Adv. Space Res. Vol. 28, No. 9, pp. 1403-1408,200l Q 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: SO273-1177(01)00446-X 0273-l 177/01 $20.00 + 0.00 INFLUENCE OF THE SPACECRAFT END-OF-LIFE RE-ORBITING ALTITUDE ON THE LONG-TERM COLLISION RISK IN THE GEOSTATIONARY RING C. Pardini and L. Anselmo CNUCE Institute, National Research Council (CNR), Via Alfieri 1,560lO Gheuano - Piss, Italy ABSTRACT A novel approach was developed to assess the contribution of satellite explosions to the object density in the geostationary ring. A low intensity explosion of a typical operational spacecraft was simulated at eight different altitudes, in between 0 and 2OCHl km above the geostationary orbit (GEO). The fragments produced were propagated for 72 years, taking into account all the relevant perturbations, and their contribution to the average object density in the GE0 ring, both short and long-term, was analyzed as a function of the end-of-life re-orbiting altitude. The explosions in geostationary orbit are the most detrimental for the GE0 ring environment. However, the average fragment density in the ring is never higher than 16 of the current background, decreasing to less than l/100 of the existent environment after 4 years, apart for the density rebound, about five decades later, due to the luni-solar perhuWions. The spacecraft end-of-life re-orbiting is a possible mitigation solution, but the long-term situation improves quite slowly, as a function of the altitude above GEO, if the explosions continue to occur. A re- orbiting 300 km above the geostationary altitude seems adequate to guarantee, after 2 - 4 years, a long-term average density of fragments in the GE0 ring at least two orders of magnitude below the present-day background, even during the density rebound observed after 54 years. However, at least 1000 km of re-orbiting are needed to stay below that l/100 threshold also in the short-W 0 2001 COSPAR.Publishedby Elsevier Science Ltd. All rights reserved. INTRODUCTION Due to the very rapid increase of spacecraft and apogee kick motors in the geosynchronous region, a growing concern matured in the 1980s regarding the possible overcrowding of the geostationary orbit (Hechler and Van der Ha, 1981; Fusco and Buratti, 1984). The collision risk was estimated, in particular to devise affordable and effective end-of-life disposal measures, such as satellite re-orbiting (Chobotov, 1990). In the following decade it became clear that also spacecraft and upper stage breakups contribute to the geostationary debris environment. Extensive investigations of its long-term evolution were carried out in Japan (Yasaka and Ishii, 1992; Yasaka, 1994, Yasaka, et al., 1999), while other authors analyzed the short (Kamprath and Jenkin, 1998) and medium-term (An&no and Pardini, 2000) collision risk represented by explosions. In order to preserve the geostationary orbit (GEO) for future use, the Inter-Agency Space Debris Coordination Committee (IADC) proposed a re-orbiting strategy for geostationary satellites at the end-of-life (IADC, 2000). They should be passivated, in order to reduce the risk of inadvertent explosions, and moved to a disposal orbit, higher than the geostationary altitude, following the relationship AH=235+C, xlOOOxA/M (1) where AH (km) is the new perigee altitude above GEO, A is the satellite average cross-sectional area (m2), M is the satellite mass (kg) and C, is the radiation pressure coefficient. A new modeling approach was implemented to investigate the effectiveness of end of life re-orbiting for long- term collision risk mitigation in GEO. Several low intensity satellite breakups, for 0 I AH I 2000 km, were simulated and accurately propagated for 72 years, including all the relevant perturbations. For each fragmentation, 1403