Hatches on a vestibule connecting Atlantis to the International Space Station officially were sealed shut at 10:28 a.m. EDT, after a shuttle crew exited the outpost for the final time.
"Never again will we see a space shuttle open through that doorway," said NASA TV commentator Josh Byerly.
Mike Fossum and Ron Garan, members of the station's Expedition 28 crew, swung the Harmony node hatch closed a short time later, revealing the U.S. flag just presented by the Atlantis astronauts.
"As we put this flag on the hatch that leads to Atlantis right now, and when we close that hatch when these guys go by, we're closing chapter in the history of our nation," Garan said during a farewell ceremony. "But in the future, when another spacecraft docks to that hatch with crew members on board, and we open that hatch, we are going to be opening a new era, and raising the flag on a new era of exploration beyond low Earth orbit."
The flag, flown on the first shuttle mission, is to be returned to Earth by the next crew to arrive at the station on a U.S. launch vehicle.
Also visible in the upper left corner of the picture is a model shuttle orbiter the Atlantis crew left behind to commemorate the shuttle's essential role in station assembly.
In all, hatches were open between Atlantis and the station for seven days, 21 hours and 41 minutes.
During all 37 shuttle flights docked at the station, hatches were open for a total of 234 days, 14 hours and 30 minutes.
The four Atlantis astronauts have begun preparations to undock at 2:28 a.m. Tuesday.
Nearly 10 full days have elapsed since Atlantis launched July 8. Landing at Kennedy Space Center is planned around 6 a.m. Thursday.
The Atlantis crew will go to sleep around 2 p.m. today.