Voyager 1's on-again, off-again relationship with the solar system would give any celebrity couple a run for their money. But with the evidence mounting, the science team in charge of the NASA spacecraft is at last ready to bid Voyager 1 farewell. What's more, they have placed a date on the probe's exit from the solar system: 25 August 2012.
The solar system is encased in a magnetic bubble blown by the constant wind of charged particles streaming from the sun, called the heliosphere. Since 1977 Voyager 1 has been cruising away from Earth towards the edge of that bubble, the heliopause. But for the past few years the spacecraft has been a bit of a tease, showing repeated but unconfirmed glimmers that it may have crossed over into interstellar space.
Now, readings taken over the past year show a definite increase in the density of charged gas around the spacecraft. This is a telltale clue, because plasma density in interstellar space should be much higher than it is inside the solar system's boundary zone. Working backwards, the Voyager team says a noticeable density increase started in August 2012 – the same time that Voyager 1 also recorded a suspicious change in its sightings of high-energy particles known as cosmic rays.
The only hitch is that the team had been expecting a third sign of a crossing: a change in the direction of the surrounding magnetic field. Although such a shift has not yet appeared, the consensus of the Voyager team is that the craft has in fact made its grand exit, and we simply do not understand how magnetic fields behave at the very edge of the solar system.
"Speaking for myself and my co-authors, we certainly believe we have crossed the boundary," says Voyager team member William Kurth at the University of Iowa in Iowa City. "The measurement of these very high densities is the smoking gun."
Highs and lows
Voyager fever began building in 2004, when the spacecraft crossed into a border zone – the heliosheath – where the solar wind suddenly slows to a crawl. Expectations began mounting that Voyager 1 was close to popping out of the sun's sphere of influence and into the open waters of interstellar space. Multiple teams on and off the mission have been scouring data from the craft's instruments looking for hints of a crossing. Some have even argued it happened before, although until now most of Voyager's lead scientists remained unconvinced.
Things changed on 25 August last year. The cosmic rays the craft saw coming from the sun seemed to just about disappear. They were replaced by galactic cosmic rays, which come from sources outside the heliosphere. That was enough for some scientists to declare that Voyager 1 had left the solar system.
But theory predicts that at the heliopause, the direction of the magnetic field carried by the solar wind should switch to that carried by the charged particles flying through interstellar space. At the time the Voyager team countered that they had not yet seen the magnetic field change direction, so Voyager 1 was still in the solar system's embrace.
Cosmic violin
The latest measurements come from an instrument on Voyager 1 that detects oscillations in the surrounding plasma, allowing it to infer density. These plasma oscillations can be triggered by extreme events on the sun that send electron beams coursing through the solar wind. When these beams interact with the interstellar plasma, they can create oscillations in a particular frequency range.
"If you think of a plasma oscillation as a violin string, it's the vibration of the string that Voyager has measured," says Kurth.
The trick is that the waves can take up to 400 days to propagate out to the heliopause, so readings have to be carefully timed to catch their effects that far away. New measurements of the oscillations taken during two periods, October to November 2012 and April to May 2013, show that plasma density has been increasing steadily and is now about 30 times higher than that inside the heliosheath. Extrapolating backwards, the Voyager team says the increase must have started on about 25 August 2012, coinciding nicely with the changes in cosmic rays.
The idea that you need to see a concrete change in the magnetic field rests on the assumption that there is a single, well-defined boundary between the heliosphere and interstellar space, but perhaps that is not necessarily so, the team says.
"What I don't think we've understood up until now is whether there is a dramatic switch or whether there could be other physics going on that involves draping of the interstellar fields around the solar boundary, or perhaps some intertwining of the field lines," says Kurth. "My interpretation is that the heliopause is not as absolute and simple as we imagined it might be."
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