BEIJING – Three Chinese astronauts’ return to Earth has been postponed owing to weather conditions.

The astronauts, Cai Xuzhe, Song Lingdong and Wang Haoze, were originally due to land Tuesday in Dongfeng, in China’s northern Inner Mongolia region, after spending six months in space. The landing was postponed to “guarantee the health and safety of the astronauts” because weather conditions were not optimal, according to the official Xinhua news agency.

The three were sent up to China’s Tiangong space station in October last year, and a new crew of astronauts recently arrived to replace them.

The three Chinese astronauts have carried out experiments and improvements to the space station during their time on board. Two of the astronauts, Cai and Song, conducted a nine-hour space walk, the world’s longest, during their mission, the Manned Space Agency said.

China built its own space station after it was excluded from the International Space Station owing to U.S. national security concerns over the control of the country’s space program by the People’s Liberation Army, the military branch of the ruling Communist Party.

The country’s space program has grown rapidly in recent years. The space agency has landed an explorer on Mars and a rover on the far side of the moon. It aims to put a person on the moon before 2030.

Last year, two American astronauts wound up stuck in space for nine months after a test flight with Boeing ran into problems and NASA determined it was too risky for the astronauts to come back to Earth in the same capsule.