After 40 years of zipping through the solar system, Voyager 2 appears to be close to leaving the neighborhood. Currently at a distance of about 17.7 billion km (11 billion mi) from Earth, the probe's instruments have begun picking up radiation signals that suggest it is breaking out of the Sun's protective bubble, and will soon join its sibling in interstellar space.

Voyager 1 and 2 were launched in 1977 before conducting a grand tour of Jupiter, Saturn, Uranus and Neptune, where the probes collected some of the clearest photos and data of the planets for the time. But their job wasn't done yet – given their exit trajectory, astronomers figured they could help study the very boundaries of the solar system.

In a similar way to how the Earth's magnetic field creates a shield that protects it (and us) from the deadly radiation of space, so too does the Sun create its own bubble. Known as the heliosphere, this huge bubble surrounds the entire solar system and is made up mostly of "solar wind" or plasma ejected from the Sun. Beyond that is the cold expanse of interstellar space, made up mostly of hydrogen and helium gases.

In 2012, Voyager 1 became the first human-made object to leave that bubble and go interstellar. Six years later and it now looks like Voyager 2 is about to follow suit, after journeying through the heliosheath (the very edge of the bubble) for the past decade.

NASA has now reported that the probe's onboard instruments have begun to detect more cosmic rays hitting the spacecraft. Since late August, Voyager 2's Cosmic Ray Subsystem has picked up a five percent increase in the rate of cosmic rays, while the Low-Energy Charged Particle instrument has registered a similar bump.

Since the heliosphere blocks many cosmic rays, this kind of increase was expected to be seen when the craft crossed the barrier. In fact, Voyager 1 reported a similar increase about three months before it officially entered interstellar space.

That said, Voyager 1's journey isn't a perfect model for what the second probe will experience. For starters, they whizzed off in wildly different directions, so the heliosphere could be very different in different areas. On top of that, the six-year gap could play into things too, since the Sun's 11-year activity cycle can cause the heliopause to swell inwards and outwards, as though the solar system is breathing.

"We're seeing a change in the environment around Voyager 2, there's no doubt about that," says Ed Stone, a Voyager Project scientist. "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."

Current estimates suggest Voyager 2 will finally enter interstellar space in late 2019 or early 2020. It's believed that both craft have enough power to continue transmitting data until 2025, before continuing silently outwards as symbols of human achievement, possibly long after we're gone.

Source: NASA

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