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NASA has decided to delay the launch of the first shuttle flight after the Columbia disaster by two months, citing continuing concerns about the buildup of ice on the external fuel tanks and other problems observed during a recent fueling test.

The earliest the launch for the shuttle Discovery could now occur would be the afternoon of July 13, new NASA administrator Michael Griffin said Friday.

The launch had been scheduled for May 22.

"We're going to return to flight, not rush to flight," said Griffin, who has been on the job for only two weeks. "We're going to do it right."

Griffin said that he also is ordering researchers at the Goddard Space Flight Center to begin planning for a Hubble servicing mission with the shuttle, even though such a mission has not been approved.

Two of the key problems that must be solved before Discovery can be launched involve hydrogen sensors in the external fuel tank and persistent ice buildup on external fuel lines on the tank.

During a recent fueling test, engineers observed intermittent malfunctions of two of the four hydrogen sensors in the tank. The sensors show when the hydrogen supply is exhausted so that the engines on the shuttle can be shut off. Mission rules require all four to be functioning at launch time.

William Readdy, the former astronaut who now heads NASA's space operations, said the sensors have been found to be functional. Engineers are now checking out the electrical lines that connect them -- in effect, wiggling junctions to see if they can find a loose connection. So far, they have not, he said.

Since the launch will be delayed by the search for malfunctioning sensors, "the prudent thing to do" is to address some potential icing problems, he said.

The primary area of concern involves bellows or expansion joints on the external lines that allow the lines to expand and contract in response to temperature changes during the fueling process.

Because of that motion, the bellows cannot be insulated with foam to prevent ice formation. The foam would crack and fall off, producing debris that could damage the leading edge of the orbiter wing -- the same problem that caused the cataclysmic destruction of Columbia during its re-entry on Feb. 1, 2003, killing all seven crew members.

Engineers already have installed what they call a "drip lip," a foam skirt around the joints that whisks condensed moisture away before it can freeze. The team thinks the drip lips can prevent as much as 70 percent of the ice buildup.

The repairs probably will not take the full six weeks, but July 13 marks the first available launch window when the mission -- designed to resupply the international space station -- can be conducted during daylight hours.

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