On what date did Voyager 2 enter the interstellar medium?

Voyager 2 entered the interstellar medium on November 5, 2018.

That date marks the spacecraft’s departure from the heliosphere, the vast bubble carved in interstellar space by the Sun’s solar wind. Voyager 2 crossed the heliopause roughly 119.7 astronomical units from the Sun, where measurements showed a transition to the surrounding interstellar plasma.

The milestone was announced in December 2018 by project scientist Edward C. Stone and colleagues. Voyager 1 had crossed into the interstellar medium in 2012, but Voyager 2 carried a functioning Plasma Science instrument, allowing scientists to obtain a particularly valuable direct measurement of the plasma boundary.

A common mix-up is to treat “interstellar space” as meaning that Voyager 2 had left the Solar System entirely. It had left the Sun’s heliosphere, but remained gravitationally bound to the Sun’s much larger outer region. The spacecraft continues its extended mission, sending extremely faint signals through NASA’s Deep Space Network.