The inevitable has occurred. A piece of space debris too small to be tracked has hit and damaged part of the International Space Station — namely, the Canadarm2 robotic arm.

The instrument is still operational, but the object punctured the thermal blanket and damaged the boom beneath. It’s a sobering reminder that the low-Earth orbit’s space junk problem is a ticking time bomb.

Obviously space agencies around the world are aware of the space debris problem. Over 23000 pieces are being tracked in low-Earth orbit to help satellites and the ISS avoid collisions — but they’re all about the size of a softball or larger.