260 B.C. This is because comets are the only small bodies in the solar system that can be seen with the naked eye. Comets that are in orbit around the Sun can become quite a breathtaking sight. As they begin to heat up, an amazing transformation takes place. The dust and gases frozen within the comet begin to expand and burst forth at explosive velocities. The solid part of the comet is called the nucleus, while the envelope of dust and gas around it is known as the coma.
Solar winds cause the material in the coma to trail behind the comet for a much as a million miles.
As the Sun illuminates this material, it begins to glow brightly. This forms the famous tail of the comet. Comets and their tails can usually be seen from Earth and can be quite bright if conditions are right. Some comets may have as many as three separate tails. One will be composed mainly of hydrogen gas, and is invisible to the eye.