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Tuesday, July 16, 2024

How NASA Nearly Lost the Voyager 2 Spacecraft Forever

When Suzanne Dodd’s team transmitted a routine command to Voyager 2 on July 21, the unthinkable happened: They accidentally sent the wrong version, which pointed the interstellar probe’s antenna slightly away from Earth. When they next expected to receive data, they heard nothing at all. The small error almost made humanity lose its connection with the popular spacecraft, which is now 12.4 billion miles from home. Along with its twin, Voyager 1, it is humanity’s farthest-flung spacecraft that is still collecting data.

Here’s what happened: Dodd’s team at NASA’s Jet Propulsion Laboratory had actually spotted the error in the command and corrected it—but then mistakenly sent out the flawed version. “It felt awful. It was a moment of panic, because we were 2 degrees off point, which was substantial,” says Dodd, the project manager of the Voyager interstellar mission.

The team settled on a solution: Blast a “shout” command in the probe’s direction, telling it to adjust the antenna back toward Earth. If the signal was strong enough, the craft could still receive it, even though its antenna was offset. 

On the morning of August 2, they sent the highest-power signal they could, using the high-elevation, 70-meter, 100-kilowatt S-band transmitter at the communication station in Canberra, Australia. The station is part of NASA’s Deep Space Network, an international system of giant antennas managed by JPL. (Because of Voyager 2’s trajectory, one can only communicate with it via telescopes in Earth’s southern hemisphere.) 

There was no guarantee of success, and it would take 37 hours to see if the solution had worked: The time it would take for their signal to ping the craft, and then—if they were lucky—for a signal from Voyager 2 to ping them back. 

The team spent a sleepless night waiting. And then, relief: It worked. Contact was restored on August 3 at 9:30 pm Pacific time. “We went from ‘Oh my gosh, this happened’ to ‘It’s wonderful, we’re back,’” says Linda Spilker, Voyager’s project scientist at JPL.

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Had the attempt failed, the team would have only had a single backup option left: the onboard flight software’s fault protection routine. Multiple fail-safes were programmed into the Voyagers to automatically take actions to deal with circumstances that could harm the mission. The next routine was expected to kick in in mid-October. If it worked, it would have generated a correct pointing command, hopefully adjusting the antenna in the right direction.

The Voyagers have been flying since the late 1970s—they’re turning 46 in a couple weeks—and as Spilker points out, “that was a two-week period with no science data, the longest period of time without it.” In the 2010s, they crossed the heliopause, the boundary between the solar wind and the interstellar wind. Since then, they’ve been taking data on the edge of the heliosphere, the protective bubble of particles and magnetic fields generated by the sun, which interacts in unknown ways with the interstellar medium. 

Still, that two-week period without contact didn’t interrupt the team’s scientific work. “The Voyager science isn’t something you need to monitor constantly,” Calla Cofield, a JPL spokesperson, told WIRED via email. “They’re studying this region of space over long distances, so a gap of a few weeks won’t hurt those studies.”

While the NASA team managed to restore communications with the beloved spacecraft, it won’t be the last scare. The Voyagers have aged well past their prime, and their dwindling power means that their scientific instruments can be run for only a few more years.

The Voyager probes are powered by radioisotope thermoelectric generators, which work by converting heat from the decay of radioisotope fuel, plutonium-238, into electricity. (Such far-flying spacecraft are typically designed to run on nuclear power, rather than on solar, which doesn’t work that far from the sun.) But the fuel source has been decaying over a long time. To conserve power, the Voyager team has already shut down the heaters for all of the scientific instruments—which, despite that, have continued operating normally. In the coming years, the magnetometer, plasma wave surveyor, and charged particle detector will themselves have to be shut down, one at a time, to ensure a few more years of interstellar science.

“Everybody’s exuberant that we have the spacecraft back,” Dodd says. “It makes me realize that this mission could end anytime, whether there's human error involved, or just because the spacecraft is old and it breaks. It makes you value what you have.”

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