As Voyager 2 sails ever farther from the sun, it continues to radio back information on the conditions it’s meeting in genuinely interstellar space. Despite being launched in 1977, the probe and its sibling Voyager 1 are expected to continue sending information for as long as another decade before they finally fall silent on their long glide into darkness. That astounding achievement is entirely powered by plutonium.
The incredible lifespan of the Voyager probes is due to the trio of radiothermal generators that have provided over 400 watts of power to each craft for the last 40 years. But for the last 14 years, NASA has given designers of spacecraft strict instructions: no more radiothermal generators. The reason for that instruction is that generators such as those included on Voyager contain about 10 pounds of solid plutonium. However, despite the dangers of plutonium, the instruction to limit its use didn’t result from an environmental concern. It came from something much more direct: NASA was out of plutonium.
Plutonium isn’t found in nature. It’s created only in breeder reactors that produce the silvery metal by bombarding uranium with small particles. It’s miserable stuff to handle. Exposed to air, it forms a crumbly powder. Exposed to moist air, it swells into flakes that are pyrophoric, meaning that they spontaneously burst into flames—not exactly the sort of behavior you want in a substance that’s so carcinogenic that breathing in a single particle can be tantamount to a death sentence. But again, that’s not why NASA quit using it.
The reason it stopped was because there was a plutonium shortage. With no more breeder reactors operating in the United States, the plutonium still available was needed for a Very Special Purpose. Now that purpose is almost complete, and there’s been a notable change at NASA.
As Scientific American reports, radiothermal generators are back. In its latest round of soliciting new ideas for scientific probes, NASA has explicitly put plutonium power, as well as a new generation of space-based nuclear reactors, back on the menu. Why are they able to do this? Because of something Reuters covered back in April.
In Energy Department facilities around the country, there are 54 metric tons of surplus plutonium. … There are enough cores there to cause thousands of megatons of nuclear explosions. More are added each day.
It’s the same reason that Nevada was recently the unwilling recipient of 1,000 pounds of weapons-grade plutonium—it’s coming from weapons.
The U.S. is dismantling older nuclear warheads in a program that began under President Obama. Partly that’s because a treaty between the United States and Russia sets a boundary on how much plutonium can be out there in weapon form at any one time. Partly, it’s because of this:
The United States wants to dismantle older warheads so that it can substitute some of them with newer, more lethal weapons. Russia, too, is building new, dangerous weapons.
The idea that both nations are busily dismantling the Cold War generation of nuclear weapons in order to put something “more lethal” into service may be the most sobering thought that can be thought.
That sudden accumulation of newly available plutonium, much of it stored at the Pantex Plant near Amarillo, Texas, where the weapons are being upgraded, is why the Department of Energy is so eager to start putting to use some of the storage sites it’s been creating over state objections. So eager, in fact, that it started using them without telling anyone and shipped over 1,000 pounds of plutonium to a site in Nevada while pretending to “negotiate” over the shipment with the state. That information would still be secret if it hadn’t emerged from court documents connected with a fight launched by the state to keep the DOE from doing what it already did in secret.
This clearly won’t be the last plutonium train crossing the country. In fact, there’s no reason to think there have not already been more such shipments. Since the DOE didn’t notify anyone about the shipment they admitted to in court, there seems little reason to believe it hasn’t been shifting around this plutonium surplus without feeling the need to let anyone know. Where is that pile of plutonium? Only Rick Perry knows … which may just up the ante on those sobering thoughts.
Over on the NASA side, this sudden plutonium wealth has benefits that address a genuine issue. The cloudless, endless day of interplanetary space might seem like an ideal place to use solar power, and it’s certainly possible to build space probes that are powered by solar panels and batteries—that’s just how the Juno probe now circling Jupiter operates. But solar power comes, no big surprise, from the sun. And it drops away quickly as probes travel away from the sun (check out the inverse-square law if you want the inside scoop).
The result is that a well-positioned square meter on Earth might get dunked in over 1,000 watts of solar energy (1,300 watts in orbit), but by the time a rover lands on Mars, it can expect no more than a peak of 590 watts at high Martian noon. Jupiter is over 5.2 times farther from the sun than Earth is. As a result, it gets less than 1/27th the solar energy. The solar panels on Juno, at almost 9 meters (29 feet) long and 2.7 meters (9 feet) wide, are about the biggest that NASA can get onto a probe. Large as they are, they still give Juno a peak energy budget that calls for very strict monitoring. For anything going beyond Jupiter, solar power simply doesn’t make for a viable option.
That’s why the Cassini probe, which left for Saturn in 2004, and the New Horizons probe that just swept past Ultima Thule after a journey that started in 2006, were the last NASA probes sent beyond Jupiter. The outer system is just out of reach without something other than solar power to keep the lights on so far from the sun.
NASA has two ways to address this. In addition to a new generation of radiothermal generators using plutonium, it’s also designed a compact nuclear reactor that uses uranium. By using tiny mechanical Stirling engines, the so-called “Kilopower” reactors are actually much more efficient at turning heat into electricity. They’re likely to be too big for most space probes, but might be just the ticket for powering future Martian or lunar bases. After all, while the moon does not have “dark side,” it does have nights that are almost 15 Earth days long.
There is certainly some intrinsic danger in flying nuclear-powered spacecraft. NASA’s designs for radiothermal generators limit the possible consequences, but the idea of a plutonium-powered craft potentially exploding in the upper atmosphere is still enough to have generated protests in the past. But NASA has been flying nuclear-powered probes for more than 50 years, and so have the Russians, who actually put a whole series of nuclear reactors in orbit. And if NASA’s plans include increasing use of nuclear materials, they’re not a patch on what’s coming out of Roscosmos.
According to Russian state TV network RT (no link provided), Russia is proposing a generation of not just nuclear-powered probes, but nuclear-powered rockets, a technology it expects will give it an edge not just on NASA, but on SpaceX and other upstarts.