Peak Plutonium is Jeopardizing the Future of Planetary Science

Almost as long as it's been around, NASA has been using nuclear material — specifically plutonium-238 — to power its long-duration interplanetary spacecraft. But the available store of nuclear material is drying up, putting the future of planetary science in jeopardy.

The nuclear reactors in spacecraft are called radioisotope thermoelectric generators (RTG). They harness the heat generated by decaying plutonium-238 oxide and convert it to electrical energy; the heat is harnessed by an array of thermocouples, which are pairs of conductive metal alloys that convert temperature difference between the two metals into a voltage output.

Ralph McNutt, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory and a project scientist for NASA’s Messenger mission to Mercury who also co-chaired the NRC's 2009 review board, puts plutonium's importance into perspective.

“We wouldn’t have 99.9 percent of our knowledge about the outer planets and their systems without plutonium-238,” McNutt said. “It’s just a huge amount that couldn’t have been done. If we are going to keep making the kind of advances that the space community would like to make, and the sort of advances that NASA takes leadership in, we can’t do it without this power supply. Without it, all of that is going to be for naught.”

NASA's use of nuclear power in space started in 1961 with the Satellite Transit 4A. Built for the U.S. Navy, it was one of four navigational satellites that updated the navigation systems on board submarines.

Since then, nuclear RTGs have powered both Pioneer spacecraft that went to Jupiter and Saturn, both Viking Mars landers, both Voyager spacecraft that took the scenic route through out solar system, the Galileo spacecraft that orbited Jupiter, the Cassini spacecraft currently in orbit around Saturn, and the New Horizons spacecraft currently on its way to Pluto. Most recently, the Mars Science Laboratory rover Curiosity was launched to Mars with its own little nuclear generator. It's got about eight pounds of nuclear material on board. On the manned front, Apollo 12 through Apollo 17 carried RTGs to power scientific experiments on the lunar surface. In other words, plutonium has powered just about everything that’s left Earth’s orbit.

But NASA is reaching the end of its plutonium stash. There might only be enough left for one more deep space mission. The National Research Council (NRC) made this clear in 2009 when it issued a statement asserting the importance of nuclear power in space. Plutonium-238 is, and will continue to be,“essential to the U.S. space science and exploration program.” Not much has changed since that statement was made three years ago.

Unfortunately, the U.S. — and indeed the world — is coming close to running out of plutonium-238. It's a byproduct of nuclear weapons, and was last produced during the Cold War. In the 1960s and 1970s, the Department of Energy (DOE) supplied NASA with the material. In the 1980s, NASA began relying on the former Soviet Union, but that store has since dried up.

With plutonium shortages everywhere, NASA staffers are already coming up with some crazy schemes to find more. For example, when Apollo 13 failed to land on the moon, its RTG came back to Earth with its lunar module. Somewhere at the bottom of the Pacific Ocean is a small store of plutonium that some are suggesting be recovered and reused for a future mission. Thorium enthusiasts insist that shifting to a thorium fuel cycle and liquid fuel nuclear reactors wouldn’t produce weaponizable plutonium 239, but ample supplies of plutonium-238.

The exact amount of plutonium-238 still available for interplanetary missions has not been made not public, but in any case the future doesn't look too bright. The NRC released its plan for the next ten years of planetary exploration in March 2011. Among its highest priorities are a sample return mission from Mars and a mission to Jupiter's moon Europa — recent data has confirmed the moon has vast oceans under its icy surface. Both of these missions will need nuclear power to succeed.

Restarting the production of plutonium-238 in the U.S. Is possible, but it would come with a price tag between $50 million and $75 million over five years. NASA's recent 2012 budget included a proposition for a cost-sharing plan between DOE and NASA to fund the work. But funding for NASA and the DOE come from separate congressional subcommittees, and key lawmakers have failed to reach an agreement that would grant the DOE the necessary financing to production of plutonium-238.

Although NASA has researched alternatives, no other power source currently exists that is as safe, reliable, and effective as the plutonium-powered RTGs the organization has been using for years. For now, planetary scientists are focusing on spreading the word, hoping a wider understanding of what is at stake when the material finally runs out will influence the lawmakers who have the ability to fix the problem.