- 65803 Didymos is a near-Earth asteroid system that gained international prominence as the target of the first full-scale planetary defense test.
- Discovered in 1996 by the Spacewatch survey at Kitt Peak Observatory, Didymos is a binary asteroid system, consisting of a primary body roughly 780 meters (2,560 feet) in diameter and a smaller moonlet, officially designated Dimorphos, measuring about 160 meters (525 feet) across. The name “Didymos” means “twin” in Greek—a fitting title for a binary system where the smaller object orbits the larger one. Didymos belongs to the Apollo group of near-Earth asteroids and has an orbital period around the Sun of approximately 2.1 years.
- What makes Didymos especially important is its role in the NASA Double Asteroid Redirection Test (DART) mission—the first real-world attempt to change the trajectory of an asteroid by direct kinetic impact. On September 26, 2022, NASA’s DART spacecraft deliberately collided with Dimorphos at a speed of approximately 6.6 km/s (14,700 mph), successfully altering its orbital period around Didymos. Before impact, Dimorphos orbited the larger asteroid once every 11 hours and 55 minutes. After the DART collision, this period was shortened by about 33 minutes, confirming that the impact had effectively changed the orbit of a celestial body for the first time in history—a critical proof-of-concept for planetary defense.
- Both Didymos and Dimorphos are classified as S-type (silicaceous) asteroids, suggesting a rocky composition primarily made of silicate minerals and some metal. The binary system is believed to be a “rubble pile” configuration—loose collections of rocks held together by gravity rather than a solid monolithic structure. This porous structure makes the system especially relevant for testing deflection techniques, as the impact behavior differs significantly from that of a solid rock. Data from the DART mission, including images and spectral measurements from both onboard instruments and ground-based telescopes, are helping scientists better understand the structural properties of such bodies and how they respond to kinetic impacts.
- Following the DART impact, the European Space Agency’s Hera mission—scheduled for launch in 2024 with arrival in 2026—will conduct a detailed follow-up study of the Didymos system. Hera will investigate the crater left by DART, analyze Dimorphos’s internal structure, and collect high-resolution data on the dynamics of the binary system. This multi-phase international collaboration represents a major step forward in planetary defense science, enabling humanity to prepare for the eventuality of a real asteroid threat by testing and refining mitigation strategies in space.
