The shuttle Discovery docked smoothly with the International Space Station high above the earth Thursday, carrying on after NASA grounded its other shuttles for fear of another Columbia-like disaster.

The two space behemoths, each weighing more than 100 tons, linked up with barely a bump as commander Eileen Collins slowly guided the shuttle in.

This flight was supposed to have been a triumphant return of the shuttle to the space station for the first time since November 2002, but NASA's surprise decision that it still is not safe to fly the ageing orbiters took the glow off.

The US space agency said flying debris captured on video at Discovery's launch on Tuesday was too similar to what brought down shuttle Columbia on February 1, 2003 and showed that the debris problem was not fixed after two-and-a-half years of work and more than US$1 billion in safety expenditure.

NASA officials believe Discovery is unharmed and will have no trouble coming home on August 7, but they do not know when the shuttles will fly again. A flight by Atlantis was scheduled in September.

Images showed chunks of foam missing from at least three places on Discovery's external fuel tank, including one almost as big as the piece that struck Columbia. There are also nicks in the protective tiles on Discovery's belly.

A 0.75-kilogram 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 Columbia 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.

The stray foam on Tuesday's launch, the first since the three-shuttle fleet was grounded after Columbia, meant it was back to the drawing boards, shuttle programme manager Bill Parsons said in a Wednesday briefing.

Discovery is scheduled to stay a week with the station, during which time it will hand over 15 tons of supplies ranging from food to light bulbs to new laptop computers and pick up 13 tons of junk to carry back to earth. 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.

Source: China Daily