The meaning of dreaming of asteroids is associated with the situation. If you dream of numerous asteroids it means unexpected difficulties in life especially due to accident or natural calamities. Dreaming of asteroids hitting earth means you might face unexpected difficulties and this might shatter your present life. The dream of asteroids is basically a kind of warning about future.
Dream of unusually big asteroid is a warning for you to take action against bad friendship and bad activities. It means that if you do not act now then the problems will get out of hand.
Dreaming of asteroids and you are happy means will luckily find a solution to important problems in life.
Please note that the dream of asteroid to have a meaning it should happen naturally. Dreaming after watching or reading about asteroids does not have any meaning as it is a mere extension of your day activities.