USA-193 decay predictions using public domain trajectory data and assessment of the post-intercept orbital debris cloud

Abstract

In early 2008, the need arose to predict the orbital decay of the American spacecraft USA-193, whose characteristics, function and orbit were classified information. With no orbit data and independent Italian tracking capability available, we turned our attention on the orbits determined by a worldwide network of about 20 visual satellite observers. The orbits of USA-193 obtained from their visual observations were therefore used as the sole source of orbit information. Contrary to our expectations, this exercise was extremely successful and we learned a lot in the process. The orbits provided by the visual observers were very accurate for such a low satellite (although the minimum and very stable level of solar activity helped considerably); however, data gaps of a few days were sometimes possible, due to unfavorable pass geometry or weather and light conditions. In any case, the orbital period and the semimajor axis were so accurate that it was possible for us to obtain very good decay fits using special perturbation software, including various atmospheric density models together with all the other relevant perturbing accelerations. We were therefore able to estimate accurate values of the ballistic parameter and the resulting decay and reentry predictions were extremely stable. Amateur optical observations and images of USA-193 had also led to a rough estimation of the shape and sizes of the satellite, revealing that the solar arrays had never been deployed. With this information, and taking into account our estimates of the ballistic parameter, we obtained reasonable and consistent values of the spacecraft mass. Based on previous reentry fragmentation analyses, we were then able to guess the expected USA-193 casualty area, casualty expectancy, debris ground footprint and probability of impact in Italy. Lastly, after the decision by the US Government to destroy the satellite, we independently predicted the interception time windows and the post-event ground tracks. Following the successful spacecraft breakup, we analyzed the evolution of the resulting debris cloud and assessed its (very limited) adverse impact on the circumterrestrial environment.