Reentry predictions of three massive uncontrolled spacecraft

During five months, from September 2011 to January 2012, three campaigns of reentry predictions were carried out for the Inter-Agency Space Debris Coordination Committee (IADC) and in support of the Italian civil protection authorities. The objects involved were three massive spacecraft, UARS (NASA), ROSAT (DLR) and Fobos-Grunt (Roscosmos), which received widespread attention for the marginal risk on the ground associated with their uncontrolled reentry, probably exceeding a human casualty expectancy of 1 in 10,000, i.e. the alert threshold adopted by several agencies in the United States, Europe and Japan. From the technical point of view, the three above mentioned reentry campaigns offered the occasion to model the orbital evolution, fit the semi-major axis, determine the ballistic coefficients and compare some semi-empirical thermospheric density models under varying solar and geomagnetic activity conditions, dealing with spacecraft characterized by quite different configurations, shapes, masses and attitude control. This paper describes the procedures applied to the reentry prediction problem and the results obtained in each campaign, showing the evolution of ballistic coefficients, relative and absolute residual lifetime errors, and reentry windows. For UARS, the average relative residual lifetime error over the campaign was 15%, with a maximum of 28%. For ROSAT the corresponding figures were 3% and 8%, respectively, while for Fobos-Grunt 4% and 8% were found.