Editor's note: Space blogs are abuzz about the recent discovery of comet C/2012 S1 (ISON), which has the potential to be a bright one late next year. I turned to amateur astronomer Bill Gray to ask for more details and whether I should be getting excited about this comet yet. Here's what he told me. --Emily Lakdawalla
It looks potentially very interesting indeed. The orbit is very well-determined. We can say, with complete confidence, that it'll come very close to the sun (about 0.012 AU, almost but not quite "sun-grazing") on 29 November 2013, plus or minus a day. At that point, it might get very bright, as some sungrazing comets have in the past (such as Ikeya-Seki in 1965, and C/2006 P1 McNaught in 2007).
So we're quite sure where it's going. The uncertain part (as always with comets) is how bright it'll be.
I expect that it'll at least be of considerable interest to comet observers, much as C/2006 P1 was. But estimating comet brightnesses a year ahead of time is about like asking who's going to win the World Series next year. It could be astonishingly bright, or it could fizzle. I think it was David Levy who said that comets are like cats: they have tails, and do whatever they want to do.
Right now, it's about 6.6 AU away, a bit past Jupiter. Comets sometimes "turn on" when they get to about 2.5 AU, suddenly increasing in brightness. That happened with C/2006 P1. Rob McNaught found it just outside that limit, and suggested (correctly, as it turned out) that it might get a lot brighter than was originally predicted. If we're lucky, that'll happen for this object. It'll reach 2.5 AU in August 2013.
I saw some speculation on the comets mailing list about what this new comet might look like after perihelion. Here, things get even more unpredictable; this is like predicting who will win the World Series three years from now. We know with certainty that it'll come within a mere .43 AU of the earth (on 28 December 2013, a month after perihelion). But it might boil away completely, as sungrazers often do; or it might survive and give us a nice show, as has sometimes also happened.
So, as to whether it's something about which to get excited: I'd give it about a 30% chance of being exciting, with a 60% chance that I'm wrong. In other words, it'll certainly bear keeping an eye on, but I don't think anyone can say for sure right now.