It’s reasonable to call the two Voyager spacecraft our first interstellar probes, in the sense that they are approaching the heliopause and are still transmitting data. Long before controllers shut them down — which should occur somewhere in the 2020s — Voyager 1 will have left the Solar System and we’ll have data on what happens when the solar wind gives way to the stellar winds from beyond. A case could be made for the Pioneer craft as interstellar probes as well, but while Pioneer 10 has reached a distance of 107 AU, the Pioneers are no longer transmitting data. Voyager 1 is now 123.45 AU out, for a round-trip light time of 34 hours, 15 minutes.

But does leaving the Solar System mean we’ve truly entered interstellar space? An entertaining piece called Postcards from the edge, published in early February by The Economist, notes that much depends on how we define ‘interstellar.’ Gravity, says its author, defines the universe at the largest scales, and if we’re talking about gravity, Voyager is still deeply in the grip of the Sun. In fact, Voyager 1 would have to travel another 14,000 years to reach the roughly 50,000 AU distance where the Sun’s gravity would cease to be a factor.


Image: Voyager 1’s cameras were turned back on in early 1990 to take pictures of our Solar System. The spacecraft took some 60 pictures of the Sun and 6 of the planets; the 60 frames were combined to make the mosaic seen above. The six individual shots were taken when Voyager 1 was more than 4 billion miles from Earth. Earth appears framed in brightness due to the amount of light scattered while taking the picture with Earth so close to the Sun. Credit: NASA/Caltech.

Over the weekend I’ve enjoyed playing around with deep space scenarios and thinking about what the sky might look like from Voyager 1. When I was growing up, some accounts I read of the outer Solar System implied that the Sun would be nothing more than another star from a place like Pluto. That turns out to be incorrect, as Phil Plait pointed out in a 2012 post. Working with Pluto’s average distance from the Sun — 39 AU — Plait found that the Sun would still be 250 times brighter on Pluto than the full Moon appears on Earth. Factor in the eccentricity of Pluto’s orbit and the actual numbers move from 150 to 450 times as bright as the full Moon.

So let’s ask this: How far would our Voyagers have to be from the Earth before the Sun began to look like just another star? It turns out we’d have to go a long way. From 400 AU, a distance at which we’re definitely in the local interstellar medium, the Sun still shows an apparent magnitude of -13.7, which makes it the brightest star in the sky (Sirius comes in at -1.46). This information comes from Mike Gruntman (USC), who worked it out in a paper studying what kind of instrumentation we’d like to have aboard a probe expressly designed for interstellar space.

The Sun obviously still dominates Voyager 1’s sky. According to Gruntman’s figures, a probe would have to reach a distance of 100,000 AU — 1.61 light years — before the Sun would finally be perceived as just another bright star. In these terms, the Voyagers are close to home indeed, but work continues on longer-range spacecraft. The ‘pale blue dot’ image showing our Earth as just an inconsequential blob of light is justly famous. Maybe the next perspective changer will be a glimpse of our system from beyond the heliosphere. From the paper:

As our first interstellar spacecraft leaves the solar system, a ”look back” would provide us with an unusual view of our home stellar system, a view from the outside. A view back will provide a unique opportunity for a global study of the heliosphere, a vast essentially 3-D region governed by the sun. This view back would also be a glimpse of what a truly interstellar mission of the distant future would encounter in approaching a target star. A combination of obtaining images from two vantage points, one from the outside of a stellar system and one from inside, would allow the characterization of an astrosphere.

In Centauri Skies

Thinking about the brightness of the Sun in Voyager’s sky brings up the question of how other stars look from space. But first, how bright would the Sun be if it were seen from the distance of Alpha Centauri? We have to distinguish between apparent magnitude (as seen by an observer on Earth) and absolute magnitude, which measures intrinsic brightness. The Sun’s apparent magnitude in our sky is -26.8. And we learn thanks to the good folks at Cornell University that seen at Alpha Centauri distance, the Sun’s apparent magnitude becomes +0.34. By contrast, Centauri A’s apparent magnitude in our sky is -0.01; Centauri B’s is 1.33.

And in case you’re wondering, from a planet orbiting Centauri B at 1 AU, the apparent magnitude of Centauri A ranges from -21.9 to -19.4, changing as Centauri A and B orbit around each other. During this 80-year period their separation varies from 11 to 35 AU. Jean-Louis Trudel (University of Ottawa) and Edward Guinan (Villanova University) worked the apparent magnitude figures out for science fiction writer Robert Sawyer, as presented here. From a planet orbiting Centauri A at 1 AU, the apparent magnitude of Centauri B ranges from -18.1 to -20.6.

We’ve learned that descriptions of the Sun as ‘just another star’ from distances of mere hundreds of AUs aren’t correct. Maybe the ‘just another star’ idea fits better in our neighboring stellar system, for Proxima Centauri would be anything but prominent in the night skies of Centauri A and B. Astronomers there would surely notice it sooner or later, because Proxima’s proper motion would tell them it was close. But this M5-class red dwarf would be a nondescript 5th magnitude star otherwise. From Earth, with an apparent magnitude of 11, Proxima is visible only in telescopes.

The paper by Mike Gruntman that I reference above is, “Instrumentation for Interstellar Exploration,” Advances in Space Research 34 (2004), pp. 204-212 (available online).