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I'm particularly interested in the size of the elliptic, but also its eccentricity and whether it will be coplanar with Earth's ecliptic plane.

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    $\begingroup$ It's not even planar or elliptical. It's a 3-body orbit, not a Keplerian one. $\endgroup$ Commented Jan 7, 2022 at 22:22
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    $\begingroup$ I should have figured that it's more complicated than I expected! $\endgroup$ Commented Jan 7, 2022 at 22:30
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    $\begingroup$ Don't worry, welcome to the club (me to, I didn't realize that). One thing to remember: elliptic trajectories exist only in an empty-space with exactly 2 bodies. Which means, if you don't ignore "real life" details, exact elliptic orbits do not exist at all! Secondly, even in a simplified scenario of exactly 3 mass points in which the 3rd point has comparatively very small mass, there is no known analytical solution of its motion in space vs time, except at these 5 Lagrangian points. And this being known since 1772 makes you feel humble, no? $\endgroup$
    – Ng Ph
    Commented Jan 7, 2022 at 23:04
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    $\begingroup$ I don't think any answer here can be better than the official description, which is a quite simple read. jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-orbit $\endgroup$
    – asdfex
    Commented Jan 8, 2022 at 11:51
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    $\begingroup$ There is a detailed paper by Cacolici et al here: Stability of Lagrange Points: James Webb Space Telescope. $\endgroup$
    – jpmarinier
    Commented Jan 8, 2022 at 11:59

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The orbit is more oval than elliptic since it is centered on... well, the center, rather than one focus. It is roughly perpendicular to the Sun-Earth axis. The orbital period 6 months. Radius of the orbit is 200,000km in the Z axis and 800,000km in the Y axis.

enter image description here

Sketch is very approximate, completely out of scale and meant to be conceptual, JWST only appears to orbit around L2 as an artifact of a rotating frame of reference. In an inertial frame of reference, it is in orbit around the Sun. And the Earth. Both at the same time, if you can wrap your head around that.

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    $\begingroup$ Great illustration! Maybe you can add a link to an additional answer with a plot of the actual orbit, but I really like this one :-) space.stackexchange.com/search?q=JWST+orbit $\endgroup$
    – uhoh
    Commented Jan 7, 2022 at 23:04
  • $\begingroup$ Suggest that you follow the usual convention of naming the axes for this type of orbits: Z->X, Y->Z, X->Y , for example as in here: jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-orbit $\endgroup$
    – Ng Ph
    Commented Jan 8, 2022 at 10:25
  • $\begingroup$ @Ng Ph Do you have a link for coordinate system conventions? The closest I could come was ECI, ECEF and GSE but none of these apply here. I'm sure it is buried in this site somewhere: naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/pdf/… $\endgroup$
    – Woody
    Commented Jan 8, 2022 at 16:34
  • $\begingroup$ Good question. The convention I referred to is what can be read from papers dealing with the CR3BP. But I haven't looked-up an official definition of this RLP reference. I assume that one must exist, within NASA or ESA. Otherwise people from different agencies operation centers may mess up when they have to perform orbit determination for station-keeping. $\endgroup$
    – Ng Ph
    Commented Jan 8, 2022 at 17:26
  • $\begingroup$ But now the orientation of the orbit in the figure does not correspond to your text (Y is the axis with the largest amplitude in general). $\endgroup$
    – Ng Ph
    Commented Jan 8, 2022 at 17:49

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