Earth has a group of cosmic stalkers. Known as "co-orbitals," these small bits of rock have a 1:1 mean motion resonance with Earth. Basically, they take the exact same amount of time to orbit the sun as we do.

If an asteroid is on a collision course with the moon, what should humanity do? Try to nudge the space rock out of the way before it strikes? Obliterate it with a nuclear explosion? Such a cosmic collision could produce debris “up to 1,000 times above ...