By Guy Gugliotta
Washington Post Staff Writer
Sunday, July 16, 2006; A07
HOUSTON, July 15 -- After a three-year delay, NASA engineers say they are finally ready to begin the most ambitious construction project ever attempted in space -- one that will double the size and weight of the international space station over the next 18 months.
If all goes as well as the space shuttle's current flight, at the end of 2007 the station will have redesigned air-conditioning and electrical systems, a new docking port for the shuttle and four sets of solar arrays instead of one, giving the station a rotating span an acre in size to draw power from the sun.
"You can see it now, when it goes over at night," said Paul Hill, the mission's lead flight director. "But with the new arrays, the station will become the first man-made object in orbit that you can see with the naked eye during daylight."
But first, Mission Control must assure that space shuttle Discovery makes it home Monday without mishap. After 11 days in space, the orbiter appeared Saturday to have enjoyed a near flawless journey.
Discovery, driven by Pilot Mark Kelly and carrying shuttle Commander Steven W. Lindsey and four other astronauts, undocked from the space station at 6:08 a.m. Eastern time, backed off 350 feet, then fired its thrusters to climb above and away.
"Have a safe journey back," radioed space station Flight Engineer Jeffrey Williams. "Soft landings, and we'll see you in a few months."
The shuttle crew spent Saturday stowing equipment and gear, and using infrared cameras to check the front edges of the wings and nose cap for micrometeorite damage. It was expected to be cleared Sunday for landing.
Mission managers will also decide Sunday what to do about a slow leak in the fuel tank feeding one of three auxiliary turbines powering the shuttle's hydraulic system. John Shannon, chairman of the mission management team, said the leak poses no danger, but that engineers might decide to empty the tank before landing just to be safe.
If that happens, Lindsey will use explosive triggers to lower the landing gear. Shannon said this is a standard backup procedure but that they have never been used before.
The spacecraft is scheduled to touch down at Florida's Kennedy Space Center at 9:14 a.m. Eastern time, but if the weather is bad, it will stay in orbit and try again.
In fact, shuttle planners have indicated that Discovery is prepared to stay in orbit until Wednesday waiting for Kennedy to clear, rather than divert to backup landing sites at Edwards Air Force Base, in California, or the White Sands Missile Range, in New Mexico.
A landing away from Kennedy would mean that Discovery would have to be ferried home to Florida on the back of a modified Boeing 747, an expensive and time-consuming process that would strain the preparation schedule for coming missions, an important consideration with NASA planning at least six more flights by the three orbiters by the end of 2007.
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."