Physical properties and long-term evolution of the debris clouds produced by two catastrophic collisions in Earth orbit

After two decades of slightly declining growth rate, thanks also to the advancing implementation of mitigation measures, the population of cataloged orbital debris around the Earth increased by approximately 40% in just a couple of years, from January 2007 to February 2009, due to two collisions in space involving the catastrophic destruction of three intact satellites (Fengyun 1C, Cosmos 2251 and Iridium 33) in high inclination orbits. Both events occurred in the altitude range already most affected by previous launch activity and breakup events, leading to the formation, in the following several months, of new shells of long lasting debris. In order to investigate the long-term consequences of the collisions on the orbital debris environment, the physical characteristics of the new clouds of fragments, in terms of ballistic parameters and area-to-mass (A/M) distribution, were derived from a decay analysis of representative samples of cataloged objects. Such information was then used to realistically estimate the debris lifetime and the long-term contribution of such clouds to the circumterrestrial environment. As a consequence of the analysis carried out, it was also found that all three clouds presented a significant fraction of cataloged fragments with very high area-to-mass ratios, hundreds or thousands of times greater than those of intact satellites, leading to the conclusion that the generation of such orbital debris might be more common than formerly supposed. In particular, in the case of Cosmos 2251, it was found that about 5% of the cataloged fragments had average A/M >= 1 m2/kg and slightly more than 1% had average A/M >= 10 m2/kg. Very similar results were also obtained for Fengyun 1C. In the Iridium 33 case, a quite larger fraction of high A/M objects was obtained: 26% of the cataloged fragments resulted to have, in fact, average A/M >= 1 m2/kg and 5% had average A/M >= 10 m2/kg. Due to the observed A/M distributions, all three debris clouds were characterized by an orbital decay faster than originally supposed, even though their detrimental effects will be felt in low Earth orbit for several decades.