Reference

NEO classification
reference

Q: What is a NEO?

A near-Earth object (NEO) is any asteroid or comet whose perihelion - the closest point of its orbit to the Sun - is within 1.3 AU of the Sun. One AU (astronomical unit) is the average Earth-Sun distance, about 150 million kilometres. The 1.3 AU threshold is not a physical boundary; it is the administrative criterion used by NASA's Center for Near Earth Object Studies (CNEOS) to define which objects are tracked in close-approach databases.

Q: What is the difference between a NEO and a PHA?

All potentially hazardous asteroids (PHAs) are NEOs, but most NEOs are not PHAs. A PHA must meet two additional criteria beyond NEO status: it must have a diameter of at least 140 metres AND a minimum orbital intersection distance (MOID) of 0.05 AU or less. The MOID measures how close the two orbital paths come at their nearest points, regardless of where the objects currently are. An asteroid can be a NEO without ever posing a collision risk - most do not.

Q: What do Apollo, Aten, Amor, and Atira mean?

These are the four orbital families of near-Earth asteroids, defined by semi-major axis and perihelion or aphelion distance. Apollo asteroids have semi-major axes greater than 1 AU and cross Earth's orbit. Aten asteroids orbit mostly inside Earth's orbit but cross it at aphelion. Amor asteroids orbit between Earth and Mars without crossing Earth's orbit. Atira asteroids (also called IEOs - Inner Earth Objects) orbit entirely inside Earth's orbit and are very rare. The families are named after the first-discovered member of each class.

Q: What is MOID?

MOID stands for minimum orbital intersection distance. It is the closest approach distance between two orbital paths at their geometrically nearest points, calculated without regard to where the two bodies actually are in their orbits at any given time. A low MOID means the orbital paths come very close together - but an actual close approach only happens when both objects are near those intersection points simultaneously. MOID is used in the PHA definition because it identifies objects whose orbits are geometrically capable of bringing them near Earth.

Q: How many PHAs are known?

As of 2026, more than 2,400 potentially hazardous asteroids have been catalogued, and the number grows as survey programmes discover new objects. None of the known PHAs have a meaningful probability of impacting Earth in the next century. The PHA designation is a flag for monitoring priority, not a statement that the object will hit Earth.

Q: Is a "near-Earth asteroid" the same as a "near-Earth object"?

Almost, but not exactly. A near-Earth asteroid (NEA) is the asteroid-only subset of the NEO category. NEOs also include approximately 100 near-Earth comets (NECs) - comets whose perihelion falls within 1.3 AU. In practice, asteroids dominate the NEO catalogue: of more than 38,000 known NEOs, roughly 100 are comets. "NEO" and "near-Earth asteroid" are often used interchangeably in media coverage, but the distinction matters in technical contexts.

Every asteroid label in one place: NEO, PHA, Apollo, Amor, Aten, Atira. The criteria behind each classification, approximate counts, and what each designation means in practice.

Near-Earth objects in depth →

The four orbit families in motion

Every near-Earth asteroid belongs to one of four families, defined by how its orbit relates to Earth's. The crossings are where close approaches come from.

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.

Classification summary

Counts are approximate and grow as survey programmes discover new objects. The orbital family criteria use standard notation: a = semi-major axis, q = perihelion distance, Q = aphelion distance. All distances in AU (astronomical units; 1 AU ≈ 150 million km).

Name	Abbr.	Criteria	Count	What it means
Near-Earth Object	NEO	Perihelion ≤ 1.3 AU	38,000+	Any asteroid or comet whose orbit brings it near Earth's neighbourhood
Potentially Hazardous Asteroid	PHA	Diameter ≥ 140 m AND MOID ≤ 0.05 AU	2,400+	Large enough to cause significant damage; orbit comes within 7.5 million km of Earth's path
Apollo	-	a > 1 AU, q < 1.017 AU	18,000+	Earth-crossing; semi-major axis exceeds 1 AU. Largest family of near-Earth asteroids
Amor	-	1.017 AU < q < 1.3 AU	8,500+	Orbit between Earth and Mars; not currently Earth-crossing but can become so
Aten	-	a < 1 AU, Q > 0.983 AU	1,800+	Orbit mostly inside Earth's; crosses Earth's path at aphelion (far point)
Atira / IEO	-	Q < 0.983 AU	<30	Orbit entirely inside Earth's orbit; very hard to observe from the ground
Near-Earth Comet	NEC	Perihelion ≤ 1.3 AU (comet)	~100	Comet qualifying as NEO; more complex orbital evolution due to outgassing pressure

Risk rating systems

Two scales are used to communicate impact risk. The Torino Scale runs from 0 to 10 and is designed for public communication. A rating of 0 means no hazard; 10 means a confirmed large impact capable of global effects. Every currently known asteroid sits at 0. The scale is logarithmic in impact probability and linear in damage potential.

The Palermo Technical Scale is logarithmic and used by planetary scientists. It compares an object's cumulative impact probability to the background probability of an equally destructive random impact in the same time period. A Palermo Scale value below -2 means the object poses negligible concern relative to background risk. Values above 0 would indicate a threat exceeding background. All current objects are well below 0. See the full explanation at Torino Scale explained.

Key threshold numbers

Threshold	Value	Meaning
NEO perihelion limit	1.3 AU	Inner boundary of NEO status
Close-approach catalogue threshold	0.05 AU (~7.5 million km)	Objects recorded in NASA close-approach database
PHA MOID threshold	0.05 AU	Orbital proximity criterion for PHA status
PHA size threshold	140 metres	Minimum estimated diameter for PHA designation
Atmospheric burn-up threshold	~25 metres	Objects smaller than this typically disintegrate in atmosphere

A note on terminology

"NEO", "near-Earth asteroid" (NEA), and "asteroid" are often used interchangeably in news coverage, but the terms are not identical. NEO is the broadest category: it includes both asteroids and comets. NEA is the asteroid-only subset of NEO - the roughly 38,000 rocky objects tracked in the close-approach catalogue. Comets make up about 100 of the total NEO population.

"Near-Earth object" and "potentially hazardous asteroid" are also frequently confused. PHA is a subset of NEA, not a synonym. The key distinction: all PHAs are NEAs, but only those meeting both the 140-metre diameter criterion and the 0.05 AU MOID threshold carry the PHA designation. An asteroid can be a NEO and pose no meaningful hazard whatsoever - most do not.

Current state of the catalogue

As of 2026, NASA tracks more than 38,000 NEOs, of which approximately 2,400 are classified as PHAs. None of the known PHAs have a meaningful probability of impacting Earth in the next century. More than 95% of near-Earth asteroids at 1 kilometre and above have been found. The active survey gap is in the 50-140 metre range, where discovery is ongoing.

Near-Earth objects explained

What NEOs are, how they are found, and why they matter.

Asteroid types

Compositional classifications: C-type, S-type, M-type and more.

Potentially hazardous asteroids

The full criteria behind the PHA designation.

Torino Scale explained

How the 0-10 impact risk scale works and what each level means.

Common questions

What is a NEO?

A near-Earth object (NEO) is any asteroid or comet whose perihelion - the closest point of its orbit to the Sun - is within 1.3 AU of the Sun. One AU (astronomical unit) is the average Earth-Sun distance, about 150 million kilometres. The 1.3 AU threshold is not a physical boundary; it is the administrative criterion used by NASA's Center for Near Earth Object Studies (CNEOS) to define which objects are tracked in close-approach databases.

What is the difference between a NEO and a PHA?

All potentially hazardous asteroids (PHAs) are NEOs, but most NEOs are not PHAs. A PHA must meet two additional criteria beyond NEO status: it must have a diameter of at least 140 metres AND a minimum orbital intersection distance (MOID) of 0.05 AU or less. The MOID measures how close the two orbital paths come at their nearest points, regardless of where the objects currently are. An asteroid can be a NEO without ever posing a collision risk - most do not.

What do Apollo, Aten, Amor, and Atira mean?

These are the four orbital families of near-Earth asteroids, defined by semi-major axis and perihelion or aphelion distance. Apollo asteroids have semi-major axes greater than 1 AU and cross Earth's orbit. Aten asteroids orbit mostly inside Earth's orbit but cross it at aphelion. Amor asteroids orbit between Earth and Mars without crossing Earth's orbit. Atira asteroids (also called IEOs - Inner Earth Objects) orbit entirely inside Earth's orbit and are very rare. The families are named after the first-discovered member of each class.

What is MOID?

MOID stands for minimum orbital intersection distance. It is the closest approach distance between two orbital paths at their geometrically nearest points, calculated without regard to where the two bodies actually are in their orbits at any given time. A low MOID means the orbital paths come very close together - but an actual close approach only happens when both objects are near those intersection points simultaneously. MOID is used in the PHA definition because it identifies objects whose orbits are geometrically capable of bringing them near Earth.

How many PHAs are known?

As of 2026, more than 2,400 potentially hazardous asteroids have been catalogued, and the number grows as survey programmes discover new objects. None of the known PHAs have a meaningful probability of impacting Earth in the next century. The PHA designation is a flag for monitoring priority, not a statement that the object will hit Earth.

Is a "near-Earth asteroid" the same as a "near-Earth object"?

Almost, but not exactly. A near-Earth asteroid (NEA) is the asteroid-only subset of the NEO category. NEOs also include approximately 100 near-Earth comets (NECs) - comets whose perihelion falls within 1.3 AU. In practice, asteroids dominate the NEO catalogue: of more than 38,000 known NEOs, roughly 100 are comets. "NEO" and "near-Earth asteroid" are often used interchangeably in media coverage, but the distinction matters in technical contexts.