CAPE CANAVERAL: NASA is keeping a close watch on the space shuttle Atlantis because of a leak in a vital hydraulic system used to control wing flaps, brakes and landing gear. The space agency says the six astronauts on board are not in danger, but the shuttle's planned five-day docking with Russian space station Mir may have to be shortened. The problem developed when one of the three auxiliary power units began leaking hydraulic fluid immediately after liftoff. NASA rules stipulate that after achieving its objectives, a shuttle must return to earth as soon as possible if a leak can not be isolated. It is too early to say if that will be the case on this mission. "As NASA is often forced to admit, the Shuttle is an incredibly complicated machine forced to operate in a harsh environment," says TIME's Michael Lemonick. "If NASA put the Shuttle through all the maintenance they could, it could take a year to check a shuttle before each flight. Its really inevitable that things go wrong."