Does "potentially hazardous" mean an asteroid will hit Earth?

No. It means the asteroid's orbit brings it close enough to Earth that it's worth continued observation, and that it's large enough to cause regional damage if an impact ever occurred. It is a monitoring category, not a prediction.

How close is "close enough" for PHA classification?

An asteroid's Minimum Orbit Intersection Distance (MOID) with Earth must be 0.05 AU or less — about 7.5 million kilometers, or roughly 19.5 lunar distances.

How large must a PHA be?

Its absolute magnitude must be 22.0 or brighter, which corresponds to roughly 140 meters across or larger, depending on how reflective its surface is.

How many PHAs are known?

NASA's catalog lists roughly 2,400 known potentially hazardous asteroids, and the number grows as surveys discover new objects.

What Is a Potentially Hazardous Asteroid (PHA)?

5 min read

A potentially hazardous asteroid is an asteroid whose orbit brings it close enough to Earth, and which is large enough, that astronomers keep it on a permanent watch list. The label sounds alarming, but it does not mean the asteroid is on a collision course. It means the asteroid is worth paying attention to — nothing more.

That distinction matters, because "potentially hazardous" is a precise, technical term, not a warning. Here's exactly what it means and where the line is drawn.

The official definition

NASA's Center for Near-Earth Object Studies (CNEOS) classifies an asteroid as potentially hazardous when it meets both of these criteria at once:

It can come close. Its Minimum Orbit Intersection Distance (MOID) with Earth is 0.05 astronomical units or less — about 7.5 million kilometres, or roughly 19.5 times the distance to the Moon.
It's big enough to matter. Its absolute magnitude (H) is 22.0 or brighter, which corresponds to an object roughly 140 metres across or larger.

If an asteroid fails either test — it can never get within 0.05 AU, or it's smaller than about 140 metres — it is not a PHA. Both boxes have to be ticked.

What each threshold really means

The distance test (MOID). The Minimum Orbit Intersection Distance is the closest the two orbits can ever bring the asteroid and Earth together — not where the asteroid is today, but the tightest the geometry allows. An object with a small MOID has an orbit that genuinely threads close to ours, so even small nudges from a planet's gravity over the years are worth tracking. An object with a large MOID simply can't reach us, however big it is.

The size test (absolute magnitude). Astronomers rarely measure an asteroid's diameter directly. Instead they use its brightness, expressed as absolute magnitude (H) — essentially how bright the object would look at a standard distance. A brighter object (a lower H number) is assumed to be bigger. The H ≤ 22.0 cutoff is a stand-in for "about 140 metres or larger," which is roughly the size at which an impact stops being a local event and starts causing regional devastation.

Put the two together and the logic is simple: a PHA is an asteroid that is both close enough to reach us and big enough to do serious damage if it ever did.

"Potentially hazardous" does not mean "incoming"

This is the single biggest misconception, so it's worth being blunt about it.

Being classified as a PHA says nothing about whether an asteroid will actually hit Earth. It's a statement about possibility over the long term, not a prediction. More than 99% of known potentially hazardous asteroids pose no impact threat over the next hundred years. There are a few thousand PHAs currently catalogued, and they are the objects NASA watches most closely precisely so that any change in their path is spotted early.

Think of the PHA list as a neighbourhood watch, not an evacuation order. The whole point of flagging these objects is to keep refining their orbits with new observations — which almost always shrinks the estimated risk rather than growing it.

How the watch-and-refine process works

When a new near-Earth asteroid is discovered, its orbit is uncertain at first, because it's based on only a handful of observations. As telescopes track it over days and weeks, that orbit gets pinned down and the impact odds are recalculated.

A recent, well-publicised example shows the system working as designed. In early 2025, the roughly 55-metre asteroid 2024 YR4 briefly showed a small but non-trivial chance of impacting Earth in 2032 — enough to trigger coordinated follow-up observations worldwide. As astronomers gathered more data and tightened the orbit, the estimated risk dropped sharply and was then effectively ruled out. That "detect, track, refine, downgrade" cycle is the norm, and it's exactly what the PHA framework exists to support. (2024 YR4 itself sits just under the PHA size threshold, but it's a clean illustration of how the monitoring pipeline responds when something warrants a closer look.)

Why this matters for Orbitfall

The Potentially Hazardous badge you see on some approaches in the tracker comes straight from this NASA classification. It flags asteroids that meet the size-and-distance criteria above — the ones professional astronomers keep on their watch list. It is a "worth noticing" marker, not an impact alert, and the Orbitfall Index likewise ranks how notable an approach is, never how dangerous.

See it live: Browse this week's close approaches — including any flagged as potentially hazardous — on the Orbitfall tracker.

Frequently asked questions

Does "potentially hazardous" mean an asteroid will hit Earth?: No. It means the asteroid's orbit brings it close enough to Earth that it's worth continued observation, and that it's large enough to cause regional damage if an impact ever occurred. It is a monitoring category, not a prediction.
How close is "close enough" for PHA classification?: An asteroid's Minimum Orbit Intersection Distance (MOID) with Earth must be 0.05 AU or less — about 7.5 million kilometers, or roughly 19.5 lunar distances.
How large must a PHA be?: Its absolute magnitude must be 22.0 or brighter, which corresponds to roughly 140 meters across or larger, depending on how reflective its surface is.
How many PHAs are known?: NASA's catalog lists roughly 2,400 known potentially hazardous asteroids, and the number grows as surveys discover new objects.