On April 28, the two astronaut crews of Shenzhou-17 and Shenzhou-18 held a handover ceremony in the Chinese space station, and Shenzhou-17 mission commander Tang Hongbo handed over the keys to the Chinese space station to Shenzhou-18 mission commander Ye Guangfu. The handover of the keys to the space station also marks the end of the Shenzhou-17 mission.
On April 30, three astronauts, Tang Hongbo, Tang Shengjie and Jiang Xinlin, entered the return capsule of the Shenzhou-17 spacecraft, which was evacuated from the Chinese space station, returned to the earth's atmosphere after flying around the earth for five times, and then landed at the Dongfeng landing site.
As early as 28 April, the astronauts had already held a handover ceremony, so why did they not evacuate from China's space station until 30 April? Can't they be evacuated early? Just like the situation of the spacecraft launching into space, the spacecraft also needs to take into account a "window" when returning home. Because the position of the spacecraft and space station assembly changes every moment during the flight around the earth, it needs to return to the earth at a suitable time in order to quickly land near the pre-selected landing site of the landing site.
Shenzhou-12 was the first manned spacecraft to go to the Chinese space station, when it was evacuated from the Chinese space station, it flew for about 28 hours before landing on the ground, of course, after the evacuation of the Chinese space station, the Shenzhou-12 spacecraft did not return immediately, but carried out the technical verification of radial rendezvous and docking. The current Shenzhou manned spacecraft all adopt the mode of rapid return, which has been reduced from the previous Shenzhou 12 orbiting the earth 18 times to the current 5 circles, and the return time has also been greatly shortened.
The spacecraft landed on land
The process of the spacecraft's return is actually more complicated, although the rapid return mode is adopted, and the number of circles around the earth is shortened to 5, but after the evacuation from the Chinese space station, it will also need to continue to fly for several hours before landing on the ground.
This is because the spacecraft and space station assembly operate in a circular orbit about 400 kilometers from the earth, and when the spacecraft is evacuated from the space station assembly, it will not immediately enter the atmosphere, but will need to carry out a series of operations, including the adjustment of the first flight attitude, the separation of the orbital module and the return module, the propulsion module expects the engine brake to slow down, and the separation of the return module and the propulsion module.
After completing this series of operations, the return capsule carrying the astronauts will find a suitable time to cut into the Earth's atmosphere. During the re-entry into the atmosphere, due to the rather high speed of the spacecraft's return capsule, exceeding 7 kilometers per second, the aerodynamic heating effect was strong, and the outside temperature rose to one or two thousand ° C. Under the scorching heat of such a high temperature, the exterior of the spacecraft's return capsule will also be burned black beyond recognition.
When the re-entry capsule descends to about 10 kilometers above the ground, the guidance parachute and parachute are opened, and the speed of descent is reduced with the help of a large parachute. When it is almost 1 meter above the ground, the return capsule of our Shenzhou manned spacecraft will also start the thrust reverser engine to complete the final deceleration, and finally the spacecraft return capsule will land on the ground.
I believe many friends have seen the process of the spacecraft returning to the earth, as well as the scene of the moment when the return capsule landed on the earth. When the spacecraft returns to the ground, smoke and dust billowed from the landing site, and sometimes fires can be seen. This is actually the thrust reverser engine started in the final stage of the return capsule, which allows the spacecraft to land at a smaller speed and can better protect the astronauts in the spacecraft. Such a scenario occurs when Chinese and Russian spacecraft return, but this does not occur when American spacecraft return to land.
This is because Chinese and Russian manned spacecraft all land at landing sites, while American spacecraft choose to land at sea. Compared with land, the sea has two obvious advantages, one of which is that the sea water itself can play a buffering role, when the American spacecraft hits the sea surface, the sea water absorbs the kinetic energy of the spacecraft's descent, so there is no need for the thrust reverser engine to complete the final deceleration.
Another advantage is that in the surface area of the earth, the area of the ocean is much larger than the area of the land, nearly 71% of the surface area of the earth is the sea, when the spacecraft returns from space to the earth and lands into the sea, the selection of landing sites will not be very demanding. When landing on land, you need to choose those sparsely populated places, such as the Dongfeng landing site where Shenzhou 17 landed.
Since there are many advantages to landing on the sea, why did the United States choose to land on the sea, while the Chinese and Russian spacecraft landed on land? One of the reasons is that the United States has bases all over the world, and their ships can quickly reach the sea area where the spacecraft landed to carry out search and rescue work.
The astronauts were carried out of the capsule
Whether it is the mainland's Shenzhou manned spacecraft, Russia's Soyuz spacecraft, or the United States' Manned Dragon spacecraft, and whether the spacecraft lands on land or into the sea, when astronauts leave the capsule, they are generally carried away. Exceptions are made in special circumstances, such as at the end of some missions with very short flight durations, when astronauts can walk standing and walking out of the capsule. What the hell is going on?
The reason is simple, that is, the environment in space is different from the environment on the surface of the earth. During the short-term space flight, although the astronauts also live in the weightless environment of outer space for a period of time, the impact on the astronaut's body is not very large, and when the astronaut returns to the earth after the mission, the astronaut can re-adapt to the gravity environment on the ground after a little rest. During a long time in space, the astronaut's body is more affected, and he cannot stand up immediately when he returns.
Because of being in the space environment for a long time, astronauts' bodies have changed greatly, and one of the problems is the accelerated bone loss and osteoporosis. When astronauts return to Earth, if they stand up immediately, they may fall or break bones. Therefore, in order to avoid injuries to the astronauts after leaving the capsule, the staff will carry the astronauts and let them rest on specially developed chairs to slowly recover their bodies. After a period of rehabilitation, the astronaut's body will return to the level it was before the mission.