The American and Russian pair set to take over the International Space Station (ISS) this week said on Wednesday they felt privileged to be part of the doubling of its permanent crew.

Speaking ahead of Thursday's launch from the Baikonur cosmodrome, Michael Barratt and Gennady Padalka, said their mission marked a new phase in the history of the ISS as the permanent crew is to be enlarged from three to six in May.

"The station is very large and complex and it was designed to produce a lot of science, to be a large, complex laboratory," Barratt told journalists from behind a glass screen where the crew were separated from journalists to prevent infection ahead of the launch.

"We're all looking forward to having six people.... The basic idea is that we need six people to adequately man the station," he said.

Until now the space station, which orbits 350 kilometres above the Earth, has had a permanent crew of three supplemented by additional crew for shorter visits.

But in May this year three more permanent crew are to arrive thanks to an additional launch from the Baikonur cosmodrome.

Padalka too said he felt "very lucky" to be part of the station's expansion.

The two are making the two-day journey to the station with American space tourist and software billionaire Charles Simonyi, who is making his second such voyage and has paid $35-million.

Padalka is to become the station's commander and Barratt its flight engineer, each of them spending six months aboard the station.

Simonyi will return to Earth after a 10-day stay in what may be the last voyage by a paying "tourist" aboard a Russian Soyuz rocket, officials say.

The station has gradually been expanded with additional laboratory facilities, living space and support facilities.

A new set of solar arrays to power the station was unfurled this month.

As well as being used for scientific experiments, the station is seen as a test bed for further human space travel, possibly to Mars.

"For preparing to go to Mars, it will probably be crews of six or so living together for long periods," Barratt noted.

AFP