Most spacecraft, including satellites and some Mars landers, rely on solar panels, but for rugged and complex outings like the Mars Curiosity rover mission and extremely long-term flights like the one that just took New Horizons by Pluto and its moons, only power extracted from radioactive sources can provide enough oomph.

NASA uses radioisotope thermoelectric generators (RTGs), a clever combination of material once extracted from the by-products of nuclear weapons and a 200-year-old technique for converting differences in temperature into electricity. RTGs began powering NASA missions in 1969.

How it works

An RTG harnesses heat generated by the radioactive decay of plutonium-238 dioxide, which can be refined from the by-products of the enrichment process required to produce a heavier, more fissile isotope, plutonium-239, for nuclear weapons. The lighter isotope produces very little gamma radiation, which means it requires relatively little shielding in a spacecraft.

As the Pu-238 decays over its nearly 88-year half-life—after 88 years, about half the original mass remains, then half again in each successive 88-year period—its heat is converted to electricity through thermoelectric couples (or thermocouples), which rely on the Seebeck effect. This was first described in 1821 by German physicist Thomas Johann Seebeck, although he didn’t quite realize what he was seeing: that an electric current flows between two metals maintained at different temperatures.

In the type of RTG used in New Horizons and several previous missions, the hot side of the connection averages 1,308 Kelvin (1,894 °F) at launch and the cold side about 566 Kelvin (559 °F). The maximum temperature decreases over time as the fuel decays, contributing to a gradual reduction in electrical output. But enough power remains after years in space to energize multiple science instruments, run computers, and handle long-distance data communications.

Shrinking stockpile

The U.S. Department of Energy used to produce Pu-238 at its Savannah River Site in South Carolina, but it stopped making weapons-grade plutonium in the late 1980s. The government supplemented its dwindling stock by buying plutonium from Russia, but Russia halted shipments in 2009, and may have little or none left in usable form. (Russia doesn’t use RTGs for spaceflight, and neither does the European Space Agency.)

The United States is estimated to have about 37 kilograms of aging Pu-238, but NASA’s planetary science division director, Jim Green, says only about 17 kilograms are in a form that could be used for RTGs. To put the figure in perspective, consider that New Horizons began its mission with 11 kilograms. Curiosity carries a scant 4.8 kilograms, but it also has lithium batteries that the RTG recharges during inactive periods, allowing the Mars rover a big overall energy supply during its daytime movements and experiments.

NASA is funding a Department of Energy effort to develop a new production cycle that involves three national labs—Los Alamos, Oak Ridge, and Idaho. By 2021, the operation hopes to produce 1.5 kilograms of plutonium-238 dioxide per year.

Given the short supply, and the fact that current RTGs are extremely inefficient—they convert just over 6 percent of the heat output into electricity—NASA’s upcoming mission to probe Europa, an icy moon of Jupiter, is expected to be solar-powered. Until recently, it was seen as infeasible to use solar at that distance, but improvements in terrestrial technology have helped. The European Space Agency also has a solar-powered Jupiter mission, JUpiter ICy moons Explorer (JUICE), slated to launch in 2022.

Several efforts are underway to improve thermocouple efficiency by using new materials or entirely reworked RTG designs. A short-term upgrade to the module style used in Curiosity could bump the extracted electricity to 8 percent—thus, a third more power from the same fuel—while longer-term projects might reach as much as 15 percent efficiency.

Alternative approach

Another very old idea for powering spacecraft is now on the shelf, but it may be dusted off. For years, NASA funded development of the Advanced Stirling Radioisotope Generator (ASRG), which, like the Seebeck effect, relies on a 200-year-old idea. A Stirling engine generates electricity from a piston powered by a heat differential, just like a thermocouple, but with some mechanical parts. The piston in this design floats in helium to prevent physical wear.

The ASRG could have four times the efficiency of current RTGs, or about 26 percent. This would stretch scarce Pu-238 much further. It’s not just theoretical, either: a prototype of the ASRG has been running in a lab for a decade without fail. The only trouble is that the ASRG’s budget was cut in 2013. NASA’s planetary sciences chief Green is eager to restart the work.

The Takeaway:

Solar will play a bigger part in the future of planetary exploration. But there remain many points that craft with just solar panels cannot reach, and many scientific instruments require more energy than is feasible or economical to run via solar panels. RTGs are required to reach Saturn and beyond, explore lunar craters, and visit Mercury’s dark side, and allow ambitious, expansive science on the scale of the Voyagers.

The next Pluto craft will have plutonium on board. It’s a matter of making sure there’s enough and it’s well used.

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