A single crack in a spacecraft window can ground an entire mission and leave astronauts stranded hundreds of kilometers above Earth. That is just the situation facing Zhang Lu, Wu Fei, and Zhang Hongzhang, the three taikonauts of China’s Shenzhou-21 mission, who have now been left marooned aboard the Tiangong space station without a functional return vehicle.
The chain of events started with the scheduled crew handover between Shenzhou-20 and Shenzhou-21. The Shenzhou-20 team Wang Jie, Chen Zhongrui, and Chen Dong already broke the national endurance record with 204 continuous days in orbit. Their return capsule, which has been docked since April, was to bring them home on November 5. Hours before departure, a fragment of orbital debris hit the spacecraft, leaving small cracks in its viewing window. The China Manned Space Agency assessed that it did not meet the safety requirements for the reentry of a manned spaceship, citing the risk of structural compromise during atmospheric descent.
Shenzhou spacecraft are constructed in three detachable sections: a forward orbital module with living quarters, a central reentry capsule with ablative heat shielding and parachute systems, and an aft service module housing propulsion and power systems. The cracked window was part of the orbital module, but CMSA’s risk assessment decided that even non-critical damage could escalate under reentry stresses. Consequently, the Shenzhou-20 crew was reassigned to Shenzhou-21’s return capsule the only other crew-rated vehicle docked at Tiangong and landed safely in the Gobi Desert nine days later.
That rescue mission left the Shenzhou-21 crew without a “lifeboat,” a term used in spaceflight for a docked, crew-capable spacecraft available for emergency evacuation. The International Space Station keeps several such craft on station at any given time, but Tiangong’s smaller design about one-fifth the mass of the ISS hosts only one during normal operations. CMSA does have a plan for a lifeboat, though: a standby Shenzhou capsule and Long March 2F rocket kept at near-launch readiness at Jiuquan Satellite Launch Center. In theory, it can be launched in as little as 8.5 days if an emergency occurs. Reports of airspace closures suggest Shenzhou-22 may launch unmanned as early as November 25, carrying supplies and acting as the escape vehicle for the stranded crew.
The debris strike only underlines the increasing danger of orbital debris: millions of fragments, from spent rocket stages, defunct satellites, and previous collisions, travel at speeds over 7 km/s-faster than a rifle bullet. Even millimeter-sized particles can penetrate spacecraft skins or destroy sensitive components. Tiangong itself has already suffered the strike of a solar panel in 2023; this spurred engineering to add localized shielding within the spacewalks. Similar threats face the ISS, which often maneuvers to avoid tracked debris, repairing damage it sustains-as has been the case with the Canadarm2 robotic arm.
Until Shenzhou-22 arrives, the Shenzhou-21 crew remains on its scheduled six-month mission, conducting experiments and performing maintenance work on the station. Psychologically, long-extended uncertainty about return distorts time perception-a phenomenon witnessed in NASA’s Starliner astronauts who have gone through months-long delays. Antarctic research stations are a good parallel: crews at extreme isolation report that constant engagement in complex tasks makes time fly by and reduces stress during extended waits. After the new capsule docks, the damaged Shenzhou-20 will likely be detached and deorbited into the Pacific Ocean.
The incident has also underscored the operational limits of Tiangong when two missions overlap, with sleeping quarters, oxygen supply, and waste management systems being stretched to their limits to support six astronauts on board. This is perhaps one reason why CMSA ordered Shenzhou-20 back to Earth before launching a replacement, although official explanations have been scant. For now, the safety of the Shenzhou-21 crew hangs on the accuracy of China’s contingency planning and the successful arrival of Shenzhou-22 before another fragment of high-speed debris tests Tiangong’s defenses.
