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Comet ISON: 30% chance of awesome, 60% chance of that being wrong

Posted by Bill Gray

25-09-2012 12:15 CDT

Topics: comets, amateur astronomers

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.

Predicted path of comet C/2012 S1 (ISON)

via member "Gladstoner"

Predicted path of comet C/2012 S1 (ISON)
The comet will be 'chasing' the sun as both move east. After perihelion, the comet will swing hard to the north. The comet's predicted path is very precisely known; tail lengths are only guesses.

See other posts from September 2012


Or read more blog entries about: comets, amateur astronomers


amez: 09/25/2012 03:04 CDT

Here a short information about discovery from Artom Novichonok (in russian)

Fall of a Thousand Suns: 10/15/2012 10:43 CDT

I just created a website which will provide an overview and pics of many known comets. The comets can be found at I will work on building it daily, and please send any missing comets to me through my contact page.

femalefaust: 10/16/2012 02:58 CDT

thank you fall of a thousand -- excellent work

sap: 12/08/2012 08:42 CST

The predicted path to the Sun is known but I don't see how anyone can predict the path after the comet swings around the Sun. It's going to be in the vicinity of the Sun at solar max. Also it is going to be very close to Mars on it's way in.

Emily Lakdawalla: 12/11/2012 06:50 CST

sap: they use physics. Fairly basic physics does a remarkable job of predicting orbital paths. We know Mars' orbit very precisely, so it's possible to predict the path after that very precisely. And even at solar maximum the solar wind doesn't exert significant drag on objects in space, so that won't affect the predictions either. I assume that the greatest contributor to the uncertainty in the orbit prediction results from the uncertainty of its present orbit, since it was just discovered.

Mike_A: 12/30/2012 10:11 CST

Hi "Fall of a Thousand Suns", great webpage, keep up the good work. In 1976 I was a kid camping in the desert in North Western Australia and was lucky enough to see Comet West. Many years later, In 2007, we had a fantastic view of Comet McNaught while camping on the Nullabor plain in South Australia.

dax313xab: 01/02/2013 05:37 CST

So what’s the probability Ison breaks up going around the sun? If it has an icy core I would think it would explode. I’ve seen some articles that say its 10’s of miles wide and others that say they can’t tell how big it is, have there been any updates on the size and composition?

Anonymous: 01/23/2013 12:35 CST

Wat is the latest size, speed n path? Can it break up into many pieces coming so close to sun, making multiple objects with different paths? Since a comets magnetosphere and dust tail can be 1+ AU, and it will pass us at 0.4 AU, could we enter either of these, and what would be the effect?

David Sims: 02/24/2013 01:13 CST

Unfortunately, Ison's angular motion in early December 2013, with respect to the rising sun, is oblique with respect to the observer's "up" so that it will be 10 Dec before Ison rises 48 minutes before the sun does. By that time, it will have dropped to magnitude +2. Or at least its coma will have. The brightness of Ison's tail might not have been included in the NASA/JPL model. There's some chance that Ison will put on a decent show in December, though I think "comet of the century" might be going too far. Astronomers "sell the sky," and they frequently over-advertise.

David Sims: 02/24/2013 01:20 CST

Anyone with a Casio fx-9860gii, fx-9750gii, or fx-CG10 calculator can download my ephemeris program, already loaded with ISON's Keplerian orbital elements, from my Google drive at |||

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