Predictions for the Dynamical States of the Didymos System before and after the Planned DART Impact

Abstract

NASA’s Double Asteroid Redirection Test (DART) spacecraft is planned to impact the natural satellite of (65803) Didymos, Dimorphos, at around 23:14 UTC on 2022 September 26, causing a reduction in its orbital period that will be measurable with ground-based observations. This test of kinetic impactor technology will provide the first estimate of the momentum transfer enhancement factor β at a realistic scale, wherein the ejecta from the impact provide an additional deflection to the target. Earth-based observations, the LICIACube spacecraft (to be detached from DART prior to impact), and ESA’s follow-up Hera mission, to launch in 2024, will provide additional characterizations of the deflection test. Together, Hera and DART comprise the Asteroid Impact and Deflection Assessment cooperation between NASA and ESA. Here, the predicted dynamical states of the binary system upon arrival and after impact are presented. The assumed dynamically relaxed state of the system will be excited by the impact, leading to an increase in eccentricity and a slight tilt of the orbit, together with enhanced libration of Dimorphos, with the amplitude dependent on the currently poorly known target shape. Free rotation around the moon’s long axis may also be triggered, and the orbital period will experience variations from seconds to minutes over timescales of days to months. Shape change of either body, due to cratering or mass wasting triggered by crater formation and ejecta, may affect β, but can be constrained through additional measurements. Both BYORP and gravity tides may cause measurable orbital changes on the timescale of Hera’s rendezvous.

Dynamical Evolution of the Didymos−Dimorphos Binary Asteroid as Rubble Piles following the DART Impact

“…However, both the shape and spin state of Dimorphos are poorly constrained. It is often assumed that Dimorphos is in a relaxed, tidally locked state, meaning that its spin period matches the measured mutual orbit period of ∼11.9216 hr, and its libration amplitude is small, although this has not yet been directly confirmed (Richardson et al 2022). Regardless of Dimorphos's preimpact dynamical state, DART's perturbation to the mutual orbit will likely excite Dimorphos's spin state as a result of the high degree of spin-orbit coupling due to the irregular shapes of both components and their close proximity.…”

Rotation-induced granular motion on the secondary component of binary asteroids: Application to the DART impact on Dimorphos

“…Due to a high degree of spinorbit coupling, the dynamical evolution of Dimorphos strongly depends on the initial conditions at the time of impact and the body's shape, which are currently unknown (Agrusa et al 2020). Depending on β and Dimorphos's shape, it is possible that Dimorphos may enter a chaotic rotation state following the DART impact (Agrusa et al 2021;Richardson et al 2022). Furthermore, numerical simulations that treat Dimorphos as a rubble pile indicate that boulders may move on the surface, depending on Dimorphos's spin state, bulk shape, and material properties (Agrusa et al 2022).…”

“…The body diameters are the volume-equivalent spherical diameters. A synchronous spin state for Dimorphos is assumed, and we refer the reader to Richardson et al (2022)…”

“…There are no observations that show Dimorphos is spin locked, but circumstantial evidence indicates that this is likely. We refer the reader toRichardson et al (2022) for a detailed discussion on this assumption.…”

“…The DART impact will not be ideally head-on and centered, but recent work indicates that these effects should be negligible in terms of determining the system's bulk dynamical properties(Richardson et al 2022). 3 ∆v is dependent on β, the impactor mass and velocity, as well as the secondary's mass.…”