Controlled re-entering spacecraft like the Space Shuttle are designed to withstand the mechanical and thermal stresses during atmospheric re-entry. But objects like rocket stages or space debris re-enter uncontrolled. Usually these objects demise during re-entry, but heavy and compact fragments may survive and reach the ground, as happened for re-entries of decommissioned space stations (Skylab, Kosmos 1686, Mir).
So far these fragments did come down in sparsely populated areas only, but for the continuously rising number objects in Earth orbit an increasing number of potential ground impacts has to be expected. To assess the on-ground risk of uncontrolled re-entry, especially the number and distribution of the fragments, a dedicated software tool is important. Especially for nuclear powered satellites this is essential. For spacecraft that are controllable at the end of their mission, such a tool can give advice on the preferable re-entry conditions to achieve complete demise, if possible.
The re-entry of a satellite is a complex physical process and not easy to be calculated. The full mathematical approach to approximately calculate this process would need using super computers. To be able to use the limited computing power of personal computers for these calculations, simplifications are necessary to apply. There are two approached to reach this: 1. maximum utilization of available resources, 2. applying further simplifications to enable parametric studies. SESAM uses the second approach.
The software SESAM ("Spacecraft Entry Survival Analysis Module") was developed to determine those fragments of a satellite that survive the atmospheric re-entry and may pose a risk to the world population. SESAM is part of the DRAMA ("Debris Risk Assessment and Mitigation Analysis") software suite to handle the many aspects of space debris. For the on-ground risk assessment the software SERAM ("Spacecraft Entry Risk Analysis Module") analyses the results of the SESAM simulation.
For the calculation of the satellite's re-entry a model of the satellite is created and it's initial state (position, epoch and velocity) is defined. The satellite's motion inside the Earth's atmosphere is determined by numerical integration over time. SESAM uses a simplified satellite model, consisting of a compartment which represents the body of the spacecraft. The compartment contains simple geometric objects like spheres, cylinders, boxes or plates, which are characterized by their measures, mass and material properties. The typical assumption for object oriented tools like SESAM is the release of the particular simple objects from the compartment at a certain altitude. Each object is then simulated separately. Basically the release altitude is a free parameter, but due to experience from re-entry observations, usually an altitude of 78 km is chosen.
The motion of the particular objects is determined by the occurring forces. The simulation takes into account the Earth's gravity and the aerodynamic drag. The temperature of the objects is calculated using approximation formulas for the heat flow as a function of local atmospheric density, velocity and ballistic coefficient. If the temperature reaches the melting temperature of the object, it stays constant and the mass starts to deplete by melting. Thus the re-entering objects can demise complete or partially or they may survive to the ground without any demise.
The concept of SESAM is very simple. The modeling of the objects determines which kind of fragments can be generated at which altitude. The advantage of the method is that it is very fast. Thus, in relatively short time, a high number of calculations can be processed. This is advantageous, if a first quick assessment for the on-ground risk is needed, e.g. during the conceptual phase of spacecraft design or at an earlier time before the expected re-entry. For a detailed analysis, the use of more complex software tools like SCARAB, which are more time-consuming, have to be used.