For several years, scientists had been expecting NASA’s iconic Voyager 1 spacecraft to cross the boundary that separates the Sun’s magnetosphere from the rest of the galaxy and make its long-anticipated jump to interstellar space. Having long completed its epic exploration of the gas giant planets Jupiter and Saturn and its systems of dozens of moons during the late 1970s and early 1980s, and after achieving a Sun-escape velocity from its various close flybys with the outer planets, Voyager 1 was ultimately bound for the open sea of interstellar space. When that historic milestone was finally reached in 2012, scientists and space enthusiasts alike appropriately celebrated humanity’s first foray outside of the Solar System, while eagerly awaiting the spacecraft’s in-situ measurements on what lay beyond. But when that data started coming in, celebration partially gave way to confusion, as several of Voyager’s measurements couldn’t quite clarify the spacecraft’s exact whereabouts in respect to the Sun’s magnetic field. It would take yet one more year for scientists to finally confirm Voyager’s long-awaited interstellar exodus. Now, a new study by a research team in the U.S. comes to answer the reasons behind the initial confusion regarding Voyager’s passage to interstellar space and further validate that the intrepid spacecraft has indeed said farewell to our little corner of the galaxy and has already set sails for the great beyond.
The realm of the Sun’s magnetic influence, better known as the heliosphere, is a vast magnetic bubble that extends well beyond the planets of the Solar System. This realm is dominated by the outward flow of the solar wind—the steady stream of charged particles that is released by the Sun’s upper atmosphere at speeds that range between 400 and 800 km/second. Traversing interplanetary space along the magnetic field lines of our home star, the solar wind eventually slows down to subsonic speeds at a boundary called the termination shock, which is located between 84 and 94 Astronomical Units away from the Sun, until it is finally stopped altogether at the heliopause (the outer limits of the heliosphere) by the pressure of the stellar winds that stream through the interstellar medium. Yet the exact point where the Solar System ends and interstellar space begins has been the topic of debate for decades, with the two leading definitions finally emerging within the scientific community. For many, the end of the Solar System is at the heliopause where the outward flow of the solar wind is finally stopped to zero and the interstellar magnetic field becomes dominant, while for others the territory of the Solar System reaches as far as the extent of the Sun’s gravitational field, which could be up to two light-years away from Earth. Semantics aside, Voyager’s arrival at the heliopause in 2012 was heralded as the true passage into the interstellar medium, since it is that area of space where the solar wind finally fades out and galactic cosmic rays dominate and where it was expected that the force of the galactic magnetic field would also take over.
Nevertheless, the space environment that Voyager encountered at the end of the heliopause initially had cast some doubt on that. It had been speculated for years that Voyager’s journey toward interstellar space would be characterised by three distinct markers: the passage through the termination shock where the solar wind slows down to subsonic speeds, the passage through the heliopause where the velocity of the solar wind slows down to zero, and the subsequent change in the direction of the magnetic field that the spacecraft would measure, since the galactic one has an entirely different orientation from the solar one. Yet, even though Voyager 1 successfully encountered the termination shock and the heliopause in 2004 and 2012 respectively, the anticipated change in the direction of the magnetic field that it would record when out of the heliosphere wasn’t at all what had been expected. Based on previous calculations, the direction of the galaxy’s magnetic field lines were in fact off by a whopping 40 degrees from where they should be. That alone had prompted many scientists to dismiss the announcement of Voyager’s passage into interstellar space as a premature one.
“There are still naysayers out there regarding Voyager 1 crossing through the heliopause – the edge of the heliosphere,” says Dr. Nathan Schwadron, an associate professor of physics at the University of New Hampshire and lead author of a new study which was published earlier this week in the Astrophysical Journal Letters. “And the reason for this doubt is that when the spacecraft supposedly broke through the heliopause we should have seen some sort of distinctive shift in the magnetic field from one medium to the other.”
Video Credit: NASA/Goddard
In their study, Schwadron and his team set out to investigate the reasons behind this discrepancy, by taking advantage of independent measurements of the interstellar magnetic field that had been conducted by other spacecraft like Ulysses, the Solar & Heliospheric Observatory, or SOHO, and the Interstellar Boundary Explorer, or IBEX. The latter, in particular, which has been studying the interactions that occur between the heliosphere and interstellar space at the edge of the Solar System since it was launched in 2008, had made the startling discovery of a previously undetected energy “ribbon“ that was consisted of energetic neutral atoms and ran perpendicular to the direction of the galactic magnetic field just outside the heliosphere. The existence of this enigmatic ribbon, which caught scientists literally off guard, had gone unnoticed by both Voyager 1 and Voyager 2 as the latter made their way out of the Solar System. “This ribbon winds between the two Voyager spacecraft and was not observed by either of them,” says Eric Christian, deputy mission scientist for IBEX at NASA’s Goddard Space Flight Center, in Greenbelt, Md. “It’s like having two weather stations, but missing the big storm that runs between them.”
By analysing and triangulating the data gathered by IBEX, Ulysses, SOHO, as well as Voyager 1 itself, Schwadron and his team were able to determine that the center of this mysterious energy ribbon just outside of the Solar System actually points toward the north pole of the galaxy’s magnetic field. “All of these different data sets that have been collected over the last 25 years have been pointing toward the same meeting point in the field,” says Schwadron.
Furthermore, the researchers’ analysis showed that the reason behind the discrepancies in the Voyager data were due to the interactions that occur between the solar and the galactic magnetic field at the edge of the heliopause. More specifically, at Voyager 1’s current location of more than 130 Astronomical Units away or 20 billion km from the Sun, the interstellar magnetic field is deflected by the heliopause itself and is oriented dozens of degrees away from the point of galactic magnetic north. These results show that even though Voyager 1 has indeed exited the heliosphere, it still measures the latter’s effects on the interstellar medium. “Our analysis confirms two things for the first time: that the center of the IBEX ribbon is the direction of the interstellar magnetic field and, secondly, that Voyager 1 is now beyond the heliopause,” adds Schwadron.
In addition to solving the mystery of Voyager’s interstellar passage, the study by Schwadron’s team also predicts that as the spacecraft recedes farther away into the galaxy, these last effects of the solar wind into the interstellar medium will eventually quiet down, allowing Voyager to observe the “real” interstellar magnetic field just before it goes out of operation during the mid-2020s. “The magnetic field that’s [now] coming in from the galaxy, is essentially warped and deflected around the protective boundaries [of the heliosphere] like a rubber band [that’s] being bent around a beach ball,” comments Schwadron. “As Voyager 1 gets further and further out into interstellar space, eventually it will see the magnetic field that is no longer affected by these protective boundaries. And that point, roughly a decade from now, will be when Voyager really enters what we call ‘pristine’ interstellar space.”
Meanwhile, Voyager 2, which is on a Sun-escape trajectory in the opposite direction from its robotic sibling, will also pass the boundary of the heliopause in the next couple of years,. When that comes to pass, scientists will have a new set of measurements from an entirely different vantage point in space than Voyager 1 upon which to further their understanding of the intricate interplay between the heliosphere with the rest of the galaxy.
With their place in history already secured, the Voyagers’ final contribution to humanity will be to provide a greater insight into the physical processes that take place as the small pond of the Solar System meets the great open sea of interstellar space.