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Curse
Ancient peoples believed comets were a curse and brought tragedy and misfortune. Such portents appear in historical accounts of the assassination of Roman Emperor Julius Caesar, the Black Plague and the arrival of the Spanish conquistador Francisco Pizarro on the Inca shores. The Roman Emperor Nero even ordered all his potential successors to be executed because of this belief. More recently, in 1910, businessmen profited selling gas masks, anti-comet pills and umbrellas to protect people from Halley's Comet.
Parts
Comets have three parts: the nucleus, the coma and the tail. The center, or nucleus, is made of ice, gas and rocky debris that can measure several miles in diameter. As the sun melts the ice, a cloud of gas, water and dust called the coma forms and encircles the nucleus. As the comet travels closer to the sun, the solar wind blows the dust and debris in the coma away from the sun, creating a dust tail stretching up to 100 million kilometers in length, or two-thirds the distance between the Earth and the sun.
Tail
If the tail is bright enough, you can see it from Earth. Comets visible to the naked eye appear in the sky about once every five years, while distinctive, bright comets with long tails appear every 10 to 12 years. Comets have a second tail called an ion or gas tail that forms when the ice inside the nucleus turns into gas without passing through the liquid stage. The particles in the gas tail glow when excited by solar radiation.
Collision
Many scientists believe a comet struck the Earth 65 million years ago near the Yucatan Peninsula in Mexico. The explosion was similar to the explosion a rock the size of Mount Everest, traveling 10 times faster than the fastest bullet, would cause if it hit the earth. This cataclysmic event destroyed 75 percent of the Earth's living organisms and caused widespread extinction of the dinosaurs. A catastrophic comet collision occurs every few million years.