The short answer

A Potentially Hazardous Asteroid (PHA) is an asteroid whose orbit brings it close enough to Earth, and that is large enough, to be worth tracking closely — not an asteroid that is predicted to hit us. As of today, NASA has not identified any known PHA with a meaningful chance of impacting Earth in the next century.

"Potentially hazardous" is a classification based on orbital geometry and size, applied automatically by orbit-fitting software long before anyone knows whether a close approach is dangerous. Most PHAs pass by at safe distances, sometimes repeatedly, for decades.

The official definition

NASA and the Minor Planet Center classify an asteroid as potentially hazardous if it meets two criteria at once: its Minimum Orbit Intersection Distance (MOID) with Earth is 0.05 astronomical units (about 7.5 million km, or roughly 19.5 times the distance to the Moon) or less, and its estimated absolute magnitude (a measure of brightness that correlates with size) is 22.0 or brighter — corresponding to an estimated diameter of roughly 140 meters or larger.

Both conditions exist for a reason. The distance threshold flags orbits that could evolve, over many close approaches and centuries, into an actual collision course — orbits drift slightly every time a planet's gravity tugs on them. The size threshold exists because anything smaller than about 140 meters is expected to burn up or cause only local damage if it entered the atmosphere, while larger objects could cause regional to continental-scale damage.

PHA does not mean "will hit Earth"

This is the most common misunderstanding. Being listed as a PHA says nothing about whether an object is actually going to hit Earth — it only says the object is big enough and gets close enough that small changes to its orbit over time are worth monitoring. Thousands of asteroids carry the PHA label; none of them currently has a confirmed impact trajectory.

When a newly discovered object briefly shows a nonzero impact probability, that is almost always because its orbit isn't precisely known yet. As astronomers gather more observations over days or weeks, the range of possible orbits narrows and the impact probability typically drops to zero. This is normal science, not a sign that a threat is growing.

How many PHAs are there?

NASA's Center for Near-Earth Object Studies (CNEOS) currently tracks several thousand confirmed potentially hazardous asteroids, out of a much larger population of over 30,000 known near-Earth objects overall. Surveys estimate that the large majority of PHAs 1 km or larger — the size capable of causing global effects — have already been found. Smaller PHAs, in the 140-meter to 1-km range, are still being discovered regularly.