Physical properties of asteroid Dimorphos as derived from the DART impact

The Double Asteroid Redirection Test (DART) mission aimed to demonstrate the feasibility of using a kinetic impactor to alter an asteroid's trajectory. The impact successfully reduced Dimorphos's orbital period around Didymos by 33 ± 1 min. Observations from LICIACube and ground-based telescopes revealed extensive ejecta and a significant brightening of the Didymos system. The momentum transfer to Dimorphos exceeded the DART spacecraft's incident momentum by a factor (β) of 2.2 to 4.9, depending on Dimorphos's unknown mass. This β parameter is crucial for understanding the impact process and refining kinetic impactor techniques for planetary defense. Determining Dimorphos's mass and material properties is essential for this purpose, as well as for understanding the formation and evolution of binary asteroid systems.

Previous research has investigated the role of asteroid strength, porosity, and internal friction in impact momentum transfer. Studies have modeled the effects of impact and target parameters on kinetic impactor outcomes, specifically for the DART mission. Photometric observations of the Didymos system prior to the DART impact provided estimates of Dimorphos's bulk density. The spectral properties of Didymos suggested potential meteorite analogues for Dimorphos's composition.

This study used numerical simulations of the DART impact to infer Dimorphos's properties. The Bern SPH shock physics code simulated the impact over a range of assumed material properties and internal structures. The DART spacecraft was represented as a low-density spherical projectile of equivalent mass. The asteroid's response was modeled for up to 1 hour post-impact. Realistic boulder distributions were generated using simulations of gravitational collapse. The macroporosity was calculated from the boulder size-frequency distribution (SFD) using a method previously applied to Ryugu. Material properties of the boulders were based on ordinary chondrite measurements. The target matrix material's response to shear deformation was described using a pressure-dependent strength model. The momentum enhancement (β) was calculated using two methods: summing the momentum of ejected particles and tracking the asteroid's center-of-mass velocity. The ejecta curtain opening angle and morphology were also analyzed and compared to LICIACube and Hubble Space Telescope observations. Resolution tests were conducted to assess the simulation's accuracy. Parameter studies investigated the effects of boulder volume fraction, grain density, cohesion, and coefficient of internal friction on the results.

Simulations indicate that β is relatively insensitive to boulder volume fractions up to 30 vol%. Higher boulder fractions reduce crater efficiency due to interlocking and armouring. The measured β constrains Dimorphos's bulk density to less than 2,400 kg m⁻³, suggesting a porous, rubble-pile structure. For a fixed boulder distribution, varying matrix cohesion and internal friction coefficient yielded several possible combinations that matched the observed momentum enhancement. However, a cohesion lower than ~50 Pa is likely. The ejecta cone opening angle shows no significant dependence on the friction coefficient but is influenced by target cohesion. Cohesionless targets produce more massive ejecta plumes with wider cone angles than cohesive targets. LICIACube images support the low-cohesion scenario. The estimated macroporosity of Dimorphos's surface (~35%) is roughly twice that of Ryugu but comparable to Itokawa. Dimorphos's low cohesive strength (less than a few pascals) contrasts with Didymos, suggesting a different formation process involving fine grain escape during accretion from Didymos. The DART impact likely caused global deformation of Dimorphos, implying easily reshaped asteroid moons with relatively young surfaces.

The findings address the research question by characterizing Dimorphos's physical properties and providing insights into its formation. The low strength and porosity of Dimorphos, similar to Ryugu and Bennu, suggest that these properties are common among rubble-pile asteroids. However, the contrast between Dimorphos and its parent body Didymos highlights potential differences in the formation processes of secondary bodies in binary asteroid systems. The global resurfacing caused by the impact suggests that similarly formed moons are easily deformed and their surfaces are frequently rejuvenated. These results improve our understanding of asteroid characteristics and inform planetary defense strategies.

This study provides valuable information on Dimorphos's physical properties, supporting its classification as a low-strength, porous rubble-pile asteroid likely formed by mass shedding from Didymos. The DART impact significantly altered Dimorphos’s surface, underscoring the potential for significant global-scale deformation of similar small bodies. Future research should focus on acquiring more detailed observational data from the Hera mission to further refine our understanding of Dimorphos's internal structure and the implications for binary asteroid formation and planetary defense strategies.

The study's simulations simplified the DART spacecraft geometry and did not account for Didymos's gravitational influence on ejecta. The limited resolution of some simulations might have influenced the accuracy of ejected mass and momentum calculations at high velocities. The model's assumptions about material properties, such as cohesion and internal friction, may introduce uncertainties in the results. The macroporosity estimate relies on currently available data and could be refined with future observations.