The International Space Station has survived disputes with Russia before. Will it this time?

After more than four-and-a-half decades of cooperation between the United States and Russia in space, tensions over Ukraine threaten to compromise the future of the International Space Station (ISS).

On Thursday, Russia's space agency Roscosmos tweeted that they will no longer co-operate with Germany on joint experiments on the Russian section of the station.

Roscosmos's head Dmitry Rogozin, known for his brash statements, has warned on Twitter that if Russia pulls out, the station could fall uncontrollably out of orbit and come down over populated areas such as China or India. 

It's not an entirely empty threat. Even though the space station orbits 400 kilometres above Earth, it still runs into thin traces of the atmosphere that constantly slow it down by small amounts.

To keep it aloft, Russian Soyuz and Progress supply ships visiting the station have used their thrusters to give it regular boosts to maintain its orbit. Without those boosts, its orbit could conceivably degrade to the point where it would be at risk of falling out of the sky, though that would take years.

Kathy Lueders, head of NASA's human spaceflight program, told reporters on Monday that the station was operating "nominally" and that they were "not getting any indications at a working level that our counterparts are not committed," according to The Guardian.

There may be other options. Elon Musk hinted that his company SpaceX could take on the task of boosting the station. The Cygnus supply ship, made by the U.S.'s Orbital Sciences Corporation, has flown to the station since 2014, and it has tested its powerful booster to see if it can provide the critical boost. 

But many in the space business hope that these alternatives won't be necessary and that the Russians will continue to be part of the program until the station is scheduled to be intentionally decommissioned and brought down around 2030.

"As a team, we are looking at where we may have operational flexibilities, but … it would be a sad day for international operations if we can't continue to peacefully operate in space," Lueders said.

This isn't the first time in recent years that international tensions have spilled out into low orbit.

When Russia annexed Crimea in 2014 and sanctions were imposed by the U.S., there was concern that the friendly relations between the two superpowers would end, and that Russia could prevent the U.S. from even reaching the station. At that time space shuttles had been retired, so Americans were totally dependent on Russian Soyuz spacecraft to reach the orbiting laboratory.

At the time, Rogozin quipped that Americans would need a trampoline to get there. 

But despite political tensions, the ISS program continued without interruption.

In 1975, American astronaut Tom Stafford shook hands with Russian Cosmonaut Alexei Leonov through a module, connecting their two space capsules in the symbolic Apollo-Soyuz Project. It marked the end of the Cold War space race to the moon and the beginning of a more co-operative approach to space exploration.