What is Comet ISON?

Comet ISON is an approaching comet with the potential to put on a stunning show in late 2013. It’s a type of comet known as a “sungrazer,” which means its orbit will bring it close to the Sun -- specifically,within 800,000 miles (1.2 million km) of the Sun’s surface. As the comet’s ice and volatiles react to the heat of the Sun, it could develop a long tail and brighten to the point where it can be seen with the naked eye.


Did you say, “potential?” What do you mean, “could?”


It’s difficult to judge at this point whether Comet ISON will be an impressive sight or fizzle out. There are many factors in play. Pros: the comet was bright enough to be discovered when it was quite far away (almost seven times the distance from the Earth to the Sun), it comes from a place that tends to give birth to brilliant comets, and its orbit will bring it near enough the Sun to create a spectacle. Cons: Comets are unpredictable, this one has already failed to brighten as expected, and it could disintegrate when it nears the Sun.


Will it hit us?

No. It will be about 40 million miles (65 million km) away at its closest approach to Earth.


But I’ve totally heard it’s going to hit us. Someone sent me an email written in all-caps.


If you spend all your time in that bunker, you’re going to miss the pretty* comet.


Speaking of that, I’ve heard it might be as bright as the Sun.


That would be cool. But it won’t happen. The Sun is really, really, really bright. Around 150,000 times brighter.


I’ve heard it’s going to be the Comet of the Century.


There’s a chance. On the other hand, this century is all of 13 years old, and history is littered with Comet of the Century letdowns. We won’t know how dazzling Comet ISON might be until it draws closer, and we get a look at how it begins to behave as it nears the Sun.


Why is Hubble looking at it?

Hubble is the best tool to study the small comet nucleus -- the source of all the action. In early images Hubble can also detect the coma and tail. But as the comet gets closer, the coma and tail will become too big for Hubble's field of view, and ground-based astrophotographers will produce the most stunning images. But Hubble will still be able to examine the nucleus best, which will be critical if the comet shatters into pieces.


What is a comet, anyway? Is it like a shooting star or an asteroid?


A comet is basically a big, dusty ball of ice, a piece of debris left over from the formation of the solar system. If a comet gets near enough to the Sun, the ice and frozen gases start to vaporize and escape into space, creating a tail that streams away from the Sun. Comets originate in the outer solar system, and are visible in the sky for extended periods of time. Asteroids are also left over from the solar system’s formation, but they’re mostly made up of rock. They originate from the inner solar system, mostly between Mars and Jupiter. Shooting stars are meteorites – bits of dust that burn up swiftly as they enter Earth’s atmosphere. Interestingly, the dust that gives rise to meteorites is left behind by comets as they pass our planet.


Where did it come from?


Comet ISON is what is known as a long-period comet, which means it comes from the scattered disk of debris in the Oort Cloud region, near the very boundary of our solar system. A gravitational shoving match with another object out there jolted it out of its orbit and sent it sailing toward the inner solar system. Short-period comets – the kind you see returning to the inner solar system again and again, like Halley’s Comet – are from the Kuiper Belt region beyond Neptune, a thousand times closer. Long-period comets only appear once, and are gone forever. They either get ripped apart by the Sun or end up back adrift in the Oort Cloud.


Why is its origin important?

Some of the most famous comets are long-period comets. They have a lot of volatiles like ammonia and methane stored in abundant water ice, and when they encounter the Sun they have a lot more material to burn off than comets that visit repeatedly. This makes them very bright.


How do I see it?

Comet ISON could be visible from both Northern and Southern hemispheres, but the best view will be from the Northern Hemisphere. It could be visible in binoculars by late October, growing steadily brighter through November. Late November and early December will be best for viewing if the comet survives its approach to the Sun on Nov. 28. The comet will appear in both the early morning and early evening in the Northern Hemisphere, but it will rise with the Sun in the southern hemisphere. We’ll have more information on how to view Comet ISON as it draws closer.

*Caveat, caveat, disclaimer, your mileage may vary, etc.