Voyager 1 is the Barbra Streisand of spacecraft: the farewell tour is eternal, but it never actually leaves. Babs is very much still with us, as my colleague Joel Stein demonstrated when he and The Divine Miss S took a one-hour road trip from Malibu to Beverly Hills last year. Voyager 1 is very much still with us too, but it’s been on the road a little longer (more than 35 years now) and has put a few more clicks on the odometer—11.5 billion miles (18.5 billion km) so far.
It’s been a long time since Voyager 1 actually had a planet to look at—since 1980, when it reconnoitered Saturn, then whipped up above the plane of the plane of the solar system and began heading into the void. Its companion ship, Voyager 2, flew on the flat until it passed Neptune in 1989, then it too headed into the true deep waters of space. It was at that point that the title of the Voyager program was changed from the Voyager Outer Planets Mission to the Voyager Interstellar Mission. One thing was certain, both spacecraft would eventually leave the solar system and indeed head into the realm of the stars, but what was unclear was when. The answer is: not just yet, but Voyager 1 is getting very, very close, as a new paper in Science has just revealed.
The boundary of the solar system is by no means defined by the orbit of the last planet or even the more remote Kuiper Belt and Oort Cloud, the vast swarm of icy bodies that lie at an even further remove. Rather, it’s defined by the heliosphere, the vast windstorm of charged particles that stream outward from the sun. The end of the helioshpere is known as the heliosheath—where the outward-streaming charged particles bump up ageainst the inward streaming particles of deep space and come to a stop, and where the lines of solar magnetism change their orientation to match those of interstellar space.
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As I reported in March, there was a brief dust-up in the astronomical community when the American Geophysical Union (AGU) announced that Elvis had at last left the building—that Voyager 1 had indeed punched through the heliosheath. But the AGU quickly had to pull back from the claim. NASA, which built, launched and has operated the Voyagers throughout their long lives, acknowledged that new data showed that the ship had entered a significant new region of the solar system where the outward-flowing particles effectively stopped and the ones flowing in from interstellar space had taken over. But the magnetic field lines had not yet made the necessary shift. Said Ed Stone, Voyager’s longtime project manager during that debate:
“The Voyager team is aware of reports today that NASA’s Voyager 1 has left the solar system. It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space.”
That status hasn’t changed, but the new Science paper—actually a collection of three papers—better explains the ambiguous region in which Voyager 1 now finds itself. Reanalyzing data from last year, scientists determined that the spacecraft is now in a region it calls the heliosphere depletion region. The previously unidentified zone is characterized by a dramatic spike in cosmic rays—the exotic stuff that streams in from the stars—and an extreme decline in particles from the sun. So far, about what was expected—even if the measurements are better than ever.
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Much more significantly, the magnetic field lines at last began to change significantly, with particles following the magnetic lines flowing out from the sun all but disappearing, meaning that solar magnetism was at last losing its influence and being overtaken by the interstellar magnetism. But the magnetic lines still did not change their direction, as they would have to if they were truly locking into the grid of deeper space. In a statement, Leonard Burlaga of NASA’s Goddard Space Flight Center in Greenbelt, Md., the lead author of one of the papers said:
“…the magnetic field suddenly [doubled], becoming extraordinarily smooth. But since there was no significant change in the magnetic field direction, we’re still observing the field lines originating at the sun.”
Making things messier still, the spacecraft crossed into and out of the heliosphere depletion region no fewer than five times in 2012. Since the spacecraft can hardly turn or go in reverse, that means the region itself was changing; Voyager was traveling through it the way an airplane flies through a ragged storm front, passing through clear and turbulent areas intermittently.
So when will the depletion region truly deplete, fully giving way to cosmos beyond? Could be tomorrow — or could be a year or more from now. The answer, as it has been for 30 years, is no one knows. But it’s a certainty that the Voyagers will eventually cross the invisible boundary, and that day, humanity will at last become a species of the stars.