CAPE CANAVERAL, Florida, USA (AP) – An astronaut on a spacewalk on Friday added to the millions of pieces of junk that orbit Earth, missing a small mirror when he left the International Space Station to repair a battery.

Commander Chris Cassidy said the mirror floated away at one foot (about 35 centimeters) per second.

Mission Control said the mirror detached from Cassidy’s spacesuit. The lost object does not pose a threat to the spacewalk or the station, NASA said.

Although millions of pieces of space junk float around Earth, more than 20,000 objects, including parts of old rockets and disabled satellites, are large enough to be tracked to protect the safety of the international station and active satellites.

Spacewalking astronauts wear a mirror on each sleeve for a wider view when working. The mirror measures just 12.7 x 7.2 centimeters (5 x 3 inches).

The mirror came off in the dark. Later, when it was in the light, Cassidy inspected her sleeve for clues that could explain how it came off. « There is no tissue damage or anything like that, » he told Mission Control.

Cassidy and Bob Behnken, who followed without incident, completed the first of at least four walks to replace traveling batteries.

Once the new lithium-ion batteries are installed, the space laboratory should run smoothly for the rest of its operational life, according to NASA. The batteries, more efficient than the nickel-hydrogen batteries they replace — power the station when it is on the night side of Earth.

Battery replacement began in 2017, when previous crews installed 18 lithium-ion batteries, half of the old ones they replaced.

Cassidy and Behnken are going to connect six more to complete the job, a cumbersome task: each battery is about a meter tall and one meter wide, with a mass of 180 kilos (400 pounds).

The hikes are expected to continue until the end of July, before Behnken returns to Earth in August aboard a SpaceX Dragon capsule.