Astronaut Susan Helms and Russian cosmonaut Yuri Usachev opened the hatch to the US module Unity at 2203 GMT and entered the Russian module Zarya some 55 minutes later.

After examining the modules, the seven member crew set to work on their extensive to-do list.

Ten smoke detectors will be changed aboard Zarya, and fans installed to improve air flow aboard the module to remove the risk of exposure to toxic fumes experienced by previous crews.

The crew will also work on Zarya's sound proofing, after criticism that the noise level onboard was too high.

Their most important task is to replace a charger and four of the six batteries needed to generate electricity for the ISS that no longer work.

They are also to load more than a tonne of material onto the ISS for use by future crews, including clothing, computers and exercise equipment.

If another module, Zvezda is launched in July as planned, there will be at least four more space flights to the station by the end of the year -- three by US shuttles and one by a Russian Soyuz spacecraft.

Work to finish the giant laboratory will require some 40 space missions between now and 2005.

The ISS will eventually house six and seven-member crews that will rotate after stays of about five months each, with the first crew -- comprising one US astronaut and two Russians -- to arrive late this year.

Three astronauts scheduled to be the second crew rotation on the ISS in 2001 are currently aboard Atlantis: Usachev, Helms and US astronaut James Voss.

Earlier Monday, two Atlantis crew members completed an almost seven-hour spacewalk, taking time to admire the view from some 330 kilometers (209 miles) above the earth.

"It's quite a planet we live on," exclaimed Voss, before he and fellow astronaut Jeffrey Williams finished repairing and modifying the outside of International Space Station's (ISS) two modules in six hours and 44 minutes.

A total of 1,700 hours of work will be needed to complete construction on ISS by the 2005 deadline.

Atlantis was launched Friday from the Kennedy Space Center, after many delays, on a 10-day mission to repair the ISS and load equipment onto it.

The 60-billion-dollar (67-billion-euro) ISS has been subject to criticism that the Russian modules are not up to US safety standards and insufficiently protected from meteorites and space debris.

While this is Atlantis' 21st mission, the upgraded space shuttle has not flown since 1997. It is scheduled to return to Earth on May 29.

The ISS project is made up of around 100 parts, and on completion will measure 108 meters (354 feet) by 74 meters (243 feet).

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