Astronauts conducting a routine inspection of the International Space Station found a puncture hole in the station’s robotic arm. The Canadian Space Agency said the damage was caused by a piece of space junk and called the collision a “lucky strike.”

Officials do not know when the unidentified piece of space junk collided with the arm and have launched an investigation into the collision.

The space junk caused a 0.2-inch puncture in the arm’s protective thermal covering but did not cause the arm to stop working.

“Despite the impact, results of the ongoing analysis indicate that the arm’s performance remains unaffected,” the Canadian Space Agency said in a statement.

Orbiting space junk poses a safety risk to the station and the astronauts living onboard. The Department of Defense is currently tracking over 27,000 pieces of debris that are hurdling in orbit at speeds of 18,000 miles per hour. At those speeds, even tiny pieces of debris can cause serious damage. In 2016, a paint chip with a diameter of about 7 millimeters cracked one of the windows on the space station.

Over the years, NASA has performed 26 special maneuvers to avoid being struck by pieces of debris.