HUNTSVILLE, Alabama -- It's one small step off a bus for men, women and children and one giant leap for space history in Huntsville. Forty-three years after men stepped foot on the moon on July 20, 1969, visitors to the U.S. Space & Rocket Center will be able to walk on the ground at the Marshall Space Flight Center beginning Friday.

Bus tours of Marshall, where Wernher von Braun built the rockets that carried man into space and to the moon, were routine until the terrorist attacks of Sept. 11, 2001. That's when commanders at Redstone Arsenal, where Marshall is located, stopped tours to secure the base.

Marshall and the space center museum have wanted to bring the tours back ever since. Marshall is home to several National Historic Landmarksand a key NASA center of today.

The most historic site - and a tour stop - is the Redstone test stand, a primitive structure and underground bunker where von Braun turned the German V-2 rocket into an American spaceship. The tour will also pass the giant Dynamic Test Stand where the Saturn V was shaken to test its strength.

The modern stop is NASA's International Space Station Payload Operations Center, which manages the science experiments aboard the space station.

Passengers will leave the bus here and walk into a viewing room where they can see Marshall technicians in the Ops Center monitoring state-of-the-art computers and talking to the astronauts. Giant wall screens show the station's interior at all times.

"We want to showcase the work that Marshall is doing," Deborah Barnhart,