Two Voyager program spacecraft explored the outer reaches of the heliosphere, passing through the termination shock and the heliosheath. "Voyager 1" encountered the heliopause on 25 August 2012, when the spacecraft measured a forty-fold sudden increase in plasma density. "Voyager 2" traversed the heliopause on 5 November 2018. Because the heliopause marks the boundary between matter originating from the Sun and matter originating from the rest of the galaxy, spacecraft that depart the heliosphere (such as the two Voyagers) are in interstellar space.