Comet Interceptor: AOCS/GNC Design Challenges for Flying through the Dust Environment of a Comet

Comet Interceptor is an ESA mission with payload contributions from ESA member states and with an international participation by JAXA. It was selected by the Science Programme Committee (SPC) in 2019 as a Fast track (F) mission. Following a preliminary design phase, a consortium led by OHB Italia has been selected for the implementation phase, with OHB Sweden leading the development of the AOCS/GNC and propulsion subsystems. The mission aims to intercept a Dynamically New Comet or an interstellar body in a fly-by scenario using the main spacecraft complemented by two smaller probes where one of these probes is provided by JAXA, in this way gathering multi-point observations of the comet and its coma. The spacecraft will be launched towards the Sun-Earth L2 (SEL2) Lagrange point where it will wait up to typically 3 years until an interesting target object is identified. The spacecraft will then initiate a transfer phase which will last between 0.5 and 4 years. The final approach starts approximately 60 days before encounter and will include ground-based navigation using on-board cameras. The encounter for the main spacecraft occurs at nominal distance of 1000 km from the comet nucleus with a relative velocity between 10 and 70 km/s. The fly-by of the comet through the central coma region involves a dynamic dust environment, which highly affects the spacecraft design to ensure that the spacecraft survives and fulfills its performance requirements. Considerations include potential blinding or reduced performance of star trackers from straylight, sufficient control authority and bandwidth to react to dust impacts, and autonomous navigation challenges given the uncertainty in target shape and illumination. The general functionality of the Comet-I AOCS/GNC is to a large extent based on functions with flight heritage from different OHB LEO and GEO missions. The functionality required during the encounter with the comet requires however new development of several functions as well as significant modification of existing designs. The relative navigation function is a new design that uses measurements from the navigation cameras to determine the direction to the comet such that the derived guidance of the spacecraft and payloads allow observation of the target during fly-by. The attitude control functionality will require significant modifications to be able to fulfill the performance requirements as well as ensuring the safety of the spacecraft under the mechanical impact from the dust environment. The modifications include control laws that allow simultaneous use of reaction wheels and thrusters for attitude control, a robust high bandwidth controller, and the use of hot redundant equipment to ensure seamless operation in the presence of failures in AOCS/GNC sensors and actuators. This paper describes the different challenges to design the AOCS/GNC that allows flying through the cometary dust environment. The key driving requirements are identified, and the resulting subsystem architecture is outlined. The paper also describes the approach for performance verification and provides some preliminary simulation results.