"We have contact and capture," a NASA official said when the delicate coupling was completed at around 1118 GMT.

The maneuver came at the end of Discovery's gradual approach to the space station, orbiting some 357 kilometers (222 miles) over the Pacific Ocean west of Chile. The final 200 meters (650 feet) took more than an hour.

The astronauts are scheduled to open the hatches between the vessels at

Before docking, when the Discovery was some 183 meters (600 feet) below the space station, it did a slow back flip, exposing its underlying thermal shield to the space station's astronauts, who took pictures with digital cameras.

The images were transmitted from the space station for analysis at NASA's Johnson Space Center, in Houston, Texas, where experts will examine the shuttle's exterior for signs of damage that might compromise its return August

Discovery blasted off from Florida's Kennedy Space Center on Tuesday on the first shuttle mission since the 2003 Columbia disaster.

NASA's decision to ground the flights once more came after astronauts on Wednesday inspected Discovery's exterior for damage during launch.

A chunk of insulating foam struck Columbia on takeoff in January 2003, puncturing its protective outer skin and allowing superheated gases to enter and cause the shuttle to break apart upon re-entering Earth's atmosphere.

All seven astronauts aboard Columbia were killed.

The space agency stressed, however, that it believes Discovery was not damaged in the latest incident.

"We didn't expect this to happen and it did," said Bill Parsons, space shuttle program manager, "It's a setback, ... and until this is closed we won't be ready to fly.

"The fact is it didn't cause any damage to the orbiter that we're aware of at this time. It didn't impact the orbiter at all," Parsons said.

Still, despite the healthy assessment, Parsons said future flights have been put on hold until the problem is corrected.

"We are going to go and carry out a thorough evaluation, and then we'll determine when it's safe to fly," he said. "I don't know if it's a month; I don't know if it's three months. We have a lot of work to do."

The grounding decision, however, will not alter Discovery's 12-day mission.

On Wednesday, the crew monitored the nose and leading edges on the wings of Discovery with special cameras and laser scanners, in an operation that lasted several hours.

What appeared to be a small piece of tile and a larger piece of debris were seen in video images falling off the external fuel tank as the shuttle blasted off Tuesday.

But NASA said there was no sign of danger so far. "The engineering community doesn't think that this is going to be a significant issue," shuttle flight director Paul Hill said.

Earlier, NASA flight operations manager John Shannon said the piece of tile was believed to be about 1.5 inches (3.8 centimeters) in size and appeared to have come off the front landing gear.

NASA quickly alerted Commander Eileen Collins about the debris, but there was no sign of concern among the seven-member crew, which includes Japanese astronaut Soichi Noguchi.

The shuttle fleet had been grounded ever since the Columbia was destroyed, amid painstaking inquiries into the cause of the disaster and ways to correct the problems.

During its mission, Discovery will take supplies to the space station and the crew will perform some risky operations designed to test new safety procedures.

During one of three planned spacewalks by the Discovery crew, Noguchi and his US counterpart Stephen Robinson will test repair techniques adopted after the Columbia disaster.

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