- Dimorphos is a small, near-Earth asteroid that serves as the moonlet in the binary asteroid system 65803 Didymos. It has an estimated diameter of approximately 160 meters (about 525 feet), making it comparable in size to the Great Pyramid of Giza.
- Dimorphos gained worldwide recognition as the first celestial body whose orbit was intentionally altered by human technology—a milestone in the history of planetary defense. It orbits the larger primary, Didymos, at a distance of roughly 1.2 kilometers (0.75 miles) and was discovered in 2003 when observations revealed that Didymos was in fact a binary system.
- The name Dimorphos, which means “two forms” in Greek, reflects its unique role in planetary defense: before impact, it was a naturally orbiting satellite; after impact, it became a transformed object—its motion altered by human intervention. On September 26, 2022, NASA’s DART (Double Asteroid Redirection Test) spacecraft intentionally collided with Dimorphos in a high-velocity impact. The goal of the mission was to test the feasibility of kinetic impact as a method of asteroid deflection, should a future object pose a collision threat to Earth. DART struck Dimorphos at about 6.6 km/s (14,700 mph), resulting in a measurable change in its orbital period around Didymos—from about 11 hours and 55 minutes to 11 hours and 22 minutes—a 33-minute reduction that far exceeded pre-impact expectations.
- Dimorphos is classified as a stony (S-type) asteroid, composed mainly of silicate rocks and metal. It is thought to be a “rubble pile” structure—a loosely bound aggregation of boulders, dust, and smaller rock fragments held together by gravity. The DART impact confirmed this hypothesis, as the collision produced a large cloud of ejecta that streamed into space, forming an elongated tail similar to a comet’s. This ejected material not only helped reduce Dimorphos’s orbital momentum but also provided rich data for scientists to analyze the physical properties and internal cohesion of small asteroid bodies.
- The impact and its aftermath were observed by telescopes worldwide and by the LICIACube (Light Italian CubeSat for Imaging of Asteroids), a small satellite deployed by DART before the collision. LICIACube captured close-up images of the impact and the resulting ejecta plume, offering a unique vantage point. Further detailed analysis of the system is expected to be carried out by the European Space Agency’s Hera mission, set to arrive at Didymos and Dimorphos in 2026. Hera will provide high-resolution imaging, radar mapping, and data on the crater left by DART, helping to refine models of asteroid deflection and surface mechanics.
