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All it took was a loose speck of metal tinier than a human eyelash to disable for three months the Canadian-made robot arm aboard the International Space Station.

In an illustration of the proverbial grain of sand that derails an entire train, it was a stray bit of wire barely three millimetres long and as thin as a human hair that put out of commission the most complex robotic device yet flown in space.

The $1.4-billion Canadarm2 became fully functional again only in June after astronauts on a special spacewalk dismantled its ailing wrist joint and replaced it with a spare.

Engineers at Canadarm2's manufacturer, MD Robotics of Brampton, Ont., examined the faulty part the astronauts brought back from orbit and found the minute piece of wire between the pins of an electrical connector.

The bit of wire wasn't a problem on Earth, but in orbit "the bit of wire floated around between the connectors," said Benoît Marcotte, the Canadian Space Agency director of operations.

The loose metal short-circuited one of the connectors, and the resulting electrical arc welded the wire into the pins.

The piece probably came from the protective mesh wrapped around the arm's electrical wires, similar to the sheathing one finds around a television's coaxial cable, Mr. Marcotte said.

Space components are assembled in so-called clean rooms, areas similar to operating rooms, by technicians clad in coats and hair bonnets.

"There are clear rules to follow in clean rooms. There's a high level of cleanliness, and you have to follow protocols," an engineer said.

"Despite all the precautions, there is always the likelihood that a little thing like that could ruin your day," Mr. Marcotte said.

The arm's troubles postponed the launch of Shuttle Flight STS-111 by a month, so two crew members could train for the repair spacewalk.

Canadarm2's joints are designed to be replaced in orbit, so the episode gave engineers a chance to test its modular construction.

The defective joint has been refurbished and can be used for a future repair.

The space station cannot be completed without the robot arm.

The arm sits atop a mobile base that runs on rails along the width of the station. Its next mission will come in October when it will be used to unload from a space shuttle a 14-tonne aluminum-scaffolding segment of the station's backbone.

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