The shuttle descended through a slightly cloudy sky and touched down on the runway a little before noon.
"Welcome back to Earth, and congratulations on a truly spectacular mission expanding our new home in space," Mission Control told commander Jeffrey Ashby and his crew.
During their 11-day mission, the shuttle astronauts successfully outfitted the space station with a $390 million girder that features a sophisticated cooling system. It was an extensive addition: The aluminum structure is 45 feet long and 15 feet wide, and its unfolded radiator reaches 75 feet.
This framework eventually will stretch 356 feet and hold more radiators and electricity-generating solar wings needed to operate future laboratories.
As Atlantis glided to a smooth landing, the space station was soaring high above the Gulf of Mexico, with its occupants marking their 135th day aloft.
Atlantis' weeklong space station visit was welcomed by the three people living aboard the orbiting complex, who had not seen anyone face-to-face since they moved in last June. Wednesday's farewell was emotional, even tearful.
"We had such a wonderful time together. ... It was really hard to say goodbye," said shuttle astronaut Sandra Magnus, who is a good friend of space station resident Peggy Whitson.
The 14-ton girder delivered by Atlantis - and installed over three spacewalks - boosted the mass of the space station to more than 180 tons.
The space station will grow 14 more tons when another girder is ferried up next month by shuttle Endeavour. An even more important payload will be the replacements for Whitson and her two Russian crewmates.
Before launching Endeavour, though, NASA wants to understand a failure in Atlantis' pyrotechnic system back on launch day.
Half of the explosive charges intended to release bolts holding the shuttle down on the pad did not fire when the countdown reached zero on Oct. 7. Only one set of explosives is needed to shatter the bolts; the other serves as a backup.
Officials said it will take a couple of weeks to access and test the suspect parts inside Atlantis.
Before heading home, Atlantis flew around the space station on a photo-documentation tour, reports CBS News Space Consultant Bill Harwood.
The goal of the fly around was two-fold: To provide photo documentation of the station in its current state, which includes the new S1 solar array truss segment; and to give Melroy hands-on flying time with the shuttle.
The photo documentation is not mandatory, but "it's highly desirable," flight director Phil Engelauf said.
"It also serves to some degree as real, hands-on-the-stick piloting training time," he said. "We typically have the pilot fly the fly-around procedure. Usually, the commanders do the bulk of the critical flying during the missions and this gives the pilot the opportunity to get some real stick time before they eventually become commanders and are doing critical operations themselves."
CBS News Space Consultant William Harwood has covered America's space program full time for more than 15 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.