Still within the sun’s grasp, but how long for? (Image: NASA)

Earth’s second emissary to interstellar space, Voyager 2, is phoning home with new views of the solar system’s ragged edge. But what it sees could be very different to what its predecessor glimpsed, revealing new details of the sun’s immediate neighbourhood.

Voyager 2 has reached the heliosheath, the beginning of the end of the solar system. If the experience of its twin, Voyager 1, is anything to go by, Voyager 2 is about two-thirds of the way to the heliopause – the outer edge of the sun’s influence, also considered to be where interstellar space begins. Voyager 1 crossed this boundary two years ago this week, according to NASA and most Voyager scientists. Not everyone agrees, though, because readings sent back by Voyager 1 left a little room for doubt.

One clue that Voyager 1 had passed the heliopause was that its instruments measured a slowing, sparser solar wind. That’s not happening yet for Voyager 2, says Rob Decker at Johns Hopkins University in Maryland.


Windsock-shaped sphere

That could be because the sun’s sphere of influence isn’t a sphere. Solar radiation blows a bubble of charged particles about 15 billion kilometres in radius, but the sun’s motion through the galaxy gives that bubble a windsock shape, with a rounded part in the direction of travel and a tail trailing behind. Voyager 1 is moving in the same direction as the sun, but Voyager 2 – 3 billion kilometres behind – is headed more sideways and down.

In addition to the sun’s motion, particles and plasma from interstellar space might be deforming the bubble, Decker says. As a result, it could take longer for Voyager 2 to reach interstellar space – or it could happen sooner, notes Ed Stone, chief Voyager scientist at NASA. Voyager 2 crossed the termination shock, another physical boundary signifying the heliosheath, about 1.5 billion kilometres before anyone expected, he says, so it’s hard to make firm predictions about what it will do in the future.

When Voyager 2 does cross the heliopause, its exit will be definitive, Decker and Stone say. Voyager 1’s plasma sensor broke down sometime in the 1980s, but the younger probe’s still works. The sensor will detect the change between the sun’s sphere of influence, which is warm and less dense, to the interstellar medium, which is cold and denser by a factor of 40. That means Voyager 2’s observations will be much clearer.

“We’re very fortunate to have a second spacecraft,” Stone says.

Journal reference: The Astrophysical Journal, DOI: 10.1088/0004-637X/792/2/126