NEAs

Near-Earth Asteroids (NEAs) are a class of asteroids whose orbits bring them into proximity with Earth's orbit, making close approaches possible. By convention, NEAs have perihelion distances of 1.3 astronomical units (AU) or less. NEAs are a subset of near-Earth objects (NEOs), which also includes comets.

NEAs are commonly divided into four dynamical groups: Atiras (or interior-Earth objects) whose orbits lie entirely inside Earth's; Atens, whose orbits cross Earth's orbit from inside; Amors, which approach Earth but do not cross it; and Apollos, whose orbits cross Earth’s from outside. These classifications describe the way the asteroid’s path relates to Earth’s orbit rather than precise size.

Sizes among NEAs range from meters to kilometers. They are composed primarily of rock and metal, with a range of densities and structures. Some NEAs are binary or contact-binary systems. Their relatively small sizes and proximity to Earth make them frequent subjects of observation and study, contributing to understanding of the solar system’s formation and evolution.

Discovery and tracking of NEAs are driven largely by ground-based surveys initiated in the late 20th century, including LINEAR, Spacewatch, the Catalina Sky Survey, Pan-STARRS, and the NEOWISE mission. Astronomical networks continually track NEAs to refine their orbits and assess potential impact risks. The Jet Propulsion Laboratory’s Sentry system continually monitors for possible Earth impacts.

A subset of NEAs is classified as potentially hazardous asteroids (PHAs) if they are large enough and come within about 0.05 AU of Earth. NEAs are also the focus of planetary defense research and, in some cases, missions to study their properties or return samples. Notable exploration includes OSIRIS-REx at Bennu and Hayabusa2 at Ryugu.