JPL tries to stop Voyager space probes from disconnecting world’s longest phone call
It’s been 45 years since NASA’s Voyager spacecraft lifted off from Earth, but the twin explorers are still calling home billions of miles away.
“We go ‘Hello, how are you?’ call once a week,” said Suzanne Dodd, project manager for the long-duration mission at La Cañada Flintridge’s Jet Propulsion Laboratory.
The recordings give Voyager 1 and Voyager 2 a chance to share their exact locations on the far side of the heliosphere, a remote region of the solar system whose magnetic field shields Earth and other planets from galactic cosmic rays.
It was during one such call in May that Voyager 1 sent a confusing signal.
Data from the computer that controls its orientation “came back as jumbled bits, jumbled ones and zeros,” Dodd said. And it continued to sound like gibberish.
“It’s as if the check engine light had come on,” added Bruce Waggoner, a JPL engineer who oversees Voyager mission operations. “We couldn’t isolate it to a specific area.”
This computer is essential because it keeps Voyager 1’s communications antenna pointed firmly in the direction of Earth. Any malfunction or loss of power would cut off mankind’s longest phone call forever.
Voyager 1 is over 22 billion kilometers from Earth. It launched from Kennedy Space Center in 1977, arrived at Jupiter in 1979, and reached Saturn in 1980. And then it kept going.
By 1998, it had become the most distant man-made object, flying farther from the sun than the Pioneer 10 space probe. It left the heliosphere and entered interstellar space in 2012 (although scientists haven’t could confirm this only in 2013). Voyager 2 followed in 2018.
Bill Nye, chief executive of the Planetary Society, called the two spacecraft “the vanguard of human intellect and treasure,” ranking them alongside decoding the human genome and formulating the theory of general relativity as leading scientific achievements.
“What’s unique about the Voyager missions is how much they’ve inspired people for half a century,” he said.
Voyager 1 is now so far away that it takes nearly 22 hours for transmissions from the craft to reach us – traveling at the speed of light.
They are worth the wait. The dispatches contain valuable scientific data on interstellar magnetic fields, cosmic rays and plasma waves.
Transmissions from the Voyagers are received by the Deep Space Network, a trio of colossal radio antennae in the Mojave Desert of California, Australia, and Spain. They are spread across the world to ensure that at least one of them can aim for any point in the sky.
All three sites have a 230-foot antenna specially designed to listen to Voyagers. The farther they go, the harder they are to hear.
The Voyagers’ radios transmit signals at only 23 watts of power. By the time these signals reach Earth, they are reduced to the faintest of whispers, barely a billionth of a watt.
Spaceships are also weakening. Each year, their batteries lose up to 4 watts of power due to the decay of plutonium-238, the radioisotope that powers them. (Solar power is not an option because the sun is so far away.)
Survival is a series of compromises. With a finite source of energy, what can be sacrificed? What can we keep?
From their remote locations, the two Voyagers form the perfect laboratory – in fact the only laboratory — to study the interface between a star and its environment.
“It’s really intriguing for me to be able to go into this medium and make measurements and understand what’s going on there,” said Bill Kurth, a heliophysicist at the University of Iowa who studies interstellar plasma waves. “It helps us understand what the environment around other stars might look like.”
But it’s not easy. The signals are so weak that even the Deep Space Network’s 230-foot antennas are too small to hear them. To capture his plasma wave data, Kurth uses it in tandem with three 112-foot antennas, “and that’s barely enough,” he said.
To add to the challenge, the network also juggles dozens of other spacecraft, Mars rovers and now the James Webb Space Telescope. He only communicates with the Voyagers for a few hours at a time.
This is not optimal since the Voyagers have a limited capacity to store the measurements they make, forcing them to transmit their data in a continuous stream. Missing information can be filled in based on context, such as how a baseball fan who missed a few strikes can tell what happened based on who is on the field, score, number of outs, and the reaction of the crowd.
All conversations with Voyagers are weak, intermittent, and slow. If there was an emergency, Wagoner said, “we would have to send out an order, wait two or more days, and then see what it did.”
So when anomalies like the one that started in May occur, scientists need to be extra vigilant.
The first erratic signals suggested something had gone wrong with the attitude control computer that steers Voyager 1 through space. But the team knew the computer was actually doing its job – if the antenna had been pointed in the wrong direction, the Deep Space Network would have seen a degradation in signal strength.
That’s why Dodd wasn’t too bothered by this problem. Nevertheless, sorting was still a high priority.
Perhaps high-energy galactic cosmic rays from which Voyager 1 is no longer shielded knocked atoms off its semiconductor chips and affected the electronics, Dodd said. Or maybe the decades-old computer system is down due to degradation over time.
After several months of investigation, the JPL team identified the culprit this week: Voyager 1’s attitude control system began sending its transmission data through a crashed computer that scrambled the data. The issue was resolved by having the spacecraft revert to using the correct computer.
Why Voyager 1 made the switch in the first place remains a mystery, and one worth resolving as it suggests something else isn’t quite right aboard the spacecraft.
Even if this problem turns out to be inconsequential, Dodd and his colleagues at JPL are aware of an inevitable end-of-mission problem: loss of power. This forces them to play a deep space survival remote game.
“There’s a constant tension between instrument power and thermal management systems on Voyager,” Dodd said.
Voyager 1’s instrument heaters consume a lot of power, so in 2012 scientists began turning off some of them to preserve power for communications and other critical systems. Fortunately, the instruments continued to return data despite operating in much colder conditions than they were designed for.
Over time, mission scientists had to get more creative. They discontinued thrusters that make extremely fine adjustments to control antenna orientation in 2019 and used the more energy-efficient spacecraft navigation thrusters to move the entire probe instead.
“We have tougher decisions to make,” Dodd said. Individual science instruments may need to be turned on and off, operating a few at a time.
Voyager 2, which is 20 billion miles away, faces the same battery issues. In 2020, a safety mechanism triggered by two power-hungry systems operating unexpectedly at the same time shut down all of its scientific instruments for several days. (They work fine now.)
Dodd hopes the spacecraft will continue to speak to us for another five years. “I want to throw this big party for the 50th,” she said, an anniversary the mission would hit in 2027.
It will be a difficult day when the queue for the last Voyager is cut.
“We’ll do everything we can to figure out what’s wrong,” said JPL planetary scientist Linda Spilker, assistant project scientist for the Voyager mission. “But at some point we’ll just have to realize, OK, maybe something really broke this time.”