Is one cracked window enough to decide the fate of an entire space mission? For three Chinese astronauts on board the Tiangong space station, that question is no longer theoretical, but the definition of their reality. Commander Zhang Lu, along with Zhang Hongzhang and Wu Fei of the Shenzhou-21 crew, are now orbiting Earth without a dedicated return craft after their own spacecraft was employed to rescue another team whose capsule got damaged by space debris.
Events started to unfold when the Shenzhou-20 crew, comprising Wang Jie, Chen Zhongrui, and Chen Dong, prepared to return to Earth on November 5 after their 204-day mission. Unfortunately, its return capsule was found with a crack in the heat-resistant glass of a small viewport window. According to the China Manned Space Engineering Office (CMSEO), “Based on preliminary analysis of photographs, design review, simulation analysis, and wind tunnel testing, a comprehensive assessment determined that the Shenzhou-20 manned spacecraft’s return capsule window glass had developed a minor crack, most likely caused by an external impact from space debris, thus failing to meet the requirements for a safe crewed return.” The risk was clear: during re-entry, plasma heating could compromise the integrity of the window, leading to decompression.
With Shenzhou-20 not safe to fly, CMSEO decided to use the newly arrived Shenzhou-21 capsule to return the Shenzhou-20 crew on November 14. That left the incoming crew without a “lifeboat” an essential safety redundancy for any crewed station. While Tiangong does have emergency evacuation procedures, the only crew-rated vehicle currently docked is the damaged Shenzhou-20, which is unfit for human return.
China’s human spaceflight program keeps a Long March 2F rocket and Shenzhou spacecraft in near-readiness at the Jiuquan Satellite Launch Center, and can be readied in as little as 8.5 days. However, orbital mechanics are very constraining for phasing of a rendezvous, so launch timing can be delayed even if the hardware is ready. An airspace closure notice now suggests an uncrewed Shenzhou-22 launch around November 25, carrying both supplies and serving as the crew’s eventual return vehicle.
The Shenzhou return capsule is a three-module spacecraft similar to Russia’s Soyuz, carrying a reentry module designed to withstand severe thermal and mechanical loads. Its safety systems include redundant parachutes, solid-fuel braking thrusters, and a pressure vessel rated for micrometeoroid impacts. Yet, as this incident shows, even small debris often less than a centimeter in diameter can inflict mission-ending damage when traveling at relative velocities near 8 km/s.
The design of Tiangong also has a bearing on the decision-making. With a space station about a fifth the size of the International Space Station, it has a total of six sleeping quarters-three in the core module Tianhe and three in the lab module Wentian-to accommodate two crews during handover. However, prolonged occupation by double crews would put extra loads on life-support systems such as water recycling, oxygen generation, and carbon dioxide scrubbing. The extended stay of the Shenzhou-20 crew before their rescue probably hastened consumable depletion, while cargo delivery aboard the Shenzhou-22 became indispensable.
Of course, space debris mitigation is a challenge that every orbital operator faces. Tiangong and the ISS frequently execute avoidance maneuvers, but the tiniest fragments avoid detection. Conversely, the implementation of shielding protection such as the debris protection panels installed during an eight-and-a-half-hour record-breaking spacewalk just this year does lower risk, but it cannot cover every exposed surface. The Shenzhou-20 window’s vulnerability underlined the deficiency in current shielding strategies.
As the Shenzhou-21 astronauts undertake their six-month mission, the echoes of similarity with past incidents await. In 2024, NASA astronauts Butch Wilmore and Suni Williams got into a somewhat similar predicament: Boeing returned its Starliner capsule uncrewed to Earth because of technical problems, and they were forced to wait for rescue aboard the ISS. The difference, however, was that the ISS had other functioning crew vehicles that were docked with immediate evacuation capability.
Tiangong’s does not, in its current configuration. The damaged Shenzhou-20 will eventually be detached and deorbited into the Pacific Ocean. Until Shenzhou-22 arrives, Zhang Lu, Zhang Hongzhang, and Wu Fei depend upon Tiangong’s systems and CMSA’s launch schedule. Every day without a lifeboat is a calculated risk one that highlights both the engineering resilience and the operational vulnerabilities of China’s rapidly maturing space station program.
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