The two Voyager spacecraft are the longest-running missions humans have sent into the deep. They’ve been traveling outward from Earth for more than 30 years, passing Neptune’s orbit more than 20 years ago. Now, data accumulated over the past few months from Voyager 1 indicate very strongly that this venerable probe is about to leave the solar system and step into true interstellar space.
Holy. Wow. Think on that: We are about to taste the space between the stars.
What does all this mean?
The Sun is one star of billions in the Milky Way galaxy. In between the stars is a very thin soup of gas, really just individual atoms and subatomic particles. The Sun is blowing off a solar wind, a far denser stream of subatomic particles that flows past the Earth, past all the planets, and into deep space. But at a distance somewhere beyond 18 billion kilometers (11 billion miles, or four times the distance of Neptune from the Sun) the solar wind slams into that stuff between the stars, slows, and then stops.
That point—called the “heliopause”— is what astronomers consider to be the end of the solar system and the beginning of interstellar space. We haven’t known exactly how far out that is because it’s so far that no instruments from Earth can detect it.
But Voyager 1 is nearly there. And it’s seeing three things that show it’s about to breach that final frontier.
For one, interstellar space is full of cosmic rays, which are subatomic particles created in supernova explosions and zipping around at high speed. They have all different energies and speeds. The slower, lower-energy ones are stopped by the Sun’s magnetic field. But the Sun’s magnetism essentially stops at the heliopause. For a while now, Voyager 1 has been inside the Sun’s magnetic protection, so it hasn’t seen these low-energy cosmic rays. However, starting just a few months ago, the number it detected jumped way up.
Second, the Sun’s own solar wind doesn’t go beyond the heliopause. Once Voyager 1 gets outside that point, the number of solar wind particles it detects should drop … and yup: That started a few months ago, too. Not only that, but the number of solar particles dropped at the same time the cosmic ray numbers jumped up, indicating Voyager 1 has entered a new region of the solar system. It’s not quite at the heliopause, but in a place a bit like a highway interchange, allowing outside particles in, and inside particles out.
Voyager 1 is on the exit ramp.
Finally, the Sun rotates, and that winds up the solar magnetic field around it like a spiral. Until recently, Voyager 1 has been able to see this; the Sun’s magnetic field has an east-west direction to it. But just recently, it’s detecting a north-south component to the magnetism, just what you’d expect as the solar magnetic field strength drops and the interstellar magnetic field picks up.
In other words, the evidence really points to Voyager 1 being on the doorstep of the galaxy.
Some of these data have been around a while now, but I’ve been reticent to write about it because it just wasn’t quite convincing enough to me. But together, these three lines of evidence look very good. It’s hard to say just how long it will be before Voyager 1 is officially and totally out of the solar system, but this is the first clear indication that we’re entering the final boundary of the Sun’s sway.
And when it does, humanity will truly be an interstellar civilization. That’s a milestone worth pondering, and definitely worth celebrating.