NASA’s Voyager 2 spacecraft has finally entered interstellar space. It is the second human-made object to do so, thus, joining its predecessor Voyager 1.

According to NASA, Voyager 2 crossed over the heliopause, the boundary between the heliosphere and interstellar space, on November 5th. So, the probe is now more than 11 billion miles from Earth.

“For me, this is an extremely exciting time for the history of space exploration,” Georgia Denolfo, a NASA astrophysicist, said in a press conference today. “We’re able to look at the galaxy through the clouded lens of our heliosphere, and now we can take a step outside with Voyager.”

Voyager 1 had been detecting increasing amounts of cosmic rays, something Voyager 1 experienced in 2012. But Voyager 2 has something its partner didn’t when it left the heliosphere — a functional Plasma Science Experiment. Voyager 1’s instrument stopped working in 1980. So, Voyager 2 is will be able to take measurements of the solar wind.

Inside the heliosphere, solar wind rules. But farther away from the Sun, speedy subatomic particles called cosmic rays start to dominate. The boundary between the two is known as the heliopause, which Voyager 2 has already passed.

Scientists use the heliopause to mark where interstellar space begins, although depending on some other factors it can stretch all the way to the Oort Cloud, which begins 1,000 times farther away from the sun than Earth’s orbit.

On November 5th, Voyager 2 detected a sharp drop in the speed of solar wind particles. Since then, it hasn’t measured any solar wind flow at all, meaning it has definitely exited the sun’s protective bubble.

Voyager 2

Voyager 2 is the only spacecraft to have visited all four gas giant planets — Jupiter, Saturn, Uranus, and Neptune. The probe has also discovered 16 moons, as well as phenomena like Neptune’s mysteriously transient Great Dark Spot, the cracks in Europa’s ice shell, and ring features at every planet.

NASA launched the spacecraft in 1977 and it has become NASA’s longest-running mission.

The probe will continue beaming home updates to its scientists here on Earth for as long as it can. Eventually, the plutonium supply that powers the spacecraft will run out. Therefore, the probe will shut down instruments. Sometime after 2025, the team expects the probe will go quiet, entirely.

However, both spacecraft are still floating inside our solar system. The edge of the Solar System is delineated by the Oort Cloud, a large collection of distant objects that are still affected by the sun’s gravity and orbit the Sun far beyond Pluto. NASA estimates that Voyager 2 will reach the near edge of the Oort Cloud in about 300 years, and it could take up to 30,000 years for it to exit the Solar System completely.

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Thumbnail image: This graphic shows the position of the Voyager 1 and Voyager 2 probes relative to the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in 2012. Voyager 2 is still in the heliosheath or the outermost part of the heliosphere. Credit: NASA/JPL-Caltech