The United States has rejected a Russian request to extend the length of International Space Station missions from six months to one year.

Russia is the only nation that can fly crews to the space station for the time being. U.S. shuttles are grounded for safety modifications for at least another year following the loss of the orbiter "Columbia" last year. Russian Soyuz craft will continue to ferry station crews through 2005.

The crews have been exchanged twice a year since station occupation began in 2000, but Moscow has suggested sending fresh crews only once a year. It wants to take two seats now allocated on one of the two annual flights and sell them to space tourists or astronauts sponsored by other countries. The U.S. space agency NASA has turned this request down.

"It's not a matter of saying, 'No, we're not planning to do this.' It's a matter of not being able to do this right now," said NASA spokesman Allard Beutel.

He adds that NASA is not ready to leap into longer duration flights because the station is in a reduced state of operations while shuttles are grounded. Furthermore, he says NASA is not yet prepared to counter the damage extended space tours wreak on the human body.

"There's radiation exposure," he said. "That would obviously be doubled from six months to one year. The more time you spend in space, you also get muscle and bone loss, so you'd have to have what we call countermeasures, other things to offset those harmful effects. We don't feel we're in a position now to actually put those procedures in place to make it by what the Russians were proposing, six months from now when the next expedition crew, Expedition 10, would go up."

In the meantime, the ninth space station crew is beginning its six-month stay at the outpost Wednesday, after Monday's in a Soyuz from Kazakhstan. Russian commander Gennady Padalka and U.S. astronaut Michael Fincke are accompanied by Dutch astronaut Andre Kuipers, who will return to Earth with the outgoing crew next week.

NASA spokesman Beutel said the agency must eventually undertake one year or longer space station visits so it can learn how to conduct long duration missions to Mars.

"We know that we will eventually have to go off and look at expanding time on the station for crews because missions that are outlined in the new vision for space exploration offer longer missions in space," he explained. "So we have people off studying what it would take to implement a longer expedition mission."

The new space exploration vision Mr. Beutel mentioned was outlined by President Bush in January. He set a long term U.S. goal of putting people on Mars after first returning to the moon by 2020.