An increase in the rate of cosmic rays detected by Voyager 2 indicates that the spacecraft is approaching the heliopause, where the solar wind gives way to the interstellar medium.
Humanity’s second ambassador for points beyond the solar system is getting ready to cross into interstellar space.
In 1977, the two Voyager spacecraft left Earth on grand tours of the giant planets. Both probes cozied up to Jupiter and Saturn, while Voyager 2 went on to become the first (and only) craft to zoom past Uranus and Neptune.
Since then, both probes have been on trajectories that will take them out of the solar system. In 2012, Voyager 1 took a significant step on that journey when it traversed the heliopause, the point where the gentle breeze of particles from the Sun finally yields to the tenuous pressure of gas that fills interstellar space.
Now Voyager 2 — roughly 18 billion kilometers (11 billion miles) from home, or nearly four times as far as Pluto — is inching toward the same milestone
Since August, the spacecraft has detected an uptick in cosmic rays, high-energy particles such as protons impinging on the solar system from deep space. Cosmic rays are usually blocked by the heliosphere, a bubble of space dominated by the solar wind and magnetic field. An increase in cosmic rays means Voyager 2 is approaching the heliopause, traversing a region where the Sun’s influence is beginning to wane.
“We’re seeing a change in the environment around Voyager 2, there’s no doubt about that,” said Voyager project scientist Ed Stone (Caltech) in an October 5th press release. “We’re going to learn a lot in the coming months, but we still don’t know when we’ll reach the heliopause.”
When Voyager 1 saw a similar increase in cosmic rays in 2012, it took another three months before it crossed the heliopause — a point widely described as the boundary of interstellar space. However, that doesn’t mean Voyager 2 is on the same timeline. The two spacecraft are on different trajectories out of the solar system and so are bound to encounter different conditions.
Voyager 1 is on a northward trek out of the solar system, exiting above the plane of Earth's orbit in the constellation Ophiuchus. Its sister craft is southbound, currently in the southern constellation of Pavo, the peacock. These diverging treks have already revealed a north-south asymmetry in the heliosphere. Voyager 2 encountered the termination shock, where the solar wind abruptly slows to below the speed of sound, roughly 2.7 billion kilometers (1.7 billion miles) closer to the Sun than Voyager 1 did. It appears that at least part of the southern heliosphere is pushed in relative to its northern half.
However long Voyager 2 takes to cross the heliopause, it will still have quite some ways to go before it leaves the solar system entirely. In about 300 years, the Voyagers will encounter the Oort Cloud, a field of comet-like debris encircling the Sun and the planets. Traversing the Oort Cloud could take another 30,000 years.
Eventually, both Voyagers will be free of the Sun’s grasp. About 40,000 years from now, Voyager 1 will be closer to a star other than the Sun when it passes within 1.6 light-years of GJ 445, a red dwarf currently in the constellation Camelopardalis. Around the same time, Voyager 2 will pass about 1.7 light-years from Ross 248, another M dwarf currently in Andromeda.
Barring a highly improbable run-in with interstellar debris or an alien salvage team, the Voyager spacecraft will then go on to silently wander the galaxy.