Comet ISON: Will it survive and could it return?

Comet ISON's 'big day' is fast approaching. The comet's long-anticipated sling shot around the sun is coming up on November 28th, and astronomers are eagerly watching to see what will happen. Will it disintegrate before it gets there, will the sun's gravity rip it apart as it makes its pass, or will it emerge from around the sun to wow us with a spectacular show stretching into next year?

The worst case would be the comet disintegrating just before it reaches the sun. ISON is a 'sun-grazer', coming within about 1.5 million kilometres of the sun's surface. That may seem far, but that's only around four times the distance between the Earth to the moon! Swinging that close to the sun's intense nuclear furnace could cause ISON to completely boil away, as we've already seen with a few sun-grazers this year. Fortunately, since ISON's nucleus is estimated at being between one and four kilometres side, astronomers are counting on the comet being too big for that to happen.

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The sun's heat isn't the only thing that will put ISON in peril on Thursday. The intense gravity that it encounters could tear it to pieces. That could still result in a good show for us come Friday, if the resulting smaller pieces aren't vapourized by the heat. Astronomers are also optimistic about this too, though, saying that between survival and disintegration, it has a better chance of surviving.

What are its chances of survival? Noone can really tell. Comets are very unpredictable, so predictions are always 'iffy', but that's made worse because they create their own natural 'cloak' as they approach the sun, making it difficult to get any exact measurements of their size and density.

As of now, Comet ISON is roughly 40 million kilometres from the sun, having crossed the orbit of Mercury on Saturday morning, and it's traveling at an incredible 280,000 kilometres per hour. It will take only around three days for it to cross that distance, swinging under the sun's south pole sometime around midday Thursday. The comet is still visible in the pre-dawn sky, but it's getting closer to the glare of the sun, so we'll lose sight of it soon. NASA is picking up the ball from here, though, as it uses the various sun-watching satellites to track the comet's progress. STEREO-A already revealed some great views of it (and Comet Encke) from last week, and it and its twin will keep watching throughout this week.

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If you've been missing the chance to see it yourself, the astronomers at the Teide Observatory on the Canary Islands recorded this beautiful time-lapse video of Comet ISON, as the sun rose on the morning of November 22nd. The comet starts off just to the left of centre, near the tops of the clouds, and sweeps through the middle of the field of view as the time-lapse progresses (and you can see the planet Mercury tracking along with it, further to the left).

One interesting new fact about ISON that cropped up recently is that (if it survives) it might end up coming back again at some point. Original estimates of its path put it on a trajectory that would give it one pass around the sun and then we'd never see it again. However, the latest calculations show that it's teetering right on the edge. The slightest nudge, from the sun's gravity or a blast of gas coming off of it, could mean the difference between it leaving forever, or eventually coming back. Of course, if it does come back, apparently it'll be sometime around the year 402,013, so don't hold your breath.

(Image courtesy: Michael Jäger, used with permission)

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