CAPE CANAVERAL, Fla. -- Shuttle Atlantis' crew successfully carried out the mission's third spacewalk on Sunday, setting the stage for the first key movement test of a railroad-like device that is critical to the International Space Station's future growth.

STS-110

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Atlantis astronauts Steve Smith and Rex Walheim, who made the first spacewalk of the mission last Thursday, spent another six hours and 27 minutes working outside to continue fully installing the $600 million S-Zero truss and its attached $190 million Mobile Transporter onto the frontier outpost.

The truss is the centerpiece of a planned utility beam for the station that is to grow some 360 feet (110 meters) long, while the transporter is a platform meant to hold the station's Canadarm2 and move the entire length of the beam on a pair of rails.

A set of bolts that prevented the transporter from inadvertently moving during Atlantis' launch into orbit and docking with the station were removed by Smith and Walheim during Sunday's spacewalk. Other impediments to movement were removed and power and data cables were installed during spacewalks on Thursday and Saturday.

Now the way is literally clear for the first test of the system, which must work now and far into the future in order for the station's robot arm to be able to carry future truss segments or other equipment from the shuttle's cargo bay to the far ends of the growing truss.

"It's a big day when we're moving the Mobile Transporter," said Paul Geery, a Boeing manager who helped oversee the effort by TRW Astro Aerospace of Carpenteria, Calif., to design, develop and build the device.

Flight controllers at Mission Control in Houston overnight are scheduled to power up the transporter and make an initial check of its systems while the ten humans in space sleep.

Then at about 7:30 a.m. EDT (1130 GMT), with the entire crew complement watching on television monitors, the transporter will be commanded to move down to one end of the 44.2-foot (13.5 meter) long S-Zero truss and then backtrack to the other end, before returning roughly to the middle of the truss.

If at any point during the exercise the hardware becomes stuck, astronauts Jerry Ross and Lee Morin will be sent outside to investigate and fix the problem during the shuttle mission's fourth spacewalk, already scheduled for Tuesday.

Tasks complete

In addition to removing the restraining bolts on the Mobile Transporter, Smith and Walheim took care of some other truss installation chores that included:

• Releasing a metal claw on the top of the Destiny science module that had initially held the S-Zero truss in place until four weight-bearing struts could be bolted to studs in the laboratory's walls during the first two spacewalks.

• Turning off power to the station's robot arm and then re-routing the arm's source of power and data from the Destiny module to the S-Zero truss and eventual distribution to the Mobile Transporter. The arm was then powered up again.

For the most part the spacewalkers went about their business with little distraction, but a couple of times the overwhelming beauty of their home planet turning 240 miles (386 kilometers) was too much to keep from noticing.

"Is that the blue tongue of the Bahamas and Florida?" Walheim asked as the complex flew above Cuba and the southern tip of Florida.

"That would be it," Smith confirmed.

"Look at that!" Walheim said.

"Cuba down there below us," Ross volunteered from inside Atlantis.

"See that dark blue part, Rex, that kind of invades the rest of that green area," Smith said.

"Oh that's beautiful. Look at that. That's incredible," Walheim exclaimed.

"Yup. Boy what a pass," Smith said.

A couple of spacewalk tasks from could not get done and are likley to be added to Tuesday's final excursion outside.

Those tasks included moving a set of tools to a storage area on the outside of the space station, as well as installing a 14-foot (four-meter) "ladder" that would create a direct spacewalking path from the Quest airlock to the S-Zero truss.

Meanwhile, shuttle Atlantis was used to pull the space station into a mile higher orbit Sunday night, completing the second of three planned orbit-raising maneuvers for this mission. The final firing of jet thrusters is expected on Wednesday just before the shuttle undocks.

Atlantis remains scheduled to land back at the Kennedy Space Center on Friday.