Citing too many uncertainties about mission safety, NASA late Friday delayed the launch of the next shuttle, Discovery, because of worries about faulty fuel valves.
A tentative date of February 27 had been set for the launch, and NASA officials did not set a new date as they briefed reporters late Friday night. They indicated, however, that April 7 could be a target date.
After meeting for 13 hours to decide flight readiness, John Shannon, space shuttle programme manager, said in webcast remarks that the team had come "really, really close" to reaching a consensus to launch.
But he said there was a "sense of unease we did not quite have the rigor" of criteria or questions answered that NASA "typically" expects.
The flight readiness review involved top managers who were looking over the results of extensive testing of fuel valves and concerns that a piece could break off and strike the shuttle on takeoff.
The worry cropped up during the launch of the Endeavour mission in November, when one of three flow control valves - that help control the flow of gaseous hydrogen from the main engines to the external fuel tank - was damaged, NASA has said.
That raised concerns about whether such falling material could damage the outside of the spacecraft during launch.
NASA still has fresh in mind the painful memory of the 2003 Columbia shuttle disaster, when insulating foam on the outside of the external fuel tanks hit the delicate heat-shield tiles on the outside of the shuttle during takeoff.
Mission specialists who noted the damage by reviewing video of the takeoff engaged in heated debate during the flight as to whether a rescue effort should be mounted, fearing a potential disaster.
In fact, Columbia was allowed to complete its mission at the International Space Station, but as it re-entered Earth's atmosphere, the spacecraft disintegrated as hot air exploded in through a crack left by the falling foam.
All seven astronauts on board were killed, and the shuttle space programme was grounded for years while NASA scientists worked to correct the problem on the ageing shuttles.