I’m as happy as anyone that the Curiosity rover got to Mars; it’s hard not to root for all those NASA geeks in their blue polo shirts. But before you get all American and apple pie about the achievement, there’s something you should know: Curiosity runs on plutonium from a Soviet-era nuclear-weapons plant.

Take a look at the back of Curiosity. Other rovers have solar panels, but Curiosity doesn’t. Instead, there’s a little white thing that looks cute, almost like a tail. Inside are eight boxes filled with pellets of nuclear fuel. This stuff is hot, so hot that the boxes glow bright red, and they will glow for years to come. Think of it as nuclear charcoal. The fuel will keep the rover toasty on cold Martian nights and supply it with electricity.

It’s a neat trick, and one that NASA has used before. Since the 1960s, the United States has been launching nuclear-powered spacecraft. The first were military satellites. That worked swell, except that when the mission ended, you had a radioactive pile of junk orbiting the planet. And every now and then, one would fail to launch or fall back to Earth. That was bad for PR.

These days, NASA puts nuclear fuel on things that aren’t coming back. The Voyager missions that left the solar system carried it, as did the first Martian missions, the Viking landers. It’s particularly useful when you’re going far from the sun — places where solar panels don’t work.

The particular kind of fuel inside Curiosity is called plutonium-238. It’s the perfect stuff for the job: It’s extremely radioactive, so it gives off plenty of heat, but the type of radioactive particles released by plutonium-238 can’t even penetrate a sheet of paper. As long as you don’t touch it or swallow it, plutonium-238 is safe, and with a half-life of 87.7 years, it decays slowly enough that a fairly small supply can power a spacecraft for a decade or more.

But plutonium-238 isn’t easy to come by. It doesn’t exist in nature, and only two places in the world have made serious quantities of it. Both made something else: nuclear warheads. Plutonium-238 is really a byproduct of the process for making another kind of plutonium, known as isotope 239. Plutonium-239 is the real terror: It takes just a couple of pounds of the stuff to make a bomb as powerful as many kilotons of TNT. Almost all modern warheads in the U.S. arsenal use plutoniuim-239 as a trigger. When it explodes, it sets off an even larger thermonuclear device capable of flattening a midsized city (say, Boulder, Colo., or Ann Arbor, Mich.). Russian warheads have even higher yields.

In the 1960s, the United States and Soviet Union were hungry for 239. They built secret reactors that irradiated uranium to create it. Then they dissolved the uranium-plutonium mix in acid and used a slew of toxic chemicals and solvents to isolate the plutonium. The work provided the plutonium-239 for thousands of tiny, high-efficiency warheads — many of which still sit atop missiles today.

Plutonium-238, the stuff in the rover, was an afterthought. NASA asked the Atomic Energy Commission to get some for the agency’s satellites in the 1950s, after falling behind in the space race. The eggheads at the nuke plant came up with a clever way of producing it from unwanted isotopes they were just going to throw away anyway. The Soviets had the same idea. Using a similar system of acids and solvents to dissolve their uranium fuel, the Soviets skimmed plutoniuim-238 off of their production operation at a secret bomb factory in the Ural Mountains. It went on for decades: In came uranium fuel, out went plutonium-239 for the bombs, plutonium-238 for the spacecraft and many other isotopes for other needs.