Three astronauts have boarded a Russian rocket ahead of their imminent launch into space to become the first long-term residents of the International Space Station (ISS).

Russians Sergei Krikalev and Yuri Gidzenko and their American commander Bill Shepherd are due to spend four months aboard the space platform about 368 km (230 miles) above the Earth.

Their Soyuz spacecraft is due to launch from the Baikonur cosmodrome in Kazakhstan at 1053 am Moscow time (0753 GMT).

A Progress cargo craft, now fueling tanks on the ISS's Zarya module, will be commanded to undock from the station on Wednesday, to clear the way for the crew's arrival at the platform the following day.

Harsh environment

The Progress craft will be directed to re-enter the Earth's atmosphere and will burn up during its descent.

Once they arrive at the station, Bill Shepherd and his crew will activate life-support systems. They have only two days' supply of oxygen to carry out the first essential tasks.

During the first week, they will also create a computer network that will help them run all the station's systems from laptop computers.

"I fully expect we'll spend much of our time making repairs," Bill Shepherd told a recent news conference.

Life on board the International Space Station will be cramped. Two of the crew will occupy bedrooms the size of a broom closet; the other will have to sleep where he can find space.

The passages of the station are narrow and there is only enough electricity to keep two of the three modules heated.

Terrestial outpost

The team plans to work 12-hour days for six days a week. On Sundays, they can relax, catch up with personal e-mail or use a two-way video conferencing facility to speak to family.

The crew will also spend two hours each day exercising to guard against the debilitating effects of weightlessness.

Expedition 1 is expected to remain in space until next spring, when they will be replaced by another three-member team.

Other crews will follow back-to-back, until the multi-billion-dollar space station is finished, in 2006 or later.

The station is designed to last at least 10 years, but it could last a quarter of a century or more.

The International Space Station is an international partnership, including the US, Russia, Europe, Japan, Canada and Brazil.