CAPE CANAVERAL, Fla. (AP) -- The six crew members of space shuttle Atlantis strapped into the spacecraft and ran through a practice countdown Friday in preparation for the program's first launch of the year next month. "It went seamlessly," NASA spokesman Bill Johnson said.

The countdown clock stopped with four seconds left. Almost 20 minutes later, the astronauts exited the space shuttle in pairs and ran across the launch pad structure to slidewire baskets in an emergency exit drill. In a genuine emergency, the astronauts would get into the baskets and slide from the launch pad down to the ground.

The pre-dawn launch for the mission to the international space station is set for March 15.

Atlantis' astronauts will deliver a 35,000-pound addition, the heaviest ever, along with a new pair of solar arrays. Crew members will unfurl the solar arrays, fold up an old pair and conduct at least three spacewalks during their 11 days in space.