And now, these researchers say, Voyager 1 has begun the first exploration of our galaxy beyond the Sun's influence,
says a University of Maryland-led team of researchers.
"It's a somewhat controversial view, but we think Voyager has finally left the Solar System, and is truly beginning its
travels through the Milky Way," says UMD research scientist Marc Swisdak, lead author of a
Based on a model of the outer edge of the Solar System constructed by Swisdak and fellow plasma physicists James F.
Drake, also of the UMD, and Merav Opher of Boston University
Boston University - Voyager 1 actually entered interstellar space a little more than a year ago.
This artist's concept shows NASA's two Voyager spacecraft exploring a turbulent region of
space known as the heliosheath, the outer shell of the bubble of charged particles around our sun. Image Credit: NASA/JPL-Caltech
On the other hand, recent papers by NASA and other scientists suggested the spacecraft was still in a fuzzily-defined transition
zone between the Sun's sphere of influence and the rest of the galaxy.
But why the controversy?
NASA scientists recently reported that last summer, after eight years of travel through the outermost layer of the
heliosphere, Voyager 1 recorded "multiple crossings of a boundary unlike anything previously observed."
Successive dips in, and subsequent recovery of, solar particle counts caught researchers' attention.
The dips in solar particle counts corresponded with abrupt increases in galactic electrons and protons.
Within a month, solar particle counts
disappeared, and only galactic particle counts remained. Yet Voyager 1 observed no change in the direction of the magnetic field.
To explain this unexpected observation, many scientists theorize that Voyager 1 has entered a "heliosheath depletion region,"
but that the probe is still within the confines of the heliosphere. Swisdak and colleagues, who are not part of the Voyager 1
mission science teams, say there is another explanation.
Researchers assert that the heliopause is both porous to certain particles and layered with
complex magnetic structure. Here, magnetic reconnection produces a complex set of nested magnetic "islands,"
self-contained loops which spontaneously arise in a magnetic field due to a fundamental instability.
Interstellar plasma can penetrate into the heliosphere along reconnected field lines, and galactic cosmic rays and
solar particles mix vigorously.
Most interestingly, drops in solar particle counts and surges in galactic particle counts can occur across "slopes"
in the magnetic field, which emanate from reconnection sites, while the magnetic field direction itself remains unchanged.
This model explains observed phenomena from last summer, and Swisdak and his colleagues suggest that Voyager 1 actually
crossed the heliopause on July 27, 2012.
In a NASA statement, Ed Stone, Voyager project scientist and a professor of physics of the California Institute of
"Other models envision the interstellar magnetic field draped around our solar bubble and predict that the
direction of the interstellar magnetic field is different from the solar magnetic field inside. B
y that interpretation, Voyager 1 would still be inside our solar bubble. The fine-scale magnetic
connection model [of Swisdak and colleagues] will become part of the discussion among scientists
as they try to reconcile what may be happening on a fine scale with what happens on a larger scale."