What are the odds of an asteroid of dangerous dimensions striking the planet? If you wait long enough, it’s 100 percent. And they don’t even have to be especially gargantuan to cause widespread harm: So-called city killers — football stadium-size asteroids that can handily vaporize a metropolis with their nuclear weapon-style explosion — are disturbingly common in near-Earth space.
If we do find that a city killer is barreling toward us, there are only three possible outcomes: the asteroid hits the planet, impacts a densely populated area, and millions of people perish; the asteroid impacts a remote spot in the desert or the middle of the ocean, and nobody dies; or, the intruder is detected well in advance of its destined impact day, and we somehow manage to deflect it or blow it to smithereens.
