HOUSTON — The last astronauts to fly on a space shuttle have boarded their spaceship for the return trip to Earth and closed its hatch on the International Space Station for the final time.
The four-astronaut crew of the space shuttle Atlantis' STS-135 mission has wrapped up a delivery mission to the station to drop off spare hardware and new supplies to outfit the laboratory for the years ahead.
Now Atlantis' astronauts will prepare their vehicle for one final trip down to Earth before the orbiter and its two sister shuttles are retired. Atlantis is due to land at NASA's Kennedy Space Center in Cape Canaveral, Fla., on Thursday (July 21) at 5:57 a.m. EDT (0957 GMT).
Atlantis' four-astronaut crew, led by commander Chris Ferguson, said goodbye to their six space station counterparts in a farewell ceremony before boarding their ship. The crews closed the hatches separating the vehicles at 10:28 a.m. EDT (1428 GMT), while the spacecraft were flying about 240 miles (386 km) above Earth. [Photos: NASA's Last Shuttle Mission In Pictures]
"I'd like to thank the commander of the International Space Station, AndreyBorisenko, for your hospitality," Ferguson said before departing."You’ve been absolutely fantastic to us. It's been wonderful to be here with you. You have a wonderful home, you're taking fantastic care of it."
"It's been an honor having you guys onboard," space station flight engineer Ron Garan of NASA said. "It's great being a part of this really important and historic mission."
Before floating out of the station, the shuttle astronauts left behind an American flag that they had carried up to the station with them on Atlantis' launch. But the flag's history goes back further; it was also launched on the very first shuttle mission, the STS-1 flight of Columbia in 1981.
"Since we've been here we've prominently displayed the flag on the forward flight deck [of Atlantis]," Ferguson said. "It just symbolized what we are all here for."
The shuttle astronauts will leave it behind to be hung inside the station's Harmony node. But that's not intended to be its final home. When a commercial American spacecraft is ready to replace the shuttle as a ferry to the orbiting outpost, the first crew to ride it will return that flag to Earth. Finally, the astronauts hope the same flag can be carried by U.S. spaceflyers when they finally embark on a mission beyond low-Earth orbit to the moon or Mars.
"This flag represents not just a symbol of our national pride and honor, but in this particular case it also represents a goal," Ferguson said.
The shuttle astronauts will perform final checks and get a good night's sleep inside their orbiter before undocking from the space station early Tuesday (July 19) at 2:28 a.m. EDT (0628 GMT).
Before the final goodbye both crews completed the last of the packing to make sure all the new supplies were unloaded onto the space station and the shuttle was filled to capacity with trash and broken parts to be brought back to Earth. Much of this equipment was loaded into the Raffaello multi-purpose logistics module, a storage container used to transport goods back and forth from orbit. [9 Weird Things NASA Flew on Space Shuttles]
This morning Atlantis mission specialist Sandra Magnus and pilot Doug Hurley used a robotic arm to move the fully packed Raffaello from its temporary perch on the outside of the space station to the shuttle's payload bay.
Savoring the experience
Atlantis is making the 135th flight of NASA's 30-year space shuttle program. The orbiters are being retired to allow NASA to shift its focus to building spaceships to take humans beyond low-Earth orbit, ultimately to Mars.
While most shuttle workers are mainly focused on the job at hand, they are also aware of witnessing the end of an era, NASA officials have said.
"I actually had a dream last night that I was in Mission Control and looking at the downlink video," flight director Chris Edelen said during a Sunday news briefing. "When I woke up, I realized, hey I really do need to savor these moments 'cause this will be the last time we'll see a big winged vehicle like that docked to the station. It will definitely be something to tell your grandchildren about."
Edelen said he reminded his flight controller team to stop and savor the experience.
After the shuttles are grounded, NASA will rely on Russian spacecraft to carry U.S. crews, until commercial American spaceships are ready to transport astronauts.
"Even though we're losing the shuttle, we're looking forward to seeing some new vehicles come up to the station," Edelen said. "It'll be an exciting time."
You can follow SPACE.com Senior Writer Clara Moskowitz on Twitter @ClaraMoskowitz. Visit SPACE.com for complete coverage of Atlantis' final mission STS-135 or follow us @Spacedotcom and on Facebook.