Voyager 1 and its twin, Voyager 2, launched in 1977, tasked chiefly with studying Saturn, Jupiter and the gas giants' moons. The two spacecraft made many interesting discoveries about these far-flung bodies, and then they just kept going, checking out Uranus and Neptune on their way toward interstellar space.
They're not quite out of the solar system yet, however. Both are still within a huge bubble called the heliosphere, which is made of solar plasma and solar magnetic fields. This gigantic structure is about three times wider than the orbit of Pluto, researchers have said.
Specifically, the Voyagers are plying the heliosphere's outer shell, a turbulent region called the heliosheath. But Voyager 1's new measurements — of fast-moving galactic cosmic rays hurled our way by star explosions — suggest the probe may be nearing the heliosphere's edge.
"From January 2009 to January 2012, there had been a gradual increase of about 25 percent in the amount of galactic cosmic rays Voyager was encountering," Stone said. "More recently, we have seen very rapid escalation in that part of the energy spectrum. Beginning on May 7, the cosmic ray hits have increased five percent in a week and nine percent in a month."
While it may be tough to identify the moment when Voyager 1 finally pops free into interstellar space, scientists are keeping an eye on the cosmic ray measurements and a few other possible indicators.