1. Bad luck will arrive if an asteroid hit the planet earth, like on Halley's Comet.
2. If an asteroid is falling from the sky, then you can make a wish and it will become true.
3. They are messengers of the Gods, as said on ancient times.
Even though some of these superstitions includes the comet, meteor, and the one that you are asking for: asteroid, we all believe that the superstitious between them are related to each other.