CRUSHER ATTENDANT WAS MAKING HIS ROUNDS AT THE SHUTTLE GALLERY. HE WAS DESCENDING THE STAIRS FROM THE #1 POC HEAD CHUTE TO THE SHUTTLE LEVEL. HETOOK A COUPLE STEPS TO THE WEST AND WAS SUDDENLY STRUCK ON THE HARD HAT BY A 6# ROCK. HE RECENTLY BEGAN MISSING TIME FOR CERVICAL STRAIN. EXTENSIVE REVIEW HAS NOT IDENTIFIED WHERE ROCK CAME FROM, BUT ADDITIONAL GUARDING & IMPROVEMENTS MADE.