CAPE CANAVERAL - NASA's first shuttle mission since the Columbia accident will be delayed until July because of fears that ice could shake free from Discovery's external fuel tank, triggering another deadly disaster.

NASA halted launch preparations at launch pad 39B on Thursday. Discovery will be hauled back to the Vehicle Assembly Building to fix the 15-story fuel tank as early as next week, according to a NASA official familiar with the decision. NASA will announce the decision today.

The extra time also will allow NASA to deal with other technical problems that have cropped up recently at the pad.

"I don't think we're ready to fly yet," the official said. "Is it disappointing? Yes. But is it the right thing to do? Yes."

NASA had hoped to launch Discovery on a test flight to the International Space Station during a 13-day window that extends from May 22 to June 3. The agency now will target liftoff during the next available launch period, which extends from about July 14 to July 31.

NASA engineers this week completed a review of debris sources on the external fuel tank, which was redesigned after the Columbia accident and holds about 500,000 gallons of supercold propellants.

Three areas remain a concern, including a 70-foot long liquid oxygen propellant line that runs along the outside of the tank. Engineers fear that ice could build up near the top of that line, break free during launch and smash into the orbiter's heat shield.

Columbia and its seven astronauts were lost in 2003 when a 1.6-pound wedge of foam insulation broke free from the shuttle's external tank, punching a hole in the left wing. The breach enabled hot gas to get inside the ship during re-entry, and Columbia disintegrated above Texas.

The foam wedge has been replaced by a heater system. NASA has modified foam insulation in several other areas of the tank, including the 17-inch wide propellant pipe. The changes around the pipeline were designed to keep condensation from pooling and freezing into large chunks of ice.

Ice dangers persist

Data from a recent fuel-loading test showed significant amounts of ice still could build up near the top of the pipeline in humid weather, NASA deputy shuttle program manager Wayne Hale told reporters last week.

"We will not launch if we think there is a concern for an unacceptable amount of ice to hit the orbiter," Hale said.

NASA planned to add a heater to the propellant line to eliminate the problem, but that modification was not expected to be ready until the third post-Columbia flight, which is scheduled for December.

This week, workers at the Michoud plant outside New Orleans began installing a pipeline heater on the tank slated for the third flight, although some qualification tests remain, Lockheed Martin spokesman Marion La Nasa said.

NASA will either install a pipeline heater on Discovery's tank or replace the tank with another that is equipped with one of the devices. That work can't be done at the launch pad. It must be done in the assembly building.

NASA also is wrestling with other technical problems:

Engine cutoff sensors

Sensors that serve the same purpose as an automobile gas gauge did not work during a recent external-tank fueling test. Engineers still don't understand why the sensors acted erratically, said Jessica Rye, a Kennedy Space Center spokeswoman.

The sensors gauge the amount of propellant left in the 15-story tank during a shuttle's climb to orbit. They also are designed to shut down the shuttle's three main engines if they sense the tank is out of fuel.

A malfunctioning sensor could trigger a premature engine shutdown in flight, which could force astronauts to attempt a risky and unprecedented emergency landing at either KSC or overseas runways.

"We would not launch if they were not working," Hale said last week.

Heat-shield blankets

During routine launch preps, hydraulic fluid dripped on launch pad equipment. High winds then blew the oily fluid onto thermal protection blankets that cover hump-like engine pods on the tail of the orbiter.

NASA might have to replace the blankets, a time-consuming job.

The postponement raises questions about whether NASA can complete two test flights in time to resume construction of the space station as planned in December.

Daylight launches

The space agency aims to launch its next two flights during daylight and at times when the external tank will be jettisoned on the sunlit side of Earth. The idea is to capture clear pictures of any launch debris and to make sure changes to the external tank were successful. The windows that opened in May and July meet those requirements.

The next available window after that is in September, but the scheduled launch of a fresh crew to the station on a Russian rocket that month could narrow the available shuttle launch period to a mere five days.

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