And in doing so, Usachev and his crewmates -- American flight engineers Susan Helms and Jim Voss -- officially closed out a tour that included both key construction work and the start-up of scientific research aboard the outpost."We tried to build the station -- to make it better and bigger," said Usachev. "Now it's time to pass the station from our hands to your hands, from our minds to your minds, and from our hearts to your hearts."
"We will pick up where you left off, and try to continue to improve on (the station), and to make this a place that the entire world can be proud of," Culbertson replied. "It's an honor for us -- a true burden of responsibility that we accept with both gratitude and with a serious approach. And we will do our best to continue the high standards that you have set."
Launched last March, Usachev and the so-called Expedition Two crew oversaw the delivery of the station's $600 million Canadian robot arm and a $164 million airlock that will enable resident crews to perform spacewalking work with no visiting shuttle present.
Both the 57-foot (17.5-meter) construction crane and the Quest airlock are considered crucial to future station construction, and the components provide an unprecedented level of self-sufficiency and independence for outpost crews.
Amid all the assembly work, the trio also carried out a suite of 18 experiments in the station's $1.4 billion U.S. Destiny laboratory, kicking off a science program that is expected to continue over the next 15 years. And they helped ready the outpost for the launch next month of its next major component, a Russian docking compartment.
"Our crew has been incredibly fortunate to be part of these major activities," Helms said. "But now I'm afraid it's time to pass the reins."
To that end, the 10 astronauts and cosmonauts aboard visiting shuttle Discovery and the station spent the better part of the day packing up an Italian moving van now attached the U.S. Unity module.
Some 2,300 pounds (1,035 kilograms) of luggage, surplus station gear and garbage is to be stuffed into the so-called Leonardo module before it is stowed back in Discovery's cargo bay for a return trip to Earth.
More than three tons of food, clothing and supplies were unloaded from the shuttle-borne shipping container earlier this week for Culberston and his crewmates, who took operational command of the station Monday.
A traditional ritual, the change-of-command ceremony simply formalized the handover.
With the re-packing work within Leonardo progressing smoothly, mission managers gave astronauts Daniel Barry and Patrick Forrester a go-ahead for the second and final spacewalk planned during Discovery's stay at the station.
Set to begin at 10:15 a.m. EDT (1415 GMT) Saturday, the six-hour excursion primarily will involve rigging up power cables on the outside of the Destiny lab.
The two astronauts mounted spare coolant tanks and two suitcase-sized materials science experiments during an initial sortie Wednesday.
Culbertson and his crew will watch the spacewalking work from inside the 17-story station, the habitable part of which now consists of four linked modules that provide as much living space as a standard three-bedroom house.
The sheer size of the station -- which now stretches 171 feet (52 meters) from end to end -- was an eye-opener for Culbertson, a veteran of two previous flights on relatively cramped shuttles.
"My first impression was, 'Wow, it's big,'" Culbertson told reporters in a joint crew news conference. And then he quickly noted that the outpost could use a little more closet space.
"As we go through it, you can see it's packed with experiments, hardware, replacement parts, supplies, clothing, etc. So it's actually pretty full, and stowage is still something we have to work on very hard."
Outgoing station flight engineer Voss agreed.
"We don't have a lot of stowage space onboard, and we have a lot more things on the station that we thought we would have originally," he said.
His suggested improvement: Permanently parking two $150 million Italian moving vans at the station, one to warehouse gear needed inside the station and the other to store spacesuits and other equipment required for work outside the outpost.
"I think that would be the biggest change that I would make to the station," Voss said. "Everything else has been running just great."
Voss, Usachev and Helms will leave the station aboard Discovery at 10:54 a.m. EDT (1454 GMT) Monday, freeing up some space aboard the outpost for Culbertson and his crew.
"As much as I like these folks here, I'm going to think, 'finally the station is ours, and we can operate freely without bumping into a lot of other people,'" Culbertson joked.
And while a Russian Soyuz taxi crew will make a six-day visit in mid-October, the hatchway to the shuttle's docking port will remain shut until their ride home arrives in early December.
"It's going to be an interesting feeling to have the door close because it won't open again until (shuttle Endeavour) comes up to pick us up, and so it's going to be the start of an interesting time in our lives," Culbertson said.
With the Expedition Two crew in tow, Discovery is scheduled to land at Kennedy Space Center at 12:48 p.m. EDT (1648 GMT) Wednesday.
Culbertson and his crew will remain in space until Dec. 9.
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