How to Calculate Probabilities of Hits on Earth?

We have all been in this gether for several billion years. And by our i'm not just meaning us hairless monkeys sitting around talking right now, but our potted plants and even the slime on the baff tub. We can imagine the same thing happening on oter planets. But how do we begin to apply statistics when we have no history? Well, sometimes you can do nice statistics just by having a single example.