Near-Earth asteroids (NEAs) are asteroids that follow orbits bringing them into the vicinity of Earth's path around the Sun. They range in size from metre-scale rocks to objects several kilometres across and are a subset of the wider asteroid population. Astronomers have discovered tens of thousands of NEAs, and ongoing surveys continue to find more. Interest in these objects comes from their scientific importance, possible resources, and the potential hazard they pose to Earth.

Characteristics

NEAs are defined by orbital parameters rather than composition. Their orbital shapes (eccentricities and inclinations), closest approach distances, and semimajor axes determine how and when they can come near our planet. Physical properties vary: some are rubble piles loosely held together by gravity, others are solid rock or metal. Sizes and surface properties are often measured using visible-light surveys and infrared observations that give estimates of diameter and albedo.

Classification

  • Atira (also called Apohele): orbit entirely inside Earth's orbit.
  • Aten: semimajor axis less than 1 AU and an orbit that crosses Earth's orbit.
  • Apollo: semimajor axis greater than 1 AU with perihelion less than Earth's aphelion; they cross Earth's orbit.
  • Amor: approach Earth's orbit from outside but generally do not cross it.

These dynamical groups are based on orbital elements and are used to predict encounters and plan observations. The term "asteroids" used here follows the conventional meaning of small rocky bodies in the inner Solar System (asteroids), and their trajectories are described by standard celestial mechanics (orbits).

Discovery and monitoring

Detection of NEAs relies on wide-field surveys that repeatedly image large areas of sky to find moving objects. Ground-based programs and dedicated telescopes scan for new objects and follow up to refine their trajectories. Space-based infrared telescopes complement optical surveys because thermal emissions help constrain size. Cataloguing efforts aim to determine orbits well enough to forecast future approaches and to identify objects that merit further study.

Impact risk and mitigation

Most NEAs are harmless, and small objects enter the atmosphere frequently and burn up. Larger impacts are rare but potentially destructive, so agencies maintain systems to assess risk and produce short- and long-term predictions. Techniques for predicting impacts include automated monitoring and specialized prediction services that compute probabilities and possible impact corridors (Asteroid impact prediction). Planetary defence concepts under study include deflection by a kinetic impactor, gravity tractors, and civil-protection planning for emergencies. In recent years, demonstration missions have tested deflection concepts to move or study small bodies.

Exploration and notable missions

NEAs are attractive destinations for robotic missions because of their accessibility and scientific value. Several have been visited: NASA's Near Earth Asteroid Rendezvous mission studied 433 Eros, and JAXA's Hayabusa returned samples from 25143 Itokawa. Other missions have orbited, sampled, or returned material from NEAs, advancing knowledge of their composition, structure, and history. For an overview of spacecraft encounters and mission planning, consult resources on missions and past encounters (visited by spacecraft).

NEAs remain a focus of astronomy, planetary science, and international cooperation. Improved surveys, better orbit determination, and mission experience together reduce uncertainty about potential hazards while opening opportunities for research and exploration.

For technical details on orbital elements and ongoing monitoring programs, refer to specialized observatory and planetary defense pages that track discovery statistics and predicted close approaches (orbits, Asteroid impact prediction). Additional background and outreach material is available through scientific institutions and mission archives (asteroids, visited by spacecraft, Earth).