The Voyager 1 spacecraft is about to leave our solar system, and we have no idea what to expect. We knew Voyager was getting close to the edge of the heliophere - the region beyond which the solar winds cease to "push back" against the interstellar headwind - but only in the last month has it become apparent that we are really, really close to that boundary.
There's a great piece over at The Atlantic by Rebecca Rosen that's essential reading. Some excerpts:
We're on the cusp of one of the greatest scientific accomplishments of all time, but we may not know when the moment strikes. Or, rather, there may be no moment.
Last week, in the corners of the Internet devoted to outer space, things started to get a little, well, hot. Voyager 1, the man-made object farthest away from Earth, was encountering a sharp uptick in the number of a certain kind of energetic particles around it. Had the spacecraft become the first human creation to "officially" leave the solar system?
We're not quite there yet, Voyager's project scientist and former head of NASA's Jet Propulsion Lab, Edward Stone, told me. The spacecraft is on its way out -- "it's leaving the solar system" -- but we don't know how far it has to go or what that transition to interstellar space will look like.
Now the data coming back aren't photographs but levels of different kinds of particles in the outer edge of the sun's bubble (the heliosphere), known as the heliosheath, the farthest the solar winds reach, which Voyager I entered in December 2004. And it was some of those data -- the levels of a certain cosmic-ray particle -- that provoked the recent speculation that Voyager I had finally flown the coop.
Some cosmic ray particles enter the heliosphere and we can see them here from Earth. But a slower type has a hard time entering the heliosphere. Last month, the sum of those slower particles, suddenly ticked up about 10 percent, "the fastest increase we've seen," Stone says. But an uptick does not mean Voyager has crossed over, though it does mean we're getting close. When Voyager does finally leave and enter the space "out there where all the particles are," the level will stop rising. The rising itself means that Voyager is not out there, yet. "But," cautions Stone, "we don't know. I mean this is the first time any spacecraft has been there." Since nothing's ever been there before, we don't know what it will look like, which makes it a little hard to recognize "it" at all. "That's the exciting thing," he continues.