Solar panels are next to useless in the outer solar system, so NASA’s Voyager probes rely on nuclear batteries—technology first developed in the 1960s—to keep running billions of miles from Earth.

Powering spacecraft with solar energy may not seem like a challenge, given how intense the Sun’s light can feel on Earth. Spacecraft near the Earth use large solar panels to harness the Sun for the electricity needed to run their communications systems and science instruments.

However, the farther into space you go, the weaker the Sun’s light becomes and the less useful it is for powering systems with solar panels. Even in the inner solar system, spacecraft such as lunar or Mars rovers need alternative power sources.