I recently wrote about what we’ve learned from the space probe Rosetta as it’s orbited the comet 67/P Churyumov-Gerasimenko: its structure, composition, size, mass, and more.
And while the up-close-and-personal view is amazing, that’s not to say we can’t do some stuff from back on good ol’ Earth, too.
The image above shows the comet on Jan. 19. It was taken with the 2.5 meter Isaac Newton telescope on the Canary island of La Palma. The image is pretty deep, showing quite faint structures.
Most obvious is that the comet has two tails! This is common in comets. One is the gas tail, due to ice on the comet turning to gas and getting blown back by the fierce solar wind). The other is from fine grain dust that is pushed by the pressure form sunlight and falls behind the comet in its orbit.
However, that’s not what you’re seeing here! They’re both dust tails in this case—kinda—and they have very different positions in space.
The upper tail does appear to be aligned with the comet’s orbit, so that’s dust that’s been recently liberated from the solid core (called the nucleus) of 67P. But the lower tail appears thinner, and if you look carefully, you can see it actually extends a bit to the left of the comet head!
That’s a dead giveaway that we’re seeing a geometrical effect (the dust in a tail can’t get ahead of the comet). This second, lower tail isn't really a tail. It's actually larger-grain dust emitted from the comet last year, probably around March, which formed a cloud around the nucleus. Because the particles are bigger they don't get blown back as much by sunlight pressure, but the cloud does tend to flatten out over time. From Earth, this looks like a second tail, called a “neckline.”
This is all pretty interesting to me; I’ve never heard this term before with comets! When I saw the image, I just assumed one was a dust tail and the other gas, but appearances can be deceiving. I’ll note, too, that all of this is essentially invisible to Rosetta, because it’s sitting so close to the comet nucleus that it’s surrounded by this cloud; the light is spread out so much the cloud is too faint to spot.
You can learn a lot by getting as close as you can to an object, but it can help to take a step back and get an overview. Sometimes that’s what tells the tails.