NASA's famed Voyager 2 spacecraft reaches interstellar space

By Meghan Bartels
December 10, 2018, 11:28:26 AM EST

It's time to say goodbye to one of the most storied explorers of our age: Voyager 2 has entered interstellar space, NASA announced today (Dec. 10).

The spacecraft, which launched in 1977, has spent more four decades exploring our solar system, most famously becoming the only probe ever to study Neptune and Uranus during planetary flybys. Now, it has joined its predecessor Voyager 1 beyond the bounds of our sun's influence, a milestone scientists weren't able to precisely predict when would occur. And intriguingly, humanity's second crossing doesn't look precisely like data from the first journey out.

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An artist's depiction of NASA's Voyager 2 probe on its long journey out of the solar system.Credit: NASA

"Very different times, very different places, similar in characteristics," Ed Stone, a physicist at the California Institute of Technology and project scientist for the Voyager mission, said during a scientific talk before the announcement. "The next months ahead could be very revealing as well. … More to come!"

Voyager 2 is the only spacecraft to have visited all four gas giant planets — Jupiter, Saturn, Uranus and Neptune — and 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.

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This NASA graphic shows the locations of NASA's Voyager spacecraft in interstellar space. NASA announced the arrival of Voyager 2 in interstellar space on Dec. 10, 2018. Voyager 1 reached the milestone in 2012.Credit: NASA/JPL-Caltech

Scientists have been watching for Voyager 2's grand departure since late August, when data beamed back by the probe suggested it was nearing what scientists call the heliopause, a bubble created by the solar wind of charged particles flowing out from our sun and influencing the environment within our solar system. Scientists use the heliopause to mark where interstellar space begins, although depending on how you define our solar system it can stretch all the way to the Oort Cloud, which begins 1,000 times farther away from the sun than Earth's orbit.

Beyond that bubble, spacecraft fly through many more cosmic rays — much higher-energy particles — than the lower-energy particles of our own neighborhood. Two instruments onboard the Voyager 2 probe track these particles as they collide with the spacecraft. The transition from mostly lower-energy particles to nearly none of these and a sudden surge of cosmic rays tells scientists the probe has crossed the heliopause.

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