HOUSTON (Reuters) - Astronauts prepared to dock the shuttle Discovery with the International Space Station on Thursday in what may be the last visit for some time after NASA again grounded its problem-plagued shuttle fleet.
The two spacecraft were scheduled to hook up for a week while the Discovery crew makes space-station repairs and brings food and supplies to the two-man station crew.
What was supposed to have been the shuttle's triumphant return to the space station for the first time since November 2002 was overshadowed by NASA's decision on Thursday that it is still not safe to fly.
The U.S. space agency said flying debris captured on video at Tuesday's launch was remindful of what brought down Columbia on February 1, 2003, and showed that the debris problem was not fixed after 2 1/2 years of work and more than $1 billion in expenditures to improve safety.
Images showed at least three areas on Discovery's external fuel tank where chunks of insulation foam came off, including one almost as big as the piece that struck Columbia.
The foam was not believed to have hit Discovery, but its existence meant it was back to the drawing boards for NASA, said shuttle program manager Bill Parsons.
"Until we're ready, we won't fly again," he said. "I don't know when that might be."
Tuesday's Discovery launch was the first shuttle mission since the three-shuttle fleet was grounded after Columbia.
A 1.67 pound (0.75-kg) piece of insulating foam from Columbia's external fuel tank broke loose at launch on January 16, 2003, and struck the left wing, causing a hole in the heat shield that doomed the shuttle during re-entry 16 days later.
As Colombia glided toward Florida, superheated gases from the earth's atmosphere entered the breach and caused the orbiter to disintegrate over Texas, killing its seven astronauts.
TROUBLING NICKS
Discovery has some troubling nicks in protective tiles on its belly, but deputy shuttle program director Wayne Hale said they are not believed to be a threat to the spacecraft.
"The good news is that the orbiter Discovery appears to be in good shape," he said.
Discovery's crew was supposed to take haven in the space station and await rescue by shuttle Atlantis if the orbiter was damaged beyond repair at launch,
Hale said the chances of that being needed were "remote." Parsons said if it did come to pass, NASA would have to make the "hard decision" to send Atlantis up.
NASA has more safety examinations of Discovery scheduled in the 12-day mission, including one just before the space station docking.
In a maneuver planned before launch, shuttle commander Eileen Collins will steer Discovery into a slow back flip 600 feet (182 meters) below the space station while station crewmembers Sergei Krikalev and John Phillips snap pictures using telephoto lenses.
Another extended suspension of flights will be a big blow to the still-unfinished space station, a $95 billion project which depends on the shuttle to ferry in the modules used to piece the international project together.
Since Columbia, Russian spacecraft have been used to change crewmembers and bring supplies, but construction has come to a halt.
Discovery will deliver 15 million tons of supplies ranging from food to light bulbs to new laptop computers and will bring back junk from the station.
Shuttle astronauts Steve Robinson and Japan's Soichi Noguchi will perform three spacewalks, during which they will replace and repair balky gyroscopes that keep the space station stable and attach an external platform to be used for storage.
They will also test still-experimental techniques to repair damage to the shuttle exterior.
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