Note: for lots of great background related to this question, see lagrangian points - The design of the halo orbit of the James Webb Space Telescope - Space Exploration Stack Exchange

There are numerous references saying that JWST will be in a halo orbit around L2, meaning that the period of the oscillation in the Y and Z axes of the Rotating Libration Point (RLP) coordinate system are equal. Thus, as viewed from the Earth, the orbit would describe a similar oval over time.

But according to Seasonal variations of the JWST Orbital Dynamics, launches of JWST before 13:00 UTC (Update - note this is for October launches) tend to lead to non-halo Lissajous orbits. For them, the oscillations in Y and Z have different periods, and the orbit looks from Earth more like a series of ovals offset in time, like this:

Lissajous orbit of DSCOVR

(But note that JWST orbits L2, not L1 like DSCOVR's orbit pictured above.)

Page 3 of Solar Radiation Pressure Effects on the Orbital Motion at SEL2 for the James Webb Space Telescope has a beautiful display of the various orbits based on modeling of various launch times on 2021-01-14. (Update: note that the 12:20 launchs in mid-January are clean halos)

The actual launch was 12:20 UTC on 2021-12-25, implying that it would not be a halo orbit.

So what is the expected orbit, and what are the implications for station-keeping, propellant and mission length, etc? E.g. how does the JWST avoid the dangerous region close to L2 where it would be in the Earth's umbra and encounter power and thermal challenges?

Update I see that I repeated a quote out-of-context, and it is now more clear to me that the resulting orbits vary by both month as well as time of day. I've added some updates related to that above. Based on the January 14 plots in the 2nd paper on Solar Radition effects, I'm guessing that the Dec 25 12:20 launch is actually a pretty clean halo orbit. But I'm still interested in confirmation of that and other up-to-date details on the predicted orbit.

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    $\begingroup$ @uhoh, nevertheless, it is worthwhile to investigate why they (NASA and ESA) didn't wait a bit longer to aim at halo orbits, the feasibility of which is implied by the first paper referred to by the OP (weather conditions, local constraints, ...?). Also, it would be interesting to understand how these launch windows and their consequences on the orbit shapes were calculated. Is it proper to Kourou? to Ariane launch profile? to the target time of flight? ... $\endgroup$
    – Ng Ph
    Commented Dec 31, 2021 at 11:14
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    $\begingroup$ @uhoh, you are absolutely right: nothing in this paper (Brown et.al.) allows to generalize their Figure 5, computed for Oct 2018, to any other month of any other year. Nevertheless, and I have just realized, if (a wild speculation in fact) we assume that it is applicable, i.e. Dec 2021 is similar to Oct 2018, then it would explain why the actual launch window started at 12:20 UTC. It would indeed indicate that they were aiming at a halo orbit (by not starting the window at 11:30 UTC). Some more weeks to wait to know the truth ... $\endgroup$
    – Ng Ph
    Commented Dec 31, 2021 at 14:34
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    $\begingroup$ @NgPh Note that page 3 of the second paper on SRP (Solar Radiation Pressure Effects...) shows a very clean halo orbit for a Jan 14 12:20 UTC launch, which is closer to the Dec 25th actual launch. I'm just looking for the actual situation and its implications. $\endgroup$
    – nealmcb
    Commented Dec 31, 2021 at 15:51
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    $\begingroup$ I just updated my question after looking more closely at the sets of diagrams from the two papers for various dates. $\endgroup$
    – nealmcb
    Commented Dec 31, 2021 at 17:00


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