This story was updated at 2:18 p.m. EST.
NASA has postponed the launch of space shuttle Discovery's final mission to no sooner than early February ? the latest in a long string of delays that have kept the spacecraft grounded for over than a month.
Discovery is now slated to launch no earlier than Feb. 3, giving NASA engineers more time to complete work to analyze why small cracks developed in the shuttle's huge external fuel tank. The cracks have since been repaired, but NASA wants to be sure that similar issues don't pose future concerns.
"The teams have done a very thorough job of looking at the data at hand, but we're coming to a point in the investigation where we have to do something different," NASA's space operations chief Bill Gerstenmaier said in a news briefing today.
Shuttle program managers met yesterday (Dec. 2) to evaluate the repairs made to Discovery's 15-story fuel tank following the initial scrubbed launch attempt on Nov. 5. [INFOGRAPHIC: NASA's Space Shuttle from Top to Bottom]
If Discovery is ready to launch in early February, NASA will have from Feb. 3 to Feb. 10 to attempt a liftoff within that window.
Understanding the problem
Engineers at NASA's Kennedy Space Center in Cape Canaveral, Fla., have been performing tests and gathering data to try to determine what caused the cracks that were found on the metal ribs of the shuttle's external tank. Yet, even with the extensive amount of available information, the teams have been unable to clearly identify the root cause.
"We're at a point where there's no obvious answer as to what occurred, and what that means is that we have to take the next step," said John Shannon, NASA space shuttle program manager. "We have to do that through demonstration. Analysis is not going to get us there. Analysis can only get you so far ? it's time to go test."
NASA will now perform a fueling test, in which gauges and other instrumentation are attached to Discovery's external tank as it sits on the launch pad. This will allow technicians to gather precise measurements of the type of stress exerted on certain areas of the tank as it is loaded with cryogenic propellant.
Engineering teams will be able to monitor how the repairs stand up to the harsh thermal conditions during tanking, and whether or not similar cracks form in the process.
"Understanding the thermal environment is critical to understanding the stress level in it," Shannon said. "We have to go to that next level and really understand the problem. This will also help determine what our screening criteria [are] to fly the tank confidently."
What's next for the shuttle missions
NASA officials said that the delay until February will not negatively impact space station operations, other shuttle launch opportunities, or the space agency's budget.
"We have sufficient margin to go ahead and work all the activities from a budget standpoint," Gerstenmaier said. "We haven't really pushed or upset anything from an overall budget standpoint."
If Discovery launches in early February, the final flight of space shuttle Endeavour ? the STS-134 mission ? will likely move from its original Feb. 27 date into early April.
Discovery's launch has been delayed since early November due to technical and weather-related issues. The shuttle's planned 11-day mission will deliver a storage room and Robonaut 2, a humanoid robot, to the International Space Station. Two spacewalks are also planned.
The mission will be the 39th and last flight for Discovery. It is one of NASA's two final scheduled shuttle flights before the orbiter fleet is retired in 2011. NASA is hoping to launch an additional shuttle flight around June, but is still awaiting final funding approval from a congressional appropriations committee.
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You can follow SPACE.com Staff Writer Denise Chow on Twitter @denisechow.