With precise grace, an overhead crane swings a 10-ton building block into position. Then, workers move in, climbing on to the structure and using hand and power tools to bolt the pieces together. It is a workaday scene that could be found on almost any city street corner, but this construction site is 250 miles up – in the airless reaches of space, where conditions alternate hourly between freezing and searing. The construction workers are astronauts, the cranes are a new generation of space robotics and the skyscraper taking shape is the International Space Station.

To assemble the 1-million pound International Space Station, Earth orbit will become a day-to-day construction site for five years beginning in 1998. Humankind will begin a move off of the planet Earth of unprecedented scale. Astronauts will perform more spacewalks in those years than have been conducted since space flight began, more than twice as many. They will be assisted by an "inch-worming" robotic arm; a two-fingered "Canada hand." Before the station's assembly is completed, more than 100 different components launched on about 45 space flights – using three different types of rockets – will have been bolted, latched, wired, plumbed and fastened together.

Because of the unprecedented complexity, NASA expects to encounter surprises during the orbital construction work. But to prepare for the challenges, engineers and astronauts have been methodically practicing procedures, preparing tools, testing equipment and building experience during more than a decade of spacewalking flight tests. A total of 34 Space Shuttle missions are scheduled to assemble, outfit and begin research use of the station from 1998 to 2004. Approximately 960 clock hours -- 160 total U.S. and Russian spacewalks -- will be performed during that time to assemble and maintain the station, equivalent to 1,920 crew hours of work since all spacewalks include two crew members. Since astronaut Ed White stepped out of an orbiting U.S. Gemini spacecraft in 1964 to become the first American to walk in space, NASA has conducted about 377 clock hours of spacewalks. Of the 960 total clock hours of spacewalks for assembly and maintenance of the station, 336 hours, 56 spacewalks, will be performed using Russian spacewalking equipment for Russian segment work and 624 hours, 104 spacewalks, will be conducted for U.S. and other partners' assembly activities using U.S. equipment.

A cooperative effort among 16 nations, the International Space Station will provide living quarters and science labs for long-term stays by up to seven astronauts. In building, operating, and performing research on the station, humanity will garner essential experience for future travels beyond Earth orbit.