Smallsat mission to Didymos: enhanced GNC design for the LICIA science return maximisation

The interest in near earth objects (NEOs) has widely spread among space agencies during the past decades, not only due to the valuable scientific data these objects can provide, but also for the potential hazard they represent for our planet. In particular, to date there still isn't a validated technique to deflect NEOs on a collision course with Earth. The DART mission, developed by NASA, aims at filling this gap, by testing the effectiveness of a kinetic impactor on the binary asteroid system”Didymos”. In 2022 the spacecraft will hit the smaller asteroid of the binary system, slowing down the moonlet speed with respect to the larger body, permitting the assessment of the impact effectiveness by ground observation measurements. The impact will also generate a cone of ejected particles, of great interest for current numerical impact models validation, which however will be hardly visible from ground. In this context, a bilateral agreement between NASA and Italian Space Agency (ASI) allowed to embark a 6-units CubeSat on DART, called Light Italian Cubesat for Imaging of Asteroids (LICIA). Deployed before the impact, it will fly by the asteroid at the right distance to observe the ejecta evolution and will also be exploited for observation of the asteroid's side not visible from DART, to help estimating the overall volume. The LICIA spacecraft and mission are being developed by ASI through an italian consortium composed by Istituto Nazionale di Astrofisica (INAF), Università di Bologna (UniBo), Argotec, and Politecnico di Milano (PoliMi), respectively covering the fields of mission science and payload, orbit determination, platform development, and mission analysis. The present paper covers the topic of attitude guidance and control design, to ensure the best performance for target pointing during the flyby. The most up-to-date trajectory is taken as baseline scenario to explore various strategies in terms of slew maneuvers to be performed, and optimization for the parameters of the controller. Uncertainties, for the platform and the environment, are introduced to evaluate such strategies in non-nominal, realistic scenarios, with deviations of the trajectory and changes in the flyby distance from the target. Results are compared and the best guidance and control strategy is selected as preliminary baseline. Next steps in the closed-loop architecture development, and related expected issues to be solved, are discussed.