NASA (search) has turned down a Russian request for astronauts to spend a full year aboard the international space station (search), saying it lacks the medical know-how for such excessively long stints.

The Russian Space Agency wanted to extend a future crew's mission from six to 12 months, so it could fill the seats on the ferrying capsule with tourists or other paying customers who would spend a week at the station. A Russian Soyuz (search) capsule is launched to the station every six months, and the two seats that would have been taken by the replacement crew could have been sold.

Americans have spent no more than 61/2 months at a time in space, while a handful of Russian cosmonauts have flown a year or more. The world space endurance record, held by a cosmonaut, is 141/2 months.

"We know we'll eventually need to extend the amount of time astronauts are in space to prepare for the missions outlined in the new vision for space exploration" by President Bush in January, NASA said in a statement this week.

Medical researchers are studying what it will take for lengthy expeditions to the moon and Mars, "however, we don't see a way to practically address these concerns in time to support" the space station crew that will fly this October, NASA said. Spokesman Allard Beutel stressed Tuesday that the space agency is not ruling out the possibility of a yearlong station mission down the road.

For such a major change in plans, NASA as well as all the other station partners — the space agencies of Russia, Europe, Japan and Canada — would have to agree.

The shuttle fleet has been grounded since last year's Columbia accident and is not making station deliveries. As a result, the station has a smaller than usual crew and fewer than usual supplies and replacement parts, and a yearlong mission would be an added burden.

What's more, NASA said in a letter to the Russian Space Agency last week, U.S. flight surgeons do not yet have sufficient measures to counter the debilitating effects of weightlessness from such a long mission. Side effects include weakened bones and muscles, and increased radiation exposure.

The space station is limited to two full-time residents — one fewer than normal — until shuttle flights resume, as early as next spring.

The Russian Space Agency had proposed prolonging the mission of the two-man crew due to be launched in October. It would not have affected the spacemen who will move into the orbiting complex on Wednesday for a six-month stay.

In constant need of cash, the Russians already routinely fly European astronauts for a high price on the Soyuz capsules that dock with the space station and serve as lifeboats. A Dutchman is currently on just such a flight. Two space tourists also have bought Soyuz seats, and a third has one on hold for next year.