By Todd Halvorson CAPE CANAVERAL, Fla. Shuttle Discovery is scheduled to pull in to the International Space Station early Saturday, bringing with it a fresh crew of astronauts and cosmonauts and mixed emotions for the tenants they will replace.
With the so-called Expedition Two crew aboard, Discovery is due to arrive at the frontier outpost at 12:34 a.m. EST (05:34 GMT) as the shuttle and the station link up some 235 miles (376 kilometers) above Planet Earth.
Hatches between the craft will swing open two hours later, signaling the beginning of the end of a 4.5-month "shakedown cruise" for the stations first full-time crew: U.S. astronaut Bill Shepherd and Russian cosmonauts Yuri Gidzenko and Sergei Krikalev.
"Itll be, I think, a happy-sad moment for the Expedition One crew," said incoming station flight engineer Jim Voss, who will be taking up duty aboard the complex along with veteran cosmonaut Yuri Usachev and fellow astronaut Susan Helms.
"Theyll be very happy to be going home to their families, [but] theyll be a bit sad for leaving this station that they have been the inaugural crew on."
At the same time, the changing of the guard will mark the start of a long-awaited escapade for Voss, Helms and Usachev, who will be spending the next four to six months on the outpost.
"I think itll be a very pleasant time for us," Voss said. "Well be starting a new adventure."
Having set sail from Kennedy Space Center at sunrise Thursday, Discovery is in the midst of a two-day orbital chase.
Trailing the station by more than 11,000 miles (17,600 kilometers) at liftoff, the shuttle is closing that gap at a rate of about 700 miles (1,120 kilometers) with every 90-minute spin around the globe.
With veteran astronaut Jim Wetherbee and rookie pilot Jim Kelly at the controls, the shuttle crew -- which includes mission specialists Paul Richards and Andy Thomas -- is set to begin the final stages of a ground-up station rendezvous at 10:15 p.m. EST Friday (03:15 GMT Saturday).
Flying 9.2 miles (14.7 kilometers) behind the station, Wetherbee will fire the shuttles twin maneuvering engines in a bid to bring the ship to a point directly in front of the 17-story outpost.
The ships tail will be pointing toward Earth and its black-tile belly will be facing the direction of travel as Wetherbee eases Discovery up to a shuttle docking port at the forward end of the U.S. Destiny science laboratory, which was delivered to the station last month.
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The angle of attack will make Discoverys arrival a bit trickier than previous shuttle dockings that have called for final approaches from directly below the outpost.
The stations prime shuttle docking port was repositioned at the end of Destiny last month to clear the way for future construction work. Consequently, Wetherbee will have to proceed with caution to make certain caustic exhaust from shuttle steering jets doesnt damage fragile station solar wings.
"We have to be very careful as were approaching," the former U.S. Navy test pilot said. "We dont want to plume the station with our braking jets."
The docking will follow a busy day of checkout work aboard Discovery.
Kelly and Thomas limbered up the shuttles 50-foot (15-meter) robot arm, which will be used to temporarily mount an Italian moving van to the station next week so that 5 tons of supplies and equipment can be unloaded from it.
The cylindrical cargo carrier ultimately will be filled with a ton of discarded station equipment and trash and then stowed back in Discoverys cargo bay for a return trip to Earth.
Richards, Helms and Voss carried out prep work in the shuttles airlock and checked out the spacesuits the latter two will don for an excursion outside the outpost late Saturday.
That six-hour spacewalk -- which will begin at 11:47 p.m. EST Saturday (04:47 GMT Sunday) -- will involve setting up a mounting platform in advance of the scheduled mid-April arrival of a Canadian robot arm critical to future outpost assembly.
A second spacewalk also will be performed during the shuttles eight-day stay at the station. That sortie will begin about 12:45 a.m. EST (05:45 GMT) Tuesday as Richards and Thomas set out to position a stowage platform and spare cooling system equipment outside the Destiny lab.
The official hand-over of the station helm, meanwhile, will be carried out over the course of the next four days as first Gidzenko, then Krikalev and finally Shepherd trade places with Usachev, then Voss and finally Helms, respectively.
The staged transition is being choreographed to give outgoing station commander Shepherd as much time as possible to brief incoming skipper Usachev while Voss and Helms wrap up duties aboard Discovery.
In space now for 129 days, Shepherd and company will bid adieu to their relief crew during a farewell ceremony at 8:12 p.m. EST March 17 (01:12 GMT March 18).
Discovery is scheduled to depart the station a few hours later, heading for a 2 a.m. EST (07:00 GMT) March 20 landing here at NASAs coastal Florida spaceport.
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