The piece of debris, perhaps a paint flake or a metal fragment from a satellite, was only a few thousandths of a millimeter across — not much bigger than a single cell of E. coli. 

But how can something so small cause visible damage?

"It all comes down to velocity," said Vishnu Reddy, an astronomer at the University of Arizona. Objects at the altitude of the ISS and most other satellites — around 250 miles (400 kilometers) above Earth — revolve around our planet once every 90 minutes, according to the European Space Agency. That's more than 15,600 mph (25,200 km/h), 10 times the speed of an average bullet shot on Earth, Robert Frost, an instructor and flight controller at NASA, wrote on Quora.