Reports have suggested that Russia may reduce the number of cosmonauts it keeps on the International Space Station (ISS), casting a shadow over what has been one of the most impressive cooperative projects between the US and Russia over the last two decades.

The news was reported by Russian newspaper Izvestia last week, with NASA confirming at a press conference yesterday that the move was under discussion. The exact reason for it is not clear, but funding issues seem to be the main concern.

Since November 2, 2000, the ISS has been continuously manned. At first it had just a three-person crew, but since 2009 there have almost always been six people on board, transported to and from the station by Russia’s Soyuz spacecraft.

NASA has continuously paid for seats for its own astronauts on the Soyuz, to the tune of half a billion dollars a year. However, in the next few years, NASA is expected to return to launching astronauts from American soil, via the upcoming SpaceX Dragon and Boeing CST-100 Starliner vehicles. This would take much-needed funding away from Russia’s space agency, Roscosmos, which has already had to undergo significant cuts in the last few years.

As a result, Russia may lower the number of people it launches to two, meaning it only ever has two cosmonauts on the ISS. This would potentially allow for cargo to be taken on Soyuz missions in lieu of an extra person, and it would also mean they need to launch fewer resupply missions to the ISS with their Progress spacecraft, reducing their costs of operating the ISS.

A timescale for this is not yet clear. If NASA keeps its number of astronauts on the station to three, this would mean only five people would be up there. With growing pressure to make as much use of the ISS as possible before it is retired by 2024, or possibly 2028 or even later, this will be a bit of a blow.

All hope is not lost, though. Both Dragon and Starliner will be capable of taking four astronauts each, with NASA’s chief scientist for the ISS, Julie Robinson, telling IFLScience earlier this year that NASA may make use of this increased capacity. This would mean the ISS could be kept with a crew of six – four in the US segment and two in the Russian segment.

At the moment, though, things are still very up in the air. But with the ISS expected to run for at least nine more years, it shows some unwanted cracks appearing in what has been a very fruitful relationship so far.