Workers check the crane that will raise Atlantis to a vertical position in the Vehicle Assembly Building. Photo: NASA/KSC

Space shuttle Atlantis was attached to its external fuel tank and solid rocket boosters over the weekend in preparation for its upcoming mission to make repairs and replace ailing components aboard the 16-month-old International Space Station.

On Friday, the winged-spaceship was hoisted over 250 feet high in the cavernous Vehicle Assembly Building and then gently lowered into position alongside its rust-colored fuel tank. Crews then securely bolted the Atlantis to the tank and made electrical connections.

Work also began Saturday to remove the shuttle's No.1 main engine located directly beneath the orbiter's vertical stabilizer. NASA managers ordered replacement of the engine because of concerns about seals in the high-pressure fuel turbopump.

The suspect engine will be removed today and replaced with a new engine on Wednesday. Atlantis is expected to be hauled to the Kennedy Space Center's launch pad 39A on Friday.

The shuttle and its crew of six Americans and one Russian are currently expected to lift off on April 17, but orbital mechanics may force NASA to delay the launch a day to avoid a lengthy four-day rendezvous with the space station.

Inside the Vehicle Assembly Building, orbiter Atlantis is lifted toward a vertical position in the transfer aisle. Photo: NASA

The shuttle's maintenance mission was ordered to extend the life of Russian-built Zarya module until the end of 2000 while NASA waits for next station segment to be launched -- the Russian Zvezda service module sometime this summer.

Zarya -- the first station element launched -- must continue to work and keep the station boosted to a safe altitude until Zvezda arrives. Once attached, Zvezda's engines will be used to raise the station's orbit.

Veteran shuttle commander Jim Halsell will lead the STS-101 mission, also known as ISS flight 2A.2a. The other crewmembers include pilot Scott Horowitz and mission specialists Mary Ellen Weber, Jeff Williams, Jim Voss, Susan Helms and Russian cosmonaut Yuri Usachev.

Voss, Helms and Usachev will later fly again as the second long-duration residents aboard ISS.