Voyager 1 scans interstellar space

New discoveries from an old probe
18 May 2021

Interview with 

Stella Ocker, Cornell University


A large astronomical telescope against a dark starry sky.


This week Adam Murphy spoke to Stella Ocker from Cornell University, who has been looking into new discoveries, published in Nature Astronomy, from a 40 year old probe, as Voyager 1 makes its way through interstellar space...

Adam - In 2012, the probe Voyager One passed beyond our solar system and into the emptiness of interstellar space. Except even interstellar space isn't empty, not quite. There's a very tenuous plasma surrounding our solar system, which is like a gas that's had all its electrons separated from its atoms. The stuff is called the interstellar medium. And to put it in perspective, air at sea level has more than a billion billion particles in every milliliter. In the interstellar medium that Voyager's swimming in, there's about one particle in every 10 litres, if my math is correct. We've been able to see big disruptions before in this stellar soup, but it's only now that we've managed to pick up the low underlying signal of the space between stars.

Stella - I had spent so long looking at these data that I had come to the point where I really wasn't expecting to find anything new. And then to kind of suddenly notice this, this really faint signal was almost a breath of relief that I hadn't wasted the past few months of my life.

Adam - That's Stella Ocker from Cornell University who spotted the signal. But getting new science from a probe launched in the seventies is a big ask and spotting it is no mean feat. So what is the signal? What is Voyager actually detecting?

Stella - Because a plasma consists of both electrons and protons. It's usually found in charge neutrality. The positive charges of the protons kind of cancel out the negative charges of the electrons. But if you have something perturb the plasma and cause the electrons to become displaced, then you end up setting up an electric field between the electrons and protons that tries to bring them back together and restore the plasma to charge neutrality. And so the electrons essentially try to come back from where they've been displaced and in doing so, they essentially vibrate. And so what you see is that the plasma vibrates in response to this displacement of the electrons and those vibrations are what we can detect using Voyager One.

Adam - Which means that really all Voyager needs to pick this up is a really good antenna, and someone willing to comb through all that data. But Voyager entered the interstellar medium in 2012 and the signal was only first spotted in 2017. So there was a while there where we didn't see this little hum of the interstellar medium, but that doesn't mean there was nothing.

Stella - Before we detected this really faint persistent vibration of interstellar plasma, we had been seeing these bright plasma oscillation events. These are very massive vibrations that are triggered by solar activity like flares or coronal mass ejections. And those really massive vibrations only last a short period of time. And so we saw those roughly once a year since Voyager One entered the interstellar medium. But we're not entirely sure why we've only just started to pick up this really faint persistent vibration.

Adam - And this isn't just for fun. It gives us a lot of new data as to the structure of the galaxy we live in.

Stella - So by measuring the frequency of these really faint vibrations, we can infer the density of interstellar space. And because these vibrations persist over such a long stretch of time, we can achieve an almost continuous measurement of the density over the entire three-year period we've found to the signal. So that means we can measure the density now over a straight stretch of space that's equivalent to about 10 times the distance between the earth and the sun. And that's the most complete map of the interstellar density that we've had to date. And the reason why that's useful is because knowing how the density behaves tells us about how plasma is distributed outside of our heliosphere. And it tells us what the structure of the interstellar medium actually is in our sun's local interstellar environment.

Adam - And more than that, these faint kinds of specific signals can sometimes be seen in our own atmosphere. So it might help us understand our own planet a little better. And it's pretty exceptional and pretty cool what a 40 year old probe can still do.

Stella - I think for me, the main message is that Voyager One continues to make new discoveries, even from 14 billion miles away


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