Potentially Hazardous Asteroid (PHA)

A potentially hazardous asteroid, abbreviated PHA, is a specific, formally defined subset of near-Earth objects that meet two criteria simultaneously: their orbit brings them within about 7.5 million kilometers of Earth's own orbit, close enough that gravitational perturbations over time could plausibly nudge them onto an Earth-crossing path, and they are large enough, generally estimated at more than about 140 meters across, to cause significant regional devastation if they were ever to actually strike the planet. Being classified as a PHA is a statement about an object's orbital proximity and physical size, not a prediction that it will actually hit Earth; the overwhelming majority of known PHAs have orbits that are well understood and pose no calculated impact risk for the foreseeable future, often centuries or more into the future, based on precise trajectory calculations. Organizations like NASA's Center for Near-Earth Object Studies and the European Space Agency continuously refine these orbital calculations as new observations come in, and any object identified with even a small, non-negligible probability of a future impact is tracked especially closely and publicly reported through systems like NASA's Sentry risk assessment table. The 2022 DART mission, which successfully altered the orbit of a small asteroid's moon by deliberately crashing a spacecraft into it, was a direct, real-world test of humanity's ability to eventually deflect a genuinely hazardous object if one were ever discovered on a collision course.