According to the US spaceflightnow.com outlet, Cassidy, while coming out of the airlock to begin works on the ISS outer surface, reported that the small mirror had somehow come off the sleeve of his spacesuit and floated into space at a speed of about a mile per hour.
The lost mirror posed no threat to the ISS and its crew, and Cassidy had a spare, spaceflightnow.com said.
Wrist mirrors are used by astronauts to read displays and examine places that are not fully visible due to the limited view in a spacesuit.
Cassidy and another NASA astronaut, Robert Behnken, went into outer space on Friday to complete the work on replacing batteries that was started in January 2017.