This is Detailed Secondary Objective 469, aka the In-flight Radiation Dose Distribution experiment, aka a human skull that was flown on three space shuttle missions and used to measure radiation in space. So, what do you do if you're a bored astronaut and you have this creepy thing floating around?
Why, you scare the crap out of other astronauts with it, of course.
The 11-pound female skull was sliced into 10 layers, with radiation dosimeters inside to measure brain exposure at different orbital inclinations. The outside of the skull was covered in some plastic stuff that was partially there to keep the thing from looking too much like, you know, a decapitated head, but it didn't help that much. Mike Mullane, who flew with the skull on STS-36 in 1990, describes some of the shenanigans that you can get in to when you have a skull floating around your space shuttle:
"Another Mission Specialist and I decide that we can "borrow" the skull for a minute to have some fun. I get in one of our sleeping bags and pull my head below the opening. The other Mission Specialist then tapes the skull to the top of the bag so that it looks like the head of whoever is in the bag. The disguise is really scary. The face of the skull has evil looking eyes and there are two bolts sticking up from the back of the head that look like horns. Even Captain Kirk would have run if he had seen this thing floating around his spaceship!
Ever so quietly, my accomplice floats me to the upper deck where the rest of the crew are working. I have my arms through the arm holes in the sides of the sleeping bags. Carefully, I push myself to float to the back of an unsuspecting crewmember. Hovering behind him, I breath loud and slow, making a deep, raspy, evil sound. The victim turns to investigate and..... aieeeeeeeeeeeeeeeeeeeeeee! He screams as I grab him! It's the attack of the horned alien from planet X!
Later, we stuff the sleeping bag with some clothes and buckle our alien onto the toilet seat to surprise another crewmember. See, astronauts like to joke around, too."
Mullane has written a memoir of his time in NASA called Riding Rockets: The Outrageous Tales of a Space Shuttle Astronaut that includes more stories like these, which you can acquire through his website at the link below.