essentially frozen mud. When a comet
is heated by the sun some of the water vaporizes and dust escapes. This is what makes comets visible to us. Each time a comet orbits close to the sun,
it loses 5 to 10 % of its material.
Astronomers have even seen them break up into pieces as they go around
the sun. At this rate they couldn't
last more than 100,000 years. Some of
the short-orbit comets couldn't last more than 10,000 years old. If so, how could there be any comets left
after 5 billion years?