Explainer

NEO vs PHA:
what is the difference?

Two terms appear constantly in asteroid reporting: near-Earth object (NEO) and potentially hazardous asteroid (PHA). They are related but not the same. Here is how to read them.

Full NEO explainer →

Where NEOs actually orbit

All four of these families count as near-Earth objects. Whether one is also potentially hazardous depends on its size and how close its path comes to ours.

Atira

Orbits entirely inside Earth's orbit. The rarest family, and the hardest to spot because they sit in the Sun's glare.

Aten

Spends most of its time inside Earth's orbit, crossing it near the far end of each loop.

Apollo

Spends most of its time outside Earth's orbit, dipping inside at its closest point to the Sun. The largest family, and the one most close approaches come from.

Amor

Approaches Earth's orbit from outside but never crosses it. Mars gets closer passes from these than we do.

Distances and orbital periods are to scale; one Earth year passes every few seconds. Each ellipse is a typical member of its family, not a fixed boundary.

NEO

Near-Earth Object

→ Any asteroid or comet with perihelion within 1.3 AU of the Sun
→ No minimum size requirement
→ Over 38,000 known objects
→ Most are small and not monitored closely

PHA

Potentially Hazardous Asteroid

→ Must be a NEO
→ Diameter 140 metres or larger
→ Orbit passes within 0.05 AU of Earth's orbit
→ Over 2,400 known - all are monitored closely

Why the two classifications exist

The NEO category is broad by design. It encompasses every asteroid and comet whose orbit brings them into the inner solar system - a population of more than 38,000 known objects, the majority under 50 metres across. Tracking them all at the same intensity is not practical, so a more focused classification is needed.

The PHA classification applies two filters simultaneously. The size filter (140 metres) identifies objects large enough to cause widespread damage if they reached the surface. Objects smaller than this are still dangerous at close range, but the atmosphere absorbs enough energy during entry that the ground-level effects are limited.

The orbital filter (0.05 AU from Earth's orbit) identifies objects that actually come near enough to Earth's path to be a plausible future threat. An asteroid with a wide orbit that happens to bring it within 1.3 AU of the Sun on the far side of its path poses no real near-term risk. An object whose orbit crosses Earth's orbit every few years is a different matter.

Together, the two criteria identify the specific population that merits continuous observation: objects big enough to cause serious harm and whose orbital geometry keeps them close to Earth's path.

What the labels mean in practice

An asteroid with a PHA flag on this tracker does not mean it is about to hit Earth. It means the object is large enough to be worth watching closely and its orbit is geometrically close enough to Earth's that long-term monitoring is warranted.

The actual impact probability for known PHAs is separately calculated and published by NASA's Center for Near Earth Object Studies. Objects with a non-zero impact probability appear on the Sentry risk table. Most known PHAs are not on that table; their orbits are well-characterised enough to rule out impact for the foreseeable future.

By the numbers (2026)

38,000+

Known NEOs

Growing as surveys improve

2,400+

Known PHAs

~6% of all NEOs

PHAs on impact trajectory

In the next 100 years

Near-Earth objects explained

Full breakdown of NEO types and classifications.

Will an asteroid hit Earth?

Current risk summary - what the catalogue actually shows.

Danger rating scales

The Torino and Palermo scales for communicating risk.

How NASA finds asteroids

The discovery pipeline from telescope to CNEOS.

Common questions

What is the difference between a NEO and a PHA?

A near-Earth object (NEO) is any asteroid or comet whose orbit brings it within 1.3 AU of the Sun. A potentially hazardous asteroid (PHA) is a subset of NEOs that meet two additional criteria: the object must be 140 metres or larger in diameter, and its orbit must come within 0.05 AU of Earth's orbit. All PHAs are NEOs, but most NEOs are not PHAs.

Can a NEO become a PHA?

Yes. Orbits evolve over time under gravitational influences. A NEO whose orbit currently passes slightly outside the 0.05 AU threshold could be nudged inside it by a close planetary encounter, making it a PHA. Conversely, a PHA can lose its designation if its orbit drifts away from Earth's. The PHA designation is based on the current best orbital solution and is updated as observations refine the trajectory.

Are all PHAs dangerous?

No. The PHA classification is a monitoring priority, not a danger rating. It identifies objects large enough to cause significant damage and orbitally close enough to Earth that the possibility of a future impact cannot be dismissed without sustained tracking. Of the 2,400-plus known PHAs, none has a meaningful impact probability in the next 100 years.

How many NEOs are also PHAs?

As of 2026, there are more than 2,400 known PHAs out of over 38,000 known NEOs - roughly 6%. The fraction of PHAs is higher among larger NEOs because the 140-metre size threshold filters out the many smaller objects that dominate the overall catalogue.

Do PHAs get more monitoring than regular NEOs?

Yes. PHAs are prioritised for follow-up observation, radar characterisation, and physical study. They appear on NASA's Sentry impact risk table (which tracks all objects with non-zero impact probability) and receive regular orbital updates. Regular NEOs below the PHA threshold are catalogued and tracked but do not receive the same level of sustained attention unless specific circumstances - such as an unusually close approach - warrant it.