The first step for Shenzhou XIII astronauts to return to Earth is to evacuate from the space station to the manned spacecraft and close the doors of the two-way pressure chamber. Prior to this, astronauts have completed all the sequencing work of the space station and set relevant parameters. Just like turning off the lights and closing the door before going out, the "lights and doors" of the space station should also be closed.

Astronauts who enter the manned spacecraft will put on the cabin pressure suit they passed during the expedition and be on duty in the return cabin for the first time. Astronauts should also make some preparations before returning to the return capsule, such as setting the return state and sending on-orbit instructions. After that, under the control of the ground control center, the Shenzhou 13 manned spacecraft was operated to separate from the space station assembly and evacuated from the space station.

The return module first retreated to the position of 19 meters below the radial direction of the core module of the space station, and then retreated to the position of 200 meters, becoming a flight attitude. After that, he flew about five times in orbit. At this stage, Shenzhou XII circled the earth by 1 1 circle, and this quick return technology saved about 9 hours at this stage.

In the process of separation and evacuation, there may be a risk that if there is pressure leakage when the two-way pressure chamber is opened, the threat to astronauts will be fatal. Once the pressure chamber leaks, the air in the chamber will disappear instantly and become a vacuum environment, and astronauts will have decompression sickness, and even body fluids will boil in severe cases.

2. Step 2: Brake derailment.

After the manned spacecraft is separated from the core module of the space station, it needs to adjust its attitude for the first time. For example, posture adjustment, just like when we drive home, turn the front of the car in the direction of going home.

After the direction adjustment, the orbital module and the return module will be separated. After separation, the attitude will be adjusted again. After adjustment, it will be in an inverted state. After that, the braking system of the propulsion module is started, and the return module enters the return orbit from the original flight orbit. This process is called braking derailment, and the return capsule can only return to the earth if it enters the predetermined return orbit and forms an appropriate angle with the earth's atmosphere.

After entering the return orbit, the spacecraft will enter the inertial sliding phase. Under the action of atmospheric resistance and braking system, the flight speed and altitude of the return cabin are gradually decreasing. After descending to a certain extent, the propulsion module will be separated from the return module, the propulsion module will be completely disintegrated under atmospheric friction and burned in the atmosphere, and the return module will continue to fall at high speed towards the earth.

The key point at this stage is that the angle between the flying direction of the spacecraft and the earth's atmosphere must be well controlled. If the included angle is too small, the spacecraft flying at high speed is likely to pass by the atmosphere; If the included angle is too big and the spacecraft falls too fast, the astronauts in the cabin can't bear such a fast acceleration.

Step 3: Re-enter the atmosphere.

When the spacecraft is about 145 km from the ground, the return module and the propulsion module begin to separate. During the separation process, the return cabin will vibrate violently and roll slowly, so the astronauts don't feel very good in the cabin.

When the return capsule is 0/00km away from the ground, it will re-enter the atmosphere. At this time, the air density is increasing, and the temperature at the bottom of the return capsule is as high as thousands of degrees Celsius, and the whole return capsule is surrounded by flames.

At the same time, the shock wave heating of the return capsule to the atmosphere and the ablation of the thermal insulation layer will produce plasma, which will absorb and reflect electromagnetic waves and interrupt the radio communication between the return capsule and the outside world. This effect is called black barrier. The return capsule will enter the "black barrier", where it will be completely lost. This state lasts about four to five minutes.

This stage is the most dangerous step in the whole process of reunification. The astronauts in the return capsule have to bear the pressure of four times their own weight, bear severe vibration and enter the state of losing contact, which requires very high physical and psychological requirements for astronauts.

Step 4: Landing safely.

At a distance of 10 km from the ground, the return cabin will open the guide umbrella, the deceleration umbrella and the main umbrella in turn. The main parachute is as big as 1200 square meters, but the return capsule cannot be opened at once when landing, otherwise the parachute will be broken by air.

The designer tailored a three-level parachute opening program for the spacecraft. First, open two series-connected guide umbrellas, and then pull out a deceleration umbrella from the guide umbrellas. After working for a period of time, the deceleration parachute is separated from the return cabin and the main parachute is pulled out at the same time.

In order to prevent the deceleration parachute and the main parachute from being stressed too much at the moment of opening, both umbrellas adopt the umbrella retraction technology. This technology can make the main umbrella of 1200 square meters open in stages, and ensure that the astronauts' bodies can withstand overload during the whole opening process.

When the altitude reaches 1 m from the ground, the return capsule starts the reverse propulsion system, and the descending speed is reduced to about 2 meters per second, so that the return capsule finally lands safely. When we returned to Shenzhou 12, the fire we saw when the return capsule landed was the effect of the start of the thrust reverser system.

During the last 10 km, the biggest challenge that astronauts need to face is overload and impact. At the moment when the parachute of the return capsule opens, the astronauts' bodies will bear greater overload instantly, and the astronauts will bear an impact when landing, which is no less than a small car accident.

Step 5: Open the hatch.

After the return of Shenzhou XIII, we still chose to land at the Dongfeng landing site. The search and rescue team has already had the search and rescue experience of Shenzhou 12, and I believe that the search and rescue speed will be faster and more accurate.

After the return capsule lands, the search and rescue team will arrive at the same time, and the first one to rush to the return capsule is of course the pilot. Opening a cabin is definitely a technical activity, which requires not only a comprehensive mastery of opening skills, but also a strong psychological quality.

In the process of returning, the return capsule will have fierce friction with the initiator, and the shell will produce high temperature, which will lead to the imbalance between the pressure inside the capsule and the pressure outside the capsule. Then when opening the cabin, we must control the pressure balance inside and outside the return cabin, and we must not open the cabin too fast, otherwise the astronauts will not adapt. Of course, you can't open the cabin too slowly, otherwise it will delay the opening time.

When Shenzhou 12 returned, it was Yi Feng who opened the cabin. This time, Yi Feng's apprentice Su Liming was responsible for opening the hatch of Shenzhou 13. When Shenzhou 12 returned to the search and rescue mission, Su Liming was the flag bearer. This time, he and Yi Feng exchanged positions as pilots, while Yi Feng served as standard bearer.

After the hatch is successfully opened, the medical staff will check the physical condition of the three astronauts at the first time. After confirming that there is no problem, the staff will help the three astronauts to get out of the cabin in turn and carry the three space heroes to the chairs prepared in advance.