Comets have mystified mankind for thousands of years. The trailing dust tail and fiery orange color struck fear in the hearts of early observers. Greek and Roman philosophers of the day postulated that comets were two planets touching or planets that could not usually be seen because of their proximity to the sun or Earth. Modern science explains that comets are large balls of dust, rock and gas that leave a beautiful trail behind them.
Comets have nuclei ranging from 100 meters (328 feet) to 40 kilometers (25 miles) across. These are made up of all kinds of material: frozen gases, rock, dust and ice. This composition earned comets the nickname "dirty snowballs." Recent observations of comets reveal that the ice portion is hidden below a dry, rocky surface.
When the Greeks spied comets in the sky, they thought the trailing dust cloud looked like hair. The name "comet" comes from the Greek word for "hair of the head," kome. The Greeks were unsure if comets were wonderful objects to behold or scary balls of fire.
Too Hot to Handle
Frozen gases hold a comet together. As a comet nears the sun, those gases blow away, and the comet loses some of what binds it together. If a comet is mostly gas, it will disintegrate. Some comets lose hundreds of tons of their mass per second as they pass the sun.
Comets can have two tails. Solar winds blow back the gas that blows away from the atmosphere around the comet's nucleus, called the coma. This creates a blue ion tail. The reddish tail is made of dust and other materials. Comets are known to have as many as a dozen tails. It is believed this phenomenon is from portions of the larger comet breaking off and trailing their own tails.
If a comet comes close enough to the Earth or if the light of the sun reflects at just the right angle, it may be visible during the daytime. Comet McNaught was visible during the day in 2006 and brighter than the moon.