Ejecta cloud from the AIDA space project kinetic impact on the secondary of a binary asteroid: II. fates and evolutionary dependencies

TL;DR

This study models the post-impact evolution of ejecta from a binary asteroid, revealing how launch site and ejection speed influence whether ejecta re-accretes or escapes, with implications for asteroid impact missions.

Contribution

It provides a detailed numerical analysis of ejecta fate dependencies on impact site, speed, and material properties in a binary asteroid system, advancing understanding of impact outcomes.

Findings

Ejecta on resonance orbits cause prolonged re-impact on Didymoon.

Non-resonant ejecta tend to escape quickly after impact.

Ejecta evolution has a violent initial phase followed by a site-dependent second phase.

Abstract

This paper presents a quantitative study of the evolution of the ejecta cloud released from a hypervelocity impact on a binary asteroid. We performed numerical simulations of the post-impact dynamics of the ejecta cloud in the framework of the current mission scenario of AIDA mission project. A grid search of launching sites of ejecta was defined over the globe of Didymoon, and considering a wide range of possible ejection speeds, we determined the dependency of ejecta fate on launching sites (projectile impact sites) and speeds. This range allows us to track all the complex cases that include different types of dynamical fates. Two major mechanisms are found to be working broadly during the post-ejection evolution of the ejecta cloud: 1) ejecta on mean motion resonance orbits with Didymoon produce long-term quasi-periodic showers onto Didymoon over at least a couple of weeks after the projectile impact, 2) ejecta on non-resonant orbits produce a rapid and high re-accretion flux. This rapid and high flux occurs just once because ejecta on such orbits leave the system unless they experience a collision during their first encounter. For the second part of this study, we performed full-scale simulations of the ejecta cloud released from 6 hypothetical impact sites. We considered two kinds of material composing Didymoon's subsurface and then combined a power-law size distribution of the ejecta with an ejection speed distribution. We find that the ejecta cloud evolution can be divided in two periods. It starts with a first violent period (<10 hr) with fast re-accretion or ejection from the system. A second period is found to be more sensitive to the launching site than the first one. During this second period, ejecta will either re-accrete or being ejected from the system, depending both on their sizes and on their average survival time in close proximity of the binary components.