The International Space Station has usually been home to six astronauts since May 29, 2009, when Roman Romanenko, Frank de Winne, and Bob Thirsk arrived on the station, expanding the crew to its full complement for the first time. But that will change in March, as Russia scales back its involvement in the partnership by flying two fewer cosmonauts to the station per year.

NASA announced Tuesday that a March launch of its Expedition 51/52 to the station will contain just two crew members, NASA astronaut Jack Fischer and cosmonaut Fyodor Yurchikhin of the Russian space agency, instead of the more typical three. They will join the crew of Peggy Whitson, Thomas Pesquet, and Oleg Novitskiy aboard the station to make for a semi-permanent crew complement of five.

Russia has scaled back its crew complement to save expenses, as sending fewer of its Progress supply ships to the orbiting laboratory will help Roscosmos close a budgetary hole. Roscosmos has said the reduced crew complement will be "temporary" but has not said when it might resume full staffing. For this year, rookie cosmonauts Nikolai Tikhonov and Ivan Vagner appear to have been pulled from their flights in favor of veterans.

One big question raised by the Russian decision to cut its crew size is how committed Roscomos is to the future of the station. Although NASA and its partners have signed agreements to continue flying and supporting the station until 2024, the US space agency has talked openly about the possibility of flying it until 2028, or possibly even beyond then. Russia, however, has made noises about breaking with the international partnership no later than 2024, if not earlier, and possibly building its own orbiting station.

Crew size will go back up

At present, the only way for Russian, NASA, and international partner astronauts to reach the station for the approximately six-month missions is via the Soyuz spacecraft, which launches from Kazakhstan. There is typically an “A-line” Soyuz and, three months later, a “B-line” spacecraft, with a total of four launches a year. The A-line vehicle usually carries two Russians and one American, and the B-line vehicle generally has one NASA astronaut, one Russian, and one international partner.

For now, the A-line missions will carry just two passengers, a NASA astronaut and a cosmonaut. Although it was theoretically possible for a NASA or international partner to fill that seat, Ars understands the space agency had no interest in chumming the political waters with such a request. Congress is already loath to spend money committed to buy Soyuz seats through 2018.

The station's crew complement will return to six in 2018 or 2019 regardless of whether the Russians decide to scale back up. When commercial spacecraft from Boeing and SpaceX begin services from Florida in two years or so, they will have the capability to carry four astronauts to the station. This will allow NASA and its partners in Europe, Canada, and Japan to fly the equivalent of two more people per year to the station.