For the first time in three decades, the U.S. has produced plutonium-238, an isotope that powers NASA's rovers on Mars and fuels missions into the outer solar system and deep space beyond.

NASA is close to running out of the element, so scientists at Tennessee's Oak Ridge National Laboratory have produced a golf-ball sized lump of the radioactive material, 50 grams in weight.

The space agency possesses just enough of the isotope for its planned 2020 Mars mission; after that a proposed mission to Jupiter's moon Europa would use up the rest.

Pu-238 produces heat — as much as 450 degrees Fahrenheit — as it decays, and can be used in thermoelectric generators in spacecraft for long-range missions where solar panels would be impractical.

The New Horizons spacecraft that flew by Pluto earlier this year is carrying around 24 pounds of plutonium dioxide generating around 245 Watts of electricity, according to NASA.

"Radioisotope power systems are a key tool to power the next generation of planetary orbiters, landers and rovers in our quest to unravel the mysteries of the universe," said John Grunsfeld, associate administrator for the agency's Science Mission Directorate.

The new chunk of the isotope produced for NASA is the first Pu-238 created in America since the Department of Energy's Savannah River Plant in South Carolina stopped producing it in the late 1980s.

The renewed production "signals a new renaissance in the exploration of our solar system," Grunsfeld said.

Under an agreement between NASA and the DOE, around 3 pounds of the plutonium isotope will be produced annually at a cost of $15 million, sufficient for one or possibly two deep space exploratory missions every decade.

The small lump of the isotope recently produced will be analyzed for chemical purity and Pu-238 richness before scientists scale up production to the levels NASA requires, Oak Ridge officials said.

"Once we automate and scale up the process, the nation will have a long-range capability to produce radioisotope power systems such as those used by NASA for deep space exploration," said Bob Wham, who leads the project for the lab's Nuclear Security and Isotope Technology Division.