Planetary Scientist; Hypervelocity Impact Physicist
Johns Hopkins University Applied Physics Laboratory (JHU APL)
“Design, Impact Modeling, and Results of NASA’s Double Asteroid Redirection Test (DART) Mission”
Abstract: 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/m3. 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. Here, I will discuss the DART mission, requirements, design, results, and how we modeled the impact to better understand what happened.
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 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. 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 (e.g., rubble pile, boulder arrangement and packing, subsurface structure) can match critical DART observations. No single simulation has uniquely explained every key observation from DART because many of Dimorphos’s properties remain currently unconstrained (subsurface structure, etc.) or are highly uncertain (e.g., density and mass of Dimorphos) . In general, the rubble-strewn surface (and hypothesized near-subsurface) provide significant complications for modeling the DART impact. Simulations show that boulder position and size can substantially affect the ejecta mass-velocity distribution and momentum transfer efficiency of the impact. Further, there is no evidence for fines at the surface of Dimorphos, so discussions of the “strength” of the surface remain complicated. The “strength” of Dimorphos is an important parameter for accurately simulating the DART impact using numerical hydrocodes, and we’ll discuss what the team knows, how we currently model things, and what might be the most appropriate model for the spacecraft materials and rubble-strewn asteroid material. Despite remaining uncertainties, initial models of DART’s kinetic impact do provide important information about the results of DART and the properties of Dimorphos, which will also be highlighted.
BIO: Dr. Angela Stickle is a planetary geologist with a background in Aerospace and Mechanical Engineering, magnetospheric physics, and impact processes on planetary surfaces. She specializes in hypervelocity impact processes and dynamic failure of materials. Dr. Stickle is currently a senior research scientist at the Johns Hopkins University Applied Physics Laboratory. She is the Deputy Principal-Investigator for the Mini-RF radar, a Co-I for the LRO-LAMP instrument aboard the Lunar Reconnaissance Orbiter, the impact modeling working group lead for the Double Asteroid Redirection Test (DART) mission, and a Co-I on the Dragonfly mission. Her research includes analyzing young impact craters on the Moon and Mars to better understand ejecta emplacement processes, impact modeling on asteroids and rocky/icy bodies, planetary defense testing, and working to understand and evaluate available technology for future lunar surface missions. Asteroid 36986 Stickle is named in her honor.