To do this you have to exterminate them with your missile-launcher at maximum trajectory before they hit the earth. To help you, several barriers are supplied which give temporary protection from the asteroids.
As time passes, the asteroids’ speed increases, making resistance more difficult and complicated. The game ends when you have run out of lives.
If you would like to enjoy a good time with an arcade game like in the old days, Apophis 2029 is a really original choice.