spinjector
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James Webb Space Telescope - replaceable mirrors?
Since one of the JWST mirrors was damaged by a micrometeorite, the internet is aflame with angst. Because of this, it's nearly impossible to search
Google for useful information about the mirrors that's not related to the damage.
And so I find myself coming here to the science-heads of Science Madness to ask: are the individual mirror segments of JWST replaceable? Did NASA
already have a plan to replace them periodically, or as a result of specific damage such as what happened recently?
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j_sum1
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YT channel Anton Petrov has all your answers and more. As does DrBecky (I can't remember her yt handle and I like her presentations less.)
In short.
Damage to mirrors from micrometeoroids was expected and planned for.
This particular hit was statistically quite big and not expected this early on.
Having before and after data enables image correction using software. This is happening.
The effect is still really small.
JWST is working better than expected overall even with this hit.
Replacement of parts is well outside design parameters. This thing is a long way away. One of the reasons for the delayed launch is that the team knew
that no repairs or hardware adjustments would be possible and so they needed to get everything right before launch. Repair would be slightly easier
than replacing the power supply on Voyager 1 but not by much.
[Edited on 30-8-2022 by j_sum1]
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spinjector
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Wow, that's an answer I wasn't expecting. I had no idea JWST was a one-shot deal. No wonder it took so long. Thank you for the information.
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Texium
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Yeah, while Hubble is in low Earth orbit and thus easy to access and repair, JWST is at the L2 Lagrange point- beyond the orbit of the moon! So a
mission to go out and repair it would likely require resources akin to a full lunar mission, unless it was possible for it to fly itself closer in and
meet in the middle somewhere.
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j_sum1
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Quote: Originally posted by Texium  | | Yeah, while Hubble is in low Earth orbit and thus easy to access and repair, JWST is at the L2 Lagrange point- beyond the orbit of the moon! So a
mission to go out and repair it would likely require resources akin to a full lunar mission, unless it was possible for it to fly itself closer in and
meet in the middle somewhere. |
L2 is five times the distance of the moon.
L2 is not a particularly stable point and is perturbed by the presence of the moon. For this reason, JWST is carefully positioned so that it orbits
an empty point in space. This is the most stable state for it to be. Nevertheless, it still requires course corrections that consume fuel.
For this reason, it would not be sensible to drop it to a lower elevation and then reposition it. The cost in fuel (even if possible, which I doubt)
would seriously shorten the life of the telescope.
If a repair was to be done my assumption is that it would be done robotically. A service vehicle would be sent to the location, docked, repairs done,
and then jettisoned: probably via impact on the moon.
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Tsjerk
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The telescope could never fly towards earth, all trusters are pointing towards earth. If the telescope would turn around the equipment would be fried.
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PirateDocBrown
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It would be cheaper to build a new one than repair this one.
Phlogiston manufacturer/supplier.
For all your phlogiston needs.
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Texium
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| Quote: Originally posted by Tsjerk | | The telescope could never fly towards earth, all trusters are pointing towards earth. If the telescope would turn around the equipment would be fried.
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Wasn’t sure if it had the capability of flying backwards, keeping the instruments pointed away from the
Earth/sun. If it could, I figured refueling could also be accomplished on rendezvous. Either way, I also didn’t realize that L2 was 5x the distance
of the moon’s orbit! I knew it was further than the moon but I didn’t think that it was THAT far. Definitely puts the impracticality into
perspective.
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j_sum1
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I am really surprised that they were able to do it at all. The L2 is a feature of the sun-earth system and is not the most stable of the Lagrange
points.
But then you have an additional orbiting body, the moon varying in distance on its monthly orbit: varying between roughly 800000 miles at its closest
point and 1800000 miles at its furthest. That exerts a considerable gravitational tug that fluctuates. Trying to find any stable locus within those
constraints I would have guessed to be impossible. (Turns out I was wrong. There's plenty good brains behind this project.)
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woelen
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Well, actually they sometimes need small corrections. There is some fuel, which can keep the telescope in the right place. Only little fuel is needed
and there is enough for its entire expected life time plus some extra.
Of the Lagrange points, only L4 and L5 are stable (provided the ratio of masses, which determine the position of the Lagrange points is sufficiently
large, appr. 25). For L1, L2, and L3, one needs some control system to keep objects in the right position.
[Edited on 31-8-22 by woelen]
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Tsjerk
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Actually, JWST orbits a point a bit away from L2, this way it falls towards earth. Because it falls towards earth it only needs trustors directed
towards it, otherwise soot would contaminate the mirrors
https://www.youtube.com/watch?v=ybn8-_QV8Tg&t=601s&a...
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