyager spacecraft. Iridium is also good for platinum jewelry. I made my wife's engagement ring out of platinum-iridium. Electron beam melted it.

Specific instance of the iridium-plutonium application — Voyager's 30-to-40-year continuous operation in deep space.

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.