Unfortunately for NASA’s Curiosity rover, you can’t call a mechanic on Mars. A NASA image (above), taken 19 March and posted online yesterday, shows that one of the zig-zag treads, or grousers, on Curiosity’s left middle wheel has broken (arrow). Two such tread fractures have occurred in the past 8 weeks, the space agency reports. Because the treads carry the rover’s weight, damage to them is more significant than the other holes and tears that have been punched into the thin aluminum wheels as the rover crossed terrain peppered with sharp rocks. Though the broken treads spell the beginning of the end for the rover, the end is not yet nigh. Ground tests on identical wheels suggest that when three grousers on a single wheel have broken, that wheel has reached 60% of its useful life. The rover is aging in other ways, too. Problems with its drill means Curiosity may not get to perform key experiments with its main chemistry instrument.