"There is a triangular mark, not very large in size. When we first saw the photos, it was difficult to determine exactly what it was—it looked as if something like paint was stuck to it." On Nov. 5, the return mission of Shenzhou-20 was urgently paused. During the routine inspection before reentry, the Shenzhou-20 crew spotted an anomaly on the porthole's edge.
Jia Shijin, chief designer of the crewed spaceship system at the China Aerospace Science and Technology Corporation, revealed: "After analysis, we have confirmed that penetrating cracks have indeed appeared on the porthole, meaning the entire glass structure has been penetrated from the inner to the outer surface. This was caused by a space debris impact. The space debris responsible is preliminarily judged to be less than a millimeter in size." Within 12 hours of confirmation, the mission command headquarters decided to postpone the return of Shenzhou-20. Faced with the uncertainties of space exploration, China's space program demonstrated a precise and efficient emergency response in this real-world scenario. It showcases not only its systematic safety assurance capabilities, but also a forward-thinking mindset in turning challenges into opportunities.
According to Xinhua Daily, the spacecraft's porthole is a three-layer structure, with the outermost serving as the thermal protection layer (part of the thermal protection system). During atmospheric re-entry, this layer acts as the primary defense against frictional heat exceeding 1,000°C. The penetrating crack breaches this critical safety barrier.
External imaging from the station's robotic arm cameras assists experts in damage assessment
The Shenzhou-20 crew photographed the cracks from multiple angles and under different lighting conditions, and the station's robotic arm cameras were used to take supplemental external pictures. More than a dozen domestic glass industry experts assessed the damage based on the transmitted images.
Jia noted that during the discussion, two issues needed to be confirmed: first, whether the observed feature was indeed a crack; second, whether it was a penetrating crack or merely a surface crack. After discussion, a consensus was ultimately reached: it was a penetrating crack.
Jia stated that the worst-case scenario would be crack propagation, eventually leading to the complete detachment of the glass panel. In such a situation, the inner sealing glass would malfunction under high-temperature conditions. During re-entry, the entire porthole could then fail, resulting in cabin depressurization and the ingress of high-speed airflow into the cabin, which could cause unpredictable damage.
Low-probability event: Suspected debris impact on porthole side
"It's quite extraordinary, as striking our spacecraft is already highly improbable, and hitting the porthole area makes it even less likely," stated Jia. It was essential to determine whether any defects existed in the initial product design and whether such defects might have caused the in-orbit crack in the glass. "The design at the time was sound, as evidenced by the design philosophy, the design scheme, and the ground-based testing and verification work. Moreover, the glass used was of high hardness and considerable thickness. Therefore, the most probable cause of the crack should be space debris. Our current analysis suggests it struck the side of the porthole—in areas such as the chamfered edges—where stress concentration is particularly prone to occur compared to the main planar surface."
"If there is any concern, it is acceptable to postpone."
Since the Shenzhou-12 mission, China Manned Space has implemented a "one operational, one standby" strategy. This means that for each spacecraft launched, a backup spacecraft is kept ready on the ground. If an emergency launch is activated, the standby spacecraft immediately enters testing procedures for contingency operations. When the unexpected issue arose with Shenzhou-20, the backup spacecraft for Shenzhou-21, Shenzhou-22, was already on standby duty at the Jiuquan Satellite Launch Center. Within less than 12 hours of confirming the presence of the crack, the mission command headquarters decided to postpone the return mission of Shenzhou-20.
Ji Qiming, assistant director and spokesperson of the China Manned Space Agency, stated, "Safety always comes first. If we lack confidence or feel uncertain, delaying is perfectly acceptable."
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