A window in the spacecraft only inches from disaster exposed China’s first in-orbit landing of a manned spacecraft due to the unyielding laws of physics in debris-related atmospheric return.
On 5th November, the return capsule of Shenzhou-20 was supposed to carry three astronauts back home after staying in space for six months in the Tiangong space station. Just before undocking, the crew members had noticed a linear defect in one of the return capsule’s porthole windows. Close analysis showed that there was indeed a triangular pattern of damage in the external debris layer, due possibly to the impact of debris from space that was less than 1 millimeter in size but moving at hypervelocity. According to Jia Shijin, designer of the Shenzhou spacecraft, “Our preliminary judgement is that the piece of space debris was smaller than 1 millimetre, but it was travelling incredibly fast. The resulting crack extends over a centimetre.”
The strike mechanism corresponds to known micrometeoroid and orbital debris (MMOD) physics: Submillimeter debris in LEO can strike with velocities of 7–11 km/s, imparting sufficient kinetic energy to shatter multiple-tier spacecraft window compositions.
The portholes in the Chinese station and the iss cupola feature layered window compositions consisting of debris pane,squeeze panes, and scratch panes.
However, engineers modeling the strike’s propagation during re-entry anticipated cabin depressurization and lethal volumes of gas entering the compartment due to ablation testing in the wind tunnels. The strike zone remained in the debris pane.
The China Manned Space Agency guidelines recommended that the affected spacecraft be removed from use. The astronauts from the Shenzhou-20 mission returned from orbit after nine days through the new Shenzhou-21 spacecraft. This period exposed the station with no “lifeboat.” This situation raised apprehensions among the safety professionals in space because of possible secondary failures such as fire-related risks, health-related problems, or debris collision. This period lasted for 11 days.
The contingency situation triggered the “one launch, one in standby” mode in CMSA’s contingency plans. The engineers were able to compress the test-to-launch timeline of more than 30 days in the backup Long March 2F rocket to only 16 days. According to Zeng Yaoxiang of the China Aerospace Science and Technology Corporation, this process demanded “full concentration and strict adherence to the plan, and uncompromising quality control.” On November 25, uncrewed Shenzhou-22 launched and docked with Tiangong 3.5 hours later.
The faulty window would now be investigated on Earth after the departures of Shenzhou-20 from the uncrewed Tiangong. According to CMSA’s spokesperson Ji Qiming, “the spacecraft would obtain the most authentic experimental data during the return.” This seems like a hint regarding the sensing meant to capture the propagation of stress and temperature in the faulty window upon re-entry. This information would be able to validate the ballistic limit equations in debris risk modeling software such as NASA’s ‘BUMPER,’ based on particle size, velocity, and material type.
MMOD protection plans are classified under passive protection plans, active protection plans, and operational plans. The passive plans include shelters and redundant systems. Active plans include debris avoidance. The operational plans entail crew exposure. For Shenzhou-20’s particular situation, the passive protection plan delayed the catastrophic failure. The operational procedures in this case ensured crew safety through return via alternate spacecraft. This particular case illustrates the shortcomings in debris tracking through current systems in place. The U.S. Space Surveillance Network currently provides debris tracking up to 10 cm in LEO, although sizes below 1 cm cannot be detected and may result in the end of the mission. The engineering community has known the significance of the shape of debris and the orientation in which they strike in terms of damage levels.
Even today, most ballistic limit assessments are carried out by considering spherical debris. However, in actuality, there may be many irregular pieces of debris due to the shattering of satellites in orbit. The use of such factors in risk analysis may allow more detailed analyses and may result in the redesign of debris panes or the use of more durable material. The case puts the Chinese system’s redundancy in human spaceflight under pressure.
The case also draws parallels with current events in the ISS orbit like the Soyuz MS-22 coolant leak or Boeing’s Starliner’s uncrewed safely back home due to hardware not being suitable for crew return. For both situations that put crew members’ safety in peril, the urgent deployment of another craft saw the return of crew safety margins. The uncrewed return of Shenzhou-20 would not only address China’s first in-orbit safety problem but would also contribute precious insight regarding in-orbit debris impacts in the range of submillimeter debris.
