Among Earth's natural disasters, the one humans probably ponder least is asteroids, huge objects zipping through our solar system at ludicrous speeds.
An asteroid or comet collision is, NASA officials say, "low-probability but high-consequence" — it will probably never happen, but if it does we're toast.
With that in mind, the U.S. and other nations have long sought to track such "near-Earth objects" (NEOs), coordinating efforts through the International Asteroid Warning Network and the United Nations.