Unity and Zarya are one

24 pins lock the two modules together

The first two components of the International Space Station (ISS) have been linked together.

Two cylindrical modules, Zarya and Unity, were shoved together by the space shuttle Endeavour. The procedure took place over the southern Pacific Ocean shortly after 2100 EST (0200 GMT).

The shuttle had chased Zarya through space for two days before grabbing the 19,960-kilo (44,000-pound) object with its robot arm. The shuttle then gently pulled the Russian-built module into a position over the Unity component which was held in Endeavour's cargo bay.

A short burst from Endeavour's thrusters pushed the two units together.

Nancy Currie did not have a direct view of the docking

It was another half an hour, however, before the crew could confirm that the docking had been completed.

Twenty-four pins that hold the two cylinders together at first refused to slot into place. But this problem was soon overcome after several small adjustments were made.

Two of Endeavour's astronauts must now undertake three spacewalks to connect cables. They must also install handrails and other tools for future crews, and check the airtightness of compartments.

Only when their work is complete will Zarya and Unity be released from Endeavour's grip.

Endeavour had to capture Zarya

Mission Control thought the docking work might take hours longer than planned and gave the six astronauts on Endeavour plenty of time for their tasks. In the end, all the procedures were completed relatively easily.

There had been a major concern over the ability of the shuttle's robot arm to handle an object the size of Zarya, which launched from the Baikonur cosmodrome in Kazakhstan on 20 November. But the arm, under the guidance of astronaut Nancy Currie, appeared to have little difficulty grabbing Zarya and moving it into a position where it could be joined to Unity.

Computerised vision system

It was an impressive performance from Currie who could not see the docking directly. She moved the two modules to within centimetres of each other with the aid of a computerised vision system and camera images.

Once she was sure she had Zarya in the right position, Endeavour fired its thrusters and the brief burst raised the shuttle, pushing Unity and Zarya together.

"We have capture of Zarya," Endeavour commander Robert Cabana announced at the moment the two pieces came together. "Congratulations to the crew of the good ship Endeavour," replied Mission Control. "That's terrific."

Spacewalks will complete the connections

The two modules now reach 23 metres (75 feet) out of Endeavour's cargo bay. They have a combined mass of 31,750 kilos (70,000 pounds).

Zarya is a power and propulsion module. Unity will become the primary docking port for future shuttle missions during construction of the multi-billion-dollar space station.

A third, Russian module will be sent up in mid-1999. This will be the crucial mission in the early construction of the space station because the power from the third module will allow the growing structure to maintain a constant orbit.

Nasa estimates that 43 more launches and 159 more spacewalks will be needed after this mission to assemble the entire orbiting complex.

The first crew should arrive in 2000, with the station becoming fully operational in 2004.

Once completed, the 16-nation space station will have a mass of 453,597 kilos (one million pounds) and house up to seven astronauts and cosmonauts.

The station will be used as an orbiting laboratory for scientific inquiry. It may also provide a launching pad for future manned missions to the Moon and even Mars.