With the spacewalks complete, the combined shuttle-station crew plan to take time off on flight day 10, to complete logistics transfer operations and to transfer spacesuits and other hardware back to the shuttle for return to earth.

"Sandy Magnus is our transfer queen," said Melroy. "She is very detail oriented and very organized. And so, she's going to help us. She's going to be the general on transfer days. She carries all this in her head."

The next day, flight day 11, Atlantis will undock from the international space station, following a now-standard departure profile. With Melroy at the controls, the shuttle will back away to a point about 450 feet directly in front of the station. Melroy then will guide Atlantis through a fly-around of the station for routine photo documentation before departing the area for good.

An illustration of the space station at the completion of Atlantis' mission. NASA

The space station will have an unusually asymmetric appearance as the shuttle pulls away, with the S1 truss sticking 45 out to one side.

The station maintains its orientation in space using control moment gyroscopes in the Z1 truss. Despite the unbalanced load represented by the S1 truss, the CMGs should have no problem compensating.

"As we build up the truss segment, we kind of have a back-and-forth situation where the truss is balanced and unbalanced," Algate said. "It is a bit more difficult to control, but our momentum management system with our control moment gyros can deal with that."

Likewise, the unbalanced load will cause no major impacts to routine procedures in which the shuttle's maneuvering jets are used to reboost the station's altitude. Two such reboost maneuvers are planned during Atlantis' mission, one on flight day five and one on flight day seven.

"On the shuttle side, looking at shuttle reboost, they did find some asymmetry in the jet firings and some of the jets get a little warmer because of the asymmetrical thrusts," Algate said. "As a result of that, in some of the reboost modes we had to cut back the maximum duration that we could do a shuttle reboost. As it turns out, for the reboost modes we're planning to use on this mission it won't have an impact."

The day after undocking, the astronauts will work through a standard pre-entry timeline, stowing equipment, testing the shuttle's entry systems and gearing up for the fiery plunge back to Earth.

"After we've undocked, we have the normal shuttle entry preparation activities, but in addition we have a secondary payload on this flight called SHIMMER," Engelauf said. "I confess, I even have to look up the name of this particular payload. It stands for 'Spatial Heterodyne Imager for Mesospheric Radicals.' It's an Air Force-sponsored payload that requires a little bit of out-the-window pointing for the shuttle to take some spectral images.

This payload is essentially a demonstration of a new remote sensing technique using ultraviolet wavelengths in mapping hydroxyl in density distributions, which would be useful mapping the ozone layer and the chemical hydroxyl, which is largely responsible for the breakdown ozone."

If all goes well, Ashby and Melroy will fire Atlantis' twin orbital maneuvering system braking rockets and glide to an afternoon landing at the Kennedy Space Center on Oct. 13 to close out an 11-day mission spanning 170 orbits and 4.5 million miles.

Atlantis' landing at KSC in April. NASA

The next two shuttle flights are critical to the continued assembly of the space station. But the work is relatively straight forward. Next year, however, "when we start bringing up the outboard solar arrays, that's where things are going to get very, very challenging for us," Gerstenmaier said.

"When we get to the 2003 timeframe, we're not only doing assembly, we're doing reconfiguration," he said. "That's new for us. And by reconfiguration, we're going to pull a radiator off of P6 and it's going to be moved outboard to one of the solar arrays and attached out there. We're going to also fold up the entire P6 solar arrays and then move them out on the mobile transporter and attach them out on the end of the truss.

"Those are major activities in terms of reconfiguration, those have a lot more uncertainty in them because the hardware's been up and operating in orbit, it's not flying in a pristine condition, it's got some runtime on it, we're going to be operating out quick-disconnects and doing those things. So that's a new series of challenges for us that we're really got to stay focused on.

"The other thing that happens in that timeframe is we go from our power system now, which is essentially P6 providing us power, to where we're going to get the permanent power system," Gerstenmaier said. "It's a full two-string power system, full two-string communications system, full two-string thermal system. A lot of those systems right now are single string systems. So we have to shut off our power system and then bring up our new power system, our new thermal system, our new communications system. That also concerns me, because that's a very critical activity."

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