SNAP-10A, the only satellite the US launched with a nuclear reactor. DOE/Wikimedia Commons At the height of our adoration of atomic energy, space agencies experimented with launching nuclear-powered spacecraft into orbit around the earth.

It makes sense if you think about it.

Radioactive materials, like uranium-235, can power a tiny satellite for years. They're more reliable than batteries and provide more energy than solar panels.

But back then, space-faring nations weren't as concerned with radioactive waste. Nuclear disasters like Three Mile Island and Chernobyl hadn't happened yet, and now we're much more worried about radiation exposure.

That's why the last nuclear-powered satellite, launched by the Soviet Union, blasted into orbit in 1988.

More than 30 different nuclear-reactor-powered satellites still orbit the earth. The US launched only one while the USSR launched all the rest.

Those nuclear reactors are similar to the ones in nuclear power plants on the ground. Uranium-235 undergoes fission, where its nucleus splits, giving off energy. This energy can be converted into electricity to power satellite instruments, or your house.

America's uranium-fueled SNAP-10A entered into an orbit of 575 miles above the earth in 1965. It operated for 43 days before it stopped responding. It's now in a slow trajectory to hit the ground in about 3,000 years. By then, hopefully, its radioactive cargo will be mostly harmless.

But if any of these nuclear-reactor-powered satellites collide with another object in space, or suddenly crash to the ground, they could release radioactivity.

The Soviet Union had a few such mishaps since it launched all those nuclear satellites. In 1978, its spy satellite, Kosmos 954, crashed into the Northwest Territories, scattering radioactivity across almost 48,000 square miles. The USSR had to pay Canada $10 million for the damage.

Debris from the Soviet Union's Kosmos 954 when it crashed into Canada in 1978. Wikimedia Commons And in 1995, NASA scientists found a cloud of liquid, radioactive sodium and potassium coolant in orbit. The space agency eventually figured out that it came from the Soviet satellite Kosmos 1900. Something else in space crashed into it, causing the nuclear reactor to leak. The cloud of radioactive fluids is still floating up there, and space agencies continue to monitor it.

The good news is that all of these dead nuclear-reactor-powered satellites are in orbits higher than 430 miles. There's barely any air molecules at that height to slow down the satellites, so it should take them hundreds or thousands of years to wind their way back to the earth — at which point much of their radioactive contents will have significantly decayed.

But NASA and Roscosmos, Russia's space agency, are reportedly looking into building nuclear engines again. This time, they want to build hyperefficient rockets that might one day take humans to Mars.

If this sounds like science-fiction, it's not. NASA built several perfectly functional nuclear rocket engines from 1955 through 1973.

Here's one called NERVA being test-fired in the desert:

Those programs ended abruptly, however, because of environmental and budget concerns.

It remains to be seen if NASA or Roscosmos can keep funding, public support, and safety moving in its favor.