BBC's James Wilkinson describes how the two parts of the station will be joined

Zooming through space at eight kilometres a second (five miles per second), crew member Nancy Currie used the Shuttle's robotic arm to raise the Unity module from the Shuttle payload bay with just 2.5cms (1in) clearance on either side.

Unity is now locked into an upright position in a docking ring at one end of the bay. Mission control in Houston praised the crew for their "beautiful and delicate work".

Docking port

Unity (right) and Zarya (left) will form the basis of the space station

Unity will serve as the primary docking port for future Shuttle missions during construction of the space station and as a connecting passageway within the complex once more modules arrive over the next few years.

Zarya, launched last month from the Baikonur Cosmodrome in Kazakhstan contains the intial power and propulsion systems for the space station.

Watch Nasa's simulation of the Endeavour mission

Together they will form the embryonic base of the International Space Station due for completion in 2004 at an estimated cost of about $40bn. Sixteen nations are involved in the project.

All systems go

Astronaut Nancy Currie will control the link up

On Sunday Ms Currie will use the 15-metre (50ft) robotic arm to grab the orbiting Zarya - meaning 'sunrise' - module, and bring it into alignment with Unity and the Shuttle. This is expected at about 1850 EST (2350 GMT) on Sunday.

The crew spent all day Saturday testing the equipment for Sunday's operation and told mission control they were "all set" to go ahead.

BBC's Mike Crook: The station will require at least 40 more launches

The two modules are so large that Ms Currie will not have a direct line of sight to where they meet and she will have to rely on a computerised vision system to line them up.

"It's not so much a case of how difficult it is, just how careful we have to be and how time-consuming it is," she told reporters before the flight.

Handmade in space

Astronaut Sergei Krikalyov on board Endeavour: Everything going to plan

The final electrical connections between Unity and Zarya will be made by astronauts during a series of three space walks as they can only be done by human hand.

Astronauts Jerry Ross and Jim Newman, who will conduct the spacewalks - or extravehicular activities (EVAs) in Nasa-speak - have been testing the equipment that will enable them to propel themselves independently if they are cut off from the shuttle.

"Everything has been checked and works very well," Commander Robert Cabana said during a press conference.