Ed Campbell reports on one of the most complicated feats of engineering ever undertaken

In a six-hour spacewalk that ended at 0325 GMT on Thursday the two astronauts also removed hatch covers from the American-made Unity module so that future modules can be attached to the station.

This work set the stage for the astronauts to enter the ISS for the first time, later on Thursday, a Nasa spokesman said.

The astronauts also use a grappling hook to pry open a stuck antenna on Zarya, the Russian-built module of the space station.

It was the second time in a week that astronauts Jerry Ross and James Newman had floated out of Endeavour's hatch to work on the station taking shape in the open cargo bay.

On Monday they hooked up 40 electrical connections between Zarya and Unity.

The astronauts began half an hour ahead of schedule

Three-and-a-half hours into the second spacewalk the astronauts completed their main goal - installing two 100lb (45kg) antennae on Unity.

They struggled to manoeuvre Unity's antennae out of the narrow tunnel leading into the cargo bay. They had to back out of the tunnel feet first with the bulky, swinging antennae in tow.

The pair hoisted the antennae halfway up the module on the end of the shuttle robot arm and mounted the units on opposite sides of the cylinder.

Once activated, the antennae will provide a direct, virtually uninterrupted communication link between Unity and Nasa's Mission Control. Otherwise, US flight controllers would have to rely on the sporadic coverage provided by Russian ground stations.

Astronauts had to remove four hatches from the module

The electronic and computer hookups for the antennae will be made inside Unity on Thursday, after the entire crew enters the orbiting station for the first time.

Mission Control decided just before the spacewalk to let Newman have a go at unjamming the Zarya antenna.

Newman used an extendible 10ft grappling hook to prod the antenna several times without much luck, but a few minutes later, the antenna suddenly popped out.

The antenna is one of two that failed to open properly following Zarya's 20 November launch from Kazakstan.

NASA had told BBC News Online that the spacewalk might begin earlier than planned, and it did - some 30 minutes ahead of the schedule.

During the six-hour walk the astronauts removed covers from four hatches on Unity in preparation for the future berthing of the US laboratory module, an airlock and two of the trusses which will form the backbone of the ISS.

Their final task was to dislodge the antenna on Zarya

Further tasks were to connect handrails to the outside of Unity to assist future astronauts during spacewalks and a sunshade to protect Unity's electronic systems from overheating in the full glare of the sun.

The lengthy spacewalks required to attach equipment to the outside of Unity are necessary because of its large size. Normally this type of equipment would be installed on Earth at Cape Canaveral, but Unity required stripping down in order to fit into the shuttle's cargo bay.

The third and final spacewalk is set for Saturday, when tool boxes for future assembly crews will be stowed.

In total, more than 1,100 hours of spacewalks will be required during the construction phase of the ISS - more than in the entire history of human space flight.

The ISS is the most ambitious and expensive engineering project ever undertaken, costing the 16 participating nations $60bn. NASA estimates that 43 more manned missions and 157 more space walks will be required to deliver and assemble the 100 components.