The crew of NASA's final space shuttle mission closed the hatch to the International Space Station for the final time on Monday in preparation for the shuttle's last flight back to Earth.
NASA is watching the path of Tropical Storm Bret off the Atlantic coast of Florida but it is not expected to interfere with Atlantis' planned landing at 5:57 a.m. EDT on Thursday.
Before floating through the shuttle's air lock for the final time, Atlantis' four crewmembers left behind two mementos to commemorate the shuttle program's 30-year history: a space shuttle model and an American flag that flew aboard Columbia on the first U.S. shuttle mission in 1981.
It was one in a string of many "lasts" in Atlantis' 13-day mission -- to be followed by the final undocking from the space station early on Tuesday and its scheduled predawn touch-down at Kennedy Space Center in Florida on Thursday.
"We're closing a chapter in the history of our nation," space station flight engineer Ron Garan said in a ceremony marking the hatch closing.
The flag will remain affixed to the station's air lock until the air lock swings open to admit astronauts in future years who ride to the station aboard a capsule built by U.S. commercial companies, NASA said.
"We are going to be opening a new era and raising the flag on a new era of exploration," Mr. Garan said.
"Thank you for honouring the thousands of people that have been a part of the space shuttle program over the years," Megan McArthur radioed from Mission Control.
Atlantis commander Chris Ferguson was the last shuttle astronaut to leave the station before the shuttle's hatch closed for the final time.
Atlantis is the bookend of 134 previous shuttle missions that have deployed satellites and observatories, including the Hubble Space Telescope. The shuttle's crowning accomplishment, NASA says, was carrying to orbit and constructing the space station -- a $100-billion project of 16 countries.
Mr. Ferguson and his crew delivered more than 5 tonnes of food, clothing, equipment and other supplies that are meant to tide over the station until NASA's newly hired cargo delivery firms begin flying next year.
NASA, meanwhile, wants to ramp up development of a new capsule-style spacecraft and heavy-lift booster that can ferry people into deep space, beyond the station's orbit where the shuttles cannot fly.
NASA is supporting efforts by four firms -- Boeing, Space Exploration Technologies, Sierra Nevada Corp. and Blue Origin, a space travel start-up backed by Amazon founder Jeff Bezos -- with technology development contracts worth $269-million.
NASA hopes the new vehicles will be ready to fly in about 2015. Russia charges the United States more than $50-million per person for Soyuz capsule transportation and training.
The end of the program will hit central Florida, Houston and other shuttle operational hubs hard, with thousands of engineers and technicians due to lose their jobs shortly after Atlantis lands.
"I get kind of freaked out and have this sinking feeling in my stomach that lasts five or 10 seconds," said lead shuttle flight director Kwatsi Alibaruho, who will complete his last shuttle shift on Tuesday. "Then I go back to doing an impersonation of a steely-eyed missile man."