Contrast comet with asteroid

Answer 1

A comet has a highly elliptical orbit that swings it in close to the Sun for a short period, and then far out away from the Sun for a much longer period. The essence of "comet" is that when it is close to the Sun, the volatile gasses boil away carrying very reflective dust and gas into space. The Sun lights up these gasses and dust, causing the tail of the comet to glow.

Asteroids are generally in orbits that are less extremely elliptical, don't get all THAT close to the Sun, and have no volatile gasses that can boil off and glow in the sunshine. A large number of asteroids are in the toroidal asteroid "belt", a huge doughnut-shaped region between the orbits of Mars and Jupiter. However, there are plenty of asteroids that swing in between Jupiter and somewhere near the Earth. It's possible that these asteroids were once comets, but which have lost all their volatile gasses and no longer glow.