The Object Reentry Survival Analysis Tool (ORSAT) is the primary NASA computer code for predicting the reentry survivability of satellite and launch vehicle upper stage components entering from orbital decay or from controlled entry. The prediction of survivability is required in order to determine the risk to humans on the ground. According to NASA-STD 8719.14, Process for Limiting Orbital Debris, this risk, which is based on the predicted total debris casualty area, orbit inclination, and year of reentry, should be less than 1:10,000. Frequently, ORSAT is used for a higher-fidelity survivability analysis after the NASA Debris Assessment Software (DAS) has determined that a spacecraft is possibly non-compliant with the NASA Safety Standard.
The ORSAT code uses integrated trajectory, atmospheric, aerodynamic, aerothermodynamic, and thermal/ablation models to perform a complete satellite or launch vehicle upper stage component analysis in determining the impact risk. A three-degrees-of-freedom trajectory is used with the 1976 U.S. standard atmosphere, MSISe-90 atmosphere, or the GRAM-99 atmosphere to model various types of object shapes in either spinning or tumbling modes. Drag coefficients of these objects are considered from hypersonic to subsonic speeds to obtain the kinetic energy of objects at ground impact. Stagnation point continuum heating rates are obtained for spherical objects and are adjusted for other bodies and for rarefied flow regimes. Both lumped mass and 1-D heat conduction models may be used to compute the surface temperature. The object is considered to demise when its absorbed heat reaches the material heat of ablation.
Thermal properties for 80 materials are included in a database in ORSAT, with temperature-varying properties listed for thermal conductivity, specific heat, and surface emissivity. For objects that are on the threshold of demise or survival, parameters such as oxidation efficiency, initial temperature, surface emissivity, number of layers, dimensions, or breakup altitude may be varied in a single run to obtain the critical demise/survival point of a component. Good engineering judgment is applied in the parametric analysis to compute the best predicted total debris casualty area. The impact risk is then calculated to determine whether the satellite or upper stage is compliant with the NASA Standard 8719.14.