On the way from the International Space Station (ISS) to Earth, space agencies and companies decide on priority and alternative landing sites. Most spacecraft that can land on the runway can also land on the sea or ocean. Following the evaluations, the basic landing target for the spacecraft is determined.
If the aim is to land in the ocean when returning from the ISS to Earth, values such as wind speed, wave range and height in the ocean, and the probability of rain and lightning are important. If these are not at the appropriate level, the process of returning to Earth may take longer. The visibility during the day and at night must also be within a certain range.
In addition, the operational capability of the rescue helicopter must be tested. The helicopter crew is available to respond to situations requiring faster access to shore or emergency medical intervention. Typically, this process involves two ships located near the target landing sites.
From the mission control center, changes are observed as to whether the process of leaving the ISS should continue or not. If the conditions are right, preparations will begin for the spacecraft’s departure from the ISS at the designated time.
Astronauts make preparations by checking procedures when they want to return to Earth from the ISS. During this journey, astronauts wear pressure-resistant suits.
Calculations are important to start the process
When calculating and preparing for return to Earth, the friction and aerodynamic calculations involved in entering the atmosphere are important to complete the process safely. Engineers involved in the process design the heat shield in the spacecraft according to the necessary calculations.
The process of returning to Earth begins when the gate connecting the spacecraft to the ISS closes and the vehicle begins to move away from the ISS.
Once the spacecraft leaves the ISS, it begins moving in a different orbit than the station with its engines turned off and is then pulled towards Earth by gravity. The spacecraft’s engine runs for a while and its combustion causes it to sink into orbit and enter the Earth’s atmosphere.
For example, the Russian Soyuz spacecraft breaks into three pieces as it enters the atmosphere. The orbit and service module disappears while in the denser layers of the atmosphere, and only the landing module with the astronauts returns to Earth.
The atmosphere is a natural braking mechanism
The Earth’s atmosphere acts as a natural braking mechanism for the spacecraft, slowing it down. The thermal protection of the landing module absorbs the heat generated by friction with the atmosphere and significantly reduces the speed of the module. As the module enters the atmosphere, the parachute opens, further reducing the capsule’s speed.
When the spacecraft re-enters Earth’s atmosphere, it travels faster than the speed of sound, the temperature increases and the air density decreases. For example, NASA’s Orion spacecraft travels 25 times faster than the speed of sound as it enters the atmosphere.
The size of the spacecraft used is also important in this context. For example, the biggest difference between the Space Shuttle and Soyuz is reentering the Earth’s atmosphere. While entering Earth’s atmosphere is easier on the Space Shuttle, this situation can almost be described as a car crash in the Soyuz capsule.
Astronauts may have difficulty walking
After the landing is safely completed, rescue teams head to the area. The spacecraft capsule is brought onto the ship, its entrance is opened and the astronauts are removed from the capsule.
When astronauts return to Earth, they may have difficulty walking, feel sick, and undergo necessary health checks. Exercise is also important for astronauts who rest after the mission.
On average, a spacecraft’s return to Earth can take between 4 and 12 hours.