Why damage to China’s Shenzhou-20 spacecraft has lesson on debris threat for space-faring nations
After a likely space junk strike left their capsule unusable, three astronauts needed a novel plan to get home, which included the launch of an unmanned craft on Nov 25. As the number of launched spacecraft rises, so does debris accumulation, making catastrophic collisions more probable.
What was to be a happy, heroic, and safe return for Commander Chen Dong and his fellow taikonauts (Chinese astronauts) from China's Tiangong Space Station this month, turned into a tense drama and cause for serious anxiety, not just for them, but for the crew of another spacecraft which had reached the Tiangong last month to relieve Chen Dong and team.Launched on April 24, 2025,...
What was to be a happy, heroic, and safe return for Commander Chen Dong and his fellow taikonauts (Chinese astronauts) from China's Tiangong Space Station this month, turned into a tense drama and cause for serious anxiety, not just for them, but for the crew of another spacecraft which had reached the Tiangong last month to relieve Chen Dong and team.
Launched on April 24, 2025, aboard Shenzhou-20, Commander Chen Dong, along with crewmates Wang Jie and Chen Zhongrui, arrived at the Tiangong Space Station. They were scheduled to spend over 200 days aboard, the longest duration for Chinese taikonauts. Further, the mission was to see Commander Chen Dong surpass a cumulative 400 days in space across three missions.
The team was to have had a glorious return on November 5 after handing over the station to the three-person crew of Shenzhou-21, who arrived at the Tiangong Space Station on October 31. Alas, that was not meant to be.
A small fragment of space debris, roughly the size of a cricket ball, nearly 10 centimetres in diameter, is believed to have struck the Shenzhou-20 spacecraft as it whizzed past it. It was this likely impact which caused cracks to develop in the windows of the spacecraft, rendering the vessel unsafe for the crew's safe return to Earth.
The damage was discovered as the crew was preparing the Shenzhou-20 for the return journey; during inspection, they found tiny cracks in the capsule's viewport. They feared that the blow could have also damaged the spacecraft's thermal protection system or parachute deployment structures, rendering the craft unsafe.
In a drama reminiscent of the ordeal the world witnessed last year, with the US’s Sunita Williams and Barry ‘Butch’ Wilmore stranded on the International Space Station for months owing to technical issues with their Boeing Starliner craft — they finally returned to Earth in March this year — the crew of Shenzhou-20 found themselves similarly stranded in space.
While the Shenzhou-20 crew has since been safely brought back to Earth, albeit at a few days’ delay, stuck there, without any escape route in case of an emergency, is the crew of Shenzhou-21. This is because Cheng Dong and his crewmates have been flown back on the Shenzhou-21, while the damaged craft remains docked at the Tiangong Space Station. To address this, China launched the unmanned Shenzhou-22 on November 25.
Orbiting the Earth at an altitude of approximately 390 kilometres, the Chinese space station Tiangong is nearly one-sixth the size of the International Space Station. Built upon the experience gained from its predecessors, the Tiangong-1 and Tiangong-2 experimental space stations, Tiangong has been permanently crewed since June 5, 2022. It is designed to comfortably host three astronauts for extended stays of about 180 days, while also accommodating six astronauts for short periods during crew changeovers.
The Shenzhou-2o liftoff. Photo: Wikimedia Commons
Under normal circumstances, a three-member crew staffs the station, with their Shenzhou spacecraft remaining docked and ready to serve as their lifeboat back to Earth in case of an emergency. Standard practice dictates that when a fresh crew arrives, the incumbent team completes a handover of station operations and then returns to Earth using the very spacecraft that brought them to orbit. However, with the Shenzhou-20 rendered unusable, this carefully orchestrated procedure was disrupted.
The Tiangong station found itself crowded with six taikonauts instead of the usual three. The station is equipped with three private sleeping quarters in the Wentian experiment module and three more in the Tianhe core module, making accommodation of the six manageable.
To support the requirements of the station, supplies, provision, experiments and propellants are regularly delivered by uncrewed Tianzhou cargo spacecraft, which are typically launched once every eight months. The latest such spacecraft, Tianzhou-9, arrived at Tiangong in mid-July, which means the station was well stocked with a buffer of supplies. Should the need arise, the next Tianzhou mission could be launched ahead of schedule to deliver additional resources.
Nonetheless, the prolonged presence of three additional crew members would have drained essential consumables, water, oxygen and food at a much faster rate. Furthermore, the critical life support systems, including the water and oxygen recycling systems and the carbon dioxide scrubbers that maintain a habitable cabin environment, would have been placed under considerably more strain.
China's space agency maintains a robust emergency protocol, always keeping a Long March 2F rocket and a replacement Shenzhou spacecraft on standby, ready to be launched in as few as 8.5 days. However, there was a critical logistical hitch in this instance.
The Tiangong space station has only three docking ports, where visiting spacecraft are berthed. At the time of the incident, all of these were occupied by the stricken Shenzhou-20, the newly arrived Shenzhou-21, and the cargo ship Tianzhou-9. To dispatch a rescue mission, one of these vessels had to be vacated and de-orbited first.
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Like the game of musical chairs, the Chinese authorities devised a solution. They decided to bring Commander Chen Dong and his crew back to Earth using the freshly arrived Shenzhou-21 spacecraft. This would free a docking port for an uncrewed standby spacecraft, Shenzhou-22, which would then serve as the return vehicle for the current station occupants.
Consequently, within just two weeks of its docking, as step one, the Shenzhou-21 spacecraft undocked and returned to Earth on November 14, 2025, carrying the stranded Shenzhou-20 crew. This left the present station crew, Commander Zhang Lu, Zhang Hongzhang, and Wu Fei, the youngest member of China's astronaut corps, in a precarious position. They were now aboard Tiangong, with no spacecraft docked to evacuate them in case of an onboard emergency.
The Shenzhou-21 mission patch. Photo: Wikimedia Commons
The timing of the launch of the standby spacecraft is based on astronautical computations. Much like how one must aim ahead of a flying bird to hit it, launching a spacecraft to rendezvous with a space station requires the station to be in the exact correct orbital position relative to the launch site. November 25 was identified by the China Manned Space Agency as the next available orbital window to successfully launch this critical standby craft.
Thus, in step two, the launch of Shenzhou-22, initially scheduled for April-May 2026, was advanced to November 25, 2025. Therefore, the crew of Shenzhou-21 will be without a lifeboat for only about 10 days, which is not a significant risk. Furthermore, being uncrewed, the spacecraft will offer additional capacity to carry extra food and supplies for the Shenzhou-21 crew, who had only just begun their six-month stay on October 31. While NASA took nearly nine months to bring the standard Sunita Williams and Butch Wilmore back home, the Chinese space agency accomplished the rescue mission in just over a week.
The successful launch of the uncrewed Shenzhou-22 brings a sigh of relief for the current occupants of the Tiangong space station. Yet, it serves as a potent warning to all space-faring nations. Outer space, particularly the low Earth orbit, is becoming increasingly unsafe owing to the proliferation of space junk.
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As the number of launched spacecraft continues to rise, so does the accumulation of debris, making catastrophic collisions, like the one that likely disabled the Shenzhou-20, alarmingly more probable. Ever since the Soviet Union launched Sputnik, the first artificial satellite, in 1957, nations and commercial entities have treated the cosmos as a dumping ground, discarding spent rocket stages, defunct satellites, and even bags of trash from crewed missions, along with smaller items like bolts, screws, and flecks of paint.
Current estimates indicate that over 34,000 objects larger than 10 centimetres are circling Earth, travelling at speeds exceeding 28,000 kilometres per hour. In addition to these, there may be millions of minuscule objects, each no larger than a grain of pepper, which nonetheless carry a powerful punch due to their immense velocity. This debris is mainly concentrated in low-Earth orbit, the congested altitude band between 180 and 2,000 kilometres.
Astronauts on space walks have reportedly observed the outer shells of space stations marked with minor dents from impacts with this tiny junk. Frequently, crews are warned to take refuge in their docked return spacecraft when ground controllers identify debris on a dangerous collision course, ready to undock and fly home in the event of a serious threat.
The past decade has seen record-breaking numbers of objects launched into space, with the UN recording 2,849 objects sent up in 2024 alone. With companies like Elon Musk’s SpaceX planning to deploy and replace tens of thousands of satellites, the strain on this orbital region is immense.
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For decades, the international community has relied on UN treaties, such as the Outer Space Treaty of 1967 and the Liability Convention of 1972, to govern activities in space. These frameworks hold nations responsible for commercial launches from their territory, meaning a country, not a private company, is liable for any damage caused.
However, there is no specific international treaty governing space debris, and regulations remain largely voluntary. This was evident in 2024 when debris from a SpaceX vehicle landed in Canada, and the company retrieved it without formal government intervention. Creating an effective international regime to manage the commercialised frontier of low Earth orbit is thus one of the most critical challenges of our time.
