Going Where No Probe Has Gone Before

The headlines about Voyager's achievement are confusing: "In a Breathtaking First, NASA’s Voyager 1 Exits the Solar System" says the New York Times; "NASA confirm Voyager 1 has left the solar system" says the LA Times; "Voyager 1 Reaches Interstellar Space. But Has It Left the Solar System? Wellllll…" says Slate; "Where does the solar system end? Voyager isn't officially there yet" from NBC News; and "Stop Saying Voyager 1 Has Left the Solar System" says Motherboard. As Heidi Klum says on Project Runway, you're either in or you're out. Which is it?

An artist's impression of Voyager 1 passing into interstellar space, represented by the brown haze. Credit: NASA/JPL-Caltech

According to a new paper in Science and experts at NASA, Voyager did reach an important milestone in 2012, passing through the bubble of the solar wind - fast-moving particles that blow away from the sun - into a region that scientists are calling interstellar space. A graphic from the New York Times explains this well.

Is this the same as leaving the solar system? No, because the Oort Cloud - containing comets - is part of the outer solar system and extends out to around 100,000 astronomical units (AU), where one AU is the distance from the sun to the earth. By contrast, when Voyager 1 passed into interstellar space last year it was located at a distance of only about 122 AU. So, it has a long way to go before leaving the solar system, as shown in this figure from NASA. (Note that the distance scale is logarithmic, so the Oort cloud is much larger than it appears from glancing at this figure.)

A figure showing the heliosphere (containing the solar wind), and the region outside it containing interstellar space and the Oort Cloud. The x-axis is in astronomical units (AU) when 1 AU is equal to the distance from the Sun to the Earth. Credit: NASA/JPL-Caltech

As pointed out by Matthew Francis in a blog post, the outer boundary of the solar system is much more poorly defined than the interstellar boundary discussed above, and Voyager is not going to reach it any time soon.

I respect the effort NASA & some writers took to describe Voyager's milestone accurately. With tricky concepts like this it's tough to communicate a consistent story. For example, Alan Boyle carefully explained that Voyager has not left the solar system, but the video at the top of the web-page shows Brian Williams on the National News confidently declaring that Voyager left the solar system. Similarly Joel Achenbach gets the story right but a video's caption says "Voyager 1 has crossed a new frontier, becoming the first spacecraft ever to leave the solar system, NASA said Thursday". Also, at the bottom there's a link to a story in a different part of the Washington Post that's titled: "Voyager 1 just left the solar system using less computing power than your iPhone".

How did NASA handle this communication challenge? Alan Boyle's article says

""It's a very fine point, and many people don't realize the Oort Cloud is in interstellar space and it's considered part of the solar system," Veronica McGregor, JPL's news chief, wrote during a Reddit AMA chat session. "We knew many media would make the error, and we tried to make it clear in interviews. None of our materials say we've exited the solar system."" 

This is true, although it might have helped to make an explicit comment that Voyager 1 has not left the solar system in their press release. The nearest they get is this vague comment: "Scientists do not know when Voyager 1 will reach the undisturbed part of interstellar space where there is no influence from our sun." They do make a clear statement in this article, but it’s harder to find.

An artist's impression of the Oort Cloud, drawn to scale. Credit: NASA/JPL

The idea that Voyager 1 has passed into interstellar space and is still in the solar system is definitely confusing, as it seems like these two possibilities should be mutually exclusive. The definition of interstellar space used here refers to the region outside the influence of the solar wind, where the density of plasma is much higher. The Science paper doesn't even use the term "interstellar space" but "interstellar medium". In an informal sense we are living in interstellar space, because we're in a galaxy between the Sun and other stars.

There were other chances for the Voyager team members to explain the milestone. Mike Wall did an interview at space.com with Ed Stone, the project scientist for Voyager. Wall's first question was "So how do you feel now that Voyager 1 has finally left the solar system?" and Stone answered: "It's been a goal right from the beginning of the project to reach interstellar space" as though leaving the solar system and reaching interstellar space are the same thing. He doesn't make an attempt to correct the question. Later Wall asks: "You'd been saying for a long time that you were looking for three signs that Voyager 1 had left the solar system…” Stone answers by discussing the solar bubble and again does not correct the question.

This agnostic approach might have been fine if it wasn’t for all of the other messages out there. They had to counter previous reports that Voyager had left the solar system, as described in several of the press articles mentioned above. Perhaps even more problematic was a News and Analysis article from Science titled: "It's Official - Voyager Has Left the Solar System". This article was part of Science's embargo package and was made available to reporters on Wednesday, September 11th, a day before the embargo came down and two days before the paper was published (update: an earlier version of my post said "several days before the paper was published, which was too vague). It may have influenced much of the press coverage, since a large number of articles used similar wording. Some stories were prepared during the embargo period and published as soon as the embargo went down, or not long after. Because the NASA release and press conference occurred after the embargo went down, they had less influence on the first wave of stories. I expect that NASA did not know about the News and Analysis article’s headline in advance.

Given all of these challenges, it's hardly surprising that the story got muddled. To be clear I'm not sure there was any completely satisfactory way to explain this milestone. This isn't unusual in science communication. I know astronomy is full of fuzzy terms and challenging concepts. Examples include the definition of a planet, defining the edge of a galaxy and explaining cosmological distances. This is a field where planetary nebulas have nothing to do with planets and higher magnitudes mean that a star is fainter. Other fields may be even more difficult.

Another problem is that a catchy name or impressive line can be very seductive. “Voyager enters interstellar space for the first time” is cool, but “Voyager leaves the solar system for the first time” is even cooler. It’s hard to remove a cool concept or name once it has taken root - think of the popular alternative name for the Higgs Boson, which I won’t repeat here - but, I think it’s worth trying.

There are many challenges in publicizing new science findings. It's obviously important to be accurate and interesting. There are also times when describing what you didn’t do is almost as important as describing what you did.