Le post des nouvelles des étoiles...

[tfpsly]

L1 L2 et L3 sont instables. Les forces en jeu sont les attractions gravitationnelles de la Terre et du Soleil. Je n'ai pas regardé en détail, mais déjà l'orbite terrestre n'est pas un cercle parfait, c'est une ellipse.
https://fr.wikipedia.org/wiki/Point_de_ ... ilit%C3%A9

[Robert64]

Tu trouveras des éléments dans ces explications de la Nasa.
https://webb.nasa.gov/content/about/orbit.html
En principe, le point L2 n'est pas stable et il faut que le télescope s'y maintienne, sinon il sera progressivement éjecté.
J'ai vu une carte des champs gravitationnels en ce point. Je la recherche.
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[Robert64]

Bon, voilà! Finalement, le mieux est encore Wiki:

L2_1.jpg (158.7 Kio) Vu 347 fois

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[tfpsly]

Ca ?

En rouge : quand la somme des gravités attire vers un point de Lagrange, en blue quand elles l'en éloignent.
Donc apparemment graviter autour du L2 permettrait d'éviter les deux flèches noires l'en éloignant : la zone stable est plus "épaisse" autour de L1 L2 et L3 qu'exactemenet à ces points.

EDIT : oui c'était bien ça

[SEM]

merci pour les réponses !
mais étonnant quand même que l'on puisse orbiter autour d'un point immatériel

[Robert64]

merci pour les réponses !
mais étonnant quand même que l'on puisse orbiter autour d'un point immatériel

Ah oui, c'est surprenant! La logique voudrait qu'on se cale en l2 et qu'on n'en bouge plus, mais comme c'est une zone instable, il faudrait dépenser du carburant pour s'y maintenir.
Tandis qu'en laissant le satellite aller dans des zones un peu plus stables, on fait cette économie, quitte à avoir une forme de trajectoire un peu...bizarre. D'ailleurs, ce genre d'orbite porte un nom...que j'ai oublié.
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[SEM]

j'ai lu sur le site de la NASA (merci Robert pour le lien) que l'orbite de Webb autour de L2 est équivalente en taille à l'orbite de la lune autour de la terre...et que Webb fait un tour tout les 6 mois autour de L2...

[Robert64]

Les réglages des éléments des miroirs viennent de commencer. Il y en a pour dix jours pour la première phase.
Le Webb vient juste de dépasser les 1 200 000 km. Plus que 246 000.
La température reste stable, 54°C au soleil et - 200°C à l'ombre

Webb Begins Its Months-Long Mirror Alignment
Webb has begun the detailed process of fine-tuning its individual optics into one huge, precise telescope.

Engineers first commanded actuators – 126 devices that will move and shape the primary mirror segments, and six devices that will position the secondary mirror – to verify that all are working as expected after launch. The team also commanded actuators that guide Webb’s fine steering mirror to make minor movements, confirming they are working as expected. The fine steering mirror is critical to the process of image stabilization.

Ground teams have now begun instructing the primary mirror segments and secondary mirror to move from their stowed-for-launch configuration, off of snubbers that kept them snug and safe from rattling from vibration. These movements will take at least ten days, after which engineers can begin the three-month process of aligning the segments to perform as a single mirror.
Source Nasa
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[Robert64]

Alors, le Webb, il devient quoi ?
Pour l'instant il continue sa route vers l'entrée de l'orbite autour de L2.
Il a parcouru 1,265 millions de km, soit 87,5% du trajet. Reste plus que 181 000 km. Pour l'instant, il avance à 1040 km/h (289 m/s)
Il fait toujours 55°C au soleil et -201 à l'ombre
Mais il ne reste pas inactif: tous les éléments des miroirs quittent leur position de sécurité pour rejoindre leur config de travail:

Quelques détails de la Nasa:
Most Recently Completed:
Mirror Segment Deployment Tracker
Nominal Event Time: Launch + 18-28 days

Status: Ongoing

The adjacent image tracks the progress of the individual primary mirror segments (A,B,C) and the secondary (SM) mirror as they move upward 12.5mm from their stowed launch position to a deployed state where they are ready for the mirror alignment process.

The mirrors move in very small increments over the course of ~10 days to complete their deployment. They move approximately 1mm per day each. For more detail on this process read this blog entry.

Each primary mirror segment has an ID which consists of a letter (A,B,C) followed by a number. The letter denotes one of 3 different 'prescriptions' for each group of primary mirror segments.

NOTE: Segment A3 and A6 will be moved separately at the end of the process because their position sensors are read out in a different way.

Ils donnent même une carte des miroirs pour suivre l'avancement du boulot:

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[Thierry.P]

1mm par jour ! Ca c'est de la vis micrométrique !!!!

[Robert64]

Plus de détails ici:
Mirror, Mirror…On Its Way!

With major deployments complete, Webb continues its journey to its final halo orbit around L2. In the meantime, there are several smaller deployments in the next couple of weeks, which constitute the beginning of a several-month phase of aligning the telescope’s optics. This week, we have started the process of moving the mirror segments (all primary plus secondary) out of their stowed launch positions.
For more details, here is Marshall Perrin from the Space Telescope Science Institute, home of the Webb Mission Operations Center:

“To support the movable mirrors during the ride to space, each of them has on its back three rigid metal pegs which can nestle into matching holder sockets in the telescope structure. Before launch, the mirrors were all positioned with the pegs held snug in the sockets, providing extra support. (Imagine Webb holding its mirrors tucked up close to its telescope structure, keeping them extra safe during the vibrations and accelerations of launch.) Each mirror now needs to be deployed out by 12.5 millimeters (about half an inch) to get the pegs clear from the sockets. This will give the mirrors ‘room to roam’ and let them be readied in their starting positions for alignment.

“Getting there is going to take some patience: The computer-controlled mirror actuators are designed for extremely small motions measured in nanometers. Each of the mirrors can be moved with incredibly fine precision, with adjustments as small as 10 nanometers (or about 1/10,000th of the width of a human hair). Now we’re using those same actuators instead to move over a centimeter. So these initial deployments are by far the largest moves Webb’s mirror actuators will ever make in space.

“And we don’t do them all at once. The mirror control system is designed to operate only one actuator at a time. That way is both simpler (in terms of the complexity of the control electronics) and safer (since computers and sensors can closely monitor each individual actuator as it works). Furthermore, to limit the amount of heat put into Webb’s very cold mirrors from the actuator motors, each actuator can only be operated for a short period at a time. Thus, those big 12.5-millimeter moves for each segment are split up into many, many short moves that happen one actuator at a time. Scripts sent from the Mission Operations Center will direct this process under human supervision, slowly and steadily moving one actuator at a time, taking turns between segments. At full speed, it takes about a day to move all the segments by just 1 millimeter. It’s about the same speed at which grass grows!

“This may not be the most exciting period of Webb’s commissioning, but that’s OK. We can take the time. During the days that we’re slowly deploying the mirrors, those mirrors are also continuing to slowly cool off as they radiate heat away into the cold of space. The instruments are cooling, too, in a gradual and carefully controlled manner, and Webb is also continuing to gently coast outwards toward L2. Slow and steady does it, for all these gradual processes that get us every day a little bit closer to our ultimate goal of mirror alignment.”

—Marshall Perrin, deputy telescope scientist, Space Telescope Science Institute
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[Robert64]

merci pour les réponses !
mais étonnant quand même que l'on puisse orbiter autour d'un point immatériel

.....
D'ailleurs, ce genre d'orbite porte un nom...que j'ai oublié.
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Pour Sem:
J'ai retrouvé! C'est une orbite de halo.
Des explications ici:
https://fr.wikipedia.org/wiki/Orbite_de_halo
Il y est fait également mention dans le lien qu'a donné initialement tfpsly
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[beb]

Non mais Halo quoi !

[Robert64]

Ploufff!
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[Robert64]

Oui, mais....
https://fr.wikipedia.org/wiki/Simon_Plouffe

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