Discovery flies farewell loop around space station on first part of journey back to Earth

There were hugs and handshakes as the shuttle's astronauts said goodbye to the two-man crew of the space station, before the hatches between the ship and orbiting lab were finally closed and Discovery inched back 400 feet (122 metres) from its docking point.

Before heading back for Earth, the pilot flew a farewell loop around the station.

Discovery's astronauts are scheduled to complete their 13-day mission tomorrow when the first shuttle to return to space since the 2003 Columbia tragedy is set to land in Florida.

There have been serious concerns about the fate of Discovery too, with damage to the external structure prompting an unprecedented "space walk" by an astronaut armed with forceps and a hacksaw.

Discovery's crew of seven astronauts spent a day longer than originally planned aboard the station to bring over additional supplies. Discovery is the first shuttle to visit the station since 2002.

Since the Columbia tragedy, unmanned Russian cargo ships have carried supplies to the station, where two-person crews rotate every six months using Russian Soyuz rockets. The rockets don't have the capacity of the shuttle, which is needed to complete construction of the station.

Nasa officials, meanwhile, said they were looking into whether a small crack in the foam on Discovery's fuel tank may have caused a 1lb section of insulation to break off during its climb to orbit. The foam, which was reminiscent of the one that doomed Columbia, did not hit Discovery.

However, the large piece of foam did cause the space agency to suspend future flights until it can prevent shedding of similar pieces from the external tanks, which fuel shuttle launches.

So far, space agency officials say Discovery's mission has been a success. The crew completed its goals: the resupply of the space station, bringing home the station's rubbish and intense inspections of Discovery for damage. The astronauts also tested repair techniques that were developed after Columbia.

While in orbit, spacewalking astronaut Stephen Robinson successfully completed unprecedented repairs to Discovery's belly when he removed two protruding strips of tile filler that engineers thought could lead to dangerous overheating.

Engineers feared leaving the strips could cause a repeat of Columbia's disastrous re-entry in 2003 when the spacecraft broke apart over Texas as it headed to Florida.

Columbia was doomed by a small piece of insulating foam that fell from its external tank during launch. The foam created a hole in the spacecraft's left wing and as it re-entered the Earth's atmosphere, searing gases melted the wing, causing the ship to disintegrate. All seven astronauts on board were killed.