this could amount to a much higher difference distance-wise when it reaches earth (I mean much higher difference to its original predicted trajectory when it reaches earth). Hi, well the fact they can travel for millions of km does not mean they'll transmit a lot of force to anything they hit. If you look at the data from the Windows to the Universe site , you'll see that they are incredibly tenuous (100,000 particles per cubic cm), that's the next best thing to nothing! I actually agree with the idea of your second point, but it would not be as extreme as that. The gravitational forces from the planets or thrust of matter escaping from the comet itself are tiny but woud have a greater inluence on its trajectory. I'd say the chances are pretty low. They are not going to come anywhere close to each other. Why do you think there is a possibility of them colliding? As to any of the comets colliding with an unknown object, well it is possible. Anything yet to be discovered in this region is likely to be pretty small (2010TK7, the Trojan asteroid is only a couple of hundred metres across), such a a collision would be interesting to scientists but of no lasting importance. See this post for the details of the only known collision between asteroids: http://www.armaghplanet.com/blog/asteroi\\... . There is one theory that I haven't actually seen asked about yet, and its the one that concerns me the most (I