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Abstract: Orbital debris poses a significant threat to spacecraft health and safety. Recent events such as China''s anti-satellite test and the Breeze-M rocket explosion have led to an even greater awareness and concern in the satellite community. Therefore, the National Aeronautics and Space Administration (NASA) has established requirements that routine conjunction assessment screening shall be performed for all maneuverable spacecraft having perigees <2000km or within 200km of geosynchronous altitude. NASA''s Goddard Space Flight Center (GSFC) has developed an operational collision risk assessment process to protect NASA''s high-value unmanned (robotic) assets that has been in use since January 2005. This paper provides an overview of the NASA robotic conjunction assessment process, including descriptions of the new tools developed to analyze close approach data and of the risk mitigation strategies employed. In addition, statistical data describing the number of conjunctions experienced are presented. A debris avoidance maneuver performed by Aura in June of 2008 is described in detail to illustrate the process. [Copyright &y& Elsevier]

Abstract: The intentional break-up of the Fengyun-1C spacecraft on 11 January 2007 via a hypervelocity collision with a ballistic object created the most severe artificial debris cloud in Earth orbit since the beginning of space exploration. More than 2000 debris of the order of 10cm or greater in size have been identified by the US Space Surveillance Network. The majority of these debris reside in long-lived orbits. The NASA Orbital Debris Program Office has conducted a thorough examination of the nature of the Fengyun-1C debris cloud, using Space Surveillance Network (SSN) data for larger debris and special Haystack radar observations for smaller debris. These data have been compared with the NASA standard satellite break-up model for collisions, and the results are presented in this paper. The orbital longevity of the debris have also been evaluated for both small and large debris. The consequent long-term spatial density effects on the low Earth orbit regime and the potential effect of the debris cloud on the growth of the near-Earth satellite population have been addressed. [Copyright &y& Elsevier]