So there's this comet named 168P/Hergenrother. It's one of a bazillion such iceballs orbiting the Sun, but this one turns out to be more interesting than most. For one thing, it has a short period, orbiting the Sun once every 6.8 years or so. Its orbit goes out to about that of Jupiter's, and reaches down into the inner solar system about as far as Mars. It never gets closer than about 80 million kilometers (50 million miles) to us, so it's usually relatively faint, and you need a big 'scope to observe it. It was discovered in 1998, and made a second pass down our way in 2005. This year, 2012, it came by again, and folks around the world observed it as they do any comet. But then, in September, it gave us a surprise. A big one. Lots of observers were reporting that practically overnight the comet grew hugely in brightness, getting as much as 700 times brighter than expected! Not only that, but observations showed the shape of the comet had changed, going from fairly point-like to much fuzzier. That could mean only one thing. The comet was breaking up.
The picture above is from the Faulkes Telescope North, located on the Hawaiian observatory on Haleakala. It's a composite of lots of separate exposures that were added together; you can see the stars are trailed (actually stippled; each exposure was short but then shifted to line up on the comet). The comet is the bright fuzzy blob in the upper right, and if you look just below the main part you can see a second fuzzy blob, much fainter. That's proof positive the comet calved, or had a big chunk break off. In fact, observations using the huge Gemini telescope show that the main body has broken up into at least four pieces! So what does this mean? First, don't panic. We've seen this happen to comets before, and this one is so far away from us we're in no danger at all. It literally cannot get near us. Second, it's very interesting scientifically. Comets are basically big frozen snowballs peppered with rock. Imagine scooping up a handful of gravel and snow and then packing into a loose ball. That's a comet, if your snowball is several kilometers across and the ice is actually frozen water and carbon dioxide. When they are far from the Sun comets stay frozen and are exceedingly dim. When they get closer, the ice goes directly to a gas (called sublimation), and escapes from the solid part (called the nucleus). It expands and can form a big fuzzy head around the solid nucleus that can be tens of thousands of kilometers across, bigger than planets! This is also what gets blown back by the solar wind (and the pressure of sunlight) to form the tail(s) of the comet. This means that every time a comet gets closer to the Sun and starts to sublimate, it dies a little bit. Material leaves the comet and never comes back. But that ice is what holds the comet together! So sometimes enough ice turns into a gas and escapes that the comet gets substantially weaker, and big chunks of it can dislodge, falling away. That's what appears to have happened to Hergenrother.
While we've seen this before with other comets, it's not like it happens every day, so any chance to see this occur is fascinating. In 2006 we watched as comet 73P/Schwassmann-Wachmann 3 literally disintegrated. Even more amazing, in 2007 the run-of-the-mill comet 17P/Holmes suddenly erupted, getting hugely brighter, and a huge shell of dust was seen to be expanding around it. Now we think Holmes collided with a small asteroid, and the violence of the event blasted off the material. I saw Holmes with own eyes when this happened, and even though it was past the orbit of Mars, the shell of dust was easily visible to the naked eye. It was awesome. No two comets are ever really alike. They have different sizes, shapes, compositions, and orbits. And each will behave slightly differently as they round the Sun and head back into deep space. If there's a lesson from Hergenrother, it's this: it's always a good idea to keep an eye on everything in the sky. Just because something looks routine now doesn't mean it won't try to pull a fast one later.
Tip o' the Whipple Shield to AsteroidWatchNickAstronomer. Image credits: Hergenrother: LCOGT/Giovanni Sostero, Nick Howes, Alison Tripp & Ernesto Guido; Holmes: Tamas Ladanyi