USA-193 reentry risk assessment using unclassified data sources

At the beginning of 2008, the need arose to predict the orbital decay of the American spacecraft USA-193, whose characteristics, function and orbit were classified. With no orbit data and independent Italian tracking capability, we turned our attention, for the first time, to the orbits determined by a worldwide network of about 20 visual satellite observers. The orbits of USA-193 obtained from such 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 resulted to be very accurate for a so low satellite (even though the minimum and very stable level of solar activity helped a lot), but data gaps of a few days were sometime possible, due to unfavorable pass geometry or weather and illumination conditions. Anyway, the orbital period and the semimajor axis were so accurate that very good decay fits were possible with a special perturbation software, including various atmospheric density models together with the other relevant perturbing accelerations. It was, therefore, possible 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 were also able to estimate roughly the shape and sizes of the satellite, showing that the solar arrays were never deployed. With this information, and taking into account our estimates of the ballistic parameter, we obtained reasonable and consistent values of the spacecraft mass. Then, based on previous reentry fragmentation analyses, it was possible to guess the expected USA-193 casualty area, casualty expectancy, debris ground footprint and probability of impact in Italy.