US astronauts last night began a difficult spacewalk to link power cables between the modules of the International Space Station in the first of about 75 spacewalks planned during its five-year construction.
The Endeavor's mission specialists, Mr Jerry Ross and Mr Jim Newman, were expected to take about 6 1/2 hours to connect cables carrying electrical power and avionics between the Russian-built Zarya station and the US-built Unity.
"The nervous system, if not the heart of the station, will be coming alive" when ground controllers signal the two modules to power up, said the lead flight director, Mr Bob Castle, at Mission Control in Houston.
Most of the cables on the International Space Station will run along the outside of the structure, avoiding the chaotic tangle that ran through the corridors of the Russian Mir station and hindered hatch closings in emergencies.
"The first EVA [extra vehicular activity] is by far the most critical one," Mr Ross said in a preflight interview. "It's the one that is required to hook together the elements of the station that are up there and to permit us to start activating the US-built parts of that station."
Mr Ross and Mr Newman have two other spacewalks planned for the 12-day mission. As they reach the top of the Zarya power station they will be working about seven stories above Endeavor.
Falling is not a problem in weightlessness, but working such a distance from the safe haven of the shuttle could be. "They may be the most difficult EVAs ever performed," said Mr Frank Culbertson, operations manager for the space station programme. "But they will get even harder."
More than 1,100 hours of spacewalks will be required during the construction phase of the station's 10- to 15-year life - more than in the history of human space flight.
The space station became a reality on Sunday as the first two components were brought together in orbit by Endeavor's crew.
In time, more than 100 space station components will be assembled in orbit in one of the most ambitious and expensive engineering feats ever undertaken.
Sixteen nations are involved in the $60 billion project, which will require more than 40 manned missions and hundreds of hours of high-risk space walks during the construction phase.
Endeavor's six-member crew will enter the station on Thursday, finding it much like the unfinished interior of a new home.
"It's a long corridor basically that doesn't have a lot of features from one end to the other," said Mr Culbertson.