Models reveal that when an impactor this size struck the proto-Earth at a low angle and relatively low speed ( 8 â `` 20 km\/sec ) , much material from the mantles ( and proto-crusts ) of the proto-Earth and the impactor was ejected into space , where much of it stayed in orbit around the Earth . Models reveal that when an impactor this size struck the proto-Earth at a low angle and relatively low speed ( 8 â `` 20 km\/sec ) , much material from the mantles and crusts of the proto-Earth and the impactor was ejected into space , where much of it stayed in orbit around the Earth .