The flight of Atlantis is the 100th human mission in the American space program, which began in May 1961 with the 15-minute, up-and-down flight of Alan B. Shepard. It is also the 69th flight by the nation's winged reusable spaceships, which first went aloft in 1981.

An orbital practice session for the docking was held in February when the shuttle Discovery closed to within 37 feet of Mir -- just short of actual contact but close enough for astronauts and ground managers to gain valuable experience in the difficult job of trying to mate two 100-ton craft speeding around the earth at more than 17,000 miles an hour.

Thursday's docking is scheduled about 9:05 A.M. and is considered tricky. Care is needed to make sure that no damage to Mir is done by inadvertent collisions.

The rigors of the job and the high political stakes of the mission prompted NASA to pick highly seasoned astronauts, led by Capt. Robert L. (Hoot) Gibson of the Navy, 48, a veteran of four previous space flights and the head of the astronaut office at the Johnson Space Center in Houston until he was selected to head the mission.

"It is almost unheard of to have a crew that includes all veteran astronauts," Captain Gibson said before the liftoff. Usually NASA takes at least one rookie on each shuttle mission to make sure the pool of experienced astronauts is constantly expanding.

The pilot for this flight is Lieut. Col. Charles J. Precourt, 39, of the Air Force, who, with only one previous shuttle mission, is the least-experienced crew member. Specialists include Dr. Ellen S. Baker, 42, a physician and veteran of two flights; Gregory J. Harbaugh, 39, a former space-agency manger who has flown twice, and Dr. Bonnie Dunbar, 46, a biomedical engineer and veteran of three shuttle missions.

Also on board Atlantis are Anatoly Y. Solovyev, 47, who has orbited the earth on three Russian flights, and Nikolai M. Budarin, 42, a space rookie. The two men will occupy the Mir station after the current crew leaves, with Mr. Solovyev as commander and Mr. Budarin as flight engineer.