Didymos

Didymos is a binary near-Earth asteroid system consisting of a larger primary, Didymos A, and a small natural satellite, Dimorphos (Didymos B). It orbits the Sun and is part of the population of near-Earth objects that cross or approach Earth's orbit. The system has been a focus of planetary defense research due to its accessibility and binary nature.

The primary body is roughly 780 meters in diameter, while the satellite Dimorphos measures about 160 meters. The two bodies orbit each other at a separation of around 1.2 kilometers, completing an orbital period of about 11.9 hours.

The Didymos system was discovered in 1996, and its moon Dimorphos was identified later; the satellite was officially named Dimorphos, derived from Greek for "two forms," reflecting the two-body configuration.

In September 2022, NASA conducted the Double Asteroid Redirection Test (DART), a kinetic impact experiment designed to test asteroid deflection techniques. DART collided with Dimorphos, causing the moon’s orbital period around Didymos to shorten by about 32 minutes. The mission demonstrated that a spacecraft impact could measurably alter an asteroid’s trajectory, validating a potential planetary defense approach.

The European Space Agency's Hera mission is planned to visit the Didymos system to characterize the crater, mass distribution, and surface properties of both bodies, providing a detailed post-impact assessment and advancing understanding of small-body physics.

Didymos thus serves as a key natural laboratory for studying binary asteroid dynamics and testing deflection concepts relevant to planetary defense.