NASA has said it will allow nuclear power sources, specifically generators using plutonium-238, to be used by scientists on the next batch of low-cost mission ideas.

The agency said it would reverse a decision stopping the use of radioisotope thermoelectric generators (RTGs) in their Discovery class of missions, which have a cost of several hundred million dollars, as plutonium-238 is becoming more readily available.

“NASA’s Planetary Science Division is pleased to announce that the ban on the use of Radio-isotope Power Systems (RPS) by proposers responding to the upcoming Discovery 2018 Announcement of Opportunity (AO) has been removed,” Jim Green, the director of NASA’s planetary science division, said in a statement on March 17.

Plutonium-238 has been used for a range of deep space missions, such as the Voyager probes and the Cassini spacecraft, to power spacecraft far from Earth. It’s the best power source we’ve got at the moment for deep space missions.

The US ceased production of plutonium-238 in 1988, and had to rely on stocks from Russia until about 2010, after which their stockpile started to shrink. But production has now been started again, and it’s expected to be in abundant supply by the 2020s.

NASA had only been considering plutonium-238 for its medium-class (New Frontiers) and upper-class (Flagship) missions, like the Dragonfly proposal to Titan in the former and the Mars 2020 rover in the latter. Now, however, it looks like Discovery missions can make use of it too.

“The status of production of the isotope, as well as projected demands, led [Green] to conclude that it would be feasible to allow the use of radioisotope power systems on the next Discovery mission,” SpaceNews noted.

Last year NASA picked its next two Discovery missions, called Lucy and Psyche, both of which will explore asteroids, which will launch in 2021 and 2022 respectively. In February 2019, it will seek proposals for the next Discovery mission, selecting a winner in 2021 to launch in 2026.

That’s good news for scientists who want to plan bigger and better missions. Plutonium-238 can be used to power radioisotope thermoelectric generators (RTGs), allowing for much more ambitious missions. Other power sources, like solar power, simply don’t have the same punch.

Previous Discovery missions have included the groundbreaking Kepler telescope, which has found thousands of planets outside the Solar System, and the Dawn spacecraft that’s currently in orbit around Ceres, both of which are solar powered. With nuclear now back on the table, we’ll have to wait to see what new low-cost mission ideas scientists come up with.

“Now people who want to propose those deeper space missions, or even a Mars mission that uses nuclear power, can do so,” space consultant Laura Forczyk told The Verge.