Atira asteroid

The Atira class was defined within modern planetary science after the discovery and confirmation of the first archetype designated 163693 Atira. Earlier detections of candidate objects (for example, 1998 DK36) and alternative naming proposals (notably the Hawaiian‑derived 'Apohele') predate the settled eponym. Following standard small‑body taxonomy practice, the dynamical class took its name from the first confirmed member and is defined by the orbital criterion that an object's aphelion is smaller than Earth's perihelion (0.983 astronomical units). The term 'interior‑Earth object' (IEO) is a descriptive English alternative used in some literature.

As a class Atira asteroids are small Solar System bodies whose individual physical properties (size, composition, albedo, binarity) vary by object; the class is defined by orbit rather than by uniform physical characteristics. The archetype 163693 Atira has been reported as an S‑type and is a binary system with a primary of roughly 4.8 km and a secondary of roughly 1.0 km in diameter in the cited literature. For many Atiras catalog diameter estimates are derived from assumed albedos (commonly ~0.14) rather than direct measurement, and most members are substantially smaller than that archetype.

Dynamically, an Atira's defining property is an orbit entirely interior to Earth's orbit (aphelion < 0.983 AU). Because they do not currently cross Earth's orbit they are not immediate impact threats in their present orbits, but gravitational perturbations (notably from Mercury and Venus) and secular processes such as the Kozai–Lidov mechanism can alter their orbits over time and potentially move them into Earth‑crossing trajectories. The population includes binaries and objects whose orbital parameters suggest past fragmentation events; observationally they are rare among known NEOs and are difficult to detect from the ground due to low solar elongation, typically requiring twilight searches or space‑based observations.