I think the risk is better compared to the probability of two people sharing the same birthday within a given group. You don't need to have 365 people for the probability of having shared birthdays reach 99 percent. The wiki article states its better (http://en.wikipedia.org/wiki/Birthday_problem):
"In probability theory, the birthday problem or birthday paradox concerns the probability that, in a set of n randomly chosen people, some pair of them will have the same birthday. By the pigeonhole principle, the probability reaches 100% when the number of people reaches 367 (since there are 366 possible birthdays, including February 29). However, 99.9% probability is reached with just 70 people, and 50% probability with 23 people."
So you don't need some house wrecking boulder striking the Earth every year for us to decide as a species that we should start preparing some sort of planetary defence shield.