Attitude Control of Spacecraft Swarms for Visual Mapping of Planetary Bodies

Planetary bodies such as asteroids, comets, and planetary moons are high-value science targets as they hold important information about the formation and evolution of our solar system. However, due to their low-gravity, variable sizes and shapes, dedicated orbiting spacecraft missions around these target bodies is difficult. Therefore, many planetary bodies are observed during flyby encounters, and consequently, the mapping coverage of the target body is limited. In this work, we propose the use of a spacecraft swarm to provide complete surface maps of a planetary body during a close encounter flyby. With the advancement of low-cost spacecraft technology, such a swarm can be realized by using multiple miniature spacecraft. The design of a swarm mission is a complex multi-disciplinary problem. To get started, we propose the Integrated Design Engineering & Automation of Swarms (IDEAS) software. In this work, we will introduce the development of the Automated Swarm Designer module of the software. The Automated Swarm Designer module will use evolutionary algorithms to optimize the design of swarms. The designed swarm will use 2 attitude control strategies to map the surface of a target body, namely: Nadir Pointing (NP), and Field of View Sweeping (FoV Sweeping). In the former strategy, the spacecraft are commanded to passively observe the sub-satellite point on the target body whenever the target is in the field of view of the individual spacecraft. This strategy is used when the observing instrument on board the spacecraft is large enough to capture the target from the desired encounter distance. In case the instrument is not large enough, then the spacecraft will have to maneuver their field of view to improve the coverage. The Field of View Sweeping strategy describes one such maneuver to improve coverage. In this strategy, the spacecraft in the swarm are commanded to sweep their fields of view about their principal axis normal to the swarm plane. The current work addresses different aspects of the swarm design problem. First, the coverage problem is addressed, and the parameters: instrument size, and target flyby distance are determined. Following this, the modeling of the swarm is described, where the individual spacecraft are arranged to define a circular plane around the target body during the encounter. Then the encounter trajectories of the spacecraft swarm are modeled. Here a Newton-Raphson iterative scheme using the state transition matrix of the dynamics is proposed for the entire swarm, which finds the trajectories between the desired starting points and destination points of the spacecraft within a specified flyby duration. Following this, the two proposed attitude control strategies are then described as they occur on these flyby trajectories. Then the design of these swarms is presented using as 2 optimization problems. The proposed strategies are used to develop a numerical simulator which will serve as the Automated Swarm Designer module of IDEAS for visual mapping missions. Finally, the simulator developed is demonstrated by designing a swarm using genetic algorithms for a visual mapping mission of asteroid 433 Eros.