The 16th astronauts of Shenzhou 100 exited the spacecraft safely! Some photos of the return capsule are black and some are gold. What's going on?

Shenzhou 16 returned home successfully, and many friends have watched the live broadcast or news. Many friends may have noticed that in the photos of the return capsule on the ground, some look golden, while others look black. What's going on?

Judging from this photo released by Xinhua News Agency, the return capsule of Shenzhou 15 is pitch black. The photo was taken by Xinhua News Agency reporter Ren Junchuan.

The image of the Shenzhou 16 return capsule is from Xinhua News Agency, photographed by Li Gang

The image of the Shenzhou 16 return capsule and astronauts leaving the spacecraft is from Xinhua News Agency, photographed by Li Gang

Why is only one side of the return capsule dark?

This is definitely not due to color difference, lighting and other issues. When the return capsule re-entered the atmosphere, one side was indeed "blackened", but the other side was relatively clean. By changing the angle of the shot, everyone saw different looks.

Huh? It burned unevenly. Was there a safety accident? Actually, there wasn't one. If we look through the old data, we will find that all the return capsules dating back to Shenzhou 1 have been black on one side and shiny on the other.

Shenzhou 1 return capsule

This uneven ablation is actually designed. When the spacecraft reenters the atmosphere, the speed is as high as 7.9 kilometers per second. In order to consume such a large amount of kinetic energy and bring the astronauts back safely, the atmosphere must help brake it. But on the one hand, we hope that the atmosphere can brake it, but on the other hand, we don't want it to brake too hard.

What happens if the atmospheric brakes are applied too hard?

First, it will directly harm the astronauts inside. Although astronauts are selected from the best physical fitness and have undergone various trainings, they can even withstand 8 to 9 times the gravity acceleration of the earth. However, astronauts are flesh and blood after all. If they exceed the limit, it is difficult to ensure safety. If the deceleration is too drastic, it may cause the astronauts to be overloaded or even injured.

Braking too hard could also cause the return capsule to overheat and become dangerous. Many people took photos of the spectacular scene when Shenzhou reentered the atmosphere, with the return capsule flying in front and the propulsion capsule disintegrating and chasing behind, like two fireballs, one controlled and the other uncontrolled.

Shenzhou 16 enters the atmosphere. Image from Xinhua News Agency video

The atmosphere on the windward side of the return capsule is compressed violently, instantly heating up to 1,000 to 2,000 degrees Celsius, ionizing the surrounding atmosphere and forming a plasma sheath around the return capsule (this is also the origin of the communication "blackout"). If the temperature is higher, the return capsule will most likely burn up.

Therefore, in order to decelerate gently, when the return capsule re-enters the atmosphere, its blunt-nosed heat-shielded outsole is not facing the direction of flight (because this would cause too much resistance, increasing the burden on astronauts and the risk of the return capsule burning), but is slightly tilted towards the earth.

Schematic diagram of the Apollo return capsule. Note that there is an angle between the forward direction and the bottom of the return capsule. Image from NASA

Just like a water skier, half-crashing and half-floating in the atmosphere, the return capsule will not overheat and the astronauts inside will feel much more comfortable. This reentry method is called "semi-ballistic".

Image from pixabay.com

The shape of the heat-proof outsole is almost an arc, slightly tilted downward, so that the upper part of it is more exposed to the wind, just like when we run against the strong wind, if our whole face is facing forward, we will not be able to open our eyes and breathe. If we lower our head slightly, these problems will be solved, and the forehead that is not afraid of the wind will have to bear stronger wind.

Therefore, the upward side of the return capsule that reenters the atmosphere with its head slightly lowered will be hotter and darker, while the other side will be cooler and remain golden.

What happened to the return capsule being “burned”?

What are those black things? It is not that the return capsule was burnt, but the heat-resistant coating on the surface of the capsule left behind carbide remains. Facing high temperatures, we cannot just stand it. The coating of these ablative materials sublimates and falls off, which can take away a lot of heat, thus preventing the capsule from overheating.

The reason is very simple. We all know that the pot will become burnt only when the water is boiled dry. This is because the boiling water takes away a lot of heat when it turns into steam, which can keep the temperature in the pot at boiling point.

Schematic diagram of material ablation process from reference [1]

Comparison of materials before and after ablation from reference [1]

These technologies are used every time Shenzhou returns. This time, Shenzhou 16 has gone a step further and added some innovation to the heat-resistant coating. The coating on the porthole of the return capsule has a self-cleaning function, which can prevent ablation pollution and ensure that the astronauts can observe the external situation through the window during the return process. After the return capsule lands, the ground search and rescue personnel can also observe the cabin through the window. This is a masterpiece of the Shanghai Institute of Ceramics, Chinese Academy of Sciences. Praise them!

Why didn’t the bottom of the re-entry cabin turn black?

Let me add one more thing. If we look at the photos of Shenzhou 15, we will find a strange phenomenon. As shown in the picture below, the bottom of the return capsule looks quite clean, doesn't it? It is not burned at all. What's going on?

The image of the Shenzhou 15 return capsule was taken by Ren Junchuan of Xinhua News Agency.

It turns out that this is not the original bottom of the return capsule. The original heat-resistant bottom looks like this↓

The heat shield bottom of the Shenzhou spacecraft return capsule. Image from CCTV News Channel

The heat-proof outsole is installed at the bottom of the return capsule. When the return capsule returns, it will bear the heaviest protection task and face the most severe challenges. It will indeed be burned black.

Schematic diagram of heat-resistant outsole from reference [2]

However, after the return capsule opens the parachute for a period of time, the heat shield will be thrown away.

The blurred shadow is the discarded heat-resistant outsole.

After discarding the heat shield, the return capsule will continue to descend. Before finally landing, it will ignite several solid reverse thrust rockets and finally brake to allow the astronauts to land safely.

Photo courtesy of Xinhua News Agency, Li Gang

Finally, let's review the key points of this article. The return capsule is indeed burned black in one direction, while the other direction looks much brighter. This is caused by the attitude of the return capsule when it decelerates when entering the atmosphere. The burned black part of the return capsule is actually the carbide left after the heat-resistant coating on the surface of the capsule was destroyed by high temperature. These mottled marks are proof that the various heat-resistant measures have worked successfully.

References

[1] Fang Zhou, Liang Xin, Deng Huoying, Dong Yanzhi, Dai Xiaowei. Performance of honeycomb reinforced low-density silicon-based ablative heat protection materials[J]. Aerospace Materials and Technology, 2021, 51(05): 79-83.

[2] Dong Yanzhi. Design of the heat-resistant bottom structure of Shenzhou spacecraft[J]. Spacecraft Engineering, 2002, 11(4):4. DOI:CNKI:SUN:HTGC.0.2002-04-007.

Author: Qu Jiong, popular science creator

Review丨Liu Yong, Researcher, National Space Science Center, Chinese Academy of Sciences