For the last five months, it seemed very possible that a 46-year-old conversation had finally reached its end.
Since its launch from Kennedy Space Center on Sept. 5, 1977, NASA’s Voyager 1 spacecraft has diligently sent regular updates to Earth on the health of its systems and data collected from its onboard instruments.
But in November, the craft went quiet.
Voyager 1 is now some 15 billion miles away from Earth. Somewhere in the cold interstellar space between our sun and the closest stars, its flight data system stopped communicating with the part of the probe that allows it to send signals back to Earth. Engineers at the Jet Propulsion Laboratory in La Cañada Flintridge could tell that Voyager 1 was getting its messages, but nothing was coming back.
“We’re to the point where the hardware is starting to age,” said Linda Spilker, the project scientist for Voyager mission. “It’s like working on an antique car, from 15 billion miles away.”
Week after week, engineers sent troubleshooting commands to the spacecraft, each time patiently waiting the 45 hours it takes to get a response here on Earth — 22.5 hours traveling at the speed of light to reach the probe, and 22.5 hours back.
By March, the team had figured out that a memory chip that stored some of the flight data system’s software code had failed, turning the craft’s outgoing communications into gibberish.
A long-distance repair wasn’t possible. There wasn’t enough space anywhere in the system to shift the code in its entirety. So after manually reviewing the code line by line, engineers broke it up and tucked the pieces into the available slots of memory.
They sent a command to Voyager on Thursday. In the early morning hours Saturday, the team gathered around a conference table at JPL: laptops open, coffee and boxes of doughnuts in reach.
At 6:41 a.m., data from the craft showed up on their screens. The fix had worked.
“We went from very quiet and just waiting patiently to cheers and high-fives and big smiles and sighs of relief,” Spilker said. “I’m very happy to once again have a meaningful conversation with Voyager 1.”
Voyager 1 is one of two identical space probes. Voyager 2, launched two weeks before Voyager 1, is now about 13 billion miles from Earth, the two crafts’ trajectories having diverged somewhere around Saturn. (Voyager 2 continued its weekly communications uninterrupted during Voyager 1’s outage.)
They are the farthest-flung human-made objects in the universe, having traveled farther from their home planet than anything else this species has built. The task of keeping communications going grows harder with each passing day. Every 24 hours, Voyager 1 travels 912,000 miles farther away from us. As that distance grows, the signal becomes slower and weaker.
When the probe visited Jupiter in 1979, it was sending back data at a rate of 115.2 kilobits per second, Spilker said. Today, 45 years and more than 14 billion miles later, data comes back at a rate of 40 bits per second.
The team is cautiously optimistic that the probes will stay in contact for three more years, long enough to celebrate the mission’s 50th anniversary in 2027, Spilker said. They could conceivably last until the 2030s.
The conversation can’t last forever. Microscopic bits of silica keep clogging up the thrusters that keep the probes’ antennas pointed toward Earth, which could end communications. The power is running low. Eventually, the day will come when both Voyagers stop transmitting data to Earth, and the first part of their mission ends.
But on the day each craft goes quiet, they begin a new era, one that could potentially last far longer. Each probe is equipped with a metallic album cover containing a Golden Record, a gold-plated copper disk inscribed with sounds and images meant to describe the species that built the Voyagers and the planet they came from.
Erosion in space is negligible; the images could be readable for another billion years or more. Should any other intelligent life form encounter one of the Voyager probes and have a means of retrieving the data from the record, they will at the very least have a chance to figure out who sent them — even if our species is by that time long gone.