The pre-impact search for further Didymos satellites using DART and LICIACube data

On 26 September 2022, the NASA/DART spacecraft performed the first test of a kinetic impactor in the history of human space exploration hitting Dimorphos, the small moon of the Near Earth and Potentially Hazardous Asteroid (65803) Didymos (Cheng et al., Nature 616, 2023; Rivkin Cheng, Nature Communications 14, 2023). The impact effectively reduced the orbital period of the secondary by 33 minutes (Thomas et al., Nature 616, 2023).The Italian Space Agency ASI/LICIACube cubesat witnessed the event in-situ after the release from the mothercraft DART, 15 days before impact (Dotto et al., PSS 199, 2021; Dotto Zinzi, Nature Communications 14, 2023; Dotto et al., Nature 627, 2024).We used images from the instruments DRACO (Didymos Reconnaissance and Asteroid Camera for Opnav) onboard DART and LEIA (Liciacube Explorer Imaging for Asteroid) oboard LICIACube to search for photometric detections of small objects that may be present in the Didymos sphere of gravitational influence before the impact, to provide hints on the complete dynamical characterization of the system.This portrait will be compared to the future environment the ESA/Hera mission will encounter at its arrival at Didymos in late 2026 (Michel et al., PSJ 360, 2022).Our investigation for new light sources around the primary asteroid, performed with optimal photometric source extraction techniques, produced a negative outcome. Therefore, we determined the size limits for any potential object that may be present in the system but undetected in the instruments images, using an estimate of the instruments limiting magnitude, a Monte Carlo code to populate the asteroid Hill sphere of 100,000 virtual satellites, and the asteroid photometrical Lumme-Bowell model and its adaptation to the H-G system (Bowell et al, Asteroids II, 1989). We estimated we found no unknown objects having a diameter larger than about 50 m in the entire Didymos Hill sphere. When considering closer distances from the primary asteroid during the approach phase with increasing spatial resolution, the limiting size gets smaller, reaching a minimum diameter equal to about 2 m within a radial distance of about 2 km from Didymos. Regardless the fact that the dynamical environment of the two linked asteroids is extremely perturbed and unstable for additional bodies, there can be still a small chance that large objects could find a stable niche in the phase space of the dynamical system and survive in long time orbits. Anyway, the null result of our work is a strong indication that this niche is either non-existent or was empty at the time of DART and LICIACube approach.Our negative result is also very important to the HERA mission science, since it provides a baseline for defining that there were no pre-existing small satellites larger than the derived limits. Thus, whatever HERA will find in orbit there is a strong indication that it should be a consequence of the material ejected in the DART impact, rather than previously acting ejection mechanisms.This work was supported by the Italian Space Agency (ASI) within the LICIACube project (ASI-INAF agreement AC n. 2019-31-HH.0) and by the DART mission, NASA Contract 80MSFC20D0004.