Soyuz – Returning To Earth

After six months aboard the International Space Station, French astronaut Thomas Pesquet is preparing to return to Earth on 2 June 2017.

While we often focus on an astronaut’s journey into space, this seems like a good opportunity to take a look at the final leg of an astronaut’s journey – the voyage home.

Undocking and Re-Entry

Currently, the only way to get astronauts to and from the International Space Station is on Soyuz, the ‘workhorse’ of Russian spacecraft. The journey back to Earth is much quicker than the journey into space. A Soyuz trip to the space station takes around six hours from launch to docking. A return journey takes around half the time, at just over three hours to make it safely to the ground. This is because landing is a little more straightforward – you’re working with gravity, rather than against it. The trick is controlling the return so that you don’t return too quick and burn up in the atmosphere, or return too slow and skip off the top of Earth’s atmosphere, out into space.

The start of a return to Earth can be serene. Once the Soyuz capsule is loaded with up to three astronauts, it undocks from the station and drifts away at the gentle rate of 10 centimetres a second. Three minutes later, a very short burn from the Soyuz thrusters helps increase the separation. The aim is to allow the capsule to drift more than 500 metres away from the International Space Station. This is a safety zone that is usually kept clear, unless a spacecraft is docking or undocking. Following this, Soyuz enters a ‘parking orbit’ where it stays in a stable orbit around the Earth for around two and a half hours. During this time, the crew check everything for re-entry.

Once the crew and mission control are happy that the spacecraft is working as it should, the re-entry burn can begin. The re-entry burn is a four minute burn designed to slow the spacecraft down. As Soyuz slows, its orbit drops lower. Once low enough, Soyuz’s path will enter Earth’s atmosphere. The interaction of the spacecraft with the atmosphere is what will provide most of the braking to return Soyuz to Earth. The burn itself only lowers the spacecraft’s speed by about 120 metres a second (compared to the starting speed of 7,660 metres a second). Despite this small change, it is enough to dip the spacecraft’s orbit low enough.

After the re-entry burn is completed, Soyuz’s orbit is again checked for accuracy, in case any correction burns are needed. If the teams are happy, then Soyuz splits into three separate pieces. It is the central gumdrop-shaped section of Soyuz, called the descent module, that returns to Earth. The other two sections are burnt up in the atmosphere. This separation occurs around 20 minutes after the burn. The three pieces orbit the Earth until, two hours, fifty seven minutes after undocking, when the Soyuz descent module finally re-enters the Earth’s atmosphere.

Through the Atmosphere

Once the craft enters the atmosphere, nature does most of the work slowing and controlling the descent of the spacecraft. But it isn’t a chance for the astronauts to relax. The design of the Soyuz descent capsule means that it generates a small amount of lift, giving some control to the crew aboard. As the capsule descends, the pilot can speed up and slow down the descent as required by adjusting the roll and angle of the spacecraft.

During this time, the descent capsule is subjected to extreme pressures and temperatures as the impact of the craft with the atmosphere heats the air to thousands of degrees. Special insulating materials surround Soyuz to keep the crew at safe temperatures.

Eventually Soyuz is travelling slow enough so that parachutes can be deployed to further slow the craft for its final approach. After a 15 minute parachute-assisted descent, and about three hours and twenty minutes after leaving the International Space Station, the crew and Soyuz are ready to land. There is one final act of the spacecraft before touchdown. A few metres above the ground, Soyuz fires several small thrusters to slow the speed down to just 15 miles an hour, which, while bumpy, is a safe landing speed.

Once Soyuz has touched down, the awaiting ground crew will check the capsule over and help the crew out of their craft, welcoming them back to Earth.