It was the last major step to be taken before the take-off scheduled from 12:20 GMT.
The JWST will then be the most advanced cosmos observation instrument to be put in space.
After several days of bad weather causing the postponement of final operations, the sliding doors of the final assembly hangar have been raised under the sun.
To reveal the rocket, placed on its "table", an enormous steel platform, with sides browned by the fires of previous launches.
Until the supported by jacks, the whole was placed on railway bogies, to take the 2.8 km of double track bringing it to its position at the firing point.
A truck, ballasted with 14 tons to ensure the grip of its wheels, then got down to the task of towing everything, weighing 1,500 tons, at the speed of a man in walking, for about an hour.
And in the rain, which could not help but come back.
At this point, the rocket cannot take off yet, but can already turn into fireworks.
"Please put your cell phones in airplane mode", to avoid any odds, warns Bruno Gérard, director of Arianespace Guyane, at the foot of the machine.
The rocket is indeed already equipped with its two booster thrusters, long tubes attached to the main body, containing a total of 480 tonnes of powder, and providing 90% of the thrust needed for Ariane 5 to tear itself away from the attraction. earthly.
The Ariane 5 rocket, which carries the James Webb Space Telescope (JWST), before being taken to its launch pad, on December 23, 2021 at the Kourou Space Center, in Guyana Chris GUNN NASA / AFP
Once arrived at its launch pad, the rocket will be filled, approximately 4:30 before H-hour, with its two propellants, liquid hydrogen and oxygen, the mixture of which will start the combustion of the main engine.
© 2021 AFP