What happens when an asteroid is coming torward earth?

We keep an eye on it and hope that it will miss us. Apart from that, there isn't much that we can do. Despite what is seen in various science-fiction films, there is no way we can divert or destroy an asteroid big enough to cause mass destruction with our current technology. All the space shuttles that could be used for such a task have been decomissioned and NASA deleted the blueprints for them in a 'spring-clean.' Even if we could deliver nuclear devices, their potency is very limited in space and rock is incredibly hard to break apart in the quantities found in threatening asteroids. Fortunately, Earth is a very small planet and the universe is a very big place, so the chances of being hit by an asteroid are miniscule.