The Russian letter cited Article 11a in a June 11, 1996 agreement between the participants in the ISS project, according to which Russia is obliged to provide the Soyuz rescue vehicle for the outpost only during the first 50 months of the crewed operations.

The countdown for the 50-month period started last November, when the Soyuz TM spacecraft delivered the first resident crew to the ISS. Since then at least one Soyuz has always remained attached to the outpost to provide the escape for the astronauts and cosmonauts in case of emergency.

Now, the letter from Rosaviacosmos officially informed NASA that after January 2005, Russia would only have the Soyuz available to conduct its national space program. This essentially means that unless the partners purchase the additional Soyuz spacecraft to provide emergency escape for the US and other non-Russian crewmembers, Russia will become the only country capable of supporting long-duration expeditions on the ISS. (The technical limitations prevent the US Space Shuttle from prolonged stays at the ISS.)

NASA spokesperson Kirsten Larson confirmed that the agency received the letter in question and the issues raised in it were discussed during the ISS Multilateral Control Board teleconference held on November 5.

NASA was developing its own rescue vehicle, known as X-38, however cost overruns in the ISS program forced the agency abandon the project earlier this year. Along with X-38, NASA had to axe the habitation module designed to provide life support for three additional crewmembers onboard the ISS.

Earlier this year, a joint venture between Russia's RKK Energia and the Houston-based Spacehab Inc. offered to sell NASA a Soyuz spacecraft in a "package" with the Russian-built Enterprise module. In combination with two Soyuz spacecraft -- one Russian and one purchased by NASA -- the Enterprise could provide life support and emergency escape for the international crew of six people.

NASA, however, was reluctant to accept any proposals, which would increase the program's dependence on the Russian hardware. In the past few years, Russian financial problems resulted in several delays with the assembly of the outpost. As a result, the U.S. Congress chided NASA for allowing Russia more control in the program.

As an alternative to Enterprise, NASA studied various options including one where European partners in the ISS project would contribute habitation facilities and even a rescue vehicle. However, none of the schemes could save enough funds to compensate for the ballooning budget in the program. "Whatever they do, our variant (Soyuz and Enterprise) will always cost less and will be available years before any American or European option," said one Russian official who did not wish to be identified, "Somebody had to tell (the partners): 'the king is naked'."

In the meantime, the officials at RKK Energia said they have to make a decision whether to go ahead with further development of the Enterprise module early next year in order to have the craft ready for launch by the beginning 2005. RKK Energia completed the preliminary design of the module last July and is ready to go ahead with its construction if funds are available, an RKK Energia representative told SPACE.com.

As NASA weighs its options in the program, the Russian space officials expect to push for their solution again during a routine meeting of the ISS partners at the Rosaviacosmos headquarters in Moscow on November 15.