Launch now is targeted for no earlier than 11:59 pm, Friday, Aug 28, from NASA's Kennedy Space Center in Florida.
During loading of the shuttle's external fuel tank for the launch attempt on Wednesday, Aug 26, a liquid hydrogen fill and drain valve located in Discovery's aft compartment failed to provide the proper indication when it was commanded to close.
Engineers cycled the valve five times on the evening of Aug 26, to collect data on the valve and its associated actuator and position indicator.
NASA managers decided on August 27 that more time is needed to analyze the test data and develop alternative procedures for confirming that the valve is closed if the valve fails to provide the proper closed indication during Discovery's next launch attempt.
Discovery's 13-day STS-128 flight to the International Space Station (ISS) will deliver storage racks; materials and fluids science racks; a freezer to store research samples; a new sleeping compartment; an air purification system; and a treadmill named after comedian Stephen Colbert.