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