NASA’s Voyager 1 spacecraft continues its journey into interstellar space and just got help from on-board thrusters that have not been used for 37 years. The four Aerojet Rocketdyne MR-103 trajectory correction maneuver (TCM) engines were last fired in 1980 when the spacecraft was passing Saturn. On Tuesday, they were needed again and worked just fine.

Voyager 1 was launched in 1977 and flew past Jupiter and Saturn in 1979 and 1980 respectively. It then headed out into the further reaches of our solar system and in August 2012 passed into interstellar space. It is now 13 billion miles from Earth.

The spacecraft needs to be properly oriented in order to continue communicating back to Earth. Small thrusters on the spacecraft keep it the correct position (“attitude”). NASA’s Jet Propulsion Laboratory (JPL) said today that engineers began noticing in 2014 that the attitude control thrusters were degrading. JPL operates Voyager and the Deep Space Network (DSN) that receives the signals.

As JPL said, “at 13 billion miles from Earth, there’s no mechanic shop nearby to get a tune-up.”

The Voyager team decided to try using the TCM thrusters, which were designed to accurately point the spacecraft as it passed Jupiter and Saturn and their moons. They are identical to the attitude control thrusters and located on the back of the spacecraft. The last time they were needed was when Voyager 1 passed Saturn on November 8, 1980. They have been dormant since then.

After reviewing decades-old data and software “that was coded in an outdated assembler language,” JPL engineers, led by JPL Chief Engineer Chris Jones, determined it was safe to attempt to fire them.

The one-way communication travel time between Voyager 1 and Earth is 19 hours and 35 minutes. On Tuesday, JPL engineers commanded the thrusters to fire and waited for Voyager’s response. It was received on November 29 at the DSN’s antenna in Goldstone, CA.

The thrusters worked just fine.

Suzanne Dodd, Voyager’s project manager at JPL, said that the ability to use those thrusters will extend the spacecraft’s life “by two to three years.”

Voyager 1 has a twin, Voyager 2, that flew past not only Jupiter and Saturn, but Uranus and Neptune, and is now headed out of the solar system on a different path. JPL says Voyager 2’s attitude control thrusters are still fine, but they will likely do a test of its TCM thrusters to determine their status. Voyager 2 is expected to leave the solar system and enter interstellar space in the next few years.

The Voyagers will continue on their journeys forever, but their ability to communicate back to Earth to tell the tales of their exploration is limited not only by the thrusters, but by the power sources that provide electricity and warmth to keep the instruments operating.

The spacecraft are powered by radioisotope thermal generators (RTGs) that generate heat from the decay of plutonium-238. The heat passes through a thermocouple to create electricity. (They are not nuclear reactors.) The RTGs lose about 4 watts of power each year and the spacecraft are moving further away. Eventually the power level will be too low for signals to reach Earth.