Some active asteroids have been proposed to be formed as a result of impact events1. Because active asteroids are generally discovered by chance only after their tails have fully formed, the process of how impact ejecta evolve into a tail has, to our knowledge, not been directly observed. The Double Asteroid Redirection Test (DART) mission of NASA2, in addition to having successfully changed the orbital period of Dimorphos3, demonstrated the activation process of an asteroid resulting from an impact under precisely known conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope from impact time T + 15 min to T + 18.5 days at spatial resolutions of around 2.1 km per pixel. Our observations reveal the complex evolution of the ejecta, which are first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and subsequently by solar radiation pressure. The lowest-speed ejecta dispersed through a sustained tail that had a consistent morphology with previously observed asteroid tails thought to be produced by an impact4,5. The evolution of the ejecta after the controlled impact experiment of DART thus provides a framework for understanding the fundamental mechanisms that act on asteroids disrupted by a natural impact1,6.
NASA’s Double Asteroid Redirection Test (DART) mission performed the first ever kinetic impact to deflect an asteroid1. The DART kinetic impact test artificially activated an asteroid with a hypervelocity impact, providing a unique opportunity for an extensive observing campaign to monitor the evolutionary process from the formation of the ejecta to its dispersion via a sustained tail. Here we report observations of the impact ejecta with the Hubble Space Telescope (HST) from impact time (T)+15 minutes to T+18.5 days at a resolution of 2.1 km per pixel. Our observations showed that the gravitational interaction between the binary system and dust under the influence of solar radiation pressure produced a complex morphology during the evolution of the ejecta. The dust ejected at speeds much higher than the escape speed of the binary system (0.25 m/s) is directly ejected out of the system. The dust moving at speeds just above the escape speed displayed signatures of gravitational interaction with the binary asteroid system, forming spirals and extended features. Slow ejecta is ultimately pushed in the antisolar direction (nearly opposite the impact direction) by solar radiation pressure to form a tail. These dynamical processes are highly dependent on particle size and ejection direction. The ejecta evolution following DART’s kinetic impact offers a framework for understanding the fundamental mechanisms acting on asteroids disrupted by natural impact2,3 for the first time.
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