NASA is switching off two science instruments on its long-running twin Voyager spacecraft to save power.

The space agency said Wednesday an instrument on Voyager 2 that measures charged particles and cosmic rays will shut off later this month. Last week, NASA powered down an instrument on Voyager 1 designed to study cosmic rays.

The energy-saving moves were necessary to extend their missions, Voyager project manager Suzanne Dodd at the NASA Jet Propulsion Laboratory said in a statement.

The twin spacecraft launched in 1977 and are currently in interstellar space, or the space between stars. Voyager 1 discovered a thin ring around Jupiter and several of Saturn’s moons, and Voyager 2 is the only spacecraft to visit Uranus and Neptune.

Each spacecraft still has three instruments apiece to study the sun’s protective bubble and the swath of space beyond.

Voyager 1 is over 15 billion miles (24.14 billion kilometres) from Earth and Voyager 2 is over 13 billion miles (20.92 billion kilometres) away.