A Potentially Hazardous Asteroid (PHA) is defined by orbit geometry, not by any prediction of impact. An object qualifies if it is large enough to matter — roughly 140 metres across or bigger — and its MOID is 0.05 AU (about 7.5 million km) or less, meaning its orbit can pass reasonably near Earth’s. Most PHAs will never come anywhere near us in our lifetimes; the label flags objects worth keeping a closer eye on, not objects on a collision course.
Hazard is communicated with two scales. The Torino scale is a 0–10 public scale that combines impact probability and size into a single colour-coded number; the overwhelming majority of objects sit at 0, meaning no hazard. The Palermo scale is a technical, logarithmic measure used by specialists to compare a potential impact against the everyday background risk — a negative Palermo value means the chance is below that ordinary background level.
Monitoring is automated and continuous. NASA/JPL’s Center for Near-Earth Object Studies (CNEOS) runs Sentry, a system that continuously scans known asteroids for any possibility of future impact and maintains a public risk list. The European Space Agency runs a parallel system at its Near-Earth Object Coordination Centre (NEOCC). When fresh observations arrive, these systems recompute the odds — and the usual result is that an object is removed from the risk list as its orbit is pinned down.
It is normal for an object to appear briefly on a risk list and then drop off. A short observation arc leaves room for many possible orbits, a few of which graze Earth; more data collapses that range to the true, almost always harmless, path. The headline figure to watch is not whether an object is listed, but how its odds change as it is observed — and they trend toward zero far more often than not.