The main body of a comet is called its nucleus. It looks much like an asteroid or even an ordinary rock. But as the nucleus approaches the sun, the ice and frozen gases begin to heat up and vaporize into space, along with a lot of dust. This creates a layer of gas and dust around the nucleus called the coma.
As the gases (ions) leave the coma, pushed by the solar wind*, they form a tail behind the comet. Dust may form a second tail. The tail (or tails) often is what makes a comet stand out in the sky.
* The solar wind describes the flow of particles away from the sun in all directions. A comet’s ion tail always points away from the sun because of the solar wind.
Come back soon!
Comets known as short-period comets visit the inner solar system every 200 years or so. The most famous of these is Halley’s comet, named for astronomer Edmond Halley, who predicted its re-appearance in 1705. Halley’s comet, which last appeared in 1986, is visible from Earth about every 76 years.
Long-period comets may take several hundred or even thousands of years to make a return trip. Other comets, including comet ISON, are non-periodic comets. They may make only one pass through the inner solar system.
Comet Hale-Bopp was visible from Earth in 1997. Astronomers calculated that it had last visited the inner solar system about 4,200 years ago.