In January 2003, the European Space Agency was scheduled to launch the Rosetta spacecraft on its 10 year journey to comet Wirtanen. On arrival at the comet, some 4.5 AU distant from the. sun, Rosetta was to rendezvous and deliver its cometary lander. Thereafter it will commence detailed close up scientific observations of the comet and accompany it throughout a 1 year long hurtle towards the sun. During it's time with the comet, the increasing solar intensity will transform the dormant ice 'rock', vaporizing and ablating the ice, producing a dense and extensive cloud of dust and ice crystals that will be seen from the earth as a fiery tail. During this entire time Rosetta must continue to operate flawlessly and entirely autonomously and be able to maintain the highly precise and stable pointing required by the scientific payload even while it's cometary orbit is within this cloud. The primary sensors used on the Rosetta spacecraft are the Autonomous Star Trackers (ASTR) designed and manufactured by Galileo Avionica. These are optical sensors that detect, identify and then track stars within their Field of View (FoV). These types of sensors are by their nature, highly susceptible to 'false' stars such as those that are likely to be generated by a cloud of dust reflecting the light from the sun. This paper aims to explain the problems that can be caused to an ASTR in such an environment, to characterize the expected environment around the comet - as seen from an ASTR, to detail the ways in which these problems were overcome for the Rosetta mission and finally to give an overview of the achieved performances of the sensor.
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