Screening the collision risk of the Iridium 33 - Cosmos 2251 Clouds

More than 10 years ago, in Rossi, Valsecchi and Farinella (Nature, 1999), it was shown how a near polar multi-plane constellation such as Iridium is particularly at risk of a collisional cascade if one of its satellites is first accidentally fragmented. Those results are recalled and actualized in the light of the real collision of February 2009, between Iridium 33 and Cosmos 2251. The collision risk, for the remaining Iridium satellites, arising from the two clouds of fragments generated by this impact, is analyzed and compared with the risk associated to the Fengyun 1C cloud and to the background catalog population. In particular the evolution of the flux as a function of time, related to the mutual geometry of the constellation planes and the clouds is highlighted and discussed. The flux is strongly time dependent with fluctuations of more than a factor 3 over a few months. For most of the impact energy levels considered, the flux associated to the three large clouds significantly exceeds the background catalog flux for the next few years.