The eyes of the world might be fixed upon Mars, where last week alone, the Ingenuity helicopter took flight and the Perseverance rover made oxygen. But farther—much farther—Voyager 1, one of the oldest space probes and the most distant human-made object from Earth, is still doing science.
The probe is well into the fourth decade of its mission, and it hasn’t come near a planet since it flew past Saturn in 1980. But even as it drifts farther and farther from a dimming sun, it’s still sending information back to Earth, as scientists recently reported in The Astrophysical Journal.
For decades, Voyager has been sailing away at around 11 miles (17 kilometers) every second. Each year, it travels another 3.5 AU (the distance between Earth and the sun) away from us. Now, it’s sending messages home even as it prepares to leave this solar system behind.
There are multiple ways to think about the “edge of the solar system.” One is a boundary region called the heliopause. That’s the frontier where the solar wind (the soup of charged particles continually thrown off by the sun) is too weak to hold off the interstellar medium—the plasma, dust, and radiation that fill the bulk of space.
When Voyager 1 left Earth in 1977, nobody was certain where the heliopause was, according to Bill Kurth, an astrophysicist at the University of Iowa who has been working with Voyager 1 since before it launched. Some scientists then even thought the heliopause was as close as 10 or even 5 AU—around the orbits of Jupiter, which Voyager 1 passed in 1979, or Saturn.
In reality, the heliopause is around 120 AU away. We know this partly because Voyager 1 crossed the heliopause in August 2012, a whole three and a half decades after it departed Earth. That puts the probe well and truly in interstellar space.
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Out here, space is filled with interstellar medium—but you’ll not see very much of it. A cube of air at sea level on Earth contains more than a trillion times as many molecules as an equal-sized cube of even the interstellar medium’s densest parts. The region that Voyager 1 is traversing is sparser still. And for the most part, it’s quiet.
But every few years, as Voyager 1 records more data about the plasma and dust out here, it finds something. For instance, in 2012 and again in 2014, Voyager 1 felt a shock. According to Kurth, what Voyager 1 recorded was a magnetic spike, accompanied by a burst of energetic electrons that caused intense, oscillating electric fields. These shocks are the most distant effects of the sun, rippling outwards even past the heliopause.
What Voyager 1 encountered in 2020 was another jump in magnetic field strength, but without those intense electrical oscillations. Scientists instead think it’s a pressure front, a much more subtle disturbance moving out into the interstellar medium. Voyager 1 previously encountered something like it in 2017.
According to Jon Richardson, an astrophysicist at MIT who wasn’t an author on the paper, this latest finding shows that Voyager 1 is still capable of surprising scientists. Normally, he says, the probe would need to experience a shock in the surrounding plasma to measure its density. But with observations like this one, scientists have found a way to use Voyager 1 to continually monitor that density—over 13 billion miles away from us.
Richardson also says the findings show that Voyager 1 continues to feel the sun’s tendrils, billions of miles past the heliopause. “The sun is still having a major effect,” he says, “far outside the heliosphere.”
Meanwhile, Voyager 1 is still within the sun’s gravitational influence. In about 300 years, scientists expect, Voyager 1 will start to enter the inner edge of the Oort cloud, that shroud of comets which stretches as far as several light-years away.
We’ve never actually seen evidence of the Oort cloud, but sadly, Voyager 1 likely won’t be the one to reveal it. The probe is quite literally living on borrowed time. Plutonium-238, the radioisotope that powers the probe’s generator, has a half-life of about 88 years.
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As a result, Voyager 1 is starting to lose fuel. Scientists are already having to make choices about which parts of the probe they should keep functional. By the mid-2020s, it’s likely that the probe won’t be able to power even a single instrument.
Still, scientists like Kurth hope they can eke the probe’s life out to 2027, the 50th anniversary of its launch. That, Kurth says, is a milestone that none of Voyager 1’s designers could ever have foreseen.