Artist's concept of Voyager | NASA/JPL-Caltech

Where no spacecraft has gone before

For a few years now NASA’s Voyager 1 spacecraft has been flirting with the edge of our Solar System. After nearly 36 years and over 18 billion kilometres of travel, the space probe is now way past Neptune (4.7 billion km from Earth1) and even Pluto (7.4 billion km2). Next stop: interstellar space.

Yet it seems at this point nobody knows for sure whether Voyager 1 has already crossed that boundary. And we have a rather fuzzy notion of what that boundary even looks like.

Our Solar System is located within the bounds of heliosphere, a region of space akin to a giant bubble, dominated by a magnetic field and charged particles emanating from our Sun. Solar wind, consisting mostly of protons and electrons, travels away from our star at over 1,000,000 km/h, and slows down once it hits interstellar medium, which is composed mostly of helium and hydrogen gas.

Known as “termination shock,” this slowdown is the first sign of the edge of the bubble. What’s theorised beyond it is a heliosheath where the solar wind is turbulently interacting with the interstellar gas. This area is followed by heliopause – the theoretical boundary where solar wind stops.

So, are we there yet?

A University of Maryland-led team of researchers came out with a paper in The Astrophysical Journal Letters last week claiming that actually the famous space probe left the Sun’s sphere of influence on July 27, 2012. “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,” said lead author Marc Swisdak.

It’s controversial because as recently as last year NASA still argued the opposite to be true. According to their scientists, while Voyager 1 has recorded successive dips in solar particle counts to the point when only galactic electrons and protons were detected, the direction of the magnetic field has remained the same. Hence we’re still in the bubble, albeit on the very outermost edge of it.

Swisdak and his colleagues disagree. In their previous work they have focused on “magnetic reconnection,” the breaking and reconfiguring of close and oppositely-directed magnetic field lines – a phenomenon implicated in high-energy solar events such as flares and eruptions. The complex magnetic structure of the heliopause proposed by Swisdak’s team may explain why the spacecraft has not detected a change in the magnetic field direction. In a statement released by UMD the scientists explained the intricacies of their own model of the heliopause:

“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.”

NASA, however, remains unimpressed. As Ed Stone, Voyager project scientist at California Institute of Technology, said in an online statement:

“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.”

Bottom line? The scientists don’t know yet. And we don’t know, either. But I’ll be damned if it still isn’t an absolutely fascinating journey to keep track of.

Show 2 footnotes

  1. At its farthest. It’s only 4.3 at its closest!
  2. 4.2 at its closest.
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