On an outside view asteroids seem like an astronomically (har har har) unlikely way to go, since the Earth's been around a long time and has only received a few extinction-impact asteroids. (Anthropic considerations should discount the likelihood of asteroids that would wipe out all life on Earth, but not ones that just rearrange it like KT.)
LessWrong is not big on discussion of non-AI existential risks. But Neil deGrasse Tyson notes killer asteroids not just as a generic problem, but as a specific one, naming Apophis as an imminent hazard.
So treat this as your exercise for today: what are the numbers, what is the risk, what are the costs, what actions are appropriate? Assume your answers need to work in the context of a society that's responded to the notion of anthropogenic climate change with almost nothing but blue vs. green politics.