Space: How will Thomas Pesquet's return to Earth unfold?

After almost 200 days spent observing the Earth from more than 360 km away, Thomas Pesquet will finally join it. Today is the time of the big departure for the French astronaut, who arrived on the ISS space station on April 24th. It is also for his NASA teammates Shane Kimbrough and Megan McArthur, as well as Aki Hoshide of Jaxa (Japan Aerospace Exploration Agency). After having postponed their return several times due to weather conditions, everyone must leave on board the Crew Dragon capsule this Monday evening. A long journey not really easy for the crew, which NASA broadcasts live on its official channel.

At 5:45 p.m. (4:45 p.m. GMT), the four astronauts said “farewell to their colleagues” who remain on the ISS station and boarded the capsule, the European Space Agency (ESA) said in a press release. After closing the hatches and checking the airtightness of the capsule, it can begin the first phase of the journey, which consists of separating from the station and then returning to Earth orbit. A stall scheduled for 8:05 p.m. (7:05 p.m. GMT). The capsule will gradually move away from the station before beginning phase 2 of its journey: de-orbiting.

"This consists of slowing down the capsule's speed a little by lighting retrorockets for a few minutes, to go from 27,700 to 27,300 km/h", explains Jean-François Clervoy, astronaut at ESA. This slowing down (very slight on the spatial scale) makes it possible to change the trajectory of the capsule so that it is no longer horizontal. “Otherwise, it just makes circles that permanently miss the Earth,” continues the former astronaut. “The idea is to have a slightly steeper downward trajectory so that the capsule ends up touching the atmosphere. »

This is when the third and final phase begins, that of entry into the atmosphere. The European Space Agency predicts it at 3:33 a.m. Tuesday (2:33 a.m. GMT). By its density, the air will naturally slow down the speed of the capsule to the ground. Landing scheduled for 4:33 am (3:33 GMT) off the coast of Florida, United States, ensures the ESA.

The first two phases of the flight (the longest) will be rather quiet. Everything will intensify as Crew Dragon enters the atmosphere. The capsule goes through a so-called “hypersonic” phase, that is to say at very high speeds compared to the speed of sound. "The friction with the air at this speed raises the temperature of the atmosphere around the capsule to almost 2,000 degrees and electrifies it", explains Jean-François Clervoy. A gas called plasma is then formed, with a reddish color. “Through the porthole, you can see flames, as if the ship were on fire. And on the ground, you have the impression of seeing a shooting star, ”reports the former astronaut. This phase lasts a few minutes “but can be a little stressful because the radio waves do not pass through the plasma, explains Guillaume Weerts, head of the medical service at ESA. All communications with the ground crews are therefore cut off. This is what is called in the jargon the "black-out".

After this phase, the air having sufficiently slowed down the capsule, small stabilizer parachutes are deployed. Others, larger, will then open in order to considerably slow down the arrival of the capsule on Earth. “This operation is shocking. The astronauts are heckled, because it stirs a lot, ”says Guillaume Weerts. The capsule should then land. “It's still a shock, even if, when it's a water landing, the water absorbs the energy,” he continues.

Once on the sea, the journey is not quite over yet. A boat equipped with a crane comes to recover the capsule. If he delays, the crew has water and food rations to last 24 hours. But beware of seasickness! "We have to wait and we are tossed about in all directions by the movement of the water, without really being able to see outside", reports Jean-François Clervoy. This trip, if it "is not really comfortable, remains tolerable", sums up Guillaume Weerts.

Overall, the bulk of the astronauts' work was done before departure, on the ISS station. Moreover, the crew brings with it about 240 kg of equipment and scientific research, indicates a press release from NASA. The Dragon capsule will be on autopilot throughout the journey, "but it is quite possible to pilot it to, for example, go to a specific point or change trajectory", indicates Jean-François Clervoy. On the other hand, if everything is automated on board, the machine has not completely replaced by the human. "The crew always carries out checks, sends confirmations to the ground and always checks that this or that system responds well", assures Guillaume Weerts.

Atmospheric entry accidents are rare – moreover, there have not been any for 27 years – but they are not non-existent for all that. For Jean-François Clervoy, there are three main ones. First risk: that the thermal shield which protects the capsule from the burning plasma does not work. This is what happened on February 1, 2003 to the space shuttle Columbia, claiming the lives of seven astronauts. Since then, NASA has developed a series of shuttle heat shield inspections, as well as in-orbit repair techniques.

The risk of an air leak is also not zero. "This is what happened to the crew of Soyuz 11 in 1971", reports Jean-François Clervoy. But again, steps have since been taken. “To counter this, the astronauts put on the same spacesuit as on takeoff, assures Guillaume Weerts. It is a protection that allows them to breathe in case of depressurization. It also protects them in the event of a fire, specifies the European Space Agency. Last risk, finally, “that the parachutes do not open badly as for the flight of the Soyuz 1 capsule”, in 1967, the very first flight of the Soviet space program. Here again, we are only talking about exceptions. The most plausible risk, for Guillaume Weerts, would have been that the return of the astronauts would be further delayed because of the weather. Nothing serious, then.