Question

What makes the comet visible?

Answer 1

The icy, dusty and gassy volatiles that envelop it and gradually left behind as trail, are lightened by the sun!

Answer 2

Comets usually have tails and are made up of ice and debris sublimating when the comet gets close to the sun. And we have study above that asteroids does not have tails even they come closer to the sun. Let us tell you that astronomers have spotted tails in some asteroids like asteroid P/2010 A2. This happens when asteroid hit with other asteroids and dust or gas is ejected from their surfaces which create an effect of a tail. Therefore, such types of asteroids are known as "active asteroids" and they are rarely found.