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A Long March-2F rocket carrying the Shenzhou-20 spacecraft takes off April 24 from the Jiuquan Satellite Launch Center for a crewed mission to China's Tiangong space station.

Three veteran Chinese astronauts are stuck – for now – aboard the Tiangong space station after their return capsule was hit and damaged by space debris fragments.

China's space agency has not revealed how badly damaged the capsule was, and it’s not known if the craft was hit by debris while in flight or while docked at the station. No one was injured. The Shenzhou-20 spacecraft remains attached to Tiangong, a permanently crewed space station operated by China Manned Space Agency.

Officials ordered the astronauts to remain on Tiangong while engineers troubleshoot the capsule. If it can’t be repaired, another capsule docked at the station, Shenzhou-21, would be used as the return vessel for the Shenzhou-21 crew.

A backup Shenzhou craft would be sent for the Shenzhou-20 crew.

The Shenzhou spacecraft ‒ made up of an orbital module, a reentry module and a service section ‒ carries three astronauts and equipment to Tiangong after launching via a Long March 2F rocket. The Shenzhou-20 mission launched on April 24 from Jiuquan Satellite Launch Center in northwest China.

Who are the stranded Chinese astronauts?

The three Shenzhou-20 astronauts are Chen Dong, Chen Zhongrui and Wang Jie. Combined, the three men have amassed more than 800 days in space. They were scheduled to return to Earth after a six-month mission aboard the station.

This is the third Shenzhou mission for Commander Chen Dong.

If the Shenzhou-20 vessel cannot be repaired aboard Tiangong, safety protocols would establish the Shenzhou-21 as the return vessel for the Shenzhou-21 crew, and a backup Shenzhou craft would be sent from the Jiuquan Satellite Launch Centre.

The Shenzhou program takes Chinese astronauts to and from Tiangong for rotating six-month stays, where they perform a variety of tasks.

How big is the Tiangong space station?

Both Tiangong and the International Space Station orbit the Earth at the same general altitude. Tiangong’s orbit is 210 to 280 miles high, while the space station can range from 230 to 285 miles with reboosts.

Tiangong, which means "Heavenly Palace," supports three astronauts, with room for three more during crew rotations.

How was the Chinese spacecraft damaged?

Space debris, usually old satellites and rocket fragments, moves at very high speeds. It can pose a risk to space stations, which are capable of performing avoidance maneuvers and other steps to protect equipment.

The Tiangong delay is the first time a return mission has been postponed by space debris, Reuters reported.

It comes more than a year after an ordeal involving the crew of the Boeing Starliner, who were stranded aboard the space station for 10 months. NASA astronauts Butch Wilmore and Suni Williams returned safely to Earth on March 18.

Contributing: Eric Lagatta, George Petras and Ramon Padilla

SOURCES Reuters, Agence-France Presse

This article originally appeared on USA TODAY: Space debris leaves astronauts stranded on Chinese space station.

Reporting by Stephen J. Beard, USA TODAY / USA TODAY

USA TODAY Network via Reuters Connect