NASA is running out of plutonium to use for powering spacecraft on deep space missions, far from the rays of the sun, NASA Administrator Michael Griffin said.

Very little of the nuclear material remains in supply, and it isn’t being produced to replace that already used, Griffin explained to reporters after he testified at a Capitol Hill hearing.

Many spacecraft don’t require such fuel, because they can use sunlight shining on photovoltaic arrays to generate electricity.

But some spacecraft, those used in deep space missions, venture so far from the sun that it isn’t a viable source of electrical power.