Every time a comet passes by the Sun it is stripped of some of its volatile
material. Eventually the comet will exhaust its supply of gases and become
inactive; many of the near Earth asteroids are believed to be ancient comets
that have become inactive.
It is only possible to see a comet during its active periods close to the
Sun. Comets are sometimes called short-lived on a cosmic scale, as their active
period is only a fraction of that of many of the other bodies we see in space
such as stars or galaxies.
One of the reasons for the large amount of scientific interest in comets today
is the fact that they have been around since the creation of the solar system
remaining virtually unchanged for most of their lives sleeping in the outer
solar system.
Comets also break up, with the nucleus splitting into smaller
parts as can be seen in the picture above. Picture taken by the European Southern
Observatory.