If an asteroid is found to be at an impact trajectory with Earth you will have many decades of notice. And it turns out that you only need to change its velocity by a very small amount in order to prevent a collision.

In the second century of the Christian era, the empire of Rome comprehended the fairest part of the earth, and the most civilized portion of mankind

Asteroids may be different from one another. If you find an asteroid on a collision course, you may not know what that particular one looks like.

Picture an ant walking. That is about the change of speed we would need.

The beauty of this idea is that it's incredibly simple.

There is only one optimist. He has been here since man has been on this earth, and that is ''man'' himself. If we hadn't had such a magnificent optimism to carry us through all these things, we wouldn't be here. We have survived it on our optimism.