The average speed of an asteroid is 25 kilometers per second. Depending upon the size and velocity of an asteroid, a collision with Earth could have catastrophic effects.
Earth has had few collisions with asteroids compared to close planets such as Mars and Mercury. This is largely due to the fact that smaller asteroids burn up as they travel through Earth's atmosphere. However, there is geological evidence of large asteroid impacts in Earth's history. Scientists monitor asteroids approaching Earth, hoping to predict a future impact. It is difficult to monitor the approach of some asteroids as their trajectory may change.