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SYFY WIRE Bad Astronomy

Voyager 1 Spacecraft on the Doorstep to Interstellar Space

By Phil Plait

The two Voyager spacecraft are the longest-running missions humans have sent into the deep. Theyâve been traveling outward from Earth for more than 30 years, passing Neptuneâs 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 pointâcalled the âheliopauseââ is what astronomers consider to be the end of the solar system and the beginning of interstellar space. We havenât known exactly how far out that is because itâs so far that no instruments from Earth can detect it.

But Voyager 1 is nearly there. And itâs seeing three things that show itâs 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 Sunâs magnetic field. But the Sunâs magnetism essentially stops at the heliopause. For a while now, Voyager 1 has been inside the Sunâs magnetic protection, so it hasnât seen these low-energy cosmic rays. However, starting just a few months ago, the number it detected jumped way up.

Second, the Sunâs own solar wind doesnât 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. Itâs 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 Sunâs magnetic field has an east-west direction to it. But just recently, itâs detecting a north-south component to the magnetism, just what youâd 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 Iâve been reticent to write about it because it just wasnât quite convincing enough to me. But together, these three lines of evidence look very good. Itâs 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 weâre entering the final boundary of the Sunâs sway.

And when it does, humanity will truly be an interstellar civilization. Thatâs a milestone worth pondering, and definitely worth celebrating.

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