Washington: A valve malfunction on the Delta IV rocket meant to carry NASA's new Orion spacecraft from Cape Canaveral, Florida, for an unmanned test flight forced the postponement of the launch.
Mission engineers at the Kennedy Space Center detected that a valve was not closing properly and decided they could not repair it in time for the launch to proceed Thursday.
Liftoff, originally set for 7.05 a.m., had already been delayed three times -- once when a boat strayed into the off-limits zone and two other times due to high winds.
NASA rescheduled the launch for Friday.
Orion is NASA's first spacecraft since the Apollo programme that is designed to carry humans beyond Earth orbit.
Once on its way, Orion will orbit the Earth twice, reaching an altitude of 5,793 km on a four-hour flight intended to test crucial spacecraft systems, including the heat shield that protects the capsule during re-entry.