Modeling of Risk to Aircraft from Space Vehicle Debris

*† With the breakup of the Space Shuttle Orbiter, Columbia, into possibly more than 100,000 pieces of debris and spread over a large area, it became evident that aircraft were very much at risk. Aircraft, of course, are vulnerable to much smaller debris than most of the Columbia debris (such as many pieces that bounced harmlessly off roofs). Following the Columbia accident, many pieces of debris descended simultaneously through scheduled commercial flight paths. In the future, prediction of debris risks to aircraft will have to be made routinely for launches and descents of space vehicles such as the Space Shuttle. To be useful, this requires modeling of a fast running code that computes debris risk to aircraft. This paper discusses methodology necessary to compute aircraft risk, as implemented into the footprint program, CRTF (Common Real-Time Footprint). This advantage of this code over others is that it performs the aircraft risk computation in a relatively small time frame. The key feature of the method is to represent the debris clouds into statistical distributions, thereby greatly reducing the number of Monte Carlo samples that need be taken. The vehicle breakup is modeled by a set of debris groups, packaged primarily according to small ranges of ballistic coefficient and imparted velocity. Each debris group leads to a debris cloud that is propagated through the atmosphere. The clouds are created from a set of initial breakup points found through Monte Carlo sampling of uncertainty due to velocity impulse, ballistic coefficient, and guidance and control. To evaluate the risk to aircraft passing through the debris clouds, the clouds are further enhanced by uncertainty in wind and lift and then modeled as trivariate normal distributions. The parameters affecting aircraft impact are the aircraft velocity, the fragment category descent velocity, the density of fragments, the frontal area of the aircraft, and the plan area of the aircraft. As an application of our methods, aircraft impact probabilities are computed for the Columbia reentry breakup accident. At the time of the study, over 75,000 pieces of Columbia were recovered across East Texas. For the Columbia Accident Investigation Board (CAIB), we analyzed this catalog of collected pieces as part of a study to estimate the risk to the public from the accident. In follow-on work described in this paper, we calculated probability of impact upon actual aircraft flights at the time of the accident. The gathered debris catalog was extrapolated to account for debris that may be hazardous to aircraft, but which was too small to be recovered by the ground search. The total probability of impact with commercial aircraft was found to be between 3 in 1000 and 1 in 10. However it was found that the risks to aircraft occurred between 8 and 20 minutes following the accident. This suggests that post-accident mitigation procedures are possible, and therefore should be established to protect aircraft.