A Japanese astronaut, a veteran Russian cosmonaut and a NASA flight engineer prepared their Soyuz ferry craft for departure from the International Space Station Tuesday, setting their sights on a fiery plunge back to Earth to close out a 188-day stay in space.

Koichi Wakata, Mikhail Tyurin and Rick Mastracchio are set for landing after a 188-day expedition on the International Space Station. Photo credit: NASA The international crew's return aboard a Russian spacecraft was the first such flight since Russia's annexation of Crimea, the imposition of U.S. and European sanctions and escalating Cold War rhetoric that stands in stark contrast to the close cooperation that has been the hallmark of the International Space Station program.

In the latest space-related tit for tat, Dmitry Rogozin, Russia's deputy prime minister for space and defense, told Russian news agencies future sales of RD-180 engines, which power the first stage of United Launch Alliances Atlas 5 rocket, will not be permitted for launches of U.S. military payloads.

The Atlas 5 is routinely used for Pentagon missions and its reliance on Russian engines has come under fire in the wake of the Ukraine crisis. ULA competitor SpaceX has argued that payments for the RD-180 violate Obama Administration sanctions, but a temporary injunction was lifted last week based on assurances by the departments of Treasury, State and Justice that the sales were compliant.

In any case, ULA officials say the company has a two-year supply of RD-180s in hand and it's not yet known what impact Rogozin's statements might have down the road.

Both sides say the station program is not affected by sanctions or other diplomatic hurdles and the Russians continue to honor their lucrative contract with NASA to carry U.S. and partner astronauts to and from the space station aboard Soyuz spacecraft at more than $70 million a seat.

The Soyuz landing and another Soyuz launch later this month to carry three fresh crew members to the orbital complex highlight NASA's lack of an operational crew-carrying spacecraft of its own and the agency's dependence on the Russians for basic space transportation until at least 2017, when a U.S. ferry craft should be ready for service.

That assumes the program receives the necessary funding from Congress and the station program continues to operate smoothly, with the full cooperation of all the international partners. The station cannot be safely operated by either side without the other.

The station crew flies 260 miles above the complex geopolitical landscape and from their perspective, the Ukraine crisis has had no impact on day-to-day operations. Entry preparations have proceeded normally, the Soyuz TMA-11M spacecraft has been checked out and the stage is set for three members of the station's six-man crew to return to Earth.

With commander Mikhail Tyurin at the controls, flanked on the left by flight engineer Rick Mastracchio and on the right by outgoing Expedition 39 commander Koichi Wakata, the Soyuz TMA-11M spacecraft was scheduled to undock from the Russian Rassvet module at 6:36 p.m. EDT (GMT-4).

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Space station crew ready for return to Earth