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A crew of three Chinese astronauts arrived at the country’s nearly complete Tiangong space station yesterday. As the third crewed mission to arrive at the station’s core module, the trio will spend six months in space and oversee the arrival and assembly of the station’s final two modules.

The orbital facility is regarded as China’s answer to the International Space Station, of which China is not a member. The ISS program consists of 15 member countries—including Russia—but US law prohibits bilateral cooperation with China on space efforts. Once completed, the Tiangong station will be roughly 20% the size of the ISS.

The station is one of the crown jewels of China’s rapidly advancing space program. In the past two years, the country has completed a satellite-based navigation system, uncrewed missions to Mars and the moon, and more.