"Now, voyager, sail thou forth, to seek and find"

Say hello to a brand new bit of the solar system, brought to you by that intrepid traveller Voyager 1: the heliosheath depletion region.

Ok, the name's not particularly catchy, but this is exciting news! Voyager launched over 35 years ago, but it was just before that anniversary last year that it entered this new, uncharted territory.

We've been getting hints that the furthest spacecraft from Earth has discovered something new and unexpected for a while. Nasa even said as much last December.

Today sees three new papers published on Voyager's latest adventures on the brink of interstellar space. What they show is that Voyager 1 is in a new region of the solar system -- not quite out, but not quite in at the same time. Scientists working on Voyager are calling this place the "heliosheath depletion region".

Yep, what Voyager's instruments are now showing us is so odd we need a new name for it. Voyager is, almost literally, pushing the boundaries of our knowledge about the solar system.

Which, if you think about it, is hardly surprising. As Stamatios Krimigis of Johns Hopkins University, Maryland, and his colleagues write in one of the three papers out today, our ideas about the size and shape of the bubble of plasma we call the heliosphere, created by the solar wind that continuously flows from the sun, are older than the space age.

But back to the results. Essentially, there's no headline news here that wasn't announced in the teleconference Nasa held in December last year. What's interesting, as ever, is the detail.

Anyone who's been following Voyager's journey will probably already know that there are three key signs scientists are looking out for to figure out whether the spacecraft has crossed into the interstellar medium yet. They are: a change in direction of the magnetic field, an increase in cosmic rays from outside the solar system, and a decrease in particles from the sun.

The heliosheath depletion region has two of those three. Solar particles are down, cosmic rays are up, but we're still missing that crucial change in direction from the magnetic field.

Magnetic field

In the first paper of the trio, Leonard Burlaga, Norman Ness and Ed Stone report magnetic field data that show Voyager 1 crossed a boundary into this unknown region of the universe five times between 29 May 2012 and 26 September 2012.

In other words, it dipped its toes in, scuttled back out, went in again, came out again, before finally crossing the boundary a fifth time. This time, it stayed. You can see Voyager's hokey-cokey in the figure below.

Magnetic field strength (top) and particle count (bottom) showing the period 29 May 2012 to 26 September 2012. Credit: Burlaga et al, Science, 27 June 2013. DOI: 10.1126/science.1235451

The jumps in magnetic field intensity during this dance, coupled each time with drops in the number of charged particles, were "unlike anything observed previously by [Voyager 1]," say Burlaga and colleagues. But the direction of that magnetic field stayed stubbornly still every time Voyager crossed the boundary. I'm afraid we're not in interstellar space just yet.

Cosmic rays and solar particles

Krimigis, who heads the Low Energy Charged Particle instrument team, and his colleagues, in the second paper out today, report a huge decrease in particles from the sun on 25 August 2012, along with a steady increase in cosmic rays from outside the solar system since Voyager 1 entered the newly-named heliosheath depletion region. That was the hint in October last year that got everyone (ok, me) speculating that Voyager had said it's final goodbyes. A third paper, by Ed Stone and colleagues, looks at this 25 August change using data from a different instrument, noting again that low-energy ions from the heliosphere suddenly vanished and were replaced by an influx of cosmic rays.

This is the event that was reported in March this year in the journal Geophysical Research Letters. A press release about the paper made the mistake of saying Voyager had left the solar system, and caused such a fuss Nasa eventually had to be a bit of a party pooper by saying it hadn't.

At the time I got in touch with the first author of that paper, Bill Webber of New Mexico State University (he's also an author on the third paper published today). His reply was so lovely I'm just going to reproduce a whole chunk of it here:

"First I don't think any of us on the CRS [Cosmic Ray Subsystem, an instrument on Voyager] team will ever forget watching on the computer monitors, even on an hourly basis, in one case, as some particle intensities dropped precipitously, and others increased simultaneously on several occasions in July and August, 2012. The changes were so large and totally unexpected that some of us actually wondered if the instrument was broken. But on the final decrease starting on August 25th the intensities just kept going down (or up) by 10~100 times and we could see it all happening day by day. What a triumph for Voyager and modern technology, to be able to observe these events as they were taking place (plus 17 hours of light travel-time over 11 billion miles away).

And then, lo and behold, after the heliospheric particles vanished, a faint signal persisted. It was the faint signal of low energy galactic cosmic rays with the peak energy 30-50 MeV, just about where some earlier predictions suggested! Just what the CRS instrument was designed to measure over 40 years ago."

From a particle point of view, he says, they considered Voyager 1 as good as gone. But the magnetic field data was going to take longer to interpret. Now we know that the magnetic field says no, or at least "not yet".

So, we wait. We wait while Voyager explores this new region of space. We wait to hear of any change in the magnetic field Voyager sees. We wait for news that we've, kind of, sort of, become an interstellar species.

But I hope we don't have to wait long. By 2020, Voyager's power supply will start running low, and one instrument will need to be turned off. Exactly which one is as yet undecided. Let's hope that, by that point, it doesn't matter if its the magnetometer.


1. Magnetic Field Observations as Voyager 1 Entered the Heliosheath Depletion Region

2. Search for the Exit: Voyager 1 at Heliosphere’s Border with the Galaxy

3. Voyager 1 Observes Low-Energy Galactic Cosmic Rays in a Region Depleted of Heliospheric Ions

Image: Impression of Voyager on the 'magnetic highway', now called the heliosheath depletion region. Credit: NASA/JPL-Caltech