NASA is giving scientists more choices for how to power their future spacecraft to explore the Solar System. Researchers proposing spacecraft ideas for NASA’s Discovery program — an initiative to develop deep-space missions that usually cost less than half a billion dollars — will be allowed to incorporate a special kind of radioactive battery in the designs for their vehicles. And that could potentially allow these missions to get more science done and go deeper into space.

Discovery proposals can now incorporate a type of power system known as a radioisotope thermoelectric generators, or RTGs. These generators are powered by radioactive material — a type of metal called plutonium-238. The metal naturally decays over time, producing heat that is then converted into electrical energy.

“Any kind of limitation that is placed on these missions means some science cannot be done.”

NASA has been using RTGs to power some of its spacecraft since the 1960s. However, NASA banned the use of these systems in the upcoming proposals for the Discovery program, since the United States has had a very limited supply of plutonium-238. Now it seems the US may have enough of the material to spare for the program. After consulting with the Department of Energy, NASA decided to lift the ban on RTGs for the Discovery program, according to a memo sent out over the weekend by James Green, the director of NASA’s Planetary Science Division.

The Department of Energy said that its recent success in producing more plutonium-238 in the US helped NASA make this decision. DOE’s Oak Ridge National Laboratory in Tennessee has produced 350 grams of the material, some of which will be incorporated in NASA’s next Mars rover. “These recent successes have reduced risks associated with future plutonium supplies and factored into NASA’s announcement last week to include radioisotope power systems in its Discovery 2018 Announcement of Opportunity,” a spokesperson for DOE said in a statement to The Verge. “The Department is committed to supporting NASA’s efforts to make radioisotope power systems available to support its space exploration goals.”

The move is a big deal for scientists since power is an essential ingredient of any space mission — and nuclear power could open up doors for how much more their missions can do in space. “Any kind of limitation that is placed on these missions means some science cannot be done,” Laura Forczyk, a space consultant and owner of space research and consulting firm Astralytical, tells The Verge.

Plus, there are only so many ways to keep a spacecraft alive in the first place. One option is to use onboard batteries or fuel cells, which release stored energy that’s created here on Earth. But the power from such batteries is finite, and vehicles often need to last for many years in space. That’s why most spacecraft are equipped with solar panels, which continually collect light from the Sun and convert it into electrical energy. Solar power is crucial for spacecraft operating near Earth or in the inner Solar System, where sunlight is strong and abundant.

But for vehicles going into deep space, solar just doesn’t really cut it. Sunlight gets weaker the farther you travel out into the Solar System, making it hard for distant travelers to rely on solar power alone. So many deep-space vehicles use RTGs instead. These nuclear batteries are currently powering NASA’s most far out vehicles, such as the New Horizons spacecraft, which flew by Pluto in 2015, as well as the Voyager spacecraft that are cruising beyond our Solar System. It’s proven to be a particularly efficient form of power production for interplanetary missions.

for vehicles going into deep space, solar just doesn’t really cut it

But NASA had to distance itself from RTGs a bit, as plutonium-238 has become a scarce resource over the last 30 years. The United States government used to produce its own plutonium-238, or Pu-238. Another slightly heavier version of plutonium, Pu-239, is the key ingredient in nuclear weapons. But production of Pu-238 stopped in 1988 as the Cold War came to an end. So for decades, the US had to get the material from Russia. But Russia stopped producing the metal, too, and now NASA has been faced with a dwindling precious resource.

However, things have changed in recent years. The Oak Ridge National Laboratory restarted Pu-238 production with NASA funding in 2015 to help power future space missions. Still, it will be a while before the crisis is completely over. Pu-238 takes a long time to make and the material isn’t produced in super large quantities. The US’s current goal with this new production line is to make 1.5kg of Pu-238 a year by 2026. However, a recent audit from the Government Accountability Office identified a few challenges that might hamper that goal from being realized.

Meanwhile, there is only about 35 kilograms of Pu-238 available that NASA can use for its upcoming missions, and only half of it is spaceflight-ready. The material is mostly being reserved for use in the next Martian rover, called Mars 2020, and NASA is reserving some more for the fourth mission in the New Frontiers program, which hasn’t been chosen yet. There are two finalists for the mission: a spacecraft to explore Saturn’s moon Titan and a vehicle that will return samples from a comet. One relies on RTG technology, while the other uses solar panels.

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

Not being able to use RTGs could have significantly limited the types of designs researchers came up with for Discovery. When NASA announced its intent to select a new mission for the program at the end of 2017, the agency said proposals could focus on exploring any body of the Solar System — except for the Earth and the Sun. Without an RTG, it would be tough to power the instruments on a mission going far beyond Mars. It can be done, as NASA’s Juno mission to Jupiter has demonstrated, but an RTG might allow for more options of what kind of instruments the spacecraft can hold. If you don’t have to plan for solar panels, that changes the entire design of the spacecraft.

“You want those kinds of options,” Forczyk says. “That’s the big story as to why everyone’s excited now that the ban has been lifted. Now people who want to propose those deeper space missions, or even a Mars mission that uses nuclear power, can do so.”

The change may be possible since there is a while before the new Discovery mission has to be ready, allowing time for more Pu-238 to be produced. NASA plans to post a final draft of the Discovery program’s call for submissions in 2019, and then the finalist will be chosen in 2021. The target launch date for the mission is some time before the end of 2026.