What challenges do astronauts need to overcome to return to Earth?

Chinese astronauts Tang Hongbo, Tang Shengjie and Jiang Xinlin completed their historic mission in the Shenzhou 17 manned spacecraft. With the successful landing of the spacecraft's return capsule at the Dongfeng landing site, their six-month space journey came to a successful conclusion. However, the journey back to Earth from space is not easy. The astronauts need to go through five stages: separation from the space station, braking and deceleration, re-entering the atmosphere, parachute deceleration, and landing cushioning, before they can safely embark on the journey home.

Shenzhou manned spacecraft configuration

Preparations for the return capsule before leaving space

If the astronauts want to return to Earth, they first need to adjust the "posture" of the spacecraft, just like we have to turn the front of the car in the direction of home before driving home. The spacecraft first rotates 90° counterclockwise in the horizontal direction, changing from the state of the orbital module in front, the return module in the middle, and the propulsion module in the back to a horizontal flight state. This is the first "posture adjustment". Then the orbital module and the return module are separated, and the spacecraft becomes a combination of the propulsion module and the return module. Then this combination rotates 90° counterclockwise again, so that the propulsion module is in front and the return module is in the back, and the angle is adjusted to prepare for "braking". Next, the brake engine of the propulsion module starts working, generating a force opposite to the flight direction, slowing down the spacecraft, leaving the original flight orbit, and entering the return orbit. This is like "brake" for the spacecraft, slowing it down and preparing to go home. After that, the return module and the propulsion module combination begin to fall freely.

When descending to more than 140 kilometers from the ground, the propulsion module and the return module will "break up". The propulsion module will be burned by the high temperature generated by atmospheric friction, while the return module will continue to descend. At this time, the return module must find the most correct "posture" to enter the atmosphere, that is, to establish the correct re-entry attitude angle. This angle is very important. If the angle is too small, the return module will pass the edge of the atmosphere and cannot return home; if the angle is too large, the return module will return too fast and will be burned in the atmosphere like a meteor. Therefore, the propulsion subsystem of the return module must work well to ensure that the spacecraft can return to the atmosphere safely and accurately.

The return capsule enters the atmosphere

The fiery test of crossing the atmosphere

When the return capsule is about 100 kilometers from the ground, it will begin to re-enter the atmosphere. At this time, its speed is as high as 7.9 kilometers per second. Such a fast speed will produce violent friction when colliding with the atmosphere. The heat caused by friction can instantly turn the return capsule into a glittering fireball. But don't worry, the return capsule is designed with a variety of thermal protection measures to completely solve the high temperature problem. The most worrying thing is that a layer of plasma gas is generated around the return capsule. This layer of gas will block all electromagnetic waves. At this time, the return capsule will lose contact with the ground and a "black barrier" phenomenon will occur. This situation will last for about 240 seconds, but don't worry, when the return capsule is about 40 kilometers away from the earth, this "black barrier" will magically disappear, and the ground measurement and control department will be able to find the location of the return capsule again.

Parachute smooth landing

A key role in ensuring safety: parachutes

When the return capsule descends to about 10 kilometers from the ground, the recovery and landing system begins to work. First, the parachute canopy pops up, and then the guide parachute, deceleration parachute and main parachute pop up one after another. The small guide parachute comes out first to serve as a vanguard, then the deceleration parachute comes out to help decelerate, and finally the main parachute comes out. Their area is constantly increasing, from a few square meters to dozens of square meters and then to more than 1,000 square meters.

Why do the parachutes have to be opened separately? Wouldn't it be faster to open all of them? It turns out that because the main parachute is 1,200 square meters, if it is opened all at once, the air will break the parachute, so the designers designed a three-level parachute opening procedure, which is partially opened first and then fully unfolded. This can reduce the overload when opening the parachute and reduce the discomfort of the astronauts. First, the guide parachute is opened, and then the guide parachute pulls out the deceleration parachute. After working for a period of time, the deceleration parachute separates from the return capsule, and at the same time, the 1,200 square meter main parachute is pulled out. In the process of going from about 8,000 meters to 6,000 meters above the ground, the speed of the return capsule can be reduced from 90 meters/second to about 8 meters/second. In addition, in order to ensure safety, the return capsule is also equipped with a backup parachute. If the main parachute system fails, the system will automatically determine and start the backup parachute to ensure the safety of the astronauts.

During the entire landing process, in addition to the parachute, the return capsule is also equipped with landing cushioning devices such as reverse thrust engines and seat systems. These devices come into play when the return capsule descends close to the ground, further reducing the landing impact load and providing additional shock absorption and cushioning protection for astronauts.

The astronauts' return journey is truly a thrilling adventure. From space to Earth, every step of the return capsule's return is crucial and closely linked. A successful return flight is not only a powerful testimony to the continuous progress of China's space industry, but also inspires us to continue exploring the mysteries of the vast universe and write a new glorious chapter for the development of the space industry.

Some information comes from People's Daily Online, CCTV.com, Shangguan News, etc.

(Scientific review: Liu Yong, researcher at the National Space Science Center of the Chinese Academy of Sciences)