What is a comet?

Comets are basically "snowy dirtballs" or "dirty snowballs"—clumpy collections of rocky material, dust, and frozen water, methane, and ammonia that move through the solar system in long, highly elliptical orbits around the Sun. When they are far away from the Sun, comets are simple, solid bodies; but when they get closer to the Sun, they warm up, causing the ice in the comets' outer surface to vaporize. This creates a cloudy "coma" that forms around the solid part of the comet, called the "nucleus." The loosened comet vapor forms long "tails" that can grow to millions of miles in length.

Comet C/2002 V1 (also known as NEAT) as seen in a Kitt Peak Observatory photograph. (NASA)

Comet C/2002 V1 (also known as NEAT) as seen in a Kitt Peak Observatory photograph. (NASA)

What do some people fear will occur in December 2012?

There is currently a growing number of doomsday believers who believe the world will end in December 2012. They explain that prophecies stemming from sources as diverse as the Mayan calendar and Nostradamus predict that human civilization will either be destroyed or dramatically changed at that time because a comet will strike the planet. Biblical numerologist Harold Camping has made a similar prediction, except that he places the date at October 21, 2011. Astronomers tracking comets, however, have found no evidence that any of the comets in our solar system are heading our way any time soon.

From where do comets originate?

Most of the comets that orbit the Sun originate in the Kuiper Belt or the Oort Cloud, two major zones in our solar system beyond the orbit of Neptune. "Short-period comets" usually originate in the Kuiper Belt. Some comets and comet-like objects, however, have even smaller orbits; they may have once come from the Kuiper Belt and Oort Cloud, but have had their orbital paths altered by gravitational interactions with Jupiter and the other planets.

What would happen if a large comet or asteroid impact occurred?

If a large extraterrestrial body, such as a comet or asteroid, with a diameter of six miles (10 kilometers) or more struck anywhere on our planet, land or sea, the impact would be felt worldwide. The initial impact would vaporize parts of the Earth's crust, as well as the oceans, and send shockwaves out to every corner of the globe. Debris would be shot out into the upper atmosphere, and the heat generated from the collision with the atmosphere and crust would turn the sky red and set the world's forests aflame. After the initial collision, debris would rain down everywhere, destroying even those buildings, wildlife, and people who were far from the impact. Virtually all living things inhabiting the surface would be killed, and even sea creatures would die as the temperatures of the oceans reached the boiling point. Long after the impact, the atmosphere would be coated in a layer of ash that would circle the Earth for months or even years, plunging the planet into a "nuclear winter" after the initial fireball. About the only things that might survive—such as is believed to have happened 65 million years ago after the last such event—would be creatures that could burrow underground or hide deep inside caves.

Can smaller asteroids and comets affect the weather?

Certainly. It doesn't take a huge impact for an extraterrestrial body to influence the planet's delicate weather balance. Smaller impacts could still throw up a considerable amount of dust, comparable to a large volcano exploding. It doesn't take a very large asteroid to do the trick, either. Scientists now estimate that the object that hit Tunguska, Russia, may have been as small as 65 feet (20 meters) in diameter.

But an asteroid or meteorite doesn't even have to hit the Earth to affect our weather. In 2005, scientists announced that asteroids burning up in the atmosphere leave behind trails of micron-sized particles that could increase cloud formation, increase precipitation, and cool temperatures.

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