Abstract: We show how one can model collisions of celestial bodies to form larger bodies. From planetesimals to planet-sized bodies is a difficult way which demands different methods. Normally one starts with pure collisions which lead to a complete merging of the bodies involved; several parameters like velocity, collision angle and the different masses of the bodies has to be taken into account. Because the forming of planets is a very time consuming process via numerical integration methods one tries to accelerate the growing of bodies by introducing a special value of the collision. Therefore the so called Hill radius is used which does not treat a ‘real’ collision. The introducing of this value leads to different and misleading results. Especially sensitive is the wrong determination of the impact velocity which leads to wrong outcomes with respect to the post collision bodies (number of fragments); this is very important when dealing with SPH computations. In this seminar I will discuss different problems with respect to the former mentioned parameters of the collision. 

    One of the most important points is the water deficit following such an impact of differently sized bodies and also the initial composition of the bodies involved (iron, regolith, water, etc). Besides a very short introduction to the method of SPH computations some examples will be shown using diagrams and also video films.