“Since late August, the Cosmic Ray Subsystem instrument on Voyager 2 has measured about a five percent increase in the rate of cosmic rays hitting the spacecraft compared to early August,” NASA wrote in an announcement. “The probe’s Low-Energy Charged Particle instrument has detected a similar increase in higher-energy cosmic rays.”
Cosmic rays are subatomic particles, primarily hydrogen and helium nuclei, that zoom through space at incredibly high speeds. It’s thought that many of these get stopped or slowed at the heliopause by the solar wind, creating a sort of cosmic ray build-up between the heliopause and the termination shock. In May 2012, about three months before it crossed the heliopause, Voyager 1 saw a similar increase in cosmic rays.
But the similarity between the two probes’ experiences doesn’t mean that Voyager 2 is about to cross over. This is because the Solar System slightly expands and contracts during the 11-year solar cycle. When the Sun is at minimum – which we’re currently closing in on – its activity levels are low, and the Solar wind is slower. At maximum, the solar wind is significantly stronger.
So while we can guess at a few things, there are still some surprises ahead, which means the Voyager team will be keeping a close eye on things. So far, the only prediction that can be made with a degree of certainty is that it will enter interstellar space before 2030.
“We’re seeing a change in the environment around Voyager 2, there’s no doubt about that,” said Voyager Project Scientist Ed Stone of Caltech. “We’re going to learn a lot in the coming months, but we still don’t know when we’ll reach the heliopause. We’re not there yet – that’s one thing I can say with confidence.”
We believe in you, little space probe!