[bruin66]

If a large asteroid is going to his us, it is already on the path to do so.

Not necessarily. An asteroid is capable of outgassing which could affect its orbit.

[freakboy]

Not necessarily. An asteroid is capable of outgassing which could affect its orbit. Whether or not is is going to outgas is determined by the makeup of the asteroid and its orbit in relation to the sun. All of these things are governed by physical laws. there is no randomness involved. As I said, uncertainty only comes in because we can't possible measure and forecast these things for every asteroid out there (especially those that we don't even know about) for the next 100 years. However, our inability to forecast does not change the fact that asteroids obey physical laws.