Thomas Pesquet is due to leave the international space station on Monday evening aboard the Crew Dragon capsule and arrive in Florida on Tuesday morning.
From the stall to the landing of the capsule, through its deorbiting and especially its entry into the atmosphere, we take stock of the journey that awaits the French astronaut.
A return far from being a walk in the park.
After almost 200 days spent observing the Earth from more than 360 km away, Thomas Pesquet will finally join it.
Today is the time for the big departure for the French astronaut, who arrived on the ISS space station on April 24.
It is also true for his teammates from NASA Shane Kimbrough and Megan McArthur, as well as Aki Hoshide from Jaxa (Japanese 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 trip not really easy for the crew, which NASA broadcasts live on its official channel.
What are the different phases of this return to earth?
At 5:45 p.m. (4:45 p.m. GMT), the four astronauts said “their farewells to their colleagues” who remain on the ISS station and boarded the capsule, the European Space Agency (ESA) said in a statement. After closing the hatches and checking the airtightness of the capsule, it will be able to begin the first phase of the journey, which consists of separating from the station and then ending up in a terrestrial orbit. A stall scheduled for 20:05 (19:05 GMT). The capsule will gradually move away from the station before starting phase 2 of its journey: deorbiting.
"This consists of slowing down the speed of the capsule a little by igniting retro rockets 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 a 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,” the former astronaut continues. “The idea is to have a trajectory a little more downwards so that the capsule ends up touching the atmosphere. "
This is when the third and final phase begins, that of entering the atmosphere.
The European Space Agency expects 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, in the United States, assures the ESA.
A healthy walk or a return to high turbulence?
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 nearly 2,000 degrees and electrifies it", explains Jean-François Clervoy. A gas called plasma is then formed, with a glowing color. “Through the window, we see like flames, as if the ship was on fire. And on the ground, we 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 teams are therefore cut off. This is what we call in the jargon the "blackout".
After this phase, the air having sufficiently slowed down the capsule, small stabilizing parachutes are deployed. Others, larger, will then open in order to considerably slow down the arrival of the capsule on Earth. “This operation causes a shock. The astronauts are heckled, because things stir quite a bit, ”says Guillaume Weerts. The capsule must then land. "It's still a shock, even if, when it is 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 retrieve the capsule.
If it is late, the crew has rations of water and food to last 24 hours.
But beware of seasickness!
"We have to wait and we are tossed around in all directions by the movements of the water, without really being able to see outside," reports Jean-François Clervoy.
This trip, if "it is not really comfortable, remains tolerable", summarizes Guillaume Weerts.
What are the astronauts going to do on board?
Overall, most of the work for the astronauts was done before departure, on the ISS station.
Moreover, the crew brings with them about 240 kg of equipment and scientific research, indicates a statement from NASA.
The Dragon capsule will be in autopilot throughout the journey, "but it is quite possible to pilot it to, for example, go to a specific point or change course", indicates Jean-François Clervoy.
On the other hand, if everything is automated on board, the machine has not completely replaced by humans.
"The crew always carries out checks, sends confirmations to the ground and always checks that such and such a system responds well", assures Guillaume Weerts.
Are there any risks?
Accidents entering the atmosphere are rare - there have not been any for 27 years - but they are not non-existent. For Jean-François Clervoy, there are three main ones. First risk: that the heat shield that protects the capsule from the burning plasma does not work. This is what happened on February 1, 2003 to the Columbia shuttle, 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, action has been taken since then.
“To counter this, the astronauts put on the same spacesuit as on take-off,” assures Guillaume Weerts.
It is a protection that allows them to breathe in the event of depressurization.
It also protects them in the event of fire, specifies the European Space Agency.
Last risk, finally, "that the parachutes do not open badly as for the theft 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 postponed because of the weather.
Nothing serious, then.
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