Six astronauts embark on a home improvement project this month. But there's no chance of a last-minute trip to Home Depot, since they'll be 220 miles from Earth.

The crew of the shuttle Atlantis, set to launch Aug. 27, will be delivering a 35,000-pound addition to the half-built international space station. The astronauts will get a little help from robotic arms for the heavy lifting, but gripping screwdrivers and bolts in bulky pressurized spacesuits isn't easy.

"This has been described ... as one of the most difficult tasks ever attempted by humans and I'm here to tell you that it seems like it's going to be that hard," said Mike Suffredini, NASA station program manager. "This has never been done before, the creation of a spacecraft in space."

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The 17 1/2-ton addition, costing $372 million, will be one of the heaviest payloads ever flown to space. The project is the beginning of an effort to finish work on the space station before the cargo-carrying shuttles are retired in 2010.

Construction has been delayed since the Columbia accident in 2003, which killed seven astronauts. The two space missions since that time have been considered test missions by NASA, checking out new safety improvements on the spacecraft.

With last month's highly successful flight — shuttle Discovery got smoothly through launch and landing — NASA has a laser focus on space station completion.

Atlantis Commander Brent Jett says more is at stake in finishing the international space lab than just building a place for science experiments.

"It's preparing us as an agency to take the next step back to the moon for a permanent outpost or onto Mars," said Jett, who will be making his fourth space trip.

Jett said his crew will set the tone for the next four years of construction since each mission to the station builds off the next.

The other crew members are pilot Chris Ferguson and mission specialists Joe Tanner, Dan Burbank, Heidemarie Stefanyshyn-Piper and Steve MacLean of the Canadian Space Agency.

"Like a sports team, an athletic event, you want to get off to a good start," Jett said. "If we have major problems ... that can't be resolved, it's going to change those missions significantly."

Jett and Tanner know what it's like to run into problems.

They were members of a crew in 2000 with similar construction tasks: bringing an addition to the space station and unfurling from it two wings of solar panels, each the length of about a third of a football field.

During the 2000 mission, the solar panels unexpectedly stuck to each other while they were being deployed, but the problem was fixed during the mission.

This time around, NASA has devised a new method to keep the wings from sticking together. The solar panels delivered on Atlantis eventually will generate about a quarter of the space station's power when the structure is finished.

The Atlantis crew has one of the most jam-packed schedules ever devised for a shuttle mission. They not only have to perform three complicated, highly choreographed spacewalks to install the addition during their 11 days in space, but they also must complete inspection tasks that were implemented after the Columbia disaster to look for any damage to the shuttle's thermal skin.

After docking with the space station, the 45-foot-long addition will be lifted by robotic arm from the shuttle's payload bay and handed off to the space station's robotic arm.

The next day, Tanner and Piper will go out on the first spacewalk, followed a day later by a second spacewalk by Burbank and MacLean.

The next day, the solar wings will be opened, and the following day, Tanner and Piper will go on a final spacewalk.

The opportunities for liftoff of Atlantis are from Aug. 27 through Sept. 13 — dates based on calculations necessary to get the shuttle to the space station in a set time period. But NASA hopes to get the shuttle up before Sept. 7 so the mission doesn't interfere with a trip to the space station by a Russian Soyuz vehicle in mid-September.

The Atlantis astronauts have trained together 4 1/2 years, making them the longest-trained crew ever. Their mission originally was set for mid-2003 but was delayed because of the Columbia disaster.

"We've managed to hang together," Ferguson said. "I think we're going to do it, barring a hurricane."