  This work is a theoretical study of the speed at which the material of an
impacted target is ejected during the formation of an impact crater. Our model,
starting from the first principle of thermodynamics, can describes the speed of
the ejecta recursing to considerations that include complex process in simple
calculations. The fit of the model with observations shows that the many
complex details implicit in an impact process could be included in some few
parameters. Ejecta speed could be described independent of impactor parameters.
The model is compared with subsonic and supersonic speed experiments showing
coincidence in several cases. The model works with subsonic and supersonic
impacts. We do not compare the model with hypersonic impacts (> 5 km/s),
however, as the model derivation no depends on the impactor velocity it is
likely that also work with this kind of impacts.
