NEOS

Near-Earth objects, abbreviated NEOs, are small solar-system bodies whose orbits bring them into close proximity with Earth. By convention, an object is classified as a NEO if its perihelion distance is 1.3 astronomical units (AU) or less. NEOs include both asteroids and comets, with most discoveries being asteroids. They are detected and tracked by space- and ground-based surveys that measure their positions over time to refine their orbits.

Near-Earth asteroids (NEAs) comprise several orbital classes named after their typical paths: Aten, Apollo, and Amor. A subset of NEAs is designated potentially hazardous asteroids (PHAs), defined by an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and a size above a certain threshold, commonly around 140 meters or larger. Some NEOs are also classified as near-Earth comets (NECs) when they display cometary activity while approaching Earth’s vicinity. Observatories and space agencies continuously track NEOs to assess potential impact risks and to understand their physical properties, compositions, and dynamical histories.

Monitoring and risk assessment are coordinated through programs such as NASA’s Center for Near-Earth Object Studies and the European Space Agency’s SSA-NEO efforts, supported by surveys including LINEAR, the Catalina Sky Survey, Pan-STARRS, and NEOWISE. Impact probability estimates are updated as new observations become available, and the information informs planetary defense planning as well as scientific investigations into the early solar system and the evolution of small bodies.