One day before their scheduled return to Earth, the three astronauts aboard China's Shenzhou-20 spacecraft noticed something unusual along the edge of a viewport: a small, triangular mark resembling a paint smudge.

The crew immediately documented the flaw, photographing it from multiple angles under different lighting conditions, while the space station's robotic arm cameras captured additional external images.

Once the pictures reached the ground, engineers urgently analyzed the damage. It was later identified as a penetrating crack. According to Jia Shijin, chief designer of the manned spacecraft system, with the China Academy of Space Technology, the crack was likely caused by space debris no larger than a millimeter, traveling at extremely high speed.

"The probability of a spacecraft being hit by debris is already very low," Jia told Xinhua News Agency. "The chance of it striking the viewport is even lower. So when we first saw it, we found it hard to believe."

This unexpected discovery brought the crew's scheduled homecoming to an abrupt pause.

The first-ever delay 

Ground control immediately launched a rigorous investigation. Teams scoured every design document, manufacturing record and test report related to the viewport. They also commissioned two independent research institutes to perform new wind tunnel verifications. "In the end, we confirmed that the design, the plans and all our ground tests were flawless," Jia said. "The most probable culprit was space debris."

Jia outlined a grave worst-case scenario: The cracks could spread, causing the outer pane to detach. This would compromise the inner pressure-sealing layer, leading to rapid cabin depressurization—a catastrophic event.

In fewer than 12 hours after confirming the crack's severity, mission command made a historic call on November 5: Shenzhou-20's return was called off.

This decision created an unprecedented situation in orbit. The Shenzhou-21 spacecraft had already arrived at the Tiangong space station on November 1 with a fresh crew for a standard handover. Now, with Shenzhou-20's delay, the two crews—totaling six astronauts—were sharing the space station. Their "space reunion" far exceeded the five-day overlap typical of all previous ones.

Even the mission's smallest participants were affected. Four mice, which had arrived with Shenzhou-21 for a planned five-to-seven-day experiment, found their "short business trip" extended into an open-ended stay.

"Manned spaceflight is inherently high-risk. That reality never changes," Wu Dawei from the China Astronaut Research and Training Center, told China Central Television. "The crew is in daily contact with us and has shown no special anxiety. The station is well-supplied, all systems are normal. We have a new risk point, but it has caused no substantive damage. Operations continue normally."

Concurrently, ground control urgently commenced preparations for the return of the three Shenzhou-20 astronauts. Ji Qiming, spokesperson for the program, explained that since the Shenzhou-12 mission in 2021, China's manned spaceflight has adopted a "rolling backup" model, with one combination of the spaceship and rocket for launch and another combination designated as backup for each mission. This ensures that a backup combination of launch vehicle and spaceship is always on standby, ready to be rapidly deployed for an emergency space station rescue mission.

This foresight extended to training. The Shenzhou-20 crew had rehearsed using the Shenzhou-21 spacecraft, just as the Shenzhou-21 crew was fully trained on Shenzhou-22. When Shenzhou-20's issue was confirmed, the Shenzhou-22 spacecraft—the designated backup for Shenzhou-21—was already on standby at the Jiuquan Satellite Launch Center in Gansu Province.

On November 10, the project officially activated its emergency plan and measures. The decision: The Shenzhou-20 astronauts would return in the Shenzhou-21 spacecraft, while a 16-day countdown began for the emergency launch of the vital Shenzhou-22.

Gearing up 

The Jiuquan Satellite Launch Center rushed into emergency mode. A standard launch preparation for a Long March-2F rocket typically takes over a month. The team had just launched Shenzhou-21 on October 31—a mere five days before the Shenzhou-20 anomaly was confirmed. Now, they were tasked with doing it again in just 16 days, necessitating a drastic streamlining of every procedure. 

As the emergency plan took effect, the spacecraft design and development team also accelerated to an unprecedented pace. Their challenge was to manage five critical paths in parallel: the return of Shenzhou-21, the launch of Shenzhou-22, the eventual retrieval plan for Shenzhou-20, and the expedited rollout of the Shenzhou-23 and Shenzhou-24 spacecraft.

Launching Shenzhou-22 without a crew introduced unique complexities. Jia pointed out that the absence of astronauts required significant mission redesign. "Tasks designed for in-orbit crew operation had to be re-engineered. Contingency plans that relied on human intervention needed complete overhaul," he explained.

The tension was eased significantly on November 14, when the Shenzhou-21 return capsule touched down safely at the Dongfeng Landing Site in Inner Mongolia Autonomous Region. Ground search and rescue teams, which had been drilling relentlessly with updated trajectories and response routes, executed a flawless recovery.

After nine anxious days, the three Shenzhou-20 astronauts were finally home. The four experimental mice accompanying them also emerged from the capsule in perfect health.

Then, on November 25, the Long March-2F Y22 carrier rocket roared into the sky at Jiuquan, carrying the uncrewed Shenzhou-22 spacecraft. This launch was historic: the program's first-ever emergency rescue mission. The spacecraft carried vital supplies—food, medicine, fresh produce and the specialized equipment needed to repair the cracked viewport on Shenzhou-20, which would later return to Earth autonomously.

This crisis-driven tempo rippled through the entire program. The development schedule for subsequent spacecraft was aggressively accelerated. Shenzhou-23's rollout was pulled forward by two months to January next year, and work on Shenzhou-24 was intensified for a summer delivery. 

This sequence created a unique historical footnote. Since astronaut crews are numbered for their mission spacecraft, the next team will be the "Shenzhou-23 crew." The "Shenzhou-22 crew" will remain permanently vacant in the official register. 

Chief designer Jia emphasized the ever-present uncertainties of the space environment, citing the escalating threat of space debris to spacecraft and crew. Confronting this challenge, he noted, is the critical work that lies ahead.

"Our only path forward is not only to uphold our standards, but to raise them," Jia concluded. "That is the only guarantee of success for the missions to come."

Copyedited by G.P. Wilson 

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