With Kelly and veteran mission commander Jim Wetherbee at the controls, Discovery is scheduled to lift off at 6:42 a.m. EST (11:42 GMT) Thursday here at NASA's Kennedy Space Center.

Strapped into its crew cabin along with shuttle mission specialists Paul Richards and Andy Thomas will be Russian cosmonaut Yuri Usachev and two American colleagues: astronauts Susan Helms and James Voss.

The so-called Expedition Two crew will replace U.S. astronaut Bill Shepherd and two Russian cosmonauts, Yuri Gidzenko and Sergei Krikalev, the three of whom will be returning to Earth after a 140-day "shake-down cruise" aboard the international outpost.

But NASA's 103rd shuttle mission is much more than a taxi flight.

Coming on the heels of the delivery last month of the station's first science laboratory, Discovery's crew will be hauling up a fully loaded Italian moving van dubbed "Leonardo."

Named for Leonardo da Vinci, an extraordinary Italian inventor, scientist, civil engineer, architect and artist, the can-shaped module is stuffed full of 9,663 pounds (4,348 kilograms) of supplies and equipment, including the station's first science research rack.

Now nestled in rear of Discovery's expansive cargo bay, the so-called Multipurpose Logistics Module, or MPLM, will be temporarily attached to the outpost so that all the gear can be unloaded by the joined shuttle-station crews.

Australian-born astronaut Andy Thomas will do that job with the shuttle's Canadian-built robot arm - a crane-like device he'll also use to place the module back in Discovery's cargo bay after it is filled up with 2,630 pounds (1,183.5 kilograms) of discarded equipment and station trash.

"Considering that the MPLM weighs, fully loaded, upwards of eight tons, this is a fairly delicate operation," Thomas said.

Among the equipment to be delivered to the station is NASA's Human Research Facility, a refrigerator-sized rack that will be installed in the station's $1.4 billion U.S. Destiny science laboratory, which was delivered to the outpost last month by a visiting shuttle crew.

The facility houses a variety of experiment apparatus that station crews will use to research the debilitating effect of long stays in weightlessness on the human body, scientific work deemed key to preparing for human expeditions to Mars.

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Also coming on board will be the Crew Health Care System rack, which is chock full of medical and first aid equipment and features an orbital pharmacy of sorts.

"This is essentially our doctor's office," said NASA mission manager Merri Sanchez.

"We'll have everything from what we call a crew medical restraint - which is essentially a backboard that we can put the crew on to immobilize them. We'll have their medical pack, which has all of the medicines that have been geared for each crewmember. We'll have the various doctor's office-type instruments that the crew is going to need."

And it also is equipped with a defibrillator.

Stretching some 21 feet (6.4 meters) in length, the Italian moving van also contains equipment required to activate the station's dish-shaped Ku-Band antenna, which will greatly enhance the ability of station crews to communicate with ground controllers.

What's more, the Leonardo module holds sophisticated computer workstations that will be needed to control the station's Canadian-built construction crane, which is to be launched to the outpost in mid-April.

Helms and Voss will venture outside the station to prepare the outside of the outpost for the arrival of the 57-foot (17-meter) robot arm, which is the centerpiece of the Canadian Space Agency's contribution to the 16-nation station project.

Richards and Thomas will carry out a second spacewalk to mount a stowage platform and spare cooling system equipment on the exterior of the Destiny lab, which is the first of six science research facilities that ultimately are to be launched to the station.

The pair also will scale to the top of the 17-story station in a bid to fix a metal bar that failed to latch firmly in place when an American-made electrical power tower was delivered to the outpost late last year.

Unlike NASA's past three station construction flights, no new major modules will be added to the outpost during Discovery's weeklong stay at the complex. The 12-day mission largely was created to carry out a hodge-podge of work originally scheduled for the mid-April shuttle flight.

"So that sort of makes us a cats ands dogs mission," Kelly said.

That's not to say, however, that the work is not critical. In fact, the work to be carried out on the first spacewalk is an absolute must before NASA can launch the station's Canadian-built construction crane.

"It's a pretty ambitious mission," said NASA lead flight director John Shannon. "There are some things we have to get done to keep the assembly sequence going."

The first changing of the guard at the station, meanwhile, represents a real crossroads for Shepherd and his two cosmonaut colleagues, marking the beginning of the last leg of their four-month tour of duty on the station.

"Our primary goal obviously is changing out the crew - that's what really makes this unique," said Kelly. "I'm sure that's real critical to `Shep' and his crew right now. So I think he'd call us the most critical mission in the ISS flow right now."

Discovery and the Expedition One crew now are scheduled to taxi back to NASA's coastal Florida spaceport on March 20.