Assessing the risk and the uncertainty affecting the uncontrolled re-entry of manmade space objects

After a brief review of the risk represented by the uncontrolled re-entry of sizable spacecraft and upper stages, a statistical analysis of 316 predictions issued during the first 20 IADC test campaigns was presented, in order to characterize the errors affecting the estimate of the residual lifetime in the couple of weeks preceding the re-entry. Overall, the mean prediction error was about 10%, increasing to 15% in the last 6 h. The re-entry predictions for upper stages resulted more accurate than average, with a mean error of about 5%, increasing to approximately 8% during the last day. In view of the statistical distribution of the predictions, an uncertainty window able to guarantee a confidence level of 90% should generally adopt an amplitude of about ±20% around the estimated nominal re-entry time, to be raised to about ±25% in the couple of days ahead of re-entry. An uncertainty window amplitude of ±30% would be, instead, needed to achieve a confidence level >=95%.