On Aug. 25, 2012, 35 years after the Voyager 1 mission launched, Earth’s most distant spacecraft detected a sharp change in the intensity of fast-moving charged particles called cosmic rays, suggesting it had left the outermost reaches of the heliosphere marking the edge of the solar system.
“Within just a few days, the heliospheric intensity of trapped radiation decreased, and the cosmic ray intensity went up as you would expect if it exited the heliosphere,” said Bill Webber, professor emeritus of astronomy at New Mexico State University in Las Cruces, in a statement.
Though Voyager 1 has apparently exited the sun’s sphere of influence, the scientists still don’t know for sure whether the probe has entered interstellar space or a mysterious in-between region beyond the solar system.
Webber and his colleagues noticed the dramatic cosmic ray signal drop when Voyager 1 was about 11 billion miles (17.7 billion kilometers) from the sun. Anomalous cosmic rays trapped in the heliosphere’s outer reaches dropped to 1 percent of their previous level. Meanwhile, galactic cosmic rays, which come from outside the solar system, jumped to twice their previous levels, reaching their highest levels since the spacecraft launched, researchers said.
The findings have been accepted for publication in the journal Geophysical Research Letters.
Voyager 1 was actually the second of NASA’s two Voyager spacecraft to launch on historic tours of the solar system. Voyager 2 blasted off on Aug. 20, 1977, with Voyager 1 following a few weeks later on Sept. 5 of that year.
Both spacecraft carry a gold-plated copper disc containing sounds and images of Earth. The golden record is about 12 inches (30 centimeters) across and attached to the hull of each Voyager probe. The records are engraved with a diagram that explains how to play them and where Earth can be found, just in case the Voyager probes are discovered by intelligent extraterrestrial lifeforms.