DART: kinetic impactor experiment determination of momentum transfer

The NASA Double Asteroid Redirection Test (DART) spacecraft will impact the secondary member of the [65803] Didymos binary in late September, 2022 in order to modify the trajectory of the moon through momentum transfer. In this way DART will demonstrate asteroid deflection by kinetic impact. A key objective of the DART experiment is to determine the momentum transfer to the target asteroid, which is most important for design of an asteroid mitigation strategy, but which is uncertain owing to the poorly understood contribution of momentum carried away by impact ejecta. The impact momentum transfer efficiency β depends on the asteroid physical properties and internal structure, as well as local slope of the target surface and on local structural features. We will discuss the determination of β from DART observations. DART will use ground-based telescopic observations to measure the orbital period change of Didymos induced by the DART impact, thereby determining the orbital speed change. From an estimate of the target mass, made using DART approach imaging, the momentum transfer is determined. A direct measurement of the target mass is planned by the Hera mission which will rendezvous with Didymos 4 years after the DART impact. Hera also plans to measure the DART impact crater. DART will carry a 6U CubeSat contributed by the Italian Space Agency and will release it close to Didymos in order to perform a separate flyby of Didymos. This CubeSat, called LICIACube, has primary objectives to study the DART impact ejecta plume and to image the non-impact hemisphere of the DART target so as to improve the shape and size determinations from imaging. We will discuss the estimation of ejecta momentum using LICIACube imaging of the ejecta plume structure and evolution.