Dr Stuart Grey

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Angles-only navigation of a formation in the proximity of a binary system

Jan 01, 2018

Francesco Torre and Romain Serra and Stuart Grey and Massimiliano Vasile

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The paper will focus, on the navigation and station keeping of one or more spacecraft around the dynamic equivalent of the Lagrange points L4 and L5 of a binary asteroid system. In a recent work by one of the authors (thiry2016) it was demonstrated that this location can be conveniently used by a spacecraft equipped with a small scale laser to demonstrate laser ablation as a viable asteroid deflection technology.Furthermore, it was shown that the set of periodic solutions around L4 and L5 can be of particular interest for the exploration of a binary system and to extract and analyse surface and subsurface material. Binary asteroids are systems composed of two asteroids orbiting around their common barycentre. Some of them are part of the Near Earth Object population, like 2000 DP107, or 65803 Didymos (1996 GT),an Apollo asteroid discovered on April 11, 1996, that is the target of the DART mission. Navigating in the proximity of an asteroid is in itself a challenging task due to the complex dynamics induced by the irregular gravity field of the asteroid, the gravity of the Sun and solar radiation pressure. Even more challenging is to navigate a formation of spacecraft. Recent work by one of the authors demonstrated the possibility to autonomously navigate a formation of spacecraft in the proximity of a single asteroid with a distributed fault-tolerant autonomous system (vetrisano2016b).The complexity increases even further in the case of a binary due the interaction between the primary and the secondary. However, a combination of optical and laser ranging measurement was shown to be sufficient to maintain one or more spacecraft at L4 of the Didymos system (torre2017).Different from Torre et al. 2017 (torre2017) this paper proposes an approach using optical navigation only and exploiting the knowledge of the dynamics of the secondary asteroid around the primary. The dynamical model includes the gravity of the two asteroids, the gravity of the Sun and solar radiation pressure. Each spacecraft is equipped with only a camera, a good attitude determination and control system and an inter satellite link that allows sharing information with other spacecraft in the formation and provides a complete spacecraft-to-spacecraft position vector.Two scenarios will be considered in this study: one in which one single spacecraft uses only information from the camera on both the primary and the secondary asteroid, the other in which a small formation of spacecraft autonomously navigates with cameras and inter-satellite link only. Measurements and dynamics are processed in a form of Unscented H-infinity Filter that can effectively estimate the state of the system and allows maintaining the formation at L4.