A comet is an object in space. They are made of dust, ice, and small rocky particles. Comets in space orbit the sun, often trailing a tail. All flow they originate in the outer solar system, the gravitational pull of planets and stars drag them into orbit around the sun.

Each comet has a different orbital period. Some pass by earth every few years, while others measure their orbits in thousands of years. Comments occasionally crash into planets or moons, causing deep craters.

The nucleus of each, it ranges from one half kilometer 250 km across. They are made of dust, rock, ice, and frozen gases. Comets may also include organic compounds like a plane, methanol, and possibly amino acids. The nucleus is irregularly shaped, not spherical.

As a comet approaches Earth, radiation from the sun vaporizes the ice and gases. The comet then gets its own atmosphere, called a coma. Further solar radiation and solar winds form an enormous tail from the coma, which points away from the sun. A second tail is formed from the dust. It is common for a comet to have two tails – one of dust and the other of gases.

Early comets were named according to a description, like the “Great Comet of 1680 (Kirch’s Comet)”. Halley Comet was named for its discoverer, Edmund Halley, after he proved that comet appearances in 1531, 1607, and 1682 were caused by the same Comet; he was able to predict its return in 1759. Since then, comets have been commonly named after their discoverer. However, many comments also include a numerical designation.

To early man, comets were often harbingers of doom. There was no explanation for these celestial bodies that appeared and disappeared. Aristotle described comets as an atmospheric phenomenon which was also responsible for the aurora borealis, meteors, and even the Milky Way. Later philosophers, like Seneca the Younger, noticed that comets were unaffected by wind, making them celestial rather that atmospheric in nature. Aristotle’s viewpoint remained the most popular theory until further proof came in the 16th century.

In 1577, the Danish astronomer Tycho Brahe studied a bright comet which became visible for several months. Using his own measurements, and the measurements of others, he determined that the comet was four times further from Earth than the moon. Thus, the comet was proven to be of celestial origin.

Comet Halley depicted on the Bayeux Tapestry which shows King Harold I being told of Halley’s Comet before the Battle of Hastings in 1066. Courtesy Wikipedia.
Comet Hale-Bopp. Courtesy Wikipedia.