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Following the recent insight in the material structure of comets,protoplanetesimals are assumed to have low densities and to be highly porousagglomerates. It is still unclear if planetesimals can be formed from theseobjects by collisional growth. Therefore, it is important to study numericallythe collisional outcome from low velocity impacts of equal sized porousagglomerates which are too large to be examined in a laboratory experiment. Weuse the Lagrangian particle method Smooth Particle Hydrodynamics to solve theequations that describe the dynamics of elastic and plastic bodies.Additionally, to account for the influence of porosity, we follow a previousdeveloped equation of state and certain relations between the material strengthand the relative density. Collisional growth seems possible for rather lowcollision velocities and particular material strengths. The remnants ofcollisions with impact parameters that are larger than 50% of the radius of thecolliding objects tend to rotate. For small impact parameters, the collidingobjects are effectively slowed down without a prominent compaction of theporous structure, which probably increases the possibility for growth. Theprotoplanetesimals, however, do not stick together for the most part of theemployed material strengths. An important issue in subsequent studies has to bethe influence of rotation to collisional growth. Moreover, for realisticsimulations of protoplanetesimals it is crucial to know the correct materialparameters in more detail.

Collisions between equal sized ice grain agglomerates