CAPE CANAVERAL, Fla. - 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.

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.