First detection of an asteroid's heliocentric deflection: The Didymos system after DART

NASA’s Double Asteroid Redirection Test (DART) was the world’s first planetary defense technology demonstration mission1 . In September 2022, the DART spacecraft impacted Dimorphos, the secondary asteroid in the (65803) Didymos binary asteroid system, and reduced its orbital period around Didymos2–4 . However, as anticipated, some fraction of the impulse delivered by the spacecraft and impact ejecta was also imparted onto the system barycenter, causing a change in the system’s heliocentric orbit5,6 . Here, we present the first-ever measurement of human-caused change in the heliocentric orbit of a natural body in the solar system. Thanks to stellar occultation observations in 2022-2025 combined with radar measurements in 2003 and 2022, we estimate that the DART impact applied an along-track change in velocity, or ∆V of −11.7±1.3µms−1 to the Didymos system, resulting in a measurable departure from the system’s pre-DART heliocentric orbit. This measurement further allows us to constrain the heliocentric momentum enhancement factor for the DART impact at 2.0±0.3 as well as the bulk density of Didymos at 2600±140kgm−3 and that of Dimorphos at 1540±220kgm−3 . Our result of an underdense secondary supports the theory that Dimorphos formed as a result of accretion7–9 . Finally, our results demonstrate that targeting the secondary asteroid in a binary system constitutes a viable strategy for kinetic impact-based deflection, adding to humanity’s planetary defense capabilities.