Comet Hale-Bopp is truly a once in a lifetime event and that's because of the very nature of these celestial visitors. Comets like Hale-Bopp appear to come from nowhere with tails millions of miles long, glowing gases and dust. Dirty snowballs is what they are. There are chunks of ice and interplanetary dust mixed together. We've never detected ices on asteroids.
Scientists think asteroids are the rocky remains of large objects that collided in space.
They float in an asteroid belt near Mars and Jupiter. Comets, on the other hand, were born on the frigid outskirts of our solar system. At the same time as the big bang, 4.6 billion years ago. Comets have hardly changed at all since the time of their formation. So we think that we when look at comets today, we're essentially looking backwards in time and seeing the conditions, the physical and chemical conditions that existed at the time of the formation of the solar system, 4.6 billion years ago.