By Marc Kaufman
Washington Post Staff Writer
Monday, September 4, 2006; A03

The three-day countdown for the shuttle Atlantis began yesterday as NASA continues its long-delayed efforts to launch the spacecraft.

After three years of waiting in the aftermath of the shuttle Columbia disaster, and then another delay last week because of storms and a lightning strike, Atlantis is being prepared to take off at 12:29 p.m. on Wednesday. It will carry a payload essential to the stalled construction of the international space station -- a massive set of trusses and solar panels that will provide energy to power the space laboratories that will follow.

NASA spokesman Bruce Buckingham said that the crew is working around the clock to get the vehicle ready -- a process that usually takes eight days once the vehicle arrives at the launchpad, but this time will be done in six. "There no wiggle room here at all," he said.

Driving the speed-up is the fact that NASA has a short window to launch Atlantis -- at best a three-day opportunity if all preparations are finished by Wednesday. NASA officials say the timetable is feasible because some complicated and time-consuming work was done when the shuttle was brought out to the launchpad in mid-August.

"If we can't get Atlantis off during this window, we'll have to be looking earliest to an October launch," Buckingham said.

The fuel and power systems connecting Atlantis to the launchpad were severed last Tuesday when NASA officials concluded that the coming Tropical Storm Ernesto posed a threat. The 12-million-pound launcher and orbiter were loaded onto a "crawler" to haul it back to safety, but the rollback was rolled back five hours later after a more favorable weather forecast. Atlantis was back on the pad by Tuesday night, but work on reconnecting the systems couldn't begin until the storm passed on Thursday.

The shuttle will be carrying a payload that includes a 17-ton truss and solar energy system, needed before astronauts can prepare to do many of the zero-gravity experiments the station was designed to make possible. The solar power grid that the shuttle will deliver must be in place and operating before two other large Japanese and European space labs can be ferried up, attached and put to work.

Russian spacecraft regularly go to the station, but only U.S. shuttles can deliver payloads as large as the trusses, and they have been either grounded or on safety-monitoring missions since the February 2003 Columbia explosion.

The launchpad preparations and countdown itself are so time-consuming because of all the fuels that have to be supplied, the electrical and communications systems that must be checked, and the planned holds to give time to assess computer data coming in.

An early step is the connecting of "umbilicals" to pump in liquid oxygen and liquid hydrogen for the 500,000-gallon external fuel tank for liftoff. A similar fuel line is set up into the orbiter -- where the hydrogen and oxygen are used in conjunction with fuel cells to make electricity for the vehicle and to produce water for the crew.

The orbiter also needs to be loaded with the fuel mixture of monomethylhydrazine and nitrogen tetroxide oxidizer, another hazardous combination used on the thrusters to change the shuttle's direction once it is in orbit.

In addition, the ground crew has to add explosives to the eight bolts that attach the shuttle to the booster rockets that will send it into space. Those bolts have to be quickly broken once the boosters have done their job after liftoff, and that is best done by setting off small explosions.

Although the fuels and explosives are not delivered and pressurized until the last hours before liftoff, the process of making sure they will be safely supplied and stored is time-consuming. The crew, commanded by Navy Capt. Brent W. Jett Jr., joins the shuttle only two hours before liftoff. The six-member crew has been together for almost seven years and includes one Canadian astronaut, four Navy officers (one retired) and a Coast Guard officer. The lone woman -- Navy Capt. Heidemarie M. Stefanyshyn-Piper -- is making her first flight into space.

The 11- or 12-day Atlantis mission will include three spacewalks, during which astronauts will attach the new solar energy system to the space station. Stefanyshyn-Piper will participate in two of the 6 1/2 -hour spacewalks.

When the shuttle docks three days into the mission with the space station, the crew will be greeted by three astronauts -- Russian station commander Pavel Vinogradov, American flight engineer Jeff Williams and European Space Agency astronaut Thomas Reiter. The space station, which circles 230 miles above Earth, was designed to house six, but it has support for only three because of its limited electric power.

Just before docking, the shuttle will do a back-flip 600 feet below the station that will allow station crew members to take high-resolution photos of the shuttle's underside heat shield -- the section of the shuttle that has caused the most trouble in the past.

Engineers inspecting Atlantis after Ernesto passed by found three areas of minor foam damage on the shuttle's external fuel tank -- a small cut in the liquid oxygen tank, an inch-long scratch where the tank connects to the solid rocket boosters, and some missing foam from the liquid hydrogen section. NASA officials said that the two scratches could be easily repaired and that the missing foam did not need to be replaced.

Foam insulation on the external tank is a concern for NASA engineers because of the role it played in the Columbia disaster. Foam that fell off the external tank during liftoff hit the craft's wing, causing a weakness that allowed gases to penetrate the shuttle on reentry and cause the fatal explosion.