This 6mm diameter chip was photographed by British astronaut Tim Peake from inside the Cupola module of the International Space Station. Alarmingly, it's actually in one of the windows. Image: Tim Peake/ESA
Caused by a space debris impact, it's not a big crack nor could it do any harm to the multi-layered station window — but it's still not a nice thing to see when you're in orbit. It was probably caused by "a paint flake or a small metal fragment no bigger than a few thousandths of a millimetre across," according to the European Space Agency.
It's amazing that a tiny speck of debris, one you cannot even notice on Earth, can do such damage at orbital speeds. This also indicates that the threat of space debris is something that cannot be ignored. The ESA emphasises:
While a chip like the one shown here may be minor, larger debris would pose a serious threat. An object up to 1 cm in size could disable an instrument or a critical flight system on a satellite. Anything above 1 cm could penetrate the shields of the Station's crew modules, and anything larger than 10 cm could shatter a satellite or spacecraft into pieces.