Stop Saying Voyager 1 Has Left the Solar System

Voyager 1 has sort of become the little spacecraft that cried interstellar. It seems like the 36-year-old probe is in the news every few months with reports that it's left the solar system, reports that are then pulled back immediately; nope, it's still kicking around, still on the verge of becoming an interstellar spacecraft. It's back in the news now, but this time, it's the real deal: Voyager 1 is really and truly in interstellar space.

But there's a bit of a catch: it’s still in the area governed by the Sun’s gravity. So it’s in interstellar space, but still in the Solar System.

Unfortunately, there’s no clear line in space that demarcates the beginning of interstellar space. It’s a murky line that’s not really a line, which is why we’ve seen so many false alarm announcements. The “line”, as it were, is a change of plasma, the ionized gas that permeates space. The material that comes from the Sun in the form of solar wind is a plasma and it only stretches so far, creating the bubble known as the heliosphere. Once the solar wind material no longer dominates and material from distant stars does, that’s interstellar space.

So to say that Voyager 1 is in interstellar space in this case means it’s reached an area where the Sun’s solar wind and plasma no longer have the dominant influence. It’s a definition that takes into account particles and fields, not distance.

To complicate this already murky line between Solar System and interstellar space is the fact that Voyager 1 is 35 years old and some of its instruments have long gone silent. In 1980, Voyager 1 lost its instrument that measured the temperature, density, and speed of plasma. Mission scientists were able to model what the plasma change would look like, but they didn’t have any data from Voyager 1 to back up their predictions.

But Voyager 1 does have two antennae that measured the direction of the magnetic field environment, and the science team used this data as a proxy for the source of the plasma. Solar plasma and interstellar plasma both carry magnetic field lines. Not only were these lines expected to be in a different direction once Voyager left the heliosphere. There was also expected to be an increase in material originating from cosmic rays.

The first indication of the change came in May 2012 in the form of an increase in cosmic ray particles. Voyager 1 was in a new region, but data recovered in August was inconclusive about whether this was outside the heliosphere or not. The increase of cosmic ray particles was at odds with the minor change in the direction of the magnetic field lines.

On April 9, 2013, Voyager 1’s plasma wave instrument measured oscillations in local plasma that scientists suspected stemmed from a solar burst a year previously. They went back through old data and found other, low-frequency oscillations while the spacecraft was still in this plasma-dense region. The team called a meeting to define the boundary between the heliosphere and interstellar space and determined this denser plasma was interstellar. They settled on August 2012 as the time when the spacecraft truly entered this new region. So after a year of false starts, Voyager has become an interstellar spacecraft.

But it hasn’t really left the Solar System, a detail that’s bound to cause some confusion. The beginning of interstellar space isn’t the same as the end of the Solar System. Since the 1960s, scientists have generally taken the Oort cloud, that icy rocky region from where comets fling towards the Sun, as the edge of the Solar System. It’s the last region where object are held in place by our Sun’s gravity. Any further and the gravity of other stars take over.

But we’re not going to see that happen. It’s going to take Voyager 1,300 years to reach the inner Oort cloud and possibly another 30,000 to fly beyond it. In about 40,000 years, the spacecraft will be closer to the star AC + 79 3888 than our Sun.

Formal beginnings and ends of cosmic regions aside, Voayger 1 has definitively left the heliosphere, which means it’s in truly unchartered waters. From this point on, it’s going to raise as many questions as it’s going to answer as it tears through this completely unknown region of space. And the science team is thrilled.

Free from the dominating influence of the Solar Wind, the science team is focussing on things they’ve never had a good chance to study. High on the list is interstellar cosmic rays, high energy particles that originate in distant galaxies. Now in interstellar space, Voyager 1 is getting history’s first clear look at this phenomenon, which is sure to help scientists working on a variety of deep space missions. The Voyager team is also turning its attention to understanding the interaction between the interstellar medium and the solar wind, because the two regions interact and affect one another.

Science aside, there’s the human aspect of Voyager 1’s interstellar status. That spacecraft is carrying a slice of humanity with it, the golden record with voices, sounds, and images of the world as it was in the mid-70s. It’s probably the greatest time capsule of all time. And though the odds of an alien race finding the record, figuring out how to play it, then following the directions printed on the panel to pay Earth a visit are painfully slim, it’s neat to think that this little spacecraft is taking a piece of us into interstellar space with it.