Has Voyager 1 actually reached interstellar space?

There's been some talk around the internet water-cooler today, about how a new study shows that the Voyager 1 space probe has left the solar system. However, although it may depend on your exact definition of where the edge of the solar system is, that claim is probably not true.

Back in September, as the Voyager 1 space probe celebrated its 35th anniversary, NASA scientists were saying that the spacecraft was on the very edge of the solar system, and could pass into interstellar space at any time. They were looking for three specific conditions to be met before they would know for sure — a decrease in the number of lower-energy cosmic particles from our Sun, an increase in high-energy galactic cosmic particles, and a change in the direction of the magnetic field.

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As early as July 28th, data from the spacecraft showed that something was going on, as the number of higher-energy cosmic particles increased slightly, and just hours later the number of lower-energy cosmic particles dropped by half. Then, roughly a month later, on August 25th, the number of high-energy particles jumped significantly, and the number of low-energy particles dropped to about 1% of their previous levels.

This graph shows the number of particles from the Sun (blue line A) and the number of high-energy galactic particles …

This radical change showed that Voyager 1 had definitely crossed some kind of threshold, but since there was still no indication that the magnetic field direction had changed, the spacecraft hadn't actually left the solar system.

In December, NASA scientists confirmed that the spacecraft had entered a "new region in deep space", still within the influence of our Sun, but still close enough to interstellar space that it could "taste what it's like on the outside because the particles are zipping in and out on this magnetic highway."

This new region, a 'magnetic highway' of charged particles, was something that scientists hadn't anticipated, but they determined that it came about due to the connection between the Sun's magnetic field lines and the lines of the galactic magnetic field. Although the magnetic field became stronger when Voyager 1 flew into the highway, the direction of the field didn't change, so it hadn't reached interstellar space yet.

In this latest study, written by Bill Webber and Frank McDonald, the authors discuss the particle data and state that "it appears that [Voyager 1] has exited the main solar modulation region," however according to Universe Today, NASA JPL spokesperson Jia-Rui Cook pointed out that an important detail was missing.

“Our last statement about this was the critical thing we were looking for was a change in the magnetic field data,” she said. “This paper does not appear to address the magnetic field data.”

Also, shortly thereafter, NASA issued the following statement:

"The Voyager team is aware of reports today that NASA’s Voyager 1 has left the solar system," said Edward Stone, Voyager project scientist based at the California Institute of Technology, Pasadena, Calif. "It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space. In December 2012, the Voyager science team reported that Voyager 1 is within a new region called ‘the magnetic highway’ where energetic particles changed dramatically. A change in the direction of the magnetic field is the last critical indicator of reaching interstellar space and that change of direction has not yet been observed."

So, it appears as though, as close as Voyager 1 is to the edge of the solar system, it still hasn't reached interstellar space.

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This is not to say that Webber and McDonald's paper is wrong. It isn't.

In an American Geophysical Union (AGU) press release, Webber states: "Within just a few days, the heliospheric intensity of trapped radiation decreased, and the cosmic ray intensity went up as you would expect if it exited the heliosphere."

However, although the heliosphere may be the limit of the solar wind, but as was discussed back in December, it isn't the limit of the influence of the Sun's magnetic field. Therefore, the study authors aren't making any claims about Voyager 1 actually entering interstellar space.

"It's outside the normal heliosphere, I would say that," Webber said in the AGU statement. "We're in a new region. And everything we're measuring is different and exciting."

(Images courtesy: NASA/Webber/McDonald/AGU)

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