NASA's Voyager finally phoned home with a device unused since 1981

At 15.4 billion miles away from Earth in interstellar space, Voyager 1 won't last much longer. 

In fact, NASA's flight engineers may have thought the 47-year-old mission had finally kicked the bucket when the uncrewed spacecraft recently went quiet. The probe had shut off its main radio transmitter for communicating with mission control. 

Voyager's problem began on Oct. 16, when flight controllers sent the robotic explorer a somewhat routine command to turn on a heater. Two days later, when NASA expected to receive a response from the spacecraft, the team learned something tripped Voyager's fault protection system, which turned off its X-band transmitter. By Oct. 19, communication had altogether stopped. 

The flight team was not optimistic. 

However, Voyager 1 was equipped with a backup that relies on a different, albeit significantly fainter, frequency. No one knew if the second radio transmitter could still work, given the aging spacecraft's extreme distance. Days later, engineers with the Deep Space Network, a system of three enormous radio dish arrays on Earth, found the signal whispering back over the S-band transmitter. The device hadn't been used since 1981, according to NASA. 

"The team is now working to gather information that will help them figure out what happened and return Voyager 1 to normal operations," NASA said in a recent mission update. 

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Both Voyager 1 and its twin, Voyager 2, have been bopping along for nearly a half-century, far beyond their original life expectancy. Launched in 1977, the pair was originally intended to study Jupiter and Saturn, their moons, and Saturn's rings. For the two-planet journey, they were built to last just five years.

After their initial success, engineers doubled their objectives to include two more giant planets, Uranus and Neptune. Between the two spacecraft, they've explored four planets, 48 moons, and a host of planetary magnetic fields and rings.

In August 2012, Voyager 1 made history as it entered interstellar space, the region between stars, filled with material ejected by other stars that died millions of years ago. Voyager 1 and 2 are the only spacecraft ever to operate outside of the heliosphere, the region of space affected by the sun's constant flow of material.

Voyager 1 is speeding away from the solar system at over 38,000 mph and is the farthest human-made object from Earth. It is so far away that it takes 23 hours for a command to reach the spacecraft, and another 23 hours for mission control to hear back from it.

NASA has previously said the Voyagers generate about 4 fewer watts of power annually, limiting the number of systems the spacecraft can use. Flight controllers have occasionally turned off equipment to conserve power. The goal is to keep the two running beyond 2025, according to the agency.

It's not clear yet why Voyager 1's fault protection system shut off the main radio transmitter. When onboard issues occur, such as the spacecraft overdrawing its power supply, the system will automatically turn less-critical equipment off so that it can continue flying. Based on data from Voyager 1, the spacecraft should have had enough power to flip on the heater without incident. 

Interstellar space is a high-radiation environment that nothing human-made has ever flown in before. That means the only thing the teams running the old probes can count on are surprises. 

Last month the team troubleshot a thruster problem on Voyager 1. At this point in the mission, nothing is easy or guaranteed, said Voyager project manager Suzanne Dodd in a statement. 

"All the decisions we will have to make going forward are going to require a lot more analysis and caution than they once did," she said.