Voyager 1 is one of the most successful space missions of all time. Launched in 1977, it visited Jupiter and then Saturn, providing better close-ups of the two planets than had ever been seen before.

But it sailed on, crossing the orbits of both Uranus and Neptune (a sister craft, Voyager 2, actually flew by the two planets). Over all those years, there has been one constant in the Voyager flight: the solar wind blowing past it. This stream of subatomic particles leaves the Sun at hundreds of kilometers per second, much faster than Voyager. But now, after 33 years, that has changed: at 17 billion kilometers (10.6 billion miles) from the Sun, the spacecraft has reached the point where the solar wind has slowed to a stop. Literally, the wind is no longer at Voyager's back. There is gas between the stars, which astronomers call the interstellar medium. The solar wind blows out into it, slowing. There is a region, over a billion kilometers thick, where the solar wind plows to a halt, creating a roughly spherical shell around the solar system. That's called the heliosheath, and it looks like Voyager 1 is now solidly inside it. In fact, it's been there for four months or so; the scientists measuring the solar wind speed noticed it dropped to 0 back in June, but it took a while to make sure this wasn't just some local eddy in the flow. It's not. Voyager 1 now has calm seas ahead. But the probe is still moving outward at 60,000 kph (38,000 mph). In a few more years it'll leave the heliosheath behind, and when that happens it will truly be in interstellar space, the vast and nearly empty region between the stars. At that moment it will be the first human device ever to truly leave the solar system and enter the great stretches of the galaxy beyond. Imagine! It was launched before personal computers were everywhere, before cell phones, before the internet! But it was given a powerful boost by its rocket, and another by the two largest planets in the solar system as it swung by them. And now, in just a few more years, it will have left our nest forever.