The 12-day flight was added to the shuttle's schedule last year to buy time in case NASA's newly hired cargo delivery companies have problems getting their spacecraft into orbit.
Atlantis will be delivering a year's worth of food, clothing, science equipment and supplies to the orbital outpost, a $100 billion project of 16 nations that circles 220 miles above Earth.
"This flight is incredibly important to the space station. The cargo that is coming up on this flight is really mandatory," said NASA's spaceflight chief Bill Gerstenmaier.
Earlier on Tuesday, the threat of an orbital debris impact interrupted the station's preparations for Atlantis' visit. NASA learned that an unidentified piece of space debris was likely to pass close to the station and told the crew to seek shelter in the station's two Russian Soyuz escape capsules.
Typically, the station maneuvers to avoid potential debris impacts, but the notice came just 14 hours before the closest approach, too late to plan and conduct an avoidance maneuver.
"We think it came within about 335 meters (1,100 feet) of the space station. It was probably the closest object that's actually come by (the) space station," Gerstenmaier said.
NO BACKUP SHUTTLE
The six station crewmembers divided into two groups of three and sealed themselves into the Soyuz capsules about 20 minutes before the object came closest to the station, which occurred at 8:08 a.m. EDT. It was only the second time in the station's history that crews had to seek shelter in their "lifeboats" for an orbital debris threat.
The station's two U.S. crewmembers are preparing for a spacewalk during shuttle's Atlantis' eight-day stay, a job normally undertaken by the visiting astronauts.
NASA, however, has been trying to keep the Atlantis crew's training as simple as possible, as the four shuttle astronauts already are tasked to do the work of the six or seven people normally assigned to shuttle flights.
The U.S. space agency pared down the crew size to accommodate the smaller Russian Soyuz spacecraft that would be used to fly the Atlantis astronauts home in case the shuttle is too damaged to attempt landing.
Since the 2003 Columbia accident, NASA has had a second shuttle on standby for a rescue mission if needed. Atlantis, however, is the 135th and last shuttle to fly, with no backup shuttle in waiting.
The United States is ending the shuttle program to save its $4 billion annual operating costs and use the money to develop spaceships that can travel beyond the station, such as to the moon, asteroids and eventually to Mars.
Space Exploration Technologies and Orbital Sciences Corp. are scheduled to begin cargo deliveries to the station next year. NASA is hoping commercial companies will be able to fly astronauts as well, though those spaceships are not expected to be ready for at least four to five years.
In the meantime, NASA will pay Russia to fly its astronauts to the station at a cost of more than $50 million per person.
(Reporting by Irene Klotz; editing by Anthony Boadle)