In 1977, humans sent two small machines on a journey far far away from Earth, the twin probes of the Voyager mission. In the three and a half decades since, they have traveled billions and billion of miles (Voyager 1 is 11 billion miles away, and Voyager 2 is just slightly closer, a 9 billion), and are now poised to become the first man-made objects to leave the bubble around our sun known as the heliosphere and enter what is known as interstellar space.
There is some chance Voyager 1 might be there already.
That's a chart of the rate of particles (mostly protons) from the sun hitting the probe. And as you can see, there's a steep drop-off, which began in late summer.
This is in addition to evidence from earlier in the year that Voyager 1 was picking up an increase in particles that are an indicator of cosmic rays, something the probe would see more of in interstellar space.
Two out of three ain't bad, but there's still one major piece of data that will take Voyager's scientists longer to analyze, meaning that any official announcement (even if Voyager has indeed already crossed over) may still be weeks off. And that's the data from the magnetometer, which will show changes in the magnetic field. "We have the data, pretty much up to date, it just takes a while -- quite a while -- to go through and very carefully calibrate the data," Ed Stone, project scientist of the Voyager mission, explained to me.
What are scientists looking for in the magnetic field data and why is it so hard to process? Inside the heliosphere, the magnetic field is a large spiral moving in the sun's east-west direction. Outside, the magnetic field is coming from the galaxy, not the sun, and was trapped in a cloud, which was created by supernovae five or 10 million years ago. "And so we expect that the magnetic field will have a different direction than it does inside," Stone said.
But it's very hard to measure. As Stone explained: "It turns out the field is very weak out there. Something like 1/100,000th of the strength of the field here on Earth, and the spacecraft itself generates magnetic fields, so we have to be very careful analyzing the data to be sure we're measuring the ambient magnetic field and not the field of the spacecraft for instance."
Part of what is so hard about recognizing this transition to the outside of our heliosphere is that we don't for sure know what it will look like, how the boundary will vary and finally give way to interstellar space. We don't know, because we've never been there before.