WASHINGTON • Nasa's Voyager 1 spacecraft - cruising interstellar space billions of kilometres from Earth - was back on the right track thanks to thrusters that were fired up for the first time in 37 years.

The unmanned spaceship was launched along with Voyager 2, its twin, more than 40 years ago to explore the outer planets of our solar system, travelling further than any human-made object in history.

But after decades of operation, the "attitude control thrusters" that turn the spacecraft by firing tiny "puffs" had degraded.

The small adjustments are needed to turn Voyager's antenna toward Earth, allowing it to continue sending communications.

"At 13 billion miles (20.8 billion km) from Earth, there's no mechanic shop nearby to get a tune-up," Nasa said in a news release.

Experts at the agency's Jet Propulsion Laboratory (JPL) in California decided to turn to four backup thrusters that were last used on Nov 8, 1980.

"The Voyager flight team dug up decades-old data and examined the software that was coded in an outdated assembler language, to make sure we could safely test the thrusters," said Mr Chris Jones, chief engineer at JPL.

The engineers fired up the thrusters on Tuesday and tested their ability to turn Voyager using 10-millisecond pulses. Then they waited 19 hours, 35 minutes for the test results to arrive at an antenna in Goldstone, California.

Turns out the thrusters worked just fine. Being able to use the backup thrusters means the lifespan of Voyager 1 has been extended by two or three years, said Ms Suzanne Dodd, project manager for Voyager.

Nasa plans to switch over to the formerly dormant thrusters in January. They will likely also conduct similar tests on the backup thrusters on Voyager 2.

Scientists still hear from the Voyager spacecraft daily, and expect to get data for about another decade.

AGENCE FRANCE-PRESSE