Pre-encounter Predictions of DART Impact Ejecta Behavior and Observability

Abstract

We overview various efforts within the DART Investigation Team’s Ejecta Working Group to predict the characteristics, quantity, dynamical behavior, and observability of DART impact ejecta. We discuss various methodologies for simulation of the impact/cratering process with their advantages and drawbacks in relation to initializing ejecta for subsequent dynamical propagation through and away from the Didymos system. We discuss the most relevant forces acting on ejecta once decoupled from Dimorphos’s surface and highlight various software packages we have developed and used to dynamically simulate ejecta under the action of those forces. With some additional software packages, we explore the influence of additional perturbing effects, such as interparticle collisions within true N-body codes and nonspherical and rotating particles’ interplay with solar radiation pressure. We find that early-timescale and close-proximity ejecta evolution is highly sensitive to some of these effects (e.g., collisions) while relatively insensitive to other factors. We present a methodology for turning the time-evolving size- and spatially discretized number density field output from ejecta simulations into synthetic images for multiple platforms/cameras over wide-ranging vantage points and timescales. We present such simulated images and apply preliminary analyses to them for nominal and off-nominal cases bracketing realistic total mass of ejecta and ejecta cumulative size–frequency distribution slope. Our analyses foreshadow the information content we may be able to extract from the actual images taken during and after the DART encounter by both LICIACube and Earth-vicinity telescopes.

3D Radiative-transfer Simulations of the Ejecta Plume Anticipated from DART Impact

“…Various observations suggested that the ejecta were dominated by millimeter-to centimeter-sized particles. The ejecta velocity is about 0.2 m s −1 , comparable to the bulk velocity of the expected DART ejecta (Fahnestock et al 2022). The dust size frequency distribution of the ejecta of P/2010 A2 was estimated to have a power-law index of −3.3 to −3.5 based on the modeling of radiation pressure.…”

“…But considering that it is in the main asteroid belt, where the average relative velocity is about 4-5 km s −1 (Farinella & Davis 1992;Bottke et al 1994), its impact velocity is probably comparable to the DART impact velocity. The only significant difference between the P/2010 A2 impact and the DART impact is the much larger impactor for the former, which was estimated to be tens of meters in radius based on the ejecta mass estimate of about 10 7 -10 8 kg (Jewitt et al 2010), roughly proportionally higher than the expected ejecta mass for DART impact (10 6 -10 7 kg; Fahnestock et al 2022). Therefore, the ejecta from P/2010 A2 provides the best analog to the DART ejecta in terms of the ejecta particle size and size distribution.…”

“…This assumption is realistic if the time of consideration is much longer than the crater formation time, and the distance of ejecta to consider is much larger than the crater size. Based on the calculation from scaling laws, the DART impact crater formation time is about 100 s and the crater size is <20 m for the nominal material properties of Dimorphos (Fahnestock et al 2022). We also included the gravity field of Dimorphos and Didymos when calculating the motion of ejecta particles, assuming that the ejecta moves in a direction directly opposite to Didymos.…”

Effects of Impact and Target Parameters on the Results of a Kinetic Impactor: Predictions for the Double Asteroid Redirection Test (DART) Mission

“…Importantly, ejecta curtains will look different as impact velocity and material properties are varied. The full details of the effects of ejecta curtains are well studied and presented in more detail in Fahnestock et al (2022). Ejecta plume simulations using information from shock physics codes as input conditions show that ejecta particle dynamics are sensitive to initial ejection parameters such as orientation and ejection velocity (Ivanovski et al 2020).…”

Anticipated Geological Assessment of the (65803) Didymos–Dimorphos System, Target of the DART–LICIACube Mission

“…The timing of the close approach flyby would allow LICIACube to be able to observe some if not all of the DART crater formation process, but the ejecta plume may obscure LICIACube's view of Dimorphos' surface. The rate of the observed evolution will depend mostly on the strength (or lack thereof) and porosity of the target (Fahnestock et al 2022). If the target tends stronger and crater formation is complete before closest approach (where the curtain is seen to separate from the crater rim), it will be possible to determine some of the surface geological properties from the crater diameter and possibly its depth.…”