NASA engineers seemed to have cured nagging problems with the International Space Station’s robot arm last week, but not before it had banged the side of the station—apparently of its own accord. The Canadarm2 sprang free from its fixing and struck the station during testing. Astronauts did not report any damage. They had been hoping to locate an earlier fault, which stopped the arm’s shoulder joint working properly. “The dilemma is do you have a perfectly healthy arm, or a problem that just hasn’t shown itself in a couple of weeks?