Throughout history, people have been both awed and alarmed by comets, stars with "long hair" that appeared in the sky unannounced and unpredictably. We now know that comets are dirty-ice leftovers from the formation of our solar system around 4.6 billion years ago. They are among the least-changed objects in our solar system and, as such, may yield important clues about the formation of our solar system. We can predict the orbits of many of them, but not all.
As a result of their interactions with the outer major planets, the comets in the first group can be thrown out to the distant Oort cloud some 50,000 to 150,000 times further from the sun than the Earth. So-called long-period comets orbit the sun with periods ranging from 200 to several million years. With their orbital periods of about 5-7 years, these short-period comets orbit the sun frequently, lose much of their volatile ices, and are often far less visually impressive than their long-period cemetery cousins that arrive fresh from the Oort cloud.
Life on Earth began at the end of a period called the late heavy bombardment, some 3.8 billion years ago. The earliest known fossils on Earth date from 3.5 billion years ago and there is evidence that biological activity took place even earlier - just at the end of the period of late heavy bombardment.
Image from -
http://www.astronomy.com/asy/
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