NASA Shelved Its Program Developing the Next Generation of Spaceflight Power

At the heart of every space mission, be it a planetary rover or a deep space probe to the outer reaches of the Solar System, is a power source. In the last few years, NASA’s Planetary Science Division has been working on a new power unit for far flung mission, one that will make good use of plutonium. But last week the agency announced it is halting this work, and this has some serious implications for the future of deep space missions.

Everybody’s favorite Mars rover Curiosity, the twin Voyager spacecraft, the Cassini orbiter currently investigating Saturn, and the New Horizons mission that’s a little over a year from reaching Pluto, are all powered by Radioisotope Thermal Generators (RTGs, though in Curiosity’s case it’s an upgraded Multi-Mission RTG or MMRTG). RTGs use plutonium, non-weapons grade plutonium-238 that was originally a by-product of the Cold War, for power. An onboard supply naturally decays, and the heat this reaction generates is changed into electricity by solid-state thermoelectric converters.

It’s a wonderful way to power a mission. Consider the twin MER rovers Spirit and Opportunity. Both have solar panels, and both have seen their power levels decrease as Martian dust settled on and obscured their solar arrays. Curiosity won’t have that problem. It’s got enough plutonium on board to last for more than a decade without slowing down.

The next wave of nucler power sources NASA has been working on with the Department of Energy are called Advanced Stirling Radioisotope Generators (ASRGs), and they could blow traditional RTGs out of the water. Though these units also turn heat into electricity, they’re designed with moving parts. A piston suspended in helium inside the unit moves a magnet inside a metal coil, generating an electrical current within the wire.

ASRGs, by design, can work either within an atmosphere or the vacuum of space to produce four times the power as an MMRTG from the same amount of Plutonium-238, extending the lifetime of their host rover or probe. Any residual heat can be vented through a radiator unit, and any excess electricity can be shed through electrical resistors. In spite of their moving parts, ASRGs are lighter units, a significant benefit when launching off the Earth.

But, in light of NASA’s current financial situation—the agency’s budget is steadily shrinking while the cost of space exploration remains the same—work on these ASRGs has stopped. NASA’s Planetary Science Division announced that it expects to have an adequate supply of plutonium-238 for radioisotope powered missions for the foreseeable future. For now, flight proven MMRTGs will be at the heart of deep space missions. Technology that has been developed as part of the ASRG program will be transferred to NASA’s Glenn Research Center.

The cancellation of ASRGs is a blow to deep space science. At the moment, NASA is the only space agency to have visited the outer Solar System and all these missions have relied on plutonium. Even the European Space Agency’s Huygens probe that landed on Saturn’s moon Titan reached its target courtesy of NASA’s plutonium-powered Cassini spacecraft. NASA is going back to Jupiter in 2016 with the Juno spacecraft, which is also the first probe to go to visit an outer planet without an RTG. Juno has solar panels, three absolutely massive solar panels.

The United States stopped producing plutonium-238 in 1989; the plutonium that’s powering Curiosity right now was actually bought from Russia. And although the US government announced earlier this year that production will resume, it won’t be at levels that can support a  burgeoning deep space exploration program. America’s production of plutonium for spaceflight will be about 1.5 kilograms per year. As a reference point, New Horizons has about 11 kilograms on board. So this new store combined with what little NASA has access to means we can only hope to see one or maybe two RTG-powered missions fly every decade. With ASRGs, we could see twice as many.

Long term, this means NASA’s going to have to pick and choose it’s deep space missions. We’ll get, for example, either a flagship mission to Europa or a sailing probe on Titan. With ASRGs, we could possibly have both. Of course, there are more variables preventing us from having a burgeoning deep space exploration program, like the cost of flight hardware, launch vehicles, and staff that have to see a mission through to the end. ASRGs wouldn’t solve all these problems, but at least they would be a step in the right, outward direction.