NASA Says It's Ready to Expand Space Station

Hill said engineers have known since they began laying out the station's architecture in 1994 that the next set of flights -- no matter when they occurred -- would be the most complicated and challenging of the assembly missions.

Construction was interrupted by the Columbia tragedy in February 2003, and the shuttle has flown only twice since then. Both missions, including the present one, were dedicated principally to resupplying the space station and testing shuttle safety measures.

As Discovery prepared for landing, it was clear that the key event in the mission was a tedious but critical spacewalk last week to repair the railed tram system that carries the space station's crane along the front of its forward truss.

"Without it, we're hamstrung," lead station Flight Director Rick LaBrode said late last week. "We can't continue assembly."

The next six assembly flights must unfurl in a single seamless pattern, like following the schematics for a life-size Lego: "If we interrupt the sequence, there will be downstream effects," said astronaut Michael Lopez-Alegria, commander of the three-member crew scheduled to take over the space station in September.

This danger has become more apparent since NASA Administrator Michael D. Griffin trimmed the number of remaining shuttle missions to 17 to retire the orbiters by 2010. Fifteen are devoted to space station construction.

This means most station resupply will be handled by small Russian and, eventually, somewhat larger European and, perhaps, Japanese cargo vehicles. Shuttle flights will carry major components and large spares, because it will not be possible after 2010 to bring large pieces of malfunctioning equipment back to Earth. That "changed our whole philosophy on spares," said Brad Cothran, deputy director of space station vehicle integration and operations for Boeing Corp., lead contractor for the station.

Instead of bringing home any malfunctioning machinery, such as the gyroscopes that keep the station pointed correctly in space, the shuttle must carry a replacement to store on the station before 2010. "We call it 'build and burn,' " Cothran said. The spare is installed when needed, and the old piece is tossed overboard to burn up in Earth's atmosphere.

For all of these reasons, every flight is precious. Atlantis, scheduled to launch at the end of August, will begin the crucial sequence, extending the station's port side truss by installing two new sections with one array of four solar panels.

The station is currently in a roughly T-shaped configuration, with the crew living in modules that make up the stem. The forward truss crosses the T. The current four-panel array hovers above like dragonfly wings. By the end of the six flights, the crosspiece will have doubled in length and each side will have two arrays of four panels each.

After Atlantis, the ensuing missions will build out the starboard side of the truss, installing two sets of new arrays there, then moving the current set of arrays from the centerline of the station to the end of the port truss.

Hill described this last operation, employing spacewalkers, cranes and the mobile tram, as "the single most complicated and critical orbital operation."

But "the one that keeps us awake at nights," Hill added, is Discovery's next flight, coming up in December, in which spacewalkers will reconfigure the station's electrical and air-conditioning system.

Like last week's tram repair, this spacewalk will look pedestrian, but it requires disconnecting and reconnecting hoses normally charged with ammonia refrigerant and the section-by-section shutdown and re-powering of the station's electronic control system.

"We have known for years that this was going to be the tough one," Hill said. "We have spent years going over and over it. Just imagine the choreography."