"I am happy, excited, relieved but I dread the rest, it's not over, it's only the beginning," Thomas Zurbuchen, head of scientific exploration at NASA, told AFP. , a few minutes after the separation of the telescope from the upper stage of the Ariane 5 rocket on Saturday.

A joy all the greater as in the minutes following the separation, carried out after 27 minutes of flight and at an altitude of about 1,400 km, the telescope's solar panels were deployed without incident, to thunderous applause from the engineers and audience at the Jupiter Control Center in Kourou.

This was the crucial first step, because the telescope cannot function without this source of electricity.

And it still has some way to go, 1.5 million km to cover in a month, before reaching the point of Lagrange 2, four times the distance separating the Earth from the Moon.

To reach this final vantage point, the James Webb made the first, and most important, of three course corrections.

The great impetus given to it by the rocket to reach its goal has been deliberately minimized to prevent the instrument, which cannot slow down its course, from exceeding its objective, with no real hope of return.

"Success," Nasa tweeted on Saturday night, with the announcement that the telescope's small motors had run flawlessly for 62 minutes.

They will again be called upon for the approach, then the final injection in orbit around the point of Lagrange 2.

Mr. Zurbuchen's caution was not feigned, because if NASA is very used to this kind of maneuver, it will attempt a first: deploy a very large instrument in space.

Folded to fit in Ariadne 5's headdress, like a chrysalis a little over 4 meters in diameter, the craft manufactured by Northrop Grumman must deploy a 6.5 m primary mirror, and a flexible heat shield of 14 meters by 20, the equivalent of a tennis court.

With no hope of rescue if things go wrong, because of the distance.

The James Webb Jonathan WALTER AFP telescope

The coming week will prove to be crucial.

About two and a half days after launch and a second course correction, engineers at the telescope's control center in Baltimore will supervise the release of the two "paddles" containing the solar shield.

This stacking of five large sails of a fabric as thin as a hair, is the sine qua non for the proper functioning of the James Webb and its instruments, whose working temperature requires a minimum cold of -230 ° C.

The telescopic mast carrying the mirrors, still folded, and the instruments, will then rise to make room for the two pallets to open.

"Our shield is very similar to a parachute, it must be folded perfectly to deploy perfectly", explained, before takeoff, Crystel Puga, Webb systems engineer at Northrop.

The operation, consisting of unfolding then stretching and finally hanging the five sails, will last several days and start just after the passage of the Moon.

It involves 140 canvas release mechanisms, eight small motors, around 400 pulleys and 90 cables.

In the second week after the launch, the tripod at the end of which stands the secondary mirror, which concentrates the light from the primary mirror before directing it towards a third mirror and the instruments, will unfold in turn.

Leaving room for the main mirror, folded in three, to open.

NASA's James Webb Space Telescope on September 8, 2021 at the Northrop Grumman Center in Redondo Beach, California Chris GUNN NASA / AFP

If all went well, then, once arrived at the Lagrange 2 point, the adjustment phase will take a few months.

It will require in particular to align the 18 hexagons of the primary mirror, to make the equivalent of a uniform surface, with a precision of the order of ten thousandth of the thickness of a hair.

And also to calibrate the instruments which will then be able to reveal things never before seen on the Universe.

With a first meeting scheduled for June, six months after takeoff.

© 2021 AFP