Impact Models Provide Critical Insight into Results from The Double Asteroid Redirection Test (DART) Mission and Information about Dimorphos Material Properties

NASA’s Double Asteroid Redirection Test (DART) mission is the first test of a kinetic impactor at scales relevant to planetary defense. The DART spacecraft impacted into Dimorphos, the moon of the 65803 Didymos system, on September 26, 2022. This impact slowed the orbital velocity of Dimorphos about Didymos and resulted in an orbital period change of 33 minutes, which translates to an orbital velocity change of ~2.7 mm/s and a momentum enhancement factor, β, of ~3.6 assuming the density of Dimorphos is 2400 kg/m 3 . Images from DART’s DRACO camera revealed Dimorphos to be a rubble-covered oblate spheroid. Follow-on images from the ASI-led cubesat LICIAcube, and Earth- and space-based telescopes, revealed spectacular ejecta streamers and rays immediately following impact, with a complicated ejecta structure and potentially ejected boulders.

Pre-impact simulations provided intuition leading up to the DART impact and suggested that the material properties that have the largest effects on deflection velocity and β are material cohesion (yield strength at zero pressure) and material porosity; internal friction can also play an important role. Approach observations and those following the DART impact provided

crucial knowledge to narrow the parameter space relevant to Dimorphos. Early post-DART simulations by the DART Impact Modeling Working Group suggest that multiple combinations of material properties (e.g., strength and porosity) and target structure can match critical DART observations. No single simulation has yet, or is likely to without further data, uniquely explain every key observation from DART because many properties remain currently unconstrained (subsurface structure, etc.) or highly uncertain (e.g., density and mass of Dimorphos). Despite remaining uncertainties, synthesizing results from a variety of initial simulations provides one of the best ways to evaluate potential “best-fit” properties of Dimorphos, and estimate ejecta mass and crater size. The constraints from the simulations will also provide additional information regarding how data from Hera can further constrain Dimorphos’s material properties after it arrives at the Didymos system.