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Debate continues whether Voyager has left the solar system

Voyager 1 has gone farther from Earth than any human-made object in history—that much we know for sure. But has it truly exited the solar system? Scientists from NASA and outside research institutions are debating this point.

NASA holds that Voyager 1 is within the outer limits of the heliosphere—the whole region over which the sun’s magnetic field has some influence—and therefore it is still within our solar system. But on Thursday, the Astrophysical Journal Letters published a study that argues that Voyager left the heliosphere most than a year ago.

The latter study, whose lead author is astrophysicist Mark Swisdak of the University of Maryland, concludes that Voyager 1 is now in its final and permanent foray within the Milky Way galaxy. NASA keeps a real-time odometer of both Voyager 1 and its sister spacecraft, Voyager 2, which calculates each spacecraft’s exact distance from Earth at any given second.

At the time of writing, Voyager 1 is more than 11.6 billion miles distant and counting, traveling at a speed of more than 38,000 miles per hour. Voyager 2 isn’t too far behind, at 9.46 billion miles.

The uncertainty is whether that area of space is within the heliosphere, as no astronomical test yet devised has been able to pinpoint exactly where the sun’s magnetic field ceases to hold any sway. Astronomers agree that there is a final enclosing boundary point, which they call the depletion region, beyond which the only magnetic pull is that of particles from other stars and other bodies in the galaxy.

NASA still gets some transmissions from Voyager, and that includes data on the magnetic field surrounding the craft. The space agency is now looking for any significant change in the direction of the surrounding magnetic field. This could be definitive proof that the spacecraft has just crossed the brink into interstellar space.

Swisdak and his team contend that maybe that magnetic shift already occurred but that NASA might have missed it. Like NASA, Swisdak and coauthors agree that the interstellar magnetic fields run in different directions from those of the heliosphere. But where the two magnetic energies meet, they might form into new, harmonious positions—a phenomenon called “magnetic reconnection” that is known to occur in solar flares. In that case, Voyager 1 is in the midst of interstellar magnetic energy, but it will appear to its instruments as no different, direction-wise, from that of the heliosphere.

On Thursday, NASA issued a statement saying that it would take Swisdak and colleagues’ arguments into consideration while continuing to examine data from Voyager. The data may keep coming until 2025, barring any serious system failures, according to NASA.