NASA has suspended certain activities with Russia. But the space station is exempt, and the rift highlights that, for the next few years, the US can't send astronauts into space without Russia.

The International Space Station, long a symbol of international cooperation in space even among former adversaries, could become the next pressure point as the US tries to raise the stakes for Moscow after Russia's takeover and annexation of Crimea.

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On Wednesday, Michael O'Brien, NASA's associate administrator for international and interagency relations issued a memo to the agency's staff suspending bilateral contacts with Russia unless a specific activity has been given an exemption.

"This suspension includes NASA travel to Russia and visits by Russian Government representatives to NASA facilities, bilateral meetings, email, and teleconferences or videoconferences." wrote Mr. O'Brien. A copy of the memo was posted on the website SpaceRef.com.

Bilateral contacts related to the operation of the space station are exempt, as are meetings held outside of Russia that involved additional countries such as those that might be include the participation of the station's other main partners, Canada, Europe, and Japan.

NASA's move is something of a diplomatic Post-it note; the two countries' space programs aren't engaged in many bilateral activities. Indeed, should operation of the space station come directly into play, the US and the other partners have the most to lose.

They have the heaviest investment in the $150 billion orbiting outpost, and they cannot get into space without Russia, which has served as America's taxi service for delivering and retrieving US crew members from the space station, as well as delivering cargo to the station.

So far, the Ukraine crisis has not interrupted US-Russia cooperation on the space station. But with the issuance of NASA's memo Wednesday, the question everyone is waiting to see answered is how Russian President Vladimir Putin responds.

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NASA suspends relations with Russia, but Putin holds all the cards (+video)