Sometimes it seems as though the Voyager 1 space probe is like a dog that can’t decide if it wants to be inside or out. A team of scientists led by the University of Maryland claim that the Voyager 1 space probe, which is now 11 billion miles (18 billion km) from Earth left the Solar System’s boundary last year and is not, as NASA claims, still passing through a transition zone. The controversial theory is based on models of the solar magnetic field on the edge of the system and how it interacts with interstellar space.

The controversy arises from the structure of the Solar System where it meets interstellar space. Inside, where we are, is the heliosphere, which is the area of influence where the power of the Sun dominates local space. It’s well defined and scientists have a pretty good idea as to its shape and extent.


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The tricky bit is the edge of the heliosphere, called the heliopause. This is the area where the influence of the Sun is reduced to almost nil and where interstellar space begins. The trouble is, very little is known about that area. Unfortunately, Voyager 1 has posed as many questions as it's answered. That’s because it’s the first and, so far, only probe to reach the edge of the Solar System. It’s one datum point in a very large volume of space.

Artist's impression of the solar and interstellar magnetic fields (Image: NASA)

It’s a bit like trying to figure out what the surface of the Earth is like by dropping a grab bucket from space with only one go allowed. The odds are four to one that you’d come back with a bucket of water, but you might also come back with a bit of sand, a clump of grass, or a surprised looking spaniel. What you would certainly get is a very poor picture of what’s actually there.

It also doesn't help that Voyager’s current mission isn't the one it was designed for. It’s purpose was to fly by and study the planets of the outer Solar System. Studying the furthest reaches of our system is just a lucky bonus due to the craft continuing to function for so long.

Somewhere around July 25 or 27, 2012, Voyager 1 moved into a new region of space, but what it is exactly is now the bone of contention. Among its experiments, Voyager carries a magnetometer and its Low Energy Charged Particle detector. These help to determine when the spacecraft leaves the Solar System because there should be a sudden change in what the instruments' readings. The particle detector picks up Anomalous Cosmic Rays (ACR), which are cosmic ray particles that have become trapped by the Sun’s magnetic field, and Galactic Cosmic Rays (GCR), which are cosmic rays from outside the Solar System.

As expected, while inside the Solar System, Voyager recorded more ACRs than GCRs. Then last July, after a surprising series of dips, the intensity suddenly flipped and the number of ACRs dropped by a factor of 500 while the GCRs doubled. That should have meant that Voyager had left the Solar System, but the fly in the ointment was that the direction of the magnetic field remained the same when it should have changed.

The current view is that this marks a transition zone or "heliosheath depletion region," the nature and extent of which is still unknown, and that once the direction of the magnetic field shifts, we’ll know that Voyager 1 has left the system. It will be a clean transition marked by either a sudden change in direction or a clear, measurable shift as the solar magnetic field is overwhelmed by the interstellar field or fields.

Marc Swisdak and James F. Drake of the University of Maryland, and Merav Opher of Boston University contend that Voyager 1 is not where NASA believes it to be. "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 Swisdak.

Zones of the heliosphere according to NASA (Image: NASA)

Based on their previous work on how magnetic fields interact, Swisdak and Drake theorized that the changes in solar particle counts and galactic particle counts detected by Voyager was related to magnetic reconnection, or the breaking and reconfiguring of close and oppositely-directed magnetic field lines, which is the mechanism controlling solar flares and similar phenomena.

The team believes that the heliopause isn’t a neatly defined barrier separating “outside” and "inside," but is porous to certain particles and layered with complex magnetic structure. Working on this assumption, the team says its models show a complex interaction between solar and interstellar magnetic fields that produced what they called nested magnetic islands – eddies and gaps in the field that make it porous and unstable as the magnetic inside and outside of the Solar System mix.

It’s a bit like sailing a ship to the arctic ice cap. If you start at the equator, the lack of ice is obvious and needs no explanation. If you’re at the North Pole, the presence of all that ice is pretty obvious, too. But if you’re at the edge of the ice cap, things aren't as simple. You don’t just bump into a solid sheet of ice. Instead, the water becomes slushy, patches of white might appear along with thin floes of ice. These will gradually increase and aggregate until your first definite proof that you’re in the ice is when you’re trapped for the winter.

According to the team, Voyager is in the interstellar arctic and traveling through the cosmic slush. They claim that their model accounts for the shift in charged particle counts and why the magnetic field in the vicinity of Voyager hasn't changed. By their reckoning, Voyager 1 left the Solar System on July 27, 2012.

In response to this claim, NASA's Voyager project scientist, Ed Stone of the California Institute of Technology in Pasadena issued this statement:

"Details of a new model have just been published that lead the scientists who created the model to argue that NASA's Voyager 1 spacecraft data can be consistent with entering interstellar space in 2012. In describing on a fine scale how magnetic field lines from the sun and magnetic field lines from interstellar space can connect to each other, they conclude Voyager 1 has been detecting the interstellar magnetic field since July 27, 2012. Their model would mean that the interstellar magnetic field direction is the same as that which originates from our sun.

"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. By that interpretation, Voyager 1 would still be inside our solar bubble.

"The fine-scale magnetic connection model 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.

"The Voyager 1 spacecraft is exploring a region no spacecraft has ever been to before. We will continue to look for any further developments over the coming months and years as Voyager explores an uncharted frontier."

In other words, where Voyager 1 is remains a definite “maybe.”

The results of Swisdak and Drake’s work was published in The Astrophysical Journal Letters.

Source: University of Maryland

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