Preliminary Analysis of Brightness Profiles Derived from Telescopic Observations of DART Impact Ejecta

In NASA's first full-scale test of a kinetic impactor for planetary defense purposes, the Double Asteroid Redirection Test (DART) spacecraft nominally impacted Dimorphos, the smaller secondary component of the (65803) Didymos binary asteroid system, on September 26, 2022. The Didymos system was observed before, during, and after this impact with a variety of telescopes, both ground-based and space-based. The latter include HST, JWST, and the L'LORRI instrument on Lucy. For a subset of suitable-quality images from these telescopes, spanning wide-ranging vantage points and post-impact timescales and available through the DART Science Operations Center (SOC), we apply an analysis approach earlier detailed in [1], which involves calculation of angular and radial brightness profiles from the central portion of images. The time-evolving level and shape of such profiles are a function of key properties of ejecta liberated during the impact, in particular: a) the total mass of ejecta, b) the full mass-velocity profile of ejecta, c) ejecta cumulative size-frequency distribution slope, and d) minimum ejecta particle size. In [1] we presented a methodology for turning the time-evolving size- and spatially-discretized number density field output from our high-fidelity dynamical simulations of ejecta particles into synthetic images. We calculated similar brightness profiles from those synthetic images for nominal and off-nominal ejecta simulation cases bracketing realistic total mass of ejecta and ejecta cumulative size-frequency distribution slope. Comparison of brightness profiles from such synthetic images from such ejecta simulations against similar brightness profiles from real images allows us to determine which surface material properties and assumptions going into the simulations most closely approximate the observed impact. Together with other analyses performed within the DART Investigation Team, this helps constrain whether the crater excavation producing the ejecta was gravity-limited or strength-limited, and further, helps constrain the cohesive strength of the surface impacted. The implications of our findings are discussed.

[1] Fahnestock, E.G., et al., 2022. PSJ, in press.