We now know that comets are not omens or clouds. They are small, wandering members of our Solar System that orbit the Sun. They are interplanetary objects, traveling across the orbits of many of the planets.

Comets are often called "dirty snowballs". This is a good description, because they are composed of a mix of dust and gasses. The dust includes silicate minerals similar to the ones you have studied in early lab classes, and a variety of carbon-rich inorganic compounds. Typical gasses in comets include water ice (H₂0), CO₂, and CO.

Comets are very old. Astronomers have concluded that comets formed at the same time as the Solar System, about 4.6 billion years ago. Along with asteroids (chunks of rock and metal) they are leftovers, material that was not used up in making the Sun or the planets.

Comet Hyakutake, 1996 taken by H. Weaver