The Space Shuttle Endeavour’s 3:37 p.m. launch has been postponed until at least Monday, after NASA engineers noticed that two heaters on the shuttle’s auxiliary power unit had failed. “The engineering team did not understand how this problem occurred and did not feel comfortable proceeding with a launch attempt,” NASA spokesman George Diller wrote in a statement. The units provide hydraulic power for the shuttle’s landing gear and brakes, among other things. The heaters are necessary to keep the units’ fuel from freezing. Capt. Mark Kelly was among the six astronauts heading to the International Space Station on a 14-day mission. His wife, Gabrielle Giffords, is the Democratic congresswoman was shot in Tuscon, Arizona in January. She flew to Florida for the launch, which is widely seen as a milestone in her recovery. NASA will hold a press-conference at 4 p.m. EST to provide more details.