Remember when I said back in October that Voyager 1 might have finally left the solar system? Well, it turns out that the spacecraft, which has been skirting the edge of the solar system for a long time now, is finding it difficult to say goodbye.
According to scientists working on the mission, Voyager 1 has just entered a "magnetic highway". They hope this road will lead Voyager out of the heliosphere, away from the influence of the sun's magnetic field… eventually.
Ed Stone, Voyager project scientist, based at Caltech, explained that this highway is connected to interstellar space, but is still inside the magnetic field of the Sun. It is "a new region of the heliosphere". NASA scientists are calling it a highway because it lets particles in and out of the solar system.
The heliosphere is a huge bubble the Sun creates around itself, through the solar wind. The interstellar medium is a particle wind outside of the solar system that pushes against the heliosphere. The boundary between the two is known as the heliopause. Before Voyager got close to the edge, scientists thought the boundary would be quite clear cut. Now they are realising that it is much messier than they anticipated.
Scientists working on Voyager are looking for three key signs to confirm when the spacecraft exits the heliosphere. We should see the particles created by high energy cosmic rays increase in number, and slow particles from inside the heliosphere should disappear. Lastly, scientists think the magnetic field will change direction, from East-West inside the heliosphere, to North-South outside it. There are three instruments on Voyager to measure these three signs.
This magnetic highway is within the heliopause. If you're counting that as the boundary to the solar system (and I am), that means Voyager is still within the solar system. But it is surrounded by particles from outside the heliosphere. "Now Voyager is in a region still in the magnetic highway where it's surrounded almost entirely by cosmic ray particles that have come in from outside," says Stone.
The first sign of this new region came on July 28 when the intensity of solar particles dropped by a factor of two, But, within five days of falling, the intensity of particles came back up. On August 13 there was another dip. And, shortly after, another return to the previous level. You can see these dips on the figure below.
You will probably also notice is that there was a third dip, around August 25. This was the graph that kick started the speculation back in October about Voyager possibly leaving the solar system. But the reality was more complicated than that.
For seven years, since it passed the termination shock in December 2004, Voyager had been in a region where slow-moving particles from the sun randomly move in all directions. But August 25 was the day it crossed over into the magnetic highway. Now suddenly it is in a new region where particles from the sun are streaming out towards interstellar space, and outside particles from cosmic rays are streaming in. The intensity of solar particles dropped that day, and hasn't picked up since.
"We are now at the highest intensity we have yet seen of the particles coming in from the outside," says Stone. You can see that from the below figure. The top half shows the same data as above, but the bottom is the cosmic ray data.
The vital sign that wasn't ticked off in October was changes in the magnetic field data. At the press conference today, Leonard Burlaga, Voyager magnetometer team scientist at NASA's Goddard Space Flight Center, explained how the magnetic field intensity has increased and remained higher than before since Voyager crossed the boundary into the new magnetic highway region.
But there's nothing to show that Voyager 1 is outside the heliosphere yet. "There's no evidence that we have entered the interstellar magnetic field," says Burlaga.
Voyager scientists are still not sure how long it will be before Voyager gets off this highway and finally, finally, makes it out into interstellar space. "It may take several more months, it could take several more years," says Stone. "But we do believe this may be the very last layer between us and interstellar space."
Voyager's interstellar future could look a little bleak, depending on how you see it. By 2020, Voyager scientists will start powering down some instruments to save remaining power reserves. By 2025 power will be so low that all instruments will have to be turned off. After that Voyager will drift.
We may be able to ping it, now again, to check it's still there. But it won't be able to send us any data. Eventually, even the transmitter will stop working. Voyager 1 will be on its own.
In 40,000 years it'll pass by one of our nearest neighbour stars. Though even that pass, at a distance of 1.6 light years, will hardly be close. After that it'll be billions of years before Voyager gets near again to any other stars.
On the one hand, it seems sad that Voyager will wander through the stars, not able to send us information about any new worlds it encounters. On the other hand, in billions of years, Voyager might reach another star! Something humans made, out there in the universe* forever. It's quite a nice thought.
For now though, we're still communicating with both Voyager spacecraft. "We talk to both spacecraft everyday," says Suzanne Dodd, Voyager project manager. "They're both healthy and robust and continuing on with this incredible journey."
I'm refusing to say goodbye to Voyager 1 just yet. It doesn't seem too keen to say farewell itself, either.
*Thanks to bradmiller in the comments for pointing out that I originally mistyped 'solar system' here. Hopefully not a premonition...