ESA’s process for the identification and assessment of high-risk conjunction events

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ESA’s Space Debris Office provides an operational service for the assessment of collision risks of ESA satellites. Currently, the ENVISAT and ERS-2 missions in low Earth orbits are covered by this service. If an upcoming high-risk conjunction event is predicted based on analysis of Two-Line Element (TLE) data from the US Space Surveillance Network, then independent tracking data of the potential high-risk conjunction object are acquired to improve the knowledge of its orbit. This improved knowledge and the associated small error covariances derived from the orbit determination process scale down the position error ellipsoid at the conjunction epoch. Hence, for the same miss-distance, in most cases an avoidance manoeuvre can be suppressed with an acceptable residual risk. During the past years sophisticated stand-alone tools have been developed and maintained at ESA’s Space Debris Office. The central tools for analysing conjunction events are the collision risk assessment software CRASS and the orbit determination software ODIN. ODIN is used to process tracking data and to determine orbits by least-squares fits of tracking data, or of pseudo-data in terms of osculating orbit states, which can for instance be derived from TLEs. On this basis, estimates of TLE error covariances also can be established as input for initial collision risk assessments. For ESA’s automated routine conjunction event assessments which are embedded in a daily process with 7-day predictions, the handling of high-risk events is work-intensive. This shortcoming has been addressed by the implementation of a job scheduler, and automated procedures to facilitate the processing of tracking data, the update of ephemerides and covariances, and the update of conjunction geometries and collision risk estimates. The application of the upgraded software environment is illustrated through two exemplary, recent conjunction events of ENVISAT (02009A) with Russian Cosmos satellites: the conjunction event on 2008-Jan-09 19:00 (UTC) with the chaser object COSMOS-1624 (85006A), and the conjunction event on 2007-Nov-14 13:49 (UTC) with the chaser object COSMOS-1486 (83079A).