10:15 〜 10:30
[MZZ40-06] Ejecta thickness distribution on Dimorphos after the NASA’s DART impact
キーワード：地球防衛、高速度衝突、DART/Hera、放出物堆積
A meteoroid entered the Earth’s atmosphere over Chelyabinsk Oblast, Russia, on Februaly 15 in 2013, and it caused a great deal of damage, including human casualties.. An atmospheric entry of meteoroid like the Chelyabinsk event has been considered to occur once every few decades. It is not difficult to predict a major catastrophe if a similar event were to occur over a major metropolitan area. How we should respond to the threat of collisions of small dodies has been considered as a planetary defense issue.
Currently, the Asteroid Impact and Deflection Assessment (AIDA) project, the world's first asteroid orbit change experiment, has been conducted. This project consists of two exploration programs, NASA's Double Asteroid Redirection Test (DART) and ESA's Hera. The targets are the binary asteroid (65803) Didymos (780 m in diameter) and Dimorphos (the satellite, ~150 m in diameter). The DART spacecraft collided with Dimorphos at 6.1449 km s-1 on 26 September in 2022. Observations were made by the small satellite detached from the DART spacecraft, space telescopes, including Hubble and JWST, and a number of ground-based telescopes. The change in the orbital period of Dimorphos due to the impact was measured in detail, and it was confirmed that the momentum enhancement factor, beta, is large, ranging from 2.5 to 4.5. The ESA’s Hera, which is scheduled to be launched in October 2024, will visit the binary system to study in detail how the binary asteroid was changed by the impact. The high beta value observed may be a probe of the physical properties of the small asteroid. In order to maximize the scientific results from the DART and Hera missions, it will be necessary to understand what happened during the DART impact and to predict how the surfaces of the asteroids has changed.
In this study, we propose a simple model to predict the outcome of the DART impact based on the modified Maxwell’s Z model proposed in our previous study [Kurosawa and Takada, 2019, Icarus, 317, 135–147]. We revised the model to incorporate the effects of target curvature on impact outcomes. One of the important features of our model is that it can reproduce, without any special assumptions, the behavior of the ejecta velocity-mass distribution deviating from a power law, which is derived from “the point-source theory”, at relatively low ejection velocities. This region cannot be treated with the pi-group scaling laws. In addition, it is difficult to treat the motions of the materials moving the particle velocities much slower than the sound speed with hydrocodes.
Our model predicts that 0.3–1% of the Dimorphos mass must be ejected to reproduce the observed beta value, and ~1/5 of it also escapes from the Didymos-Dimorphos system, and that the effective strength of the surface materials on Dimorphos is likely to be <100 Pa. The model also provides the velocity-mass distribution of the ejecta, its launch position, and the ejection angle measured from the surface of Dimorphos. Assuming ballistic flights, we can obtain the thickness distribution of the ejecta on the surface of Dimorphos. The surface of Dimorphos would be entirely altered due to ejecta deposition with a thickness greater than several tens centimeters.
Currently, the Asteroid Impact and Deflection Assessment (AIDA) project, the world's first asteroid orbit change experiment, has been conducted. This project consists of two exploration programs, NASA's Double Asteroid Redirection Test (DART) and ESA's Hera. The targets are the binary asteroid (65803) Didymos (780 m in diameter) and Dimorphos (the satellite, ~150 m in diameter). The DART spacecraft collided with Dimorphos at 6.1449 km s-1 on 26 September in 2022. Observations were made by the small satellite detached from the DART spacecraft, space telescopes, including Hubble and JWST, and a number of ground-based telescopes. The change in the orbital period of Dimorphos due to the impact was measured in detail, and it was confirmed that the momentum enhancement factor, beta, is large, ranging from 2.5 to 4.5. The ESA’s Hera, which is scheduled to be launched in October 2024, will visit the binary system to study in detail how the binary asteroid was changed by the impact. The high beta value observed may be a probe of the physical properties of the small asteroid. In order to maximize the scientific results from the DART and Hera missions, it will be necessary to understand what happened during the DART impact and to predict how the surfaces of the asteroids has changed.
In this study, we propose a simple model to predict the outcome of the DART impact based on the modified Maxwell’s Z model proposed in our previous study [Kurosawa and Takada, 2019, Icarus, 317, 135–147]. We revised the model to incorporate the effects of target curvature on impact outcomes. One of the important features of our model is that it can reproduce, without any special assumptions, the behavior of the ejecta velocity-mass distribution deviating from a power law, which is derived from “the point-source theory”, at relatively low ejection velocities. This region cannot be treated with the pi-group scaling laws. In addition, it is difficult to treat the motions of the materials moving the particle velocities much slower than the sound speed with hydrocodes.
Our model predicts that 0.3–1% of the Dimorphos mass must be ejected to reproduce the observed beta value, and ~1/5 of it also escapes from the Didymos-Dimorphos system, and that the effective strength of the surface materials on Dimorphos is likely to be <100 Pa. The model also provides the velocity-mass distribution of the ejecta, its launch position, and the ejection angle measured from the surface of Dimorphos. Assuming ballistic flights, we can obtain the thickness distribution of the ejecta on the surface of Dimorphos. The surface of Dimorphos would be entirely altered due to ejecta deposition with a thickness greater than several tens centimeters.