A piece of space junk travelling at 28,163km/h has punctured the International Space Station, prompting Nasa to warn of the danger posed by debris orbiting the Earth.
The ISS suffered a direct hit from a stray fragment, which pierced the thermal blanket of a robotic arm and damaged the boom beneath, leaving a 5mm-wide hole.
Experts from the Canadian Space Agency (CSA) and Nasa took detailed images of the area to assess the damage, which occurred in mid-May but was only disclosed late last week.
Their findings indicate that the arm's performance remains unaffected, but it has raised concerns of repeat events.
"The rising population of space debris increases the potential danger to all space vehicles, including to the International Space Station and other spacecraft with humans aboard, such as SpaceX's Crew Dragon," Nasa said.
The CSA added: "While the utmost precautions are taken to reduce the potential for collisions with the ISS, impacts with tiny objects do occur."
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More than 23,000 objects the size of a croquet ball or larger are tracked day and night to detect potential collisions with satellites and the space station.
A further 100 million tiny objects, ranging from rock or dust particles to flecks of paint from satellites, are too small to be monitored, but travel at up to 28,163km/h - fast enough to damage a spacecraft. Even tiny paint flecks can damage a spacecraft when travelling at these velocities.
According to Nasa, millimetre-sized orbital debris represents the highest mission-ending risk to most robotic spacecraft operating in low-Earth orbit.
In 1996, a French satellite was damaged by debris from a French rocket that had exploded a decade earlier.
In 2009, a defunct Russian spacecraft collided with and destroyed a functioning US Iridium commercial spacecraft.
China's 2007 anti-satellite test, which used a missile to destroy an old weather satellite, added more than 3500 pieces of large, trackable debris and many more smaller particles to the problem.
"When you consider that the number of [trackable] objects is approximately 22,000, that one irresponsible action represents a significant portion of the total debris in orbit today," said Air Chief Marshal Sir Mike Wigston.
Last September, the ISS was forced to change course to avoid debris from a Japanese rocket. In a separate incident, Elon Musk's SpaceX recently accused British rival OneWeb of "misleading" the public by claiming that their satellites nearly collided in orbit.
A SpaceX satellite came within 58m of a OneWeb craft in April, prompting OneWeb to make an evasive manoeuvre, the company said. Following the incident, Chris McLaughlin, the regulatory head at OneWeb, told The Wall Street Journal that Musk's business "has a gung-ho approach to space".
SpaceX forcefully denied this claim in a letter to the Federal Communications Commission.
