Three Chinese astronauts have been forced to extend their six-month stay in space over concerns their return ship may have been hit by debris, China’s space agency said.

Chen Dong, Chen Zhongrui and Wang Jie were due to return to Earth on Wednesday –– they had even handed over the keys to the Chinese space station to a new crew –– but their journey home was postponed due to a suspected impact on their Shenzhou-20 spaceship, China’s National Space Administration said Wednesday.

“The impact analysis and risk assessment are under way,” the space agency said, without specifying details of any damage or how long it might take to check the ship.

No alternative dates have been given for the return of the trio of taikonauts – Chinese astronauts – who blasted off from the country’s Inner Mongolia Autonomous Region in April.

The news came just days after China, which has rapidly ramped up its space ambition in recent years, celebrated the successful launch of Shenzhou-21.

The newer model spacecraft successfully ferried the next batch of astronauts to China’s space station Tiangong – or “Heavenly Palace” in Chinese. Among the new cohort was Wu Fei, 32, feverishly touted by local media as China’s youngest astronaut ever to make it to space.

On arrival, state-owned CCTV aired a handover ceremony involving the crews of both ships huddling in a narrow compartment to sign documents on a floating board.

“We are about to return to Earth, and now I am handing over the hatch key that symbolizes the right to maintain operations on this Chinese space station to you,” said Chen Dong. But now he and his teammates are stuck in space while their ship is assessed.

China’s space agency said the postponement was to ensure the astronauts’ safety and health.

The biannual launches of China’s Shenzhou program have been a source of pride as the country’s space program makes giant leaps forward. Recent advances have seen China break an American-held record for the longest spacewalk, with a nine-hour feat, and it’s about to open the doors of its space station to foreigners for the first time, with plans to welcome an astronaut from Pakistan to Tiangong next year.

The developments have raised alarm bells in Washington, which is racing to put an astronaut on the moon again, as the Trump administration placed bans on Chinese citizens with US visas participating in NASA programs.

The US space agency has become very familiar with the difficulties of retrieving astronauts stuck in space over the past year.

What was meant to be a short stay in space for US astronauts Suni Williams and Butch Wilmore stretched into a more than nine-month mission, after the Boeing Starliner spacecraft that was supposed to ferry them home malfunctioned.

The pair, who departed Earth in June last year, eventually returned home in a SpaceX Crew Dragon capsule in March.

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