CAPE CANAVERAL, Fla. -- Shuttle Atlantis and a crew of seven astronauts returned to Earth on Friday, safely ending an 11-day mission that began a new phase of assembly operations at the International Space Station.

STS-110

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When this final construction period ends in 2004, the station will be at what NASA now calls "core complete" and is to include the first of the science laboratories built by the station program's international partners.

Given the apparent ease with which Atlantis' mission was conducted, officials expressed happiness and cautioned not to raise expectations about future flights.

"We've had an extraordinarily successful mission," Ben Sellari, NASA's S-Zero launch package manager, said this week. "We seem to make the missions (appear) very easy. But I can assure you, nobody who has followed this flight closely believes this has been an easy mission."

Atlantis' homecoming began with Wednesday's undocking from space station Alpha and continued on Thursday with the shuttle adjusting its orbit and the astronauts checking out the systems they would need Friday.

Then at 11:19 a.m. EDT (1519 GMT) Friday the shuttle's braking rockets were fired, committing the spaceplane to an hour-long, hypersonic glide through Earth's atmosphere on a course for Central Florida.

Dropping through partly cloudy skies, the 100-ton spaceplane glided to a perfect 12:27 p.m. EDT (1627 GMT) touchdown at the Kennedy Space Center (KSC).

Mission commander Mike Bloomfield primarily was at the controls as the shuttle made its approach to Runway 33, turning in a wide, right-hand circle out over the Atlantic Ocean and then diving toward the concrete strip's centerline. Pilot Stephen Frick also briefly handled the stick.

A red, white and blue drag chute popped from the shuttle's tail a moment after Atlantis' main gear hit the runway, helping to keep the vehicle straight down the middle of the pavement as it rolled to a halt a minute or so later after a 4.5-million-mile (7.2-million-kilometer) journey.

"Houston, Atlantis, wheels stop," Bloomfield called out, marking the official end of NASA's 109th shuttle mission flown during the past 21 years.

"Bloomer, we copy wheels stop. That was a great landing and a great way to end a mission that has been superb in all respects," astronaut Charlie Hobaugh said from Mission Control in Houston.

As soon as the spaceplane stopped, a team of KSC shuttle processing technicians and engineers raced in a convoy of vehicles to greet the shuttle as it sat on the runway -- their first job to make sure Atlantis wasn't leaking any toxic gases.

Assuming not, during the next hour or so the KSC workers were to hook up cooling lines to the shuttle's tail, open the main hatch, help the seven astronauts out of Atlantis and immediately begin preparing the orbiter for its next launch: an Aug. 22 mission to carry another truss segment to the space station.

Launched on April 8, four days later than planned because of a broken pipe at the launch pad, Atlantis carried the first such truss segment to the outpost -- the $600 million S-Zero truss that will serve as the cornerstone for an additional eight segments to be added during the next couple of years.

Eventually stretching the length of a football field, the beam will carry power and other utilities from future solar arrays to the rest of the station, their distribution controlled by computers inside the S-Zero truss.

Known as Phase Three, this final construction period is targeted to last less than three years and conclude when two science laboratories built by Europe and Japan are added to the outpost.

"By the end of this year we'll have over 200 tons on orbit and be we'll on our way," said Mike Suffredini, a NASA space station manager at the Johnson Space Center. "We're doing research while we're assembling space station and we believe we're set for the upcoming elements to fly."

Other goals achieved during the STS-110 mission included:

• Testing the $190 million Mobile Transporter, the first "railroad" in space that is to be used as a moveable base for the station's Canadarm2. Officials continue to report that initial problems experienced during the tests are easily solved and there is nothing wrong with the hardware.

• Boosting the outpost's orbital altitude about 6.5 miles (10.5 kilometers) by firing Atlantis' steering jets.

• Transferring from the shuttle to the station several hundred pounds of science experiments and supplies, which included nitrogen and oxygen gas, as well as drinking water.

• Staging four spacewalks to securely attach and wire the truss and transporter to the station's systems.

For mission specialist Jerry Ross, the flight put him in the record books as the first human to fly seven spaceflights, and also extended his record for being NASA's most experienced spacewalker with nine sorties outside for a total of more than 58 hours.

Asked about his experience during a space-to-ground media interview this week, Ross wouldn't pick a favorite among his many journeys.

"They all have unique qualities. I felt the same on this one as I did on the first one," Ross said, noting that on this flight he had a "great crew to fly with, incredible mission to perform, totally excited about the entire event, and the spacewalks were indescribable.''

The crew also included mission specialists Ellen Ochoa and Steven Smith, as well as two rookies who are now spaceflight veterans: Lee Morin and Rex Walheim.