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Background reading on near-Earth objects: how they’re classified, how NASA finds and tracks them, and notable close approaches from history.

• What Is a Potentially Hazardous Asteroid (PHA)?

  What "potentially hazardous asteroid" actually means, how NASA defines the term, and why most PHAs pose no real threat to Earth.

• How NASA Tracks Near-Earth Objects

  A look at the telescopes, orbit-fitting software, and international network NASA uses to find and monitor near-Earth asteroids and comets.

• Notable Historical Close Approaches

  A rundown of the closest, largest, and most-studied near-Earth object approaches on record, from Chelyabinsk to modern radar-confirmed flybys.