FIGURE 8.1 Depiction, using ORSAT, of the reentry disintegration of the Upper Atmosphere Research Satellite (UARS). The demise altitude versus downrange impact point is evaluated for major UARS components. SOURCE: Courtesy of NASA-JSC.
For each surviving object, a single point estimate is provided for the mass remaining along with its terminal velocity, thus producing a damaging effect on people on the ground as determined by its kinetic energy. The dispersion of multiple fragments surviving to the ground from a single object’s disintegration provides an area over which the debris will potentially pose a hazard to people on the ground, which in turn produces a single probability of casualty on the ground for the reentry event. Despite the wide range of potential variations that could result in rentry, neither uncertainty estimates nor confidence bounds are provided for impact point, mass of surviving object, or probability of casualty.4 Uncertainty information is required for management to be able to place this risk in proper perspective with others to guide decisions about direction of work and funding.
Finding: NASA’s Object Reentry Survival Analysis Tool provides results as point estimates without confidence bounds or uncertainty estimates.
Recommendation: In regard to debris reentry risk, NASA should provide confidence bounds on and uncertainty estimates of the resulting risk levels for use in both the Debris Assessment Software and the Object Reentry Survival Analysis Tool.
ORSAT is developed by NASA and used only by Orbital Debris Program Office personnel, due to the complex model interfaces and complicated modeling processes portrayed. However, the ODPO has created a more conservative and simpler reentry demise module as part of DAS, which is available to the public. As indicated previously, if DAS determines that none of the material from a reentering piece of space hardware will survive to the ground, then this determination is sufficient to assure compliance with STD 8719.14. However, if DAS indicates that the reentry scenario is non-compliant, the situation may be reanalyzed using ORSAT, which provides
4 R.N. Smith, J. Dobarco-Otero, K.J. Bledsoe, and R.M. DeLaune, User’s Guide for Object Reentry Survival Analysis Tool (ORSAT)—Version 6.0, JSC-62861, NASA Johnson Space Flight Center, Houston, Tex., January 2006.