Rocky Russian relations could leave U.S. astronauts without rides to the International Space Station.
Since NASA retired its fleet of space shuttles in 2011, Russia has had a monopoly on flying crews to the orbital outpost. The only other country currently flying people in space is China, which is not a member of the 15-nation space station partnership.
That leaves the United States in a vulnerable position as it ponders options to diffuse a tense standoff between Russia and Ukraine.
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For now, the U.S.-Russian space partnership is insulated from the political whirlwind generated by Russia's decision to move troops into the Ukrainian peninsula of Crimea last week, fueling fears of a full-fledged invasion.
"We are continuing to monitor the situation," NASA administrator Charles Bolden told reporters on a conference call on Tuesday.
"Everything for us continues to be nominal," he said.
Bolden noted that the space station has been through "multiple international crises" since crews began living there full-time on Nov. 2, 2000. That includes the 2008 war between Russia and Georgia over break-away regions Abkhazia and South Ossetia.