By Richard A Lovett
It has now been confirmed that on 5 November 2018, after a 41-year cruise through the Solar System, NASA’s Voyager 2 spacecraft punched through the boundary separating the Solar System from interstellar space.
Scientists are already well advanced in crunching the numbers and considering the implications, however. And there is a lot to play with.
Voyager 2 is the second spacecraft ever to pass through that boundary – after its twin, Voyager 1 – but the first to arrive with a complete suite of functioning instruments, allowing it to gather more complete data on the transition.
The first point to make is that it didn’t actually take much of a “punch” to achieve this feat. By earthly standards, that boundary is an extremely good vacuum, with only 39 electrons per cubic metre, according to Voyager 2’s instruments.
But it’s an important demarcation, one that scientists call the heliopause.
On one side is the heliosphere, a giant bubble created by the solar wind, as it streams outward from the Sun. On the other is the interstellar medium, composed of the winds from dying stars that exploded millions of years ago.
“We are trying to understand the nature of that boundary,” says Edward Stone, of California Institute of Technology, Pasadena, the Voyager program’s project scientist.
Donald Gurnett, a space physicist at the University of Iowa compares it to a weather front.
“There was a time 50 years ago when people thought the solar wind would just gradually dissipate,” he says.
“But instead, Voyager’s transition from solar to interstellar space was so rapid that it passed through the boundary in a matter of hours. There is a very, very sharp boundary.”
Non-physicists, Gurnett adds, may be surprised to hear that the interstellar medium on the outside is 20 to 50 times denser than the plasma inside the heliosphere. “That’s because of the very large temperature difference.”
Because material inside the heliosphere is so much hotter than the interstellar medium, he and Stone say, the laws of physics say that the pressures can’t balance across the boundary unless the inside is less dense.
But the boundary is a lot more complex than a simple change in density.
To begin with, says Leonard Burlaga an astrophysicist at NASA Goddard Space Flight Center, Greenbelt, Maryland, there is a magnetic barrier just inside the heliopause, much thicker than the heliopause itself. “It took us 80 days to pass through it,” Burlaga says. “It’s almost an AU thick.” [An AU, or astronomical unit, is the distance from the Earth to the Sun, or about 150 million kilometres.] {%recommended 9158%}
That zone, Burlaga says, contains a relatively strong magnetic field, which, among other things, helps shield the Solar System from galactic cosmic rays, a type of extremely powerful (and dangerous) space radiation.
One fact that immediately emerges in comparing measurements from the two spacecraft is that they encountered the heliopause at very similar distances from the Sun, even though they were going in different directions and encountered the boundary at very different times in the Sun’s 11-year activity cycle.
In the case of Voyager 2, the boundary was 119.0 AU out from the Sun. In the case of Voyager 1, it was 121.6 AU out – a difference of only 2.2%.
But they also discovered that the heliopause is an oddly leaky boundary.
When Voyager 1 approached in 2012, Stone says, it encountered bands of plasma leaking in from the outside.
Voyager 2, on the other hand, found the reverse: material from the inside leaking out into the interstellar medium.
One possible explanation is that Voyager 2 exited through the south side of the Solar System’s magnetic field, while Voyager 1 went out the north side. But why that would make such a difference is not well understood, says Stamatios Krimigis, a space scientist from Johns Hopkins University’s Applied Physics Laboratory, Laurel, Maryland.
Another surprise, previously discovered by Voyager 1, is that solar storms create shock waves that propagate outward into the interstellar medium.
“These are explosions of the Sun, which eject material outwards,” Gurnett says. “It was a surprise that these shock waves can just keep going. They go right through the heliopause and propagate into the interstellar medium, [creating] disturbances in the cosmic ray flux…and probably other things we haven’t studied closely yet.”
Yet another mystery is that the orientations of the magnetic field inside and outside the heliopause are similar, even though they should be from two different sources: the Sun and the interstellar medium.
When that was discovered by Voyager 1, scientists didn’t know if it meant anything important. But when Voyager 2 found the same thing, it obviously meant that something unknown is linking the two fields. “We could dismiss it as coincidence in one case, but we cannot do that twice,” Burlaga says.
Meanwhile, both Voyagers are continuing to explore the interstellar medium and how it interacts with the solar bubble.
But that won’t continue forever.
“In another five years or so we may not have enough power to have scientific instruments on any longer,” Stone says. After that, he says, the spacecraft will continue to move into unexplored regions, “but not with power to send anything back.”
Nor can scientists expect new data about the heliopause from NASA’s Pluto-exploring New Horizons spacecraft, now about 50 AU out from the Sun. “The expectation is that New Horizons will run out of power at about 90 AU,” Krimigis says.
Still, the new Voyager 2 data is a pleasant surprise, because the Voyager missions were never designed to carry on this long.
“When [the Voyagers] were launched, the Space Age was only 20 years old,” Stone says. “It was hard to know that anything could last 40 years.”
