N'Sync singer Lance Bass has begun a week of Nasa training for a possible flight to the International Space Station, even though contract issues with the Russians have cast doubts over the trip.

The 23-year-old star arrived at the Johnson Space Center in Houston, Texas over the weekend, along with the rest of his crew, a Russian and a Belgian.

The trio, who have been training at Star City in Russia, are due to blast off from Kazakhstan on 28 October in a Russian Soyuz capsule which will remain at the space station as a fresh lifeboat.

Bass' participation is still in question. He has yet to reach a deal with the Russian Space Agency, or receive the backing of a panel of space station representatives.

However, Nasa have agreed to put him through essential safety briefings and simulation classes so that he is ready for the voyage if an agreement over his contract is reached.

'Unforgiving'

Bass will learn about the US side of the space station during his week at the Space Center, and will also be taught about the risks of space travel.

"Space flight can be very unforgiving if there's a mistake that's made, and those are the kinds of things that we'll make sure all three crew members are trained for," said flight director John Curry.

Lance Bass has been in training in Russia

Bass is only the second space tourist to train at Nasa, following South African tycoon Mark Shuttleworth who went into space in April this year.

He is being sponsored by a consortium of major companies and Hollywood producer David Krieff, and has signed a preliminary contract for the mission - but recent reports have suggested that he has missed deadlines to pay the $20m flight fee.

If he does go into space on 28 October, he will become the youngest person to do so, replacing Soviet cosmonaut German Titov, who was 25 when he blasted off in 1961.