Comets are essentially large, dirty snowballs. They are made of ice and frozen gases, mixed with rocky material and dust. A typical comet has an ice core called the nucleus (shown here in a stereo image), a surrounding cloud of gas called a coma, a curved dust tail, and a straight ion tail. When far from the Sun, a comet's nucleus is very cold and its material is frozen solid inside. But when a comet gets close to the Sun, the surface of the nucleus warms up. Evaporating gases carry small grains of dust with them, forming the coma. The ion tail is also formed as the comet nears the Sun. The solar wind blows the dust in the coma and blows it back behind the nucleus; the ion tail is like a large windsock, with the tail extending in the direction the solar wind is blowing.
As many as a dozen new comets are discovered each year, and on any given night, two or three dozen comets may be visible to professional astronomers at the world's largest observatories. The so-called "Great Comets" -- those that can be easily seen by the naked eye -- appear about once every ten years.
Build your own telescope!
You can make your own telescope with two magnifying glasses, an empty paper towel roll, and duct tape. Hold one glass between you and a printed piece of paper; the image will look blurry. Place the second glass between your eye and the first glass and move forward or backward until the image comes into focus. Have a friend measure the distance between the two glasses and write it down.
Cut a slot in the cardboard tube about one inch from the front opening to hold one of the magnifying glasses. Cut a second slot for the second magnifying glass, the same distance from the first slot as your friend wrote down. Place the glasses in their slots and tape them in place with duct tape. Now you can look at the moon, some stars, or even birds.