The preliminary injunction against the deal between the US Air Force and United Launch Systems was issued late Wednesday by Judge Susan Braden of the US Court of Federal Claims.
The ruling blocks ULS, its parent company United Launch Alliance, and the Air Force from making payments to any entity subject to the control of Russian Deputy Prime Minister Dmitry Rogozin.
That includes NPO Energomash, the Russian state-owned company that makes the rocket engines that power ULA launches of US government and national security satellites.
SpaceX on April 28 filed a legal protest against the contract, which guaranteed the purchase of 36 rocket cores from ULA to be used in national security launches, on the basis that it was struck without any competition from other companies.
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However, it was unclear how much money was at stake in the ruling. The injunction would not apply to any purchase orders placed or money paid prior to April 30, the court documents said.
The injunction did not address the issue Musk raised of fair competition, and could be lifted if the US Treasury, Commerce Department or State Department reviews the deal and finds it does not violate sanctions.
On Tuesday, Rogozin lashed out at US sanctions on high-tech exports to Russia and said the move could endanger US astronauts at the International Space Station.
The world's astronauts rely on Russian rockets for transport to the orbiting outpost, paying tens of million per seat ever since the retirement of the US space shuttle in 2011.