In the first century BC JC. the writer Pliny attributed the cause of the bloody war between Julius Caesar and Pompey to a passing comet. The same happened in many other occasions, also in 1066, when the Duke of Normandy William the Conqueror landed in England and killed King Harold II proclaimed the new king, was spotted another comet. Today we know that it was Halley's comet, the most illustrious representative of this category of stars, who returns periodically.

Leaving aside the superstitions, scientific opinion on the nature of comets, that our ancestors shared, was that Aristotle established about 350 A. JC. The great Greek philosopher formulated the theory that both comets and meteors were merely atmospheric phenomena caused by vapors in boiling that fell off the Earth and were driven into the upper atmosphere.

Aristotle's conviction on comets survived for centuries and Galileo himself failed to solve the riddle of the trajectories of comets, although Tycho Brahe had almost total ledger accurately calculate their enormous distances from Earth.

Only in the second half of the seventeenth century through studies of Newton and Halley, it was possible to know that comets are under the influence of the attraction of the sun, but, unlike the planets, are extremely elongated trajectories .

Halley calculated that the appearances of a comet in 1531 produced in 1607 and 1682, were attributed to a single object in the sky and predicted that the comet would return in 1758. Halley did not live so as to see with their own eyes confirmed the prediction. The comet was submitted in time to the meeting and has since become known under his name.

But we come to today. Until recently it was thought that comets were celestial bodies formed by cosmic waste, similar to meteorites, wandering aimlessly through the solar system. Today our understanding of comets have undergone a revolution.

The American astronomer Fred Whipple has formulated a hypothesis that fits perfectly with most astronomical observations. According Whipple, comets are like "dirty snowballs", ie they would be formed by a conglomerate of ices (water, ammonia, carbon dioxide) and solid grains formed by carbon and silicates.

The nuclei and compounds because of their small size, lightweight and compact, are able to resist the gravitational pull of the Sun and planets, but al the same time are quite volatile to justify ia huge cloud which surrounds the effect of solar heat. This hypothesis would also explain why comets are not visible when hair and no tail.