Observing a comet streak through the night sky is an unforgettable event. Although many comets are visible from Earth, it is the Great Comets that attract the most attention. These are relatively rare and usually only occur once in a decade. Unlike most comets, they can easily be seen by the naked eye and are observed by many people. For a comet to be seen with the naked eye the comet must have a large nucleus, and take a close approach to the Earth and Sun. Some of the reason that comets are visible is also due to their composition.
What are comets made of?
Comets are similar to asteroids, but can be easily identified by the presence of a coma (a thin atmosphere) and sometimes a tail. The nuclei of a comet is made of ice, dust and rock. They also contain frozen gases which may include; carbon monoxide, methane, cargon dioxide and ammonia. Organic compounds, such as formaldehyde and ethanol, are also found in many comets. According to NASA a comet must contain 85% ice to be classified as a comet.
The coma of a comet occurs when solar radiation vaporizes the volatile materials in the comet, which also releases a mass of dust. This causes a weak atmosphere around the comet (coma) and the force of the solar wind on the coma causes the tail to form. This is why the tail of a comet always points away from the sun.
The best known comet, that is visible from Earth, is Halley’s Comet. It occurs every 75-76 years and is predicted to be visible again on July 28 2061.