Classification
Types of
near-Earth asteroids
Near-Earth asteroids fall into four orbital families defined by the shape of their orbits relative to Earth's. A separate classification covers composition. The two systems are independent - an asteroid's orbital family says nothing about what it is made of.
What is a near-Earth object? →The four orbital families
Composition is one way to sort asteroids; orbit is the other. These are the four near-Earth orbit families, moving at their true relative speeds.
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.
Why the orbital families matter
Near-Earth asteroids (NEOs) are classified by orbital shape, not by location on any given day. Two parameters define each family: the semi-major axis (the average distance from the Sun across the full orbit) and the perihelion or aphelion distance (the closest or farthest point from the Sun).
Orbital family tells planetary scientists how often an asteroid can approach Earth and at what range. An Apollo-class asteroid crosses Earth's orbital path twice per revolution by definition. An Atira asteroid never reaches Earth's orbital distance at all. That distinction shapes monitoring priority and encounter frequency.
More than 38,000 near-Earth asteroids are currently catalogued. The Apollo family accounts for roughly two-thirds of them - the most numerous group by a significant margin.
The four orbital families
All four families share the NEO criterion: perihelion within 1.3 AU of the Sun. The boundaries between families are defined by orbital mechanics, not naming conventions.
The largest group - roughly two-thirds of all known near-Earth asteroids. Semi-major axis exceeds 1 AU but perihelion falls inside Earth's orbit, making them Earth-crossers. Named after 1862 Apollo, discovered in 1932. Most close approaches in the tracker involve Apollo-class objects.
Orbit lies between Earth's orbit and Mars. Perihelion falls just outside Earth's path, so they do not currently cross it. Gravitational perturbations from Jupiter can gradually shift Amor orbits until they become Earth-crossers over thousands to millions of years.
Semi-major axis sits inside Earth's orbit - their average position is closer to the Sun than Earth's - but their aphelion extends into Earth's orbital neighbourhood. They cross Earth's orbit twice per revolution. Named after 2062 Aten, discovered in 1976.
Orbit lies entirely within Earth's orbit. Aphelion never reaches 0.983 AU, so they are always between Earth and the Sun from our perspective. Ground-based survey telescopes struggle to observe them because the sky near the Sun is difficult to image. Also called Inner Earth Objects (IEOs). The rarest of the four families.
Near-Earth comets
Roughly 100 comets also qualify as near-Earth objects - they meet the same 1.3 AU perihelion threshold. Near-Earth comets originate in the outer solar system and have been perturbed into shorter orbits by gravitational interactions, primarily with Jupiter.
When a comet warms on approach to the Sun, subsurface ices sublimate and produce the characteristic coma and tail. That outgassing creates a non-gravitational force that makes orbital prediction more complex than for asteroids. Near-Earth comets are catalogued and monitored alongside asteroids but are tracked separately in NASA's database.
Compositional classification
Orbital family and composition are independent. A C-type asteroid can belong to any of the four orbital families. Composition is inferred from spectroscopy and albedo measurements; spacecraft visits give more precise data.
| Type | Composition |
|---|---|
| C-type | Carbonaceous, primitive |
| S-type | Silicaceous (stony) |
| M-type | Metallic iron-nickel |
| Other | Mixed or uncertain |
Fractions are approximate. S-types appear over-represented because they are brighter and easier to detect than the dark C-types.
Does orbital family predict impact risk?
Orbital family is a starting point, not a verdict. Apollo asteroids cross Earth's orbital path by definition - but "crossing" means the two paths intersect somewhere in space. Earth and the asteroid are rarely at that intersection point at the same time. Most Apollo-class passes are tens of lunar distances away.
Impact probability requires detailed orbital calculations spanning centuries, accounting for gravitational perturbations from all planets. The potentially hazardous asteroid (PHA) designation adds size as a requirement: an object needs to be 140 metres or larger and pass within 0.05 AU of Earth's orbit. No known PHA carries a meaningful impact probability in the next century.
The bottom line
Orbital family determines encounter frequency. Composition affects internal structure and how an object responds to deflection. Neither alone determines impact risk. NASA monitors all known near-Earth objects regardless of family and publishes updated impact probability tables for any object with a non-zero risk score.
Related pages
What is a near-Earth object?
The broader NEO category - the 1.3 AU threshold and what it means.
Asteroid size comparison
How size affects impact risk - from bus-sized to mountain-scale.
Asteroid danger rating
The Torino and Palermo scales used to communicate impact risk.
What is a close approach?
How NASA defines and measures close approaches.
