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.