Shuttle Discovery undocks from space station

CAPE CANAVERAL, Fla., Dec 20 (Reuters) Shuttle Discovery astronauts undocked their craft from the International Space Station yesterday and prepared to begin the journey home to Earth.

''Better go or we'll never leave,'' shuttle commander Mark Polansky told station skipper Michael Lopez-Alegria as he made his way to a connecting tunnel between Discovery and the space station.

''It has been a very exciting time, so it's obviously hard to let go,'' said Germany's Thomas Reiter, who is returning aboard Discovery after nearly six months on the station.

The crew left behind rookie astronaut Sunita Williams, who took over the slot vacated by Reiter as a member of the station crew.

The shuttle astronauts floated into Discovery, which gently backed out of the station's berthing port at 5:10 pm (local time), ending an extended eight-day mission at the orbital outpost.

After a final inspection of the shuttle's critical heat shield and a checkout of the landing systems, the astronauts are expected to land on Friday at the seaside Kennedy Space Center in Florida.

They installed a new electrical power system at the station so laboratories built by Europe and Japan can be added.

NASA was prepared with spare equipment in case cooling pumps did not start and recovery plans in case the rewiring triggered a blackout. But it was the fairly straightforward retraction of an old solar-array wing that tripped up the mission, requiring an unplanned and hastily choreographed spacewalk so astronauts could manually free tension wires, realign grommets and shake out flaps in the partly extended panel.

NASA originally allotted three to four hours for the job, which engineers had hoped to accomplish by remote command.

Instead, the work spanned four days, including a spacewalk that required veteran astronaut Robert Curbeam to venture outside for a record fourth time during a single mission.

He improvised with tools that had been wrapped in insulating tape to prevent electric shocks and worked in areas never intended for an astronaut's glove. Sweden's Christer Fuglesang accompanied Curbeam on three spacewalks and Williams was Curbeam's partner for one.

NASA needed the panel retracted so it can be repositioned on the station next year.

''I've been scared of this flight for a long time,'' said the station's lead flight director, John Curry. ''I'm very relieved.'' The work kept Discovery at the station an extra day and delayed the shuttle's homecoming from Thursday to Friday.

The US space agency is under a firm deadline to finish the 100 billion dollars International Space Station by 2010 when the shuttle fleet is scheduled to be retired. At least 13 more missions are needed to complete assembly.

Discovery's crew also delivered more than four tons of equipment, food and supplies, installed a new segment to the station's metal backbone and delivered shields to better protect the station's living quarters against micrometeoroid strikes.

The extra day in space to fix the panel forced NASA to make a difficult choice: give up a final check of the shuttle's heat shield, or use one of two days' emergency supplies reserved for landing delays due to poor weather or equipment problems.

Managers opted to dip into the reserves but mobilised backup landing sites in California and New Mexico, in addition to the shuttle's prime runway in Florida, for landing attempts on Friday.

REUTERS PKS RN0508