CAPE CANAVERAL, Fla. Three are flying, and three were left behind. But thats just the unusual nature of the business when it comes to building and maintaining the International Space Station.

Shuttle Atlantis originally was supposed to launch on a mission to repair an aging space tug at the station and outfit the so-called Russian service module dubbed "Zvezda," or "Star" -- a crucial command and control center that will double as crew living quarters.

But when the latter suffered yet another in a long string of delays, NASA split the intended mission in half, deferring all work to equip the service module until another Atlantis flight now scheduled to take off in August.

Russia's Zvezda service module under construction

In doing so, the space agency also made a relatively unusual move: Three of Atlantis original crew members were bumped to the service module outfitting flight, splitting up a team that had been training together for more than a year.

And for the astronauts and cosmonauts involved, it was a difficult turn of events.

"Having trained together for so long, we do feel like family," shuttle skipper Jim Halsell said in an interview with SPACE.com. "And to be told that you trained together, and youre an outstanding crew, but were going to have to split your family in two that emotionally was a difficult transition at first."

Still on board from the original crew will be Halsell, pilot Scott Horowitz and mission specialist Mary Ellen Weber.

and being tested with its thermal protection system.

Grounded until the August flight: U.S. astronaut Edward Lu and two Russian cosmonauts Yuri Malenchenko and Boris Morukov.

Replacing that trio on just two-months notice: Russian cosmonaut Yuri Usachev and two U.S. astronauts Susan Helms and Jim Voss.

The reasoning behind NASAs version of musical chairs is this: Lu, Malenchenko and Morukov trained to get the service module up and running, but its launch is being delayed until mid-July.

So it made sense, Halsell said, to shift them to the crew that will be doing that work on the follow-up Atlantis mission, now scheduled for launch August 19.

At the same time, Usachev, Helms and Voss were deemed perfect stand-ins. All veteran space fliers, the three have been training together since 1997 and will serve as the second resident crew of the international outpost in early 2001.

Consequently, each is considered expert in the type of maintenance work that will be carried out on the upcoming space station repair flight.

As it turns out, Lu, Malenchenko and Morukov still will have a full plate of work on the August shuttle mission.

Flying along with shuttle commander Terrence Wilcutt, pilot Scott Altman and two mission specialists Daniel Burbank and Richard Mastracchio the three men and their new crew will be responsible for:

• Performing a spacewalk to electrically connect the Zvezda service module to the rest of the station, which now is made up of the Russian space tug and a six-port U.S. docking module -- called Unity -- that will serve as a passageway to other parts of the outpost.
• Setting up critical electrical systems that will power the service module, which is an upgraded version of the core lab of Russias space station Mir.
• Activating oxygen generation and carbon dioxide removal systems that will be crucial to sustaining life in the airless vacuum of space
• Unpacking a Russian space freighter and then stowing medical and exercise equipment, as well as food, water and other supplies for the first full-time station crew which is scheduled to take up residency on the outpost in early November.

Despite slight bouts of separation anxiety, the remaining members of Atlantis original crew say the split-up ultimately was the right thing to do. And Halsell is quick to add that their feelings on the matter "in no way reflect badly" upon Usachev, Helms and Voss.

"We know all of them. Their expertise level is extremely high. And theyll be fun people to go fly in space with," Halsell said.

Thats not to say, however, that Lu, Malenchenko and Morukov wont be missed by their former crew mates.

"We had trained with them for over a year and were extremely, extremely disappointed that we wont be able to have them go on this flight," said Weber.

"But they have a great mission coming up. And I think all of us are just so excited to think that were going to get to go up to the space station and be part of this whole new era."