Recent studies had given engineers a new appreciation of the hazards of ice forming on the cold surfaces of the external fuel tank, according to NASA officials. Reviews of the hazards from the debris of launching indicated that the ice buildups that shook free in flight posed more of a danger than previously believed.

NASA is not going to rush to fly just to meet a schedule, new agency administrator Dr. Michael Griffin told a news conference.

Investigators of the space shuttle Columbia disaster which took the lives of all seven astronauts aboard said lax safety standards and a drive to maintain flight schedules contributed to the accident.

James Wetherbee, a former shuttle commander, told the Times the decision to delay lift-off "indicates a healthy change for the better in the culture."