NASA has scrubbed the launch of the Space Shuttle Endeavour at least 72 hours due to a failure in one of two heater circuits. Endeavour's launch will be no earlier than Monday at 2:33 p.m. EDT.
NASA said that "during today's countdown, engineers detected a failure in one of two heater circuits associated with Auxiliary Power Unit (APU) 1. Heaters are required to keep the APUs' hydrazine from freezing on orbit. Attempts to activate the heater were not successful and engineers now believe the problem might be associated with a Load Control Assembly, which is a switchbox, located in the aft end of Endeavour, or an electrical short in the wires leading into or out of the switchbox."
"Endeavour's external tank will be drained, technicians will enter Endeavour's aft compartment overnight and by tomorrow afternoon, will put in a platform to gain access to the avionics bay where the Load Control Assembly is located. Once there, they will assess whether they need to remove and replace the switchbox, or fix an electrical wiring short."
The President is still scheduled to arrive at Kennedy Space Center this afternoon.
The shuttle crew left for the pad just minutes before the launch was scrubbed.
And minutes later before they reached the pad the crew turned around and head back to the Operations and Checkout Building.