Shuttle Discovery leaves space station

HOUSTON, July 15 (Reuters) Shuttle Discovery departed from the International Space Station today to end a nine-day visit that included three spacewalks and a repair critical to resuming construction of the half-finished space outpost.

After astronauts closed the hatch linking the two spacecraft, Discovery gently undocked from the 100 billion dollars station and moved slowly away in the first steps of the shuttle's planned return to Earth on Monday.

''And we have physical separation,'' a Discovery crewmember radioed.

''Have a safe journey back, soft landings and we'll see you on the ground in a few months,'' space station astronaut Jeff Williams replied.

Two hours after departure, the Discovery crew was to perform a final inspection to the shuttle heat shield.

Until engineers on the ground study the inspection data and say all is well, the orbiter was to stay within 45 miles of the station so it could return in case damage is found.

The six astronauts could take refuge on the station until another shuttle is sent to rescue them.

NASA ordered extensive in-flight inspections as part of 1.3 billion dollars in safety upgrades since the Columbia disaster in February 2003.

Inspections earlier in the flight, conducted with the same cameras and sensors on a robot arm to be used in today's survey, found no damage and so far Discovery has been declared fit for the fiery return to Earth.

SMALL LEAK The only problem nagging at NASA engineers is a small chemical leak in one of the shuttle's three power units used during landing.

NASA plans to test the unit tomorrow and if the leak worsens, it may be shut down, forcing the shuttle to land with just two of the power units for the first time in the program's history.

While NASA said that would not be a problem, it would mean explosive bolts instead of hydraulic power would be used to release the shuttle's landing gear for lowering into place, deputy shuttle programme manager John Shannon said.

Also, to minimise risk, the space agency might decide to land the shuttle at Edwards Air Force Base in California where the weather is more stable and the landing area larger than at Kennedy Space Center in Florida.

Discovery currently is scheduled to land at Kennedy at 0914 hrs, local time on Monday.

NASA has said this mission was critical to the future of the shuttle program as it tries to recover from the Columbia accident.

Columbia's wing heat shield was cracked during launch by falling fuel tank foam, but because there were no in-flight inspections at the time, the damage went undetected.

Columbia disintegrated while returning to Earth 16 days later when hot gases penetrated its structure. The seven astronauts on board were killed.

On this flight, just the second since Columbia, astronauts not only conducted inspections, but also tested heat shield repair techniques during one of the mission's three spacewalks.

Spacewalkers Piers Sellers and Michael Fossum also repaired a transporter on the space station which will be be needed to install large components to finish off the space outpost that is sponsored by 16 nations.

NASA plans to fly 16 shuttle missions to complete the station.

Its next flight is scheduled for launch around August 28.

REUTERS AK KN1630