In the 1960s, the US 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.

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. You see, 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: almost all modern warheads in the US arsenal use it as a trigger. When it explodes, it sets off an even larger thermonuclear device capable of flattening a midsized city (say, 100,000 to 200,000 people). Russian warheads have even higher yields.

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.

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.

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 plutonium-238 off 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.

The factories churned out something else, too: radioactive waste. At the US plant on the South Carolina-Georgia border, workers dumped tens of millions of litres of radioactive waste a year into open-air basins. The worst of the stuff, 140 million litres of radioactive sludge, salt and liquid waste, was put into underground storage tanks, where it sits to this day. The site, known as Savannah River, is still heavily contaminated, and clean-up operations have run to many billions of dollars.

In Russia, the situation is even grimmer. In true Soviet fashion, the bomb makers secretly dumped unknown quantities of liquid waste into giant reservoirs around the plant. Nobody knows how much radioactive contamination is out there, but a single accident - the explosion of a waste tank in 1957 - is thought to have been Chernobyl-like in scale. As recently as the 1990s, the plant was spewing radioactive waste at a rate that makes the leaks of radioactive water from the melted-down Fukushima powerplant look like a bubble bath. People living around the plant have elevated rates of leukaemia and genetic mutations. Their children get cancer.

The US stopped making plutonium in the late 1980s, after it became apparent that both sides had stockpiled enough warheads to destroy civilisation. At first, NASA was able to draw on the supply of plutonium-238 left over at Savannah River, but that soon ran out. So it turned to Russia. The first shipment from the Russian plant arrived in the 1990s, and to date, NASA has received about 30 to 40 kilograms of plutonium. A few pounds of Stalin's finest plutonium-238 hitched a ride to Mars on the back of Curiosity.

There's nothing wrong with oooh-ing and aaah-ing over Curiosity's photos. The project is an incredible achievement, and the science it produces will be amazing. But remember this, too: That little rover on Mars has left a big mess back here on Earth.