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?
look at some comets ...