Ooh - let me guess... It's a stainless screw in a stainless thread and has suffered spalling?
Spalling is a really wierd effect that only happens with screws and threads made of stainless steel. You can tighten a screw with your fingers (no tools, just finger tightened), and then try to undo it and it'll suddenly be stuck. You try with tools, and it's still stuck. You use a lot of force, and it snaps off!
It happens because stainless steel can act a little like animal fur - rub in one direction and it slides easily, while when you change direction the fur meshes and refuses to move at all.
Doesn't happen reliably either - you can have 100 identical screws in 100 identical holes, and perhaps 5% will get stuck.
Normally the fix is to put vaseline or some other lubricant on the screws (before it gets stuck - after it's stuck you're sol), but I figure the NASA team doesn't want that lubricant contaminating their sample.
Spalling is a really wierd effect that only happens with screws and threads made of stainless steel. You can tighten a screw with your fingers (no tools, just finger tightened), and then try to undo it and it'll suddenly be stuck. You try with tools, and it's still stuck. You use a lot of force, and it snaps off!
It happens because stainless steel can act a little like animal fur - rub in one direction and it slides easily, while when you change direction the fur meshes and refuses to move at all.
Doesn't happen reliably either - you can have 100 identical screws in 100 identical holes, and perhaps 5% will get stuck.
Normally the fix is to put vaseline or some other lubricant on the screws (before it gets stuck - after it's stuck you're sol), but I figure the NASA team doesn't want that lubricant contaminating their sample.