CAPE CANAVERAL, Fla. -- The most hectic and critical day of shuttle Atlantis' mission to the International Space Station has begun.

On the agenda for today: using the station's robot arm to install the $790 million S-Zero truss with its attached Canadarm2 Mobile Transporter atop the U.S. Destiny science module, followed immediately by the first of four spacewalks necessary to begin the hundreds of mechanical and systems connections between the truss segment and the laboratory.

STS-110

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"Our busiest day by far is the day after we rendezvous and the first (spacewalk)," Atlantis commander Mike Bloomfield said before the launch.

The hard work began early this morning one hour after the shuttle crew was awakened at 4:44 a.m. EDT (0844 GMT) as Atlantis mission specialist Ellen Ochoa, with Expedition Four flight engineer Dan Bursch watching over her shoulder, powered up the station's Canadian robot arm and grabbed hold of the truss about 6:05 a.m. EDT (1005 GMT).

After some brief troubleshooting to make sure the arm had firmly grappled a pin on the 44.2-foot (13.5-meter) long truss segment, Ochoa lifted the S-Zero and began a three-hour procedure to move the 13-ton collection of 475,000 parts into its proper place atop Destiny.

At the same time, Atlantis mission specialists Steve Smith and Rex Walheim -- with the help of seven-time shuttle astronaut Jerry Ross -- began preparing for a six-and-a-half-hour spacewalk that is timed to begin about 11 a.m. EDT (1500 GMT).

"All that activity is going very smoothly so far this morning," said mission commentator James Hartsfield.

The exact start of the first spacewalk is to be timed within moments of a metal claw that is in a cradle on top of the Destiny module closing around a bar connected to the base of the truss.

That claw will temporarily hold S-Zero firmly in place until Smith and Welheim make the initial structural and power connections between the truss and the station.

Those power connections are considered critical because they will supply heat to the truss, which from the moment it was freed of Atlantis' cargo bay began to cool off in the vacuum of space. Power must be applied to the truss within about 16 hours or some of S-Zero's systems could be damaged.

All together four spacewalks during the next six days will be needed to make the estimated 250 mechanical and 110 electrical connections required to securely fasten the truss to Destiny and wire it up so it works.

Smith and Walheim will take on spacewalks planned for today and Sunday, while Ross and Lee Morin -- NASA's first pair of spacewalking grandfathers -- will team up for excursions outside set for Saturday and Tuesday.

S-Zero will serve as the centerpiece to which additional truss segments are to be added during the next two years, forming a rigid beam that eventually will stretch greater than the length of a football field.

To that completed keel NASA program managers plan to attach new power-generating solar wings and heat-dispelling radiators, enhanced utilities that will be required before launching new science laboratories built by Japan and the European Space Agency.