On the Use of Atmosphere Models in Re-Entry Predictions

Details On the Use of Atmosphere Models in Re-Entry Predictions On the Use of Atmosphere Models in Re-Entry Predictions

Dec 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996esasp.392..287k&link_type=abstract

Environment Modelling for Space-based Applications, Symposium Proceedings (ESA SP-392). ESTEC Noordwijk, 18-20 September 1996. E

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The catalog of the Space Surveillance Network (SSN) of US Space Command (USSpaceCom) contains more than 7600 objects larger than 10 cm. On the average, one of these objects re-enters the earth atmosphere every day, and every second day there is a re-entry of a decommissioned spacecraft or upper stage (which together account for more than 40% of the catalog population). The vast majority of these re-entries is entailing an extremely low risk potential, since most of the structures are disintegrated and burnt up during an extended heat flux and g-load exposure under shallow entry angles. In some instances, however, a non negligible risk from ground impact or ground/atmosphere pollution may arise in case of very massive objects (e.g. Skylab with 75t and Salyut-7 with 40t), objects which were designed to survive re-entry (e.g. China-40 capsule), or spacecraft with hazardous payloads (e.g. Kosmos-954 and 1402 which were equipped with reactors containing 50kg of radioactive material). In such cases, ESOC performs re-entry predictions which are communicated to international points of contact as input to their emergency plans (if necessary). The prediction of uncontrolled re-entries is based on a propagation of the perturbed orbital motion of a spacecraft up to the point of disintegration (at about 80km altitude). The drag coefficient is determined from a least squares retro-fit over a history of observations of the semi-major axis. Apart from the attitude dynamics and associated cross-section variations, the major uncertainty in re-entry predictions is due to inadequate modeling of the atmosphere, and in particular of the air density. At standard operating altitudes of LEO satellites atmosphere models can be assumed accurate to within 10% to 15% rms in density for well known atmospheric parameters. Due to the lack of underlying data, density models become less reliable below 200km altitude where the critical phase of a re-entry begins. Moreover, in case of prediction, there are no measurements available for the very important atmosphere model parameters solar activity and geomagnetic activity, and predicted densities must be based on activity forecasts. Associated uncertainties for the orbit and ambient atmosphere prediction will be addressed in this paper. Problem areas will be highlighted by means of a review of selected re-entry events.

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