I don't think a 0.1g probe would make it through an atmosphere at any speed. The targets I had in mind, were moons, rings, asteroids, much like Deep Impact; I used the term "planet" as a catch-all.

Nevertheless, your point stands, if we consider technologically advanced life a possibility. It would be real shame to clobber someone's moon colony. Perhaps, these impactor missions should not be considered on the first expedition.