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Old 3rd January 2022, 04:52 PM   #361
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Originally Posted by smartcooky View Post
Well I watched Bill Nelson's rambling "God bothering" statement live during the launch. First thought was "Who the hell is this idiot? Why is is saying all that crap?" I thought it was utterly cringeworthy.

What the hell were they thinking appointing this old fool as head of NASA - he's nearly 80 FFS. The NASA administrator needs to be someone younger with enthusiasm and fresh ideas.

Bring back Jim Bridenstine!!

.
.
Oh I totally agree with you there. I was watching NASA's official feed too, and that was the low point. Absolute cringe-inducing drivel. Bill Nelson is a career politician. The reason he got the job of course is that as a former senator he was well-known to Joe Biden, also a long-time senator. We might also add that the president should also be someone younger with enthusiasm and fresh ideas. But unfortunately this is who voters chose.
(off-topic rant about Bill Nelson follows, please ignore)
Along with Joe Lieberman he's the reason why Americans still don't have affordable health care like every other first world country. He was a senator until 2019 before he became the NASA administrator and he helped to kill the "public option" from the ACA as a payoff for private insurance companies who profit from the lack of affordable healthcare.

[/rant]
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Old 3rd January 2022, 05:19 PM   #362
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Originally Posted by RecoveringYuppy View Post
I'm not reading this as a problem. I'm reading it as them having the luxury of doing something they prefer to do rather than something they urgently need to do. So, I'm reading this as the opposite of reactionary.
Two problems they worked:

They had to reset the solar panel so it would draw more power, and they repointed the telescope to limit sunlight on overheating motors.

Both problems were solved.
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Old 3rd January 2022, 05:52 PM   #363
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Originally Posted by slyjoe View Post
Two problems they worked:

They had to reset the solar panel so it would draw more power, and they repointed the telescope to limit sunlight on overheating motors.

Both problems were solved.
Not sure why you quoted me there, did something go wrong?

ETA: https://www.space.com/news/live/jame...escope-updates
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Old 3rd January 2022, 06:12 PM   #364
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Originally Posted by RecoveringYuppy View Post
Not sure why you quoted me there, did something go wrong?
I think you need to read posts a bit more carefully. Sometimes, you don't seem to get that people quoting you are actually agreeing with you!
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Old 3rd January 2022, 06:13 PM   #365
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Originally Posted by smartcooky View Post
I think you need to read posts a bit more carefully. Sometimes, you don't seem to get that people quoting you are actually agreeing with you!
I'd love to hear that since the alternative is that something went wrong!

ETA: Oh, and I suppose I'm also demonstrating the same anxiousness Puppycow is expressing.

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Old 3rd January 2022, 06:26 PM   #366
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Glad to hear that regardless. So basically the only issue is that they needed to reorient the telescope a bit so the motors could cool down and the solar panel could draw more power.

IOW, just routine problems, not mission-threatening problems.

The blog also mentioned a conference call, which I haven’t had time to listen to yet, but plan to
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Old 3rd January 2022, 06:34 PM   #367
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Btw. I’m guessing that they will never be able to point this telescope at Earth or the moon. Because the sun shield won’t allow it.
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Old 3rd January 2022, 06:41 PM   #368
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Originally Posted by Puppycow View Post
Btw. I’m guessing that they will never be able to point this telescope at Earth or the moon. Because the sun shield won’t allow it.
I don't know about "able" but they certainly don't want to. And it won't be the sun shield that doesn't allow it, but rather the controller software.

Hubble can't be pointed at Earth or the Sun either, they are too bright and would damage the instrument (I think it may have a mode that allows it to be pointed at Earth but some instruments have to be put in a protective configuration but that's not an operating mode).

I suspect pointing JWST at either the Earth or Moon would destroy it since it would then also be pointing at the Sun. But even the brightness of the Moon or Earth without the Sun are definitely not what it is meant for. They might be damaging on their own.

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Old 3rd January 2022, 06:50 PM   #369
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Originally Posted by Puppycow View Post
Btw. I’m guessing that they will never be able to point this telescope at Earth or the moon. Because the sun shield won’t allow it.
Why would they want to? We already have tons of task-optimized stuff looking at Earth.
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Old 3rd January 2022, 07:01 PM   #370
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Originally Posted by theprestige View Post
Why would they want to? We already have tons of task-optimized stuff looking at Earth.
Indeed: although it's now known as NASA Earth Science, at one point it was poetically named the Mission to Planet Earth.
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Old 3rd January 2022, 08:14 PM   #371
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Originally Posted by theprestige View Post
Why would they want to? We already have tons of task-optimized stuff looking at Earth.
I'm just curious what the Earth, Moon and Sun would look like in a photo from the L2 point, that's all. I don't think I've ever seen a photo where all three bodies are in one shot.
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Old 3rd January 2022, 08:17 PM   #372
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Originally Posted by Puppycow View Post
Btw. I’m guessing that they will never be able to point this telescope at Earth or the moon. Because the sun shield won’t allow it.
Correct. It can only get within 85° of pointing at earth.
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Old 3rd January 2022, 08:20 PM   #373
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Originally Posted by RecoveringYuppy View Post
I'd love to hear that since the alternative is that something went wrong!

ETA: Oh, and I suppose I'm also demonstrating the same anxiousness Puppycow is expressing.
I'd be beyond astonished if a mission as complex as this one went off without a single hitch.


So far, the only things that seem to have been sub-nominal are

A bad sensor reading on a non-explosive actuator - solved
A minor power issue - solved
Some minor overheating on a sunshield deployment motor - solved


Eta: Cold side temperatures continue to fall

Primary Mirror: -132°C (down from -116°C yesterday)
Instrument Radiator: -194°C (down from -192°C yesterday)
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Old 3rd January 2022, 08:23 PM   #374
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Originally Posted by RecoveringYuppy View Post
they are too bright and would damage the instrument
Yeah, you're right. The sun would be much too bright for the delicate instrument to look at without destroying it. We can't even look directly at it with our own eyes for more than half a second without damaging them. And you certainly wouldn't want to look at the sun through binoculars.
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Old 3rd January 2022, 08:30 PM   #375
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Originally Posted by Puppycow View Post
I'm just curious what the Earth, Moon and Sun would look like in a photo from the L2 point, that's all. I don't think I've ever seen a photo where all three bodies are in one shot.
The problems go beyond pointing the camera at Earth. The difference in brightness of those three objects would be daunting for a camera. And I think the field of view of the JWST is about 3 minutes of arc which mean you couldn't even get the entire sun in to the picture. Don't quote me on that last part.
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Old 3rd January 2022, 08:31 PM   #376
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The second and third layers of the sun shield have been tensioned now:

https://blogs.nasa.gov/webb/
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Old 3rd January 2022, 08:46 PM   #377
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Originally Posted by RecoveringYuppy View Post
The problems go beyond pointing the camera at Earth. The difference in brightness of those three objects would be daunting for a camera. And I think the field of view of the JWST is about 3 minutes of arc which mean you couldn't even get the entire sun in to the picture. Don't quote me on that last part.
Earth L2, is only 1% further away from the Sun than the earth itself, so the Sun's apparent diameter will be about the same as it is from earth - half a degree (30'), so yes, 10 times too big for Webb's FoV.

Even the Earth and the Moon are too big to fit in Webb's field of view (28' and 9' respectively)
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Old 3rd January 2022, 09:23 PM   #378
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Originally Posted by smartcooky View Post
Earth L2, is only 1% further away from the Sun than the earth itself, so the Sun's apparent diameter will be about the same as it is from earth - half a degree (30'), so yes, 10 times too big for Webb's FoV.

Even the Earth and the Moon are too big to fit in Webb's field of view (28' and 9' respectively)
Just to be clear: are you sure that 3 minutes of arc is the correct field of view for JWST? That was the part I was least certain of.
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Old 3rd January 2022, 11:53 PM   #379
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Originally Posted by RecoveringYuppy View Post
Just to be clear: are you sure that 3 minutes of arc is the correct field of view for JWST? That was the part I was least certain of.
Near-Infrared Camera (NIRCam):
Two modules, each observing a 2.2’ × 2.2’ area separated by ~44" and cover 9.7²' total.
https://jwst-docs.stsci.edu/jwst-nea...nircam-imaging

Mid Infrared Instrument (MIRI):
Its imaging FoV is 74" × 113" (arcseconds)
https://jwst-docs.stsci.edu/jwst-mid...s/miri-imaging

Near Infrared Imager and Slitless Spectrograph (NIRISS):
Similar FoV to Nircam 2.2' x 2.2'
https://jwst-docs.stsci.edu/jwst-nea...niriss-imaging

Near-Infrared Spectrograph (NIRSpec):
Its 3' x 3' and is probably the one you are thinking of, but it does not have any imaging modes... its just a spectrograph.
https://jwst-docs.stsci.edu/jwst-nea...tsanddetectors
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Old 4th January 2022, 05:58 AM   #380
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Originally Posted by Puppycow View Post
I'm just curious what the Earth, Moon and Sun would look like in a photo from the L2 point, that's all. I don't think I've ever seen a photo where all three bodies are in one shot.
The Earth and the Moon wouldn't look like anything, not being illuminated by the sun.

Assuming you're properly at L2, the enshadowed disk of the moon would be invisible within the enshadowed disk of the earth, or vice versa. The larger disk would occlude some or all of the disk of the sun.

Getting all three in one shot would be a mild achievement in applied geometry, and an interesting happenstance of some other mission entirely. But it wouldn't look like much, and wouldn't be worth going out of our way for.
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Old 4th January 2022, 06:56 AM   #381
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Thanks smartcooky.
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Old 4th January 2022, 07:49 AM   #382
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NASA livestream:

https://www.youtube.com/watch?v=IBPNi7uGgWM
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Old 4th January 2022, 08:07 AM   #383
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They're explaining why there's no cameras on the telescope. There's really no light on the dark side, and it's incredibly cold too. An off-the-self camera couldn't operate in temperatures that cold, and would need artificial light to see anything anyway. On the other side, the hot side, it would be very bright and shiny, and of course very hot. You would need many cameras too if you want to see all parts of the telescope.
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Old 4th January 2022, 08:14 AM   #384
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Am I the only one who's a little depressed that they actually felt they had to explain this?
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Old 4th January 2022, 08:25 AM   #385
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The 4th layer has been fully tensioned. On to the 5th, and final layer.
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Old 4th January 2022, 08:52 AM   #386
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Originally Posted by theprestige View Post
The Earth and the Moon wouldn't look like anything, not being illuminated by the sun.
Atmospheric scattering would create a visible glow around the edges of the earth, so you could see it if you're off L2. If you're at L2, then the earth is producing an annular eclipse of the sun, and the sun itself will wash that out, but the silhouette of the earth against the sun would be quite dramatic.

Quote:
Assuming you're properly at L2, the enshadowed disk of the moon would be invisible within the enshadowed disk of the earth, or vice versa.
Only during a solar or lunar eclipse, otherwise the moon isn't in line with the sun and the earth. The earth-L2 distance is about 930k miles, and the earth-moon distance is about 240k miles. So at half moon, the moon will be at about 14 degrees away from earth (and the sun). That's enough that you should be able to see a sliver of illumination on the moon.

Quote:
The larger disk would occlude some or all of the disk of the sun.
The earth is smaller than the sun when viewed from L2.

Quote:
Getting all three in one shot would be a mild achievement in applied geometry,
Not really. Just wait for a half moon (happens twice a month). Balancing the exposure is probably more of an issue (the sun is so much brighter than the moon), but that can probably be dealt with easily enough by combining multiple exposures, as is pretty standard practice in astronomy. Alternatively, it wouldn't be hard to catch the moon and the earth both partially eclipsing the sun, with the moon above or below the earth. The tilt of the moon's orbit is why we don't get solar eclipses every lunar cycle.

Quote:
But it wouldn't look like much
Oh, I don't know. A lot of people really liked seeing the pale blue dot of the Earth against Saturn's rings from Cassini.

Quote:
and wouldn't be worth going out of our way for.
I'll agree there.
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Old 4th January 2022, 09:09 AM   #387
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Originally Posted by Ziggurat View Post
Atmospheric scattering would create a visible glow around the edges of the earth, so you could see it if you're off L2.
The orbit JWST is headed for would also see a bit of crescent Earth too wouldn't it? If I understand the orbit correctly I think Earth would be a constant crescent sliver with the crescent rotating across the planet every 6 months.

BTW there is also the issue that we are talking about infrared instruments. Neither the "dark" side of the Moon nor the Earth would be dark in that range of the spectrum.

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Old 4th January 2022, 09:30 AM   #388
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Originally Posted by RecoveringYuppy View Post
The orbit JWST is headed for would also see a bit of crescent Earth too wouldn't it? If I understand the orbit correctly I think Earth would be a constant crescent sliver with the crescent rotating across the planet every 6 months.

BTW there is also the issue that we are talking about infrared instruments. Neither the "dark" side of the Moon nor the Earth would be dark in that range of the spectrum.
Nitpick: I wasn't talking about infrared instruments, since they would be burned out by such a view. I was talking about ynot's hypothetical gopro or whatever.
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Old 4th January 2022, 11:39 AM   #389
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Originally Posted by theprestige View Post
Assuming you're properly at L2, the enshadowed disk of the moon would be invisible within the enshadowed disk of the earth, or vice versa. The larger disk would occlude some or all of the disk of the sun
Nope. JWST will never be "properly at" L2, it will be in a halo orbit "around" L2, so the Earth will never occlude any part of the sun's disk

https://jwst-docs.stsci.edu/files/97.../JWSTorbit.jpg

"The distance of JWST from the L2 point varies between 250,000 and 832,000 km, as shown in Figure 1. The period of the orbit is about 6 months. The maximum excursion above or below the ecliptic plane is 520,000 km. The maximum distance from the Earth is 1.8 million km, and the maximum Earth-Sun angle is <33°."
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Old 4th January 2022, 11:42 AM   #390
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Tensioning is complete. All layers now fully tensioned, so the sunshield is ready to do its work

Next up, deployment of the secondary mirror tomorrow or the day after.
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Old 4th January 2022, 11:53 AM   #391
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Originally Posted by smartcooky View Post
Tensioning is complete. All layers now fully tensioned, so the sunshield is ready to do its work

Next up, deployment of the secondary mirror tomorrow or the day after.
And with that accomplished, JWST will be able to do useful work even if the wing mirrors don't deploy. Better if they do, of course.
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Old 4th January 2022, 12:07 PM   #392
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Originally Posted by Trebuchet View Post
And with that accomplished, JWST will be able to do useful work even if the wing mirrors don't deploy. Better if they do, of course.
Secondary mirror still needs to deploy. Won't do anything at all without that.
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Old 4th January 2022, 12:14 PM   #393
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Old 4th January 2022, 12:41 PM   #394
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How many single-point-failure steps left to go?

(ETA: In my head at least, deploying the sunshield seemed to be just about the most complicated thing. Good to have that done.)

Last edited by crescent; 4th January 2022 at 12:42 PM.
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Old 4th January 2022, 12:53 PM   #395
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Originally Posted by crescent View Post
How many single-point-failure steps left to go?

(ETA: In my head at least, deploying the sunshield seemed to be just about the most complicated thing. Good to have that done.)
Serious engineering question: Is it still considered a single point failure if there's redundant pathways to success?

For example, many modern tank turrets have both an electrically-driven mechanism for turning the turret, and a manual mechanism for turning the turret. But there's still only one turret and one rotating joint. Would that be considered a single point of failure (to turn the turret)?
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Old 4th January 2022, 01:06 PM   #396
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Originally Posted by crescent View Post
How many single-point-failure steps left to go?
A NASA tweet I saw said 75% of the 344 have been retired.
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Old 4th January 2022, 01:26 PM   #397
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Originally Posted by Pixel42 View Post
A NASA tweet I saw said 75% of the 344 have been retired.
And at least half of that 75% are the ones they were most concerned about.
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Old 4th January 2022, 01:42 PM   #398
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Originally Posted by theprestige View Post
Serious engineering question: Is it still considered a single point failure if there's redundant pathways to success?

For example, many modern tank turrets have both an electrically-driven mechanism for turning the turret, and a manual mechanism for turning the turret. But there's still only one turret and one rotating joint. Would that be considered a single point of failure (to turn the turret)?
That depends on the failure mode.

In the example you give, a drive motor failure would not be a single point of failure if the redundancy included a manual rotation system (like landing gear on an aircraft). However, the breaking of a drive cog or worm gear that was common to both the motor drive system and the manual system of rotation would be a single point failure.

There was a great example of a design allowing a single failure to cause the failure of redundant systems in United Flight 232 that crashed in Sioux City Iowa in 1989. This was a Lockheed L1101, a Tri jet (two engines under the wings, one engine in the tail).

The hydraulic system was triple-redundant - three complete and separate systems for the control surfaces - if one failed there were two more to take over. However the problem with the design was that the hydraulic lines for all thee systems came very close to each other near the tail engine, and when a fan disintegrated in that engine, the shrapnel took out all three of those lines, and left the crew with no control of the aircraft.

This is not a true "single point failure" more a design flaw (like your tank turret) where the engineers did not foresee the possibility that a single failure might knock out all available redundancy too.
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Old 4th January 2022, 03:18 PM   #399
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Originally Posted by smartcooky View Post
That depends on the failure mode.
There are also usually hypothetically possible failure modes which we don't practically have to worry about. In the tank example, a turret rests on a sort of ring track that it can rotate on. If this ring track buckles significantly, the turret won't work, and there's no backup. But that ring track isn't going to buckle significantly under any conditions which aren't basically destroying the tank.

With the JWST, if the strut on the upper arm for the secondary mirror snaps, it's over, the whole thing fails. But the upper arm strut isn't going to just fracture. The joint might have problems, the motor that's supposed to extent it might have problems, but the strut itself just snapping isn't something they're likely including in their list of possible single point failures.
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Old 4th January 2022, 04:26 PM   #400
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Originally Posted by smartcooky View Post
Tensioning is complete. All layers now fully tensioned, so the sunshield is ready to do its work

Next up, deployment of the secondary mirror tomorrow or the day after.
I'm glad I got caught up here after the fact. I think waiting through each tensioning would have been stressful. I can only imagine what it must be like for the engineers!

I will try to check in on main mirror deployment, though. (Friday?)
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