The missions that humankind has sent farthest into space, a pair of NASA spacecraft called the Voyagers, are billions of miles from Earth. The last time one of them took a picture of its surroundings was in 1990, after flying by Jupiter, Saturn, Neptune, and Uranus on its way to interstellar space, the mysterious expanse between stars. This far beyond the planets, there’s not much to see.
But there are some things to feel, in the sense that a spacefaring machine can feel something. Even this far out, the sun can still make its presence known.
A team of scientists has detected sudden bursts of cosmic rays around the Voyagers. The bursts, they report, are caused by shock waves emanating from solar eruptions that spew particles out at a million miles an hour. The shock waves take more than a year to reach the Voyagers, but when they do, they excite cosmic-ray electrons nearby. Scientists have observed similar phenomena closer to home, around Earth and our planetary neighbors, but never in interstellar space.
“We’re really discovering that what we thought would be this quiet, pristine interstellar medium is actually disturbed considerably” by the sun, Don Gurnett, a University of Iowa professor emeritus of physics and astronomy, who led the research, told me.
More than 40 years ago, Gurnett designed and built one of the instruments on the Voyager mission that can sense such things. Voyager 1 crossed into interstellar space in 2012, and Voyager 2 followed in 2018. But the spacecraft actually haven’t left the solar system, despite many headlines over the years claiming that they have. This might seem, at first glance, a little contradictory—how can something exist in the space between stars and within the solar system at the same time? Aren’t those two different things?
From our perspective, interstellar space begins when sun particles can’t go any farther. The sun releases a steady current of high-energy particles in all directions, all the time, and this solar wind encompasses the planets, their moons, and other celestial bodies in a protective bubble called the heliosphere. Scientists had predicted that the breeze would stop where it met the cold particles of the interstellar medium, which is sprinkled with material left behind by supernovas, the deaths of other stars. But they didn’t know exactly where this sphere of the sun’s influence stopped until 2012, when Voyager 1 detected the beginning of a different cosmic environment. “It’s not impossible, but it’s very difficult for solar plasmas to cross that boundary,” Bill Kurth, a research scientist at the University of Iowa and Gurnett’s co-author on the new findings, told me.
This is where the Voyagers are, beyond the heliosphere. Kurth once published a commentary in a science journal that said leaving the heliosphere was more or less the same as leaving the solar system. “I was soundly criticized,” he said, laughing. Because while the solar wind blows quite far—120 astronomical units, with a single unit equal to the distance between the Earth and the sun—our star’s influence extends even deeper. Not through warmth, but through gravity.
The sun’s gravity can keep objects in its orbit far beyond where the heliosphere ends. As the Voyagers continue on their journey, eventually they will enter the Oort cloud, a region of icy objects past Pluto. Because those objects are gravitationally bound to the sun, they still count as ours. This is where the solar system truly ends—past the far edge of the Oort cloud, which is somewhere between 10,000 and 100,000 astronomical units away. “Even though Voyager 1’s out and beyond 150 astronomical units, it’s got a long, long ways to go before it gets beyond the Oort cloud,” Kurth says. It will be another few hundred years before the Voyagers reach this region, and tens of thousands more before they pass through to the other side.
When the Voyagers launched in 1977, the notion of doing science so far from our own planet, out in interstellar space, was a distant thought. NASA was focused on swinging by our neighboring planets and moons to collect valuable data and beautiful pictures. After the grand tour, the spacecraft just kept going. In the years since, mission managers at NASA’s Jet Propulsion Laboratory have turned off various components on the two spacecraft, from science instruments to heaters, rationing every watt of power to keep the machines going. Someday, engineers may be forced to turn off one of the elements that help the spacecraft communicate with Earth, a process that takes about 20 hours each way. It’s a risky move. “If it does work, then we gain two more watts,” Suzanne Dodd, the Voyager project manager, told me last year. “If it doesn’t, then we lose the mission."
For now, the Voyagers still call home with dispatches from the unknown. Engineers expect that the spacecraft will go silent sometime in the next decade. Even then, they will keep moving, unlikely to crash into something in the vastness of space. About 5 billion years from now, when our sun dies in a dramatic process that will engulf our entire planet, the Voyagers will probably still be coasting through the galaxy. “At some point in time, the Voyagers could be the only remaining evidence that Earth was around,” Kurth said.
When the Voyagers do finally leave the solar system, they will enter, for the first time, a stretch of space that does not play by the sun’s rules. There, the sun will become just another star in someone else’s night sky. Its warmth or gravity won’t register, but its glow will radiate for light-years.