Three Chinese astronauts whose return to Earth was delayed by space debris hitting their vessel last week landed in China on Friday afternoon, according to state broadcaster CCTV.

China Manned Space Agency (CMSA) revealed details about the debris damage for the first time on Friday, saying "tiny cracks" were found in a small window of the Shenzhou-20 spacecraft's return capsule.

"The capsule does not meet the safety requirements for a crewed return, Shenzhou-20 will remain in orbit and conduct relevant experiments," the agency said in a statement.

The astronauts were due to return to Earth nine days ago after completing a six-month mission on China's permanently inhabited Tiangong space station, a programme known as Shenzhou, or "Divine Vessel", when the crack was discovered.

The crew left Tiangong on another spacecraft, the Shenzhou-21, according to CMSA, touching down at the Dongfeng landing site in north China's Inner Mongolia Autonomous Region at 4:40 pm.

The mission began in April and went smoothly until the debris incident forced the Shenzhou-20's return, originally scheduled for November 5, to be postponed, CMSA said.

The delay, while only nine days, was highly unusual for a programme that had run like clockwork and in the past year reached new milestones, with the deployment of astronauts born in the 1990s, a world-record spacewalk and plans to send the first foreign astronaut, from Pakistan, to Tiangong next year.

Logistical Headache

Every Shenzhou mission on the Tiangong ends with a handover, where the departing crew welcomes the arriving crew that will take over the space station's operations. During the handover period of several days, two Shenzhou vessels are docked at the space station.

China's manned space programme now has to deal with another logistical headache - how to get the space station's newly arrived crew home in the event of an emergency. The Shenzhou-21 spacecraft and its three-person crew arrived at Tiangong two weeks ago.

But with the departure of the Shenzhou-20 crew on the Shenzhou-21 vessel, the Chinese space station is currently without a flightworthy vessel, meaning the Shenzhou-21 crew currently living there is stuck in space until a replacement vehicle arrives.

According to China's safety protocols, when astronauts cannot be safely flown back to Earth due to a malfunction, an unmanned emergency rescue spacecraft will be launched from the ground to take the astronauts back to the ground.

CMSA said the Shenzhou-22 spacecraft would be launched at "an appropriate time in the future".

Space junk danger

The damage to the Chinese spacecraft highlights the growing challenge of "space junk" to space exploration.

"Due to the sharp increase in orbital debris, the likelihood of damage to spacecraft and space stations of all countries has risen significantly," Igor Marinin, a member of the Russian Academy of Cosmonautics in Moscow, told Reuters.

While this is the first known debris disruption for a Shenzhou mission, junk in space has ensnared past missions to the International Space Station, the 25-year-old science lab led by the United States and Russia.

The SpaceX capsule NASA used to transport astronauts to the ISS has had to dodge suspected pieces of space debris during flight, while the football field-sized station itself has manoeuvred several times to steer clear of junk.

The disintegration of old, defunct satellites, mishaps with active ones and anti-satellite weapon tests can create vast fields of space debris that remain in orbit for years.

The sudden breakup of an old Russian satellite last year created at least 180 pieces of trackable debris, forcing ISS astronauts to prepare for evacuation. A spent Chinese rocket stage later that year created nearly 1000 pieces of junk after possibly colliding with debris.

The threat has prompted calls for rival space powers U.S. and China to work together to mitigate space debris and develop satellite traffic rules, though US law largely bars NASA from working with Beijing's space programme.

Still, the US and China have increasingly coordinated on space safety matters in recent years, largely on an ad-hoc basis. Chinese spacecraft manoeuvre alerts to US operators stepped up last year to the Pentagon and with NASA.

US space agency NASA last year saw two of its astronauts stranded for nine months in the ISS due to propulsion system issues in their vessel, a Boeing Starliner spacecraft.