Cape Canavarel, Fla. ? Six astronauts embark on a home improvement project this month.

But there’s no chance of a last-minute trip to Home Depot because 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.”

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 2003 Columbia accident, which killed seven astronauts. The two space missions since then have been considered test missions by NASA, checking out safety improvements on the spacecraft.

With last month’s highly successful flight – shuttle Discovery made it smoothly through launch and landing – NASA has a laser focus on space station completion.

Atlantis Commander Brent Jett said 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 would set the tone for the next four years of construction because each mission to the station builds off the next.

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 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.