Voyager 1 at the Threshold of Forever

A 35-year old spacecraft prepares to exit the solar system and enter interstellar space

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SSPL / Getty Images

Voyager 1, launched on 5th September 1977.

Ed Stone has held a lot of jobs in the past 40 years, both at the Jet Propulsion Laboratory (JPL) and Caltech, but there’s one he’s not giving up: project scientist for the Voyager 1 and 2 space probes. Stone was named to the post in 1972, five years before the Voyagers even left Earth, and in the decades since, he’s seen his space babies reconnoiter Jupiter, Saturn, Uranus and Neptune, and then fly on and on through the solar system—on track, eventually, to be the first human-made machines to enter interstellar space. At a news conference on Monday, JPL officials revealed that one of the ships—Voyager 1—is tantalizingly close to that goal, and Stone was on hand for the announcement.

(Photos: Voyager, 11 Billion Years Later)

Voyager 1, which flew on the flat past Jupiter and Saturn and then swung up and above the plane of the solar system in 1981, is now about 11 billion miles (18 billion km) from home. Voyager 2, which never made such a due north turn and thus flew past all four gas giants, is 9 billion miles (15 billion km distant). It was Voyager 1 that was thus the focus of Monday’s report, mostly because of what its few still-functioning instruments are telling it about the magnetic field through which its traveling.

The solar system exists within what’s known as the heliosphere—a vast bubble of charged particles given off by the sun. That high-energy gale grows weaker and weaker as it moves away from the center of the solar system, slowing from supersonic speed to what’s effectively a standstill as the solar particles bump up against the outside particles of the interstellar medium. The point at which they begin to slow is known as the termination shock, which Voyager 1 crossed in 2004. That took it into the final zone of the solar system—the heliosheath—where solar wind particles swirl about chaotically. No one knows how thick the heliosheath is, but only when Voyager 1 emerges on the other side will it have have truly sailed out among the stars.

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“We believe this is the last leg of our journey to interstellar space,” said Stone at Monday’s press conference. “Our best guess is that it’s likely just a few months to a couple of years away.”

That best guess got better, however, thanks to Voyager 1’s new magnetism reports. Even as the particles around the spacecraft continue their random dance, the magnetic field within the heliosphere is now coupling to the magnetic field outside, allowing low-energy charged particles from the solar system to exit and high-energy ones from outside to enter. The actual alignment of the sun’s magnetic field has not changed — yet — but the way it plugs into the larger universe has.

“Although Voyager 1 is still inside the sun’s environment,” said Stone, “we can now taste what it’s like on the outside because the particles are zipping in and out on this magnetic highway. The new region isn’t what we expected, but we’ve come to expect the unexpected from Voyager.”

(More: After 34 Years in Space, the Voyager Spacecraft Fly On — and On and On)

One more unknown is just what the environment surrounding the ship will look like when it does punch through the film of the heliosheath, but that’s becoming clearer too. For about a month in July and August, the solar wind came to a dead stop—the threshold known as the heliopause. That should have been it, the boundary should have been crossed. But the edge of the solar system is apparently a ragged thing, and the wind stirred and fell silent a few times before resuming its motion. “If we were judging by the charged particle data alone, I would have thought we were outside the heliosphere,” said Stamatios Krimigis, a Voyager principal investigator. But they weren’t judging by the charged particles alone; the ship remained on the newly discovered magnetic highway—still driving along the sun’s field lines, albeit ones that are beginning to merge with deeper, more distant ones.

When the first, and then the second of Stone’s Voyagers do sail into the interstellar region for good, he will still be able to remain in touch with them. The spacecrafts’ radiothermal generators — essentially on-board nuclear power plants — left Earth with a warranty good for 50 years. That means Voyagers 1 and 2 could still be sending back data in 2027, before finally going cold and dark and continuing their eons-long journey well and truly alone.

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