Penetrator proof
...a 4.4-pound (two-kilogram) prototype probe containing a soil collection drill and a circular group of eight lithium thyonal chloride cells -- forming two batteries -- was shot into the ground at more than 400 mph ...
Of course, this proof would be a tad more convincing if the mission had been a success. Even so, there is certainly an indication here that we might be able to devise some kind of effective penetrator.