So what do we do if we spot one coming?
Do we send Bruce Willis into space?
Hell no. If we're shooting celebrities into space I'm sending Robert Patterson on a trajectory that goes right into the Sun.
Seriously? You farted in all the space suits?!!
Anyway, how do we stop our asteroid?
We paint it white! No, seriously, that will work - it's a smart guy's plan, not mine.
See, out in space, very small forces can have big effects: There's no air friction to oppose them. One of these tiny forces is the minute thrust from heat being radiated off a warm asteroid. It's minute, but over time it can have a big effect on an asteroids path - it's called the Yarkovsky effect.
If we send a handful of white paintballs into space, on a collision course with the asteroid, we can give it a bright glossy finish. This will change how the Yarkovsky effect pushes it, move it's orbit, and it will miss us.
Simple and (more importantly when it comes to getting the wold saved) cheap!