This story was updated at 1:40 p.m. EDT.

NASA's first space shuttle to fly since the Columbia disaster will not liftoff until July, a drastic delay that will force mission managers to miss the first of two launch windows this summer.

Space shuttle officials are now targeting a launch window opening July 13 for the space shuttle Discovery, after it became clear that lifting off during the current window - which runs from May 22 to June 3 - would be unattainable. The new launch window closes July 31.

"We're going to return to flight, we're not going to rush to flight," said NASA chief Michael Griffin in a press conference at the space agency's Washington, D.C. headquarters. "We're going to do it right."

Unresolved debris issues, malfunctioning external tank sensors and soiled thermal protection blankets contributed to Discovery's launch delay, which will push back preparations to deliver a large cargo pod to the International Space Station (ISS). The delay will also allow more time to develop plans to service the Hubble Space Telescope, Griffin said.

The space shuttle is sitting atop launch pad 39B at NASA's Kennedy Space Center (KSC) in Florida as part of the agency's first return to flight mission STS-114. A countdown test with the crew and orbiter stack is scheduled for next week at KSC.

The STS-114 flight is slated to test a host of new tools and techniques designed to increase shuttle flight safety, a direct response to the loss of the Columbia orbiter and its seven-astronaut crew. Columbia broke up during reentry on Feb. 1, 2003, which investigators later determined was a result of wing damage caused by external tank debris shed two weeks earlier at the mission's launch.

"Every time we've established a launch date, it's been on the best data available," said William Readdy, NASA's spaceflight program chief, during the press conference. "Since we first established a launch date in June 2003, we've adjusted it a half a dozen times based on new data."

Ice debris and sensor glitches

It was new data on the potential danger of ice debris striking the shuttle that pushed shuttle managers to set the new launch date. The ice, which tests have shown can break off in chunks of up to five inches long, forms on regions of a 17-inch wide liquid oxygen feed line running down the external tank.

Tank engineers had previously installed a drip lip to the area of most concern - a bellows unit that expands and contracts - which reduced the risk of ice by about 70 percent, NASA officials said.

"A very small piece of ice can cause problems," said NASA shuttle program manager William Parsons said in a separate briefing today from Johnson Space Center (JSC) in Houston. "Ice does not cause the same damage foam...we understand foam much better because we've done a lot of testing."

The damage to Columbia's wing leading edge was caused by a briefcase-sized chunk of insulation that separated from the external tank during liftoff. Tank engineers have since redesigned portions of the tank to reduce the amount of foam shedding.

"The testing on the ice lagged behind the testing on foam, but was clearly put in place," said Wayne Hale, deputy shuttle program manager at JSC, during the briefing. "But we had plans on how to deal with it [and will] now that we know that we have to do something about it."

Shuttle engineers could install a removable heater to the bellows area to prevent ice build-up. The heating unit is already expected to ride on the third external tank to fly, and will likely be installed on another tank already inside NASA's 52-story Vehicle Assembly Building to iron out assembly processes before a unit is attached to Discovery's fuel tank.

But Hale said that heater assembly kits won't arrive at KSC until May 5, and there still remains some testing to determine if they will ultimately fly aboard Discovery.

Earlier this month, shuttle mission managers and launch workers performed a critical test of Discovery's revamped external tank, fueling it with 500,000 gallons of liquid oxygen and hydrogen which the orbiter burns to reach space.

During that test, problems were detected in two of the tank's four fuel sensors, used to measure liquid hydrogen propellant levels, for reasons engineers still do not fully understand. Four functioning fuel sensors are required in order to launch the shuttle, Readdy said.

"I assure you that four [sensors] of four is the launch commit criteria of STS-114 and STS-121," Readdy added.

The follow-up to Discovery's test flight, the STS-121 mission aboard Atlantis will also test a series of new procedures and technologies for enhanced shuttle safety.

Other concerns also included the accidental contamination of Discovery's thermal protection blankets with hydraulic fluid, though Parsons said those concerns have been lessened and the blankets could be cleaned, or even left alone, after further study.

ISS support and Hubble

Discovery's two-month launch delay will also affect space station operations, where Expedition 11 astronauts Sergei Krikalev and John Phillips were anticipating the shuttle's May arrival to bring a wealth of food, supplies and new science equipment.

NASA's space shuttle fleet has been grounded since the Columbia accidents, leaving space station crews dependent on Russian Soyuz spacecraft to reach the ISS and the automated Progress vehicles for cargo deliveries. Discovery will deliver a large cargo container to the ISS, as well as large station components that currently cannot be launched aboard any other spacecraft.

William Gerstenmaier, NASA's ISS program manager, said more water - currently the key consumable aboard the station - would be added to a Progress vehicle set to launch in June.

"Each one of our international partners was disappointed that we're not going to launch the shuttle in that first window, but I think they understood clearly why we're doing it," Gerstenmaier said.

During the earlier briefing today, Griffin told reporters that he had already informed key members of Congress Thursday evening that he would direct engineers at Goddard Spaceflight center to start preparing for a space shuttle servicing mission to Hubble on the assumption that one ultimately will go forward.

Griffin said a final decision is still pending NASA's successful return to flight with the launch of the shuttle Discovery. However, with that launch now delayed nearly two more months, Griffin said the Goddard team has to get started now to preserve the option of saving Hubble before the popular telescope is scheduled to go dark around 2008.

"[But] We're not going to allow Hubble preparations to interfere with return to flight," Griffin said.

Space News Staff Writer Brian Berger contributed to this report.

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