As astronauts Jerry Ross and Jim Newman ride the space shuttle Endeavour to the world's first free-fall construction site, they'll no doubt be hoping that all those hours of practice in the pool at Johnson Space Center will pay off. They're the stars of the first official space station assembly mission. Tucked in the shuttle's cargo hold lies station Part No. 2, Unity. Their mission: to connect it to part No. 1, Zarya.

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Unity soars into space with no shortage of connections. The six ports in its 14-foot diameter cylinder will eventually provide doorways to a U.S. laboratory, an airlock, a cupola, the Zarya service module, the Leonardo reusable cargo module, and a docking point for the exterior frame.

The section is equipped with two pressurized mating adapters -- cone-shaped tunnels -- built by Boeing. One pressurized mating adapter attaches to Zarya, which has been waiting in orbit since its launch from Russia on November 20. The second adapter locks onto the space shuttle so that astronauts can scoot into the space station without having to change clothes.

Even though Unity looks to be a simple hallway with a bunch of doors, it was difficult to construct. Two hundred people from Boeing and its subcontractor team put the module together in three years. On a rotating assembly in a clean room in Huntsville, Ala., they installed 216 lines to carry fluids and gases, six miles of wire to connect 121 internal and external electrical cables, and 50,000 mechanical items. Because Unity contains hundreds of environmental control and life support system ducts that have complex shapes, engineers and technicians relied on a 3-D computer model to guide them. The assembly was so exacting that during some welding operations, a team of inspectors reviewed the work every hour.

Constructing in space

Two days into the mission, astronauts Nancy Currie, Bob Cabana and Rick Sturckow perform some very tricky maneuvers in hooking the shuttle to Zarya. First, they open Endeavour's cargo hold. Then Currie uses the shuttle's robot arm to pluck the 18-foot-long Unity, which weighs considerably less in space than its 25,000 pounds on Earth, from the aft cargo bay and lock it tight to the docking station in Endeavour's forward cargo bay. That's the easy part.

Cabana steers the shuttle to approach Zarya from above. At this point, he can't see his target. He must use TV monitors to guide Endeavour to the point where Zarya is directly behind and slightly below the top of Unity. There will be no room for a Mir-type collision here. He gingerly edges Endeavour's cargo bay just 10 feet from Zarya. At that moment, Currie grabs the module with the shuttle's robot arm, positioning it so that the shuttle's engines can gently push the pieces together.

The next day, Astronauts Jerry Ross and Jim Newman gear up in their $10 million space suits for the first of three six-hour spacewalks. And these spacewalks aren't cakewalks. During the first one, astronauts set up the work site and put out a safety wire. Then they begin attaching eight umbilicals between the pressurized mating adapter connecting Unity and Zarya, and do some electrical work. They've repeated the maneuvers over and over in the Johnson Space Center pool, where astronauts practice putting every piece of the station together. It's still slow going -- things don't weigh much in space, but moving and attaching them is awkward, especially since the astronauts are stuck inside those cumbersome space overalls. And doing something as simple as screwing a bolt in microgravity requires that they brace themselves. It's as frustrating as pushing a plumber's snake.

Before the second spacewalk, the shuttle crew pressurizes the docking system that leads from the shuttle cargo bay to Unity, looks for leaks, and checks out the communications system. Houston mission control makes sure they can control Zarya through the Russian mission control center in Moscow. Zarya pressurizes the mating adapter between it and Unity. Moscow turns on a contaminant filter in Zarya. Houston turns on Unity's cabin fan.

During the second spacewalk, Ross and Newman do more work site preparation, including installing handrails. They remove launch restraints from the hatches of the common berthing mechanisms -- the amazing connectors that hold all space station parts together. They also install communications antennas and a sunshade. If they get hungry or thirsty while they're out, they've got a rice-paper-wrapped fruit bar and a 32-ounce container of water. If they have to go to the bathroom, they do so in a diaper.

Before the third, and final, spacewalk, the shuttle crew becomes the first human beings on the space station. They'll spend two days installing some portable fans and lights, plus some parts of the communications system.

The third space walk is devoted to tasks that will help future construction workers, such as stowing a tool box, installing a handrail on top of Zarya and attaching other equipment to the station's outer hull. Three day's later, they head back to Mother Earth. Two down -- 43 missions to go.