U.S. space shuttle Discovery has docked smoothly with the International Space Station, the last time a shuttle visits the outpost for a while because NASA has grounded the fleet again. Four pieces of insulating foam broke away from Discovery's external fuel tank during Tuesday's launch, raising fears that the problem that doomed the orbiter Columbia in 2003 could happen again. NASA says Discovery appears unharmed by the debris.

Discovery eased up under the space station and closed in on its target very slowly as engineers on the ground examined images of the orbiter to determine if any launch debris had damaged it. Shuttle pilot Jim Kelly confirmed the link-up to the station to mission control in Houston.

The shuttle's astronauts will spend eight days with the station's two-man crew conducting maintenance spacewalks, resupplying the space laboratory, and hauling out nearly three years worth of trash.

It has been that long since a shuttle connected to the station because of the flight moratorium after Columbia disintegrated in orbit. Now, NASA has reinstated the flight ban until it can figure out why separating foam continues to threaten shuttles, even after it spent two years and $1.5 billion to correct the problem. It was a piece of the material that punctured a hole in Columbia's wing upon liftoff, causing the vehicle to break up and kill seven astronauts in the searing heat of re-entry.

The space station crew used telephoto cameras to get extreme closeups of the shuttle as it approached. Shuttle operations manager John Shannon says the pictures and laser scans from the end of a shuttle crane will let technicians determine if Discovery sustained damage from the foam and if a pit seen in a protective heat tile is deep enough to pose a re-entry threat.

"The imagery guys have them and they are poring over them," said Mr. Shannon. "The initial report was that it looks extremely good and that we don't have anything to worry about on Discovery. However, I am pre-empting the six-day process that we are going to go through. We are going to continue to go through that and understand it exactly."

Mr. Shannon says the foam shed during Discovery's launch was much less than has ever come off before, thanks to the two years of redesign. He notes that the location that yielded the biggest chunk this time, less than half a kilogram, had not been a problem before, so the tank or the method of spraying the foam on was not changed in that location.

"We were wrong and we missed something," said Mr. Shannon. "We have to go figure out what it was and go fix it. Whether that's just changing techniques or redesign, we don't know. The team is off looking at that very hard."

Because of the Columbia tragedy, Discovery astronauts will take a spacewalk Saturday to practice methods to repair shuttle surface damage. They are scheduled to return to Earth on August 7.