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updated 12/16/2010 4:47:16 PM ET 2010-12-16T21:47:16

HOUSTON To test a troublesome fuel tank on the space shuttle Discovery, NASA plans to fill the tank Friday morning and run a countdown to what would normally be its last few seconds before liftoff.

But instead of launching the spacecraft on its planned mission to the International Space Station (ISS), the space agency will halt the so-called "tanking test" in an effort to learn why two of the fuel tank's support beams cracked during a previous launch attempt last month.

NASA is now planning Discovery's liftoff for no earlier than Feb. 3, 2011 on its final space voyage before the orbiter is retired. But first, engineers must figure out what caused the shuttle's fuel tank to crack.

Shuttle technicians have outfitted the tank's ribbed midsection, or intertank region, with nearly 90 instruments, including strain gauges to precisely record movement and temperatures as the tank chills and warms again during the test's fuel loading and emptying process.  

The tank holds more than 500,000 gallons of super-cold liquid oxygen at minus 297 degrees Fahrenheit and liquid hydrogen at minus 423 degrees F. Earlier measurements have shown that these cryogenic propellants cause the tank to shrink by about half an inch. [ INFOGRAPHIC: NASA's Space Shuttle from Top to Bottom ]

The test will help shuttle engineers determine if that shrinkage caused two 21-foot-long, U-shaped aluminum brackets, called stringers, to crack during the lead-up to a Nov. 5 attempt to launch Discovery on its STS-133 mission. That launch attempt was called off not because of the cracks but rather due to a gaseous hydrogen leak that emanated from a cable connecting the tank to the pad.

The cracked stringers, however, caused their overlying foam insulation to crack as well, which is how NASA initially discovered the damage in the hours following the foiled Nov. 5 countdown. In addition to the cracked beams posing a potential structural concern, cracks in the foam could cause the insulation to fall off during the shuttle's climb to orbit, creating a risk it might impact and damage the orbiter's heat shield.

In February 2003, falling foam from the fuel tank hit shuttle Columbia's left wing, damaging its leading edge. The impact ultimately led to the loss of the vehicle and its seven-person crew when Columbia re-entered the atmosphere at the conclusion of its STS-107 mission.

Though NASA has encountered cracks in the tank's stringers before, they have always shown up during its assembly. This was the first time that cracks were found while the shuttle was on the launch pad, leading engineers to wonder if they missed something during the beams' fabrication or installation. A root cause for the damage has not yet been found, leading managers to call for the tanking test.

"The idea is to help us figure out what caused these cracks in the first place to happen, which we haven't been able to do before this round of tests," NASA's Kennedy Space Center news chief Allard Beutel told SPACE.com. "And also get the data out of it that will help figure out if this could happen again and could we do something to address that and preempt it from happening again."

The test will provide more information about how the tank performs during fueling, as well as verify the repairs made to the cracked stringers. Technicians installed new sections of metal, called "doublers" because they are twice as thick as the original stringer metal, to replace the tops of the two cracked stringers.

The countdown for the test began at 9:00 p.m. EST (0200 GMT) on Wednesday at the T-minus 33 hour mark. Five planned holds are built into the countdown, which will run until T-minus 31 seconds, when during a normal launch, Discovery's onboard computers would take control.

"To make this [test] as realistic as possible, they are doing as many real launch day activities as practical," said Beutel. "There's some things we'll obviously not be doing  the crew is obviously hundreds of miles away [at Johnson Space Center in Houston] so we won't be doing anything with them."

Fueling of Discovery's external tank is scheduled to get under way at 7:00 a.m. EST (1200 GMT). The test is expected to be completed approximately three hours later.

Unless the foam cracks again, results from the test will not be known immediately. Managers and engineers will review the data that the test generates before determining the next course of action.

In the meantime, Discovery will be rolled off the launch pad and back to the 52-story-tall Vehicle Assembly Building, where its tank will undergo X-ray scans to examine the stringers. Barring any further problems, the shuttle will be readied for launch.

Discovery's next opportunity to embark on its 39th and final mission is scheduled for no earlier than 1:34 a.m. EST (0634 GMT) on Feb. 3. The shuttle's planned 11-day trip will deliver a storage room and Robonaut 2, a humanoid robot, to the International Space Station. Two spacewalks also planned.

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