Tracking Non-stellar Objects on Ground and in Space

Many space exploration missions require a fast, early and accurate detection of a specific target. E.g. missions to asteroids, x-ray source missions or interplanetary missions.A second generation star tracker may be used for accurate detection of non-stellar objects of interest for such missions, simply by listing all objects detected in an image not being identified as a star. Of course a lot of deep space objects will be listed too, especially if the detection threshold is set to let faint object pass through. Assuming a detection threshold of, say mv 7 (the Hipparcos catalogue is complete approximately down to CCD magnitude mv 7.5), the objects thus listed will include galaxies, nebulae, planets, asteroids, comets and artefacts as satellites.The angular resolution in inertial reference coordinates is a few arcseconds, allowing quite accurate tracking of these objects. Furthermore, the objects are easily divided into two classes; Stationary (galaxies, nebulae etc.), and moving object (planets, asteroids, satellite etc.).For missions targeting moving objects, detection down to mv 11 is possible without any system impacts, simply by comparing lists of objects with regular intervals, leaving out all objects that do not move. For stationary objects no straightforward procedure exists to reduce the size of the list, but in the case the user has an approximate knowledge of which area to search the amount of data may be reduced substantially. In the case of a mission to an asteroid, the above described function, may be used to guide the spacecraft towards the target. The star tracker may detect and track the target delivering the inertial coordinates for, either onboard or on ground processing for trajectory planning.The AVS instrument of TEAMSAT, a technology satellite launched by the second Ariane 5 Launcher, featured apart from autonomous star tracker operations also a non-stellar object detection module.This paper discusses the operation of this module and compares its theoretical and actual performance. Further enhancements of the module and their testing are described. Finally the applicability for future missions is addressed.