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updated 12/17/2010 10:59:35 AM ET 2010-12-17T15:59:35

NASA fueled space shuttle Discovery at the pad Friday, not for a flight but for tests to help understand mysterious cracks that appeared in the fuel tank during a launch attempt last month.

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Discovery is grounded until at least the beginning of February because, while the cracks have been fixed, engineers still do not know what caused them.

In a countdown test that began at sunrise and lasted well into the afternoon, the launch team pumped more than 500,000 gallons of liquid hydrogen and oxygen into Discovery's external fuel tank. The tank was rigged with sensors and other equipment.

"We're not committing to flying anytime soon. We've got to wait until we know we have a good answer to go fly," launch manager Mike Moses said as Discovery's 15-story tank filled up. "We want to make sure we know the risk we have in front of us."

The concern is that cracks could cause chunks of foam to pop off and, in the worst case, slam into the Discovery at liftoff. A large slab of foam doomed space shuttle Columbia in 2003.

When Discovery does fly, the trip will be its last. Just two or three missions remain before NASA ends its shuttle program next year. Endeavour is due to fly in April, and Atlantis may follow in the summer if funding is forthcoming.

Discovery is loaded with supplies for the International Space Station as well as an experimental humanoid robot.

Back on Nov. 5, NASA halted the countdown for Discovery because of leaking hydrogen gas. An unrelated problem — the cracking — later was discovered in the insulating foam of the fuel tank, in the ribbed central portion that holds instruments. When the foam was removed, cracks were found in two of the more than 100 aluminum ribs, or brackets, making up that area. The two damaged ribs — each 21 feet long — were next to each other.

Both the leak and cracks were fixed, and NASA aimed for a possible December flight. But engineers were stumped by the cracks. They now believe there may have been a buildup of stress in the brackets during assembly, which caused them to crack when the super-cold fuel was loaded into the tank, Moses said.

Besides stringing cables with gauges and sensors on the suspect portion of the tank, technicians also painted small black dots — 10,000 to 12,000 of them — on the exposed white foam over the repaired area. Technicians worked in freezing temperatures, dipping their gloved fingers in paint and then pressing them gently onto the foam.

The dots were part of an optics test. A pair of cameras provided visuals of the dots, recording the motions of the tank in that area and hopefully providing additional clues to the cracking.

There were no leaks and no immediate signs of cracking as the test concluded Friday afternoon.

Discovery will be taken back to the Vehicle Assembly Building next week so engineers can X-ray the brackets on the back of the fuel tank.

The goal is to launch Discovery as early as Feb. 3 or at least by month's end, Moses said.

In orbit, meanwhile, the space station got three new residents Friday with the arrival of a Russian Soyuz capsule. The orbiting lab is now back to a six-person crew: three Russians, two Americans and one Italian.

The Soyuz — NASA's sole means of getting astronauts to orbit once the shuttles retire — blasted off from Kazakhstan two days earlier.

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Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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