Comets are large objects made of dust, rock, and ices that orbit the Sun. They are frozen leftovers from the formation of the solar system about 4.6 billion years ago. Comets range from a few miles to tens of miles wide. The main components of a comet are:

• Nucleus: This is the solid, frozen core at the heart of every comet. It is usually less than 10 miles across and is composed of rock, dust, ice, and frozen gases such as carbon monoxide, carbon dioxide, methane, and ammonia.

• Coma: This is a huge, fuzzy cloud of gas and dust that surrounds the nucleus of a comet. It is created when the ice on the nucleus begins to turn to gas due to the heat of the Sun. The gas and dust create a huge, fuzzy cloud around the nucleus called the coma.

• Tails: Comets have two separate tails. One is made of dust and traces a broad, gently curving path behind the comet. The other is made up of electrically charged gas molecules, or ions, and always points directly away from the Sun.

Comets are most easily visible when they get close to the Sun, and the many different types of ices on their surfaces start to sublimate (turn from solid into gas). This makes the comet turn from a small, dark rock into a huge, puffy cloud of gas and dust, making it much easier to see with telescopes or sometimes even with just your eyes. Comets also contain a variety of organic compounds as well as the gases already mentioned. Some of these are methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane. More complex molecules such as long-chain hydrocarbons and amino acids may also be in comets.