"Looks like a beautiful day to go fly. Discovery's ready for you. I'd like to wish good luck to the entire crew," NASA shuttle launch director Mike Leinbach told the seven people aboard Discovery minutes before they set sail for the station.

"Okay, thanks," replied shuttle skipper Jim Wetherbee. "Expedition Two is ready to relieve Expedition One."

Now cruising at 25 times the speed of sound, Discovery will spend the next two days playing a game of orbital catch-up. The winged spaceship is due to dock at the station about 12:30 a.m. EST (05:30 GMT) Saturday.

A few hours later, Shepherd and his crew will welcome their replacements: Incoming station commander Yuri Usachev, a veteran Russian cosmonaut, and two U.S. astronauts, Susan Helms and Jim Voss.

The second station trio, known as the Expedition Two crew, was ferried to the outpost by Wetherbee and three other NASA astronauts: Discovery pilot Jim Kelly and mission specialists Paul Richards and Andy Thomas.

The shuttle quartet will ferry Shepherd and company back to Earth later this month after a hectic 140-day "shakedown cruise" aboard the international outpost.

Said NASA station project manager Tommy Holloway: "Im sure `Shep and his crew are getting to the point where theyre ready to come home."

The crews homecoming, however, will be preceded by two spacewalks outside the outpost, as well as the unloading of an Italian moving van now nestled in the shuttles cargo bay.

The first spacewalk calls for Helms and Voss to prepare the outside of the outpost for the scheduled mid-April arrival of a Canadian-built robot arm critical to future outpost assembly.

That spacewalking sortie which will be the 101st in NASA history -- is set to begin about 11:45 p.m. EST Saturday (04:47 GMT Sunday).

Richards and Thomas will carry out the second spacewalk, which is scheduled to start at about 12:45 a.m. EST (05:45 GMT) Tuesday.

Their job: Mounting a stowage platform and spare cooling system equipment on the exterior of the stations new U.S. Destiny laboratory, 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.

In between and after the spacewalks, the 10 astronauts and cosmonauts will unpack some 9,663 pounds (4,348 kilograms) of supplies and equipment from an Italian moving van dubbed "Leonardo."

Named for Leonardo da Vinci, the renowned Renaissance inventor, scientist, engineer, architect and artist, the cylindrical container is the first of three pressurized Italian logistics modules built to haul equipment and supplies to and from the station.

Among the gear to be unloaded on this mission: NASAs human research facility, a refrigerator-sized rack that will be installed in the $1.4 billion Destiny lab, which was delivered to the outpost last month by a visiting shuttle crew.

The rack houses experiment apparatus for research into the adverse effect of weightlessness on the human body, scientific work deemed key to preparing for human expeditions to Mars.

An expanded station health clinic, space-to-ground communications gear and computer workstations needed to operate the stations robot arm also are stowed in the so-called multi-purpose logistics module, or MPLM.

Usachev, Helms and Voss will spend the next four months setting up much of that equipment, including gear required to activate the stations Ku-band communications antenna.

Set up by spacewalking assembly workers last October, the dish-shaped antenna is designed to send back high-quality audio and video communications through NASA relay satellites.

Once activated, it will reduce reliance on widespread Russian ground stations, which primarily provide for audio communications, and the stations so-called U.S. Early Communications System, which only is capable of sending back grainy television signals.

What so far has been relatively sparse contact with the station also will be greatly enhanced. NASA station flight director Rick LaBrode said the Ku-band system would provide coverage during about 50 percent of each 90-minute station orbit of Earth.

Shuttle-like coverage -- which averages more than 85 percent of each orbit -- wont be possible because the stations towering solar wings frequently will block the antennas line-of-sight with the NASA relay satellites.

The Expedition Two crew also will be starting up station research in earnest, concentrating primarily on biomedical studies to gauge the amount of radiation outpost crews are exposed to.

That work will represent a key milestone for researchers who have been preparing to send experiments to the station for years.

"Were taking our research program to a new level with this expedition," said NASA project scientist John Uri. "Were kicking it into high gear with the arrival of our first rack and a diverse array of investigations. So the lab is literally open for business."

The crews most critical job, however, will involve activating systems that will be needed to control the stations robot arm, which must be working properly before further construction work can be done at the growing outpost.

Considered the centerpiece of the Canadian Space Agencys contribution to the station project, the 57-foot (17-meter) construction crane will be capable of "inch-worming" from work site to work site outside the station, crawling to places the shuttles fixed robot arm cannot reach.

The new station crew, meanwhile, expects all the work to go well.

The three have been training for their expedition for four years, and they got a test run aboard the outpost during a station maintenance mission last May. Whats more, theyve formed an unusually tight bond during that time.

"I consider myself blessed to have two people I get along with very well to be crewmates for a long stay in space," said Voss.

"Were like a little family," Helms added. "If you had two brothers and a sister that got along closely in a family, I think that would really well describe how we are as a crew."

That close relationship could prove important for the group, which could end up spending up to six months working together in the isolated confines of a frontier outpost some 235 miles (376 kilometers) above the planet.

Said Wetherbee: "If youre going to spend four or five or six months with your closest friends, they had better be your closest friends."

With Shepherd and his cosmonaut colleagues in tow, Discovery is scheduled to depart the station about 11:00 p.m. EST March 17 (04:00 GMT March 18), leaving the Expedition Two crew behind. NASAs 103rd shuttle flight is due to end with a 2:00 a.m. EST (07:00 GMT) March 20 landing here at the agencys coastal Florida spaceport.