It is to be attached to the station during four spacewalks by the seven-member crew.
The seven-member crew aboard Discovery was on a 13-day mission carrying supplies and science equipment for the space station.
That was what caused the loss of the shuttle Columbia and its seven-member crew in February 2003.
The seven-member crew included a Japanese astronaut, who was the first Japanese woman in space.
During the 15-day flight the seven-member crew conducted more than 80 experiments focusing on materials and life sciences research in microgravity.
Its seven-member crew died in the accident, the second loss of a shuttle in 113 flights.
The seven-member crew returned from the 10-day supply mission on June 6.
With any break in the weather, the seven-member crew will ride Atlantis into an 184-mile-high orbit.
The seven-member crew was killed some time after the breakup of the vehicle.
The seven-member crew of Columbia was asleep when the announcement came that its mission would continue.