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29 January 2022



I must say that I was surprised there was such a thing as an L2, but I guess that an L4 & L5 implies 1,2,3. I believe that Webb has an orbit around it.

George Rebane

scenes 1223pm - Webb will not have what one could consider a regular orbit around L2, but instead be so guided by ad hoc thrusts so as to remain in L2's vicinity. I didn't want to put too fine a point on it, but due to the many influences that dynamically distort Earth's gravitational field, L2 is not a relatively stationary point, but wanders around a bit - e.g. every time the Moon swings by in its orbit around the Earth. This requires that Webb also has to 'chase' it to remain in the L2 vicinity.


aah, thanks. I didn't pick that up from the articles about it. It's always presented in a more simplified fashion. Is the wandering predictable?

George Rebane

scenes 216pm - The wandering is predictable for relatively short future horizons. It's an estimation problem (see Kalman filters) that is affected by the error in the Webb's current state (position, velocities, and roll rates) and the accuracy of the implemented control (direction, timing, and duration of correction thrusts), all of that is combined with the prediction of the multi-body (more than three) gravitational field dynamics at L2. And if that isn't enough, then add to it the total unpredictability of the solar wind which does have an impact on Webb because of its huge sun-shade that is kept orthogonal to the sun at all times. The resulting correction trajectory is at best what technically is called chaotic. A lot of the correction calculations and position keeping can be done onboard, thereby eliminating some long distance errors to Earth. The whole control regime is continually recomputed and optimized to require a minimum expenditure of onboard fuel, since that is what we're told determines the useful life of Webb. My own feeling is that if the damn thing works half as good as its specs, then they'll make a refueling run out there when needed. No one knows how many even better space telescopes will be put into Lagrange points in the next 20-30 years. And less fuel will be needed at L4 and L5.

The Estonian Fox

George, you just HAD to do it. Couldn't just accept 1,000,000 miles because it seemed too contrived! Tony Fauci said, and I quote, "What's 56,230.62 miles between friends?" And he is the SCIENCE you know.

Anyway, good job. I've always known them as the Trojans, the captured bodies at L4-L5 around Jupiter. And Saturn doesn't seem to have many Trojans, if any, most likely because Jupiter & Saturn are sort of resonant-locked in their nearly 5:2 ratio of orbital periods. Jupiter is just too massive to allow any stray bodies 'near' Saturn, even 60 degrees away.

I just hope that the Webb doesn't have a problem similar to the one experienced by Hubble. I still have my 1986 copy of Discover magazine, which had a long article about the extensive testing/quality control procedures at Perkin-Elmer, to ensure clear-seeing in the Hubble. And we all got to literally 'see' how well that turned out at first launch.

I agree with you that if the Webb performs as expected, they will indeed make refueling (Tesla self-driving robotic??) runs to it. Don't forget, Elon already has a Tesla out there somewhere - who's to say that he didn't load the auto-driver software onto it? Just need to attach a small light-sail, and away we go.

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