Keep in mind that the cave man is pretty much in familiar territory naked and unarmed, and he is used to fighting and killing that way. Also, don't sell him short on the intelligence factor either. He and his entire species managed to not only survive, but thrive in an environment filled with animals far stronger, faster, and more aggressive than ANY astronaut who ever stepped into a space shuttle. The astronauts will be totally outclassed and overwhelmed in a stand up man to man fight.
Modern man's advantage comes with his technology, and without it. he has nothing to compensate for being weaker, slower, and less aggressive. Give the astronauts time to prepare, to fabricate weapons superior to what is available to the cave men (bows and arrows against clubs and rocks, for instance), and to develop a working strategy, and they stand a better than even chance, but without it, ther're cave man fodder, nothing else.