In May, when researchers contacted the International Sun-Earth Explorer 3 (ISEE-3) for the first time in 16 years, that a decade’s old spacecraft still had enough juice to phone home might have come as a surprise. Launched in 1978, here was an object some 36 years old that had better battery life than some of the most advanced technology in existence today.
But that’s precisely it: ISEE-3, and other spacecraft, like it, don’t run on batteries. And they likely won’t in the future, either. Sure, battery technology has certainly improved since ISEE-3’s heyday, and solar technology is certainly more efficient than it once was, too. But the truth is, the long-distance space craft of the future tasked with exploring the outer planets and beyond will likely be powered with the same thing we’ve been using for decades: plutonium-238. That's right, we're talking about nuclear power.
When sending unmanned vehicles into space, you really only have two options for power: light from the sun, or heat from a nuclear source. Obviously, the former is preferred where possible. It’s relatively cheap to harness, and there’s practically an unlimited supply. But the sun has other limits. Light can only travel so far, which means the farther you travel, the less electricity you can produce. In some places, such as parts of the moon, there are permanently shadowed regions which never receive the sun’s light. Even on planets such as Mars, which are still close enough to harness the sun’s rays, dust dramatically reduces the efficacy of solar panels over time.
For deep-space missions, and missions to hostile environments where light from the sun won’t do, NASA’s only other option is to harness the heat generated by a slowly decaying hunk of radioactive material – in this case, plutonium-238 – with a radioisotope thermoelectric generator, or RTG. This process turns heat into electricity, and in some cases there is even excess heat that can be used to warm the components of a spacecraft or rover too. Make no mistake, though, this is old technology.