Constraining the Orbital Parameters of the Didymos-Dimorphos System: Lightcurve Observations in Preparation for AIDA/DART

The binary near-Earth asteroid (65803) Didymos is the target for the Asteroid Impact and Deflection Assessment (AIDA) mission, which is a concept with two primary spacecraft: NASA's DART (Double Asteroid Redirection Test) impactor and ESA's Hera orbiter (Cheng et al. 2018; Michel et al. 2018). DART is NASA's first planetary defense mission and will be the first demonstration of asteroid deflection by a kinetic impactor. The DART spacecraft is designed to impact Dimorphos, the secondary in the Didymos system, and modify its orbit through momentum transfer. DART will launch in mid-2021 and is scheduled to impact in fall 2022. The DART spacecraft will carry ASI's LICIACube (Light Italian Cubesat for Imaging of Asteroids, Dotto et al. 2021) to observe the DART impact event and the resulting impact ejecta. A key scientific goal of the DART and Hera missions is to measure and characterize the deflection caused by the DART impact. The impact will change the satellite orbit period, which will be measured by ground-based facilities in the post-impact period. In order to correctly interpret the data from the impact epoch, we need to understand the baseline, unperturbed dynamics of the system. The DART/Hera Observations Working Group is tasked with characterizing the Didymos-Dimorphos system properties with sufficient accuracy to measure the change in the binary orbital period to within 7.3 seconds. This measurement is a small, but observable fraction of the current orbital period of the satellite (Porb=11.92 hours). The observed period change is a critical input to the calculation of the momentum transfer enhancement parameter, "Beta" (β). We are obtaining lightcurve observations during the current apparition (December 2020 to March 2021) to further characterize the system. Combining these observations with past data will provide us with the opportunity to establish the state of the system before impact to a high level of precision. We have two goals for our 2020-2021 Didymos observing effort: (1) Measure the amount of Binary YORP (BYORP) torque occurring in the system and (2) Establish whether or not the secondary is in synchronous rotation. An international group of observers affiliated with DART and Hera obtained time at 11 different facilities over the time period from December 2020 to March 2021. We will discuss our state of knowledge from previous observations (through 2019) and selected results from our 2020-2021 observations.