Voyager 1: almost ready to cross into interstellar space. via NASA/JPL
It looks like Voyager 1 is getting ready to make good on a months-old promise. More than 11 billion miles from the Sun, it’s getting ever closer to leaving the Solar System behind and jumping into the unchartered waters of interstellar space.
The interstellar mission phase wasn’t planned. When Voyager 1 was launched on September 5, 1977 (less than two weeks after its sister probe Voyager 2) it was on a mission to visit the gas giant planets Jupiter and Saturn. Anything else—if the spacecraft survived in space long enough to keep working—was gravy.
Voyager's been in an extended mission phase since December 18, 1980, when the spacecraft left the vicinity of Saturn. The massive planet whipped the spacecraft off away from the plane where all the planets orbit and on a new path towards interstellar space. Thirty-three years later, it’s still on that path. And while some of its instruments have failed over time, it’s doing remarkably well.
And it’s getting closer to passing through this metaphorical barrier, the point where the heliosphere gives way to interstellar space. The heliosphere is the bubble made from the Sun’s magnetic field that encloses the Solar System.
Voyager 1's portraits of the planets in the Solar System taken 4 billion miles from Earth. (Mercury and Mars were obscured by scattered sunlight). via NASA
Space fans' interest about Voyager’s impending departure of our Solar System was piqued last August when the spacecraft entered a region called the "magnetic highway." This is a region where charged particles from the Sun are disappearing as they zoom out along the solar magnetic field and cosmic rays from far outside zooming in towards the Solar System. In both directions, particle run in smooth magnetic field lines. The presence of these two types of radiation are a telltale sign that the spacecraft is coming up on the edge of the heliosphere.
But the data can be hard to read. The presence of cosmic rays near the interstellar boundary means Voyager 1 will be able to sample interstellar space before it actually gets there. So cosmic ray sampling isn’t an indication of interstellar space.
In fact, this murky data will make it hard for scientists to pinpoint the moment when Voyager does make the leap into interstellar space. Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena, notes that the cosmic ray and energetic particle data read on its own suggest Voyager 1 is already in interstellar space. But other data shows the spacecraft is still within the bubble of the sun's magnetic field.
There’s a specific sign scientists are keeping an eye out for: an abrupt change in the direction of the magnetic field. This would indicate the presence of the interstellar magnetic field, and give strong evidence that the spacecraft was in this new region of space.
There’s some old news in this news: no one’s totally sure how far Voyager 1 is from interstellar space, but it’s getting there. It’s been getting there for a while now. It could take months or years to reach interstellar space. Or it could leave the heliosphere tomorrow. Either way, it’s a thirty-six year old spacecraft speeding away from our Solar System but still talking to Earth. And that’s awesome.