By Andrew Grant
It’s finally official: Voyager 1 has become the first human-made object to enter interstellar space, mission scientists report September 12 in Science. On August 25, 2012, the scientists say, Voyager 1 exited a giant invisible bubble called the heliosphere that is inflated by a torrent of subatomic particles spewing from the sun. Now the probe is surrounded almost exclusively by particles produced by other stars. But whether it’s correct to say that the probe has left the solar system depends on how you define the solar system. “From my perspective, Voyager is nowhere near the edge of the solar system,” says planetary scientist Hal Levison of the Southwest Research Institute in Boulder, Colo. The sun continues to exert gravitational dominance out to hundreds of times the distance of Voyager 1 from the sun, where trillions of icy pebbles, boulders and comets orbit. In the last 36 years, Voyager has traveled an impressive 25.4 billion kilometers, but it still has a long way to go to unambiguously depart the solar system.
OORT CLOUD
Distance from the sun: 5,000–100,000 AU
The sun, planets and Voyager probes sit inside the tiny yellow dot at right, within a giant sphere called the Oort cloud. This reservoir of trillions of ice chunks extends 100,000 astronomical units out, tethered to the sun by gravity. Astronomers believe these objects got thrown out of the inner solar system as the planets took shape 4.5 billion years ago. Occasionally these castaways pass near Earth: The comet ISON, which may light up the night sky this November, started out in the Oort cloud. The Voyagers would have to travel another 30,000 years before clearing this broadest definition of the solar system.