A problem with the wheels on NASA’s Spirit rover has stopped it dead in its tracks on the surface of Mars. For some reason, the rover’s right-front and left-rear wheels stopped operating as commanded on Oct. 1. NASA engineers have performed a series of diagnostic tests to understand which systems could be affected, and they’re still analyzing the results. One fix would be to permanently disable the brakes on those wheels, but it could put the rover at an increased risk. Spirit has now traveled 3.6 km (2.2 miles) across the surface of Mars; much further than it was designed for, so it’s no surprise it’s starting to have some mechanical problems.