NASA's Voyager 1 mysteriously lost touch with Earth

NASA's farthest spacecraft gave mission control a scare when it inexplicably went silent and missed two days of planned check-ins. 

The nail-biting incident happened just two weeks ago, when the Deep Space Network didn't receive a call from Voyager 1 on either Oct. 18 or 19. But before the agency could send repair commands, the spacecraft, now outside the solar system, signaled back on Oct. 20. No communication outages have happened since. 

Engineers still don't know what caused the problem. So far, the team has discovered that one set of thrusters shut down, leaving a backup set to take over. The thrusters are small engines that point and steady the spacecraft. Voyager 1's computers do this automatically whenever they detect an issue with the active thrusters to ensure the spacecraft is in a safe mode. 

"This is the first time we've seen the spacecraft do something like this, but we are also at a point in the mission where we have to expect the unexpected," said Kareem Badaruddin, Voyager mission manager at NASA's Jet Propulsion Laboratory, in a statement. "We're relieved to be back in contact with Voyager 1 and are eager to understand what happened."

Voyager 1's trouble adds to a growing list of signs that NASA's longest-running mission is winding down. Its plutonium power source loses about four watts every year, forcing engineers to turn off two science instruments to conserve power. On Nov. 18, the spacecraft is expected to reach a historic milestone of one "light-day" from Earth. That distance, roughly 16.09 billion miles away, measures how far light travels over 24 hours.

Voyager 1 has traveled farther than any other human-made object and is racing through space at 38,000 mph.

Launched in 1977 to survey the outer planets, the spacecraft and its twin Voyager 2 have explored four planets — Jupiter, Saturn, Neptune, and Uranus — 48 moons, and a host of planetary magnetic fields and rings.

In August 2012, Voyager 1 was the first to enter interstellar space. The two remain the only spacecraft ever to leave the solar system, though New Horizons is trying to make its escape as well.  

Both Voyagers run on radioisotope thermoelectric generators, which are power units that turn heat from decaying plutonium into electricity. In September, NASA engineers stretched their shrinking power supplies by turning off three instruments on each and replacing them with three lower-powered alternatives — a plan they nicknamed "the Big Bang." The devices involved in the swap also help keep the spacecraft warm enough to work in the freezing cold of space, so the swaps had to happen at the same moment to spread heat evenly.

That strategy should keep their Cosmic Ray Subsystems, Magnetometers, and Plasma Wave Subsystems running for at least one more year, according to NASA. The devices measure high‑energy particles, the local magnetic field, and ripples in charged gas around the spacecraft.

Once Voyager 1 passes one light day, a radio message from Earth will take 24 hours to reach it, and its reply will take another 24 hours to come back.

"If you want to say 'hello' to Voyager, you send your message and then you go home and you wait two days before you finally get a response," said Suzanne Dodd, Voyager project manager.