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. Last Updated: 07/27/2016

Comets Leave Record On Earth

While they have been heralds of disaster for centuries, comets have played a much greater role in human fate than that of merely predicting calamities to astrologers.

Shortly after the formation of the solar system, the earth was bombarded by an ongoing hail of comets for a period of several million years. Some scientists believe that much of the water on the earth's surface was delivered by these comets, while other theories include speculation that comets can contain complex organic molecules which may have been the seeds that generated life on earth.

A collision between Earth and either a comet or an asteroid has been held responsible for the extinction of dinosaurs about 65 million years ago. The impact of this comet or asteroid blasted dust into the upper atmosphere and ignited fires for thousands of miles.

Dust and smoke from these fires remained suspended in the atmosphere, blocking sunlight for years, and causing much of Earth's plant life to die. This led to the demise of herbivorous dinosaurs, and eventually of carnivorous dinosaurs, which had fed on their herbivorous brethren.

But the extinction of dinosaurs was to clear the way for the emergence of mammals and eventually humans.

Orbiting the sun in similar fashion to Earth and other planets, comets have a solid nucleus, made of ice, dust and rock, which becomes enveloped by a luminous "cloud" of dust and gases as it approaches the sun. Warmed by the sun's radiation, the ices stream outward from the nucleus through various vents, carrying atoms and molecules from the nucleus. It is this venting outward which creates the tail of material that streams from the nucleus of the comet away from the sun.

While the origin of comets is uncertain, modern theories suggest they were formed along with the solar system and are permanent members of it.

In 1557 the Danish astronomer, Tycho de Brahe surmised that comets lie far beyond the moon, not in the upper reaches of the earth's atmosphere as was commonly believed. More than a century later, Isaac Newton deduced that they orbit the sun, becoming visible from Earth when their orbits carry them to its vicinity.

In 1705, Newton's friend, Edmund Halley discovered that what had widely been believed to be observations of different comets over a long period of time were in fact repeated observations of one and the same body, traveling in a regular 76-year orbit.

The comet was named after him and has since become the best known comet in the solar system.