A moment of tension as the James Webb Space Telescope stretches sunshield on way to L2 destination The James Webb Space Telescope has continued to notch up the milestones on its journey to its L2 destination with the tensioning of its sunshield. After a successful launch atop an Ariane 5 on 25 December, the observatory has begun unfurling as it hurtles towards its final location. After antennas and sunshield pallets popped …
Once upon a time I had some paper drawings of knee replacement components I needed to turn into 3D models.
Dimensions were in mm, but every one included very odd fractions of a mm. Very odd that is until I converted them to imperial and they all turned out to be nice round multiples of thousands of an inch.
Have you never wondered where the expression "Rule of thumb" came from?!?
Or tried to fathom how a foot became named. I'll give you a hand by mentioning that the etymology of the knot is also similar. I could chain a few more units together to get to the stone, but the person I feel sorry for is the one whose back yard only measured 36 inches!
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"[...] 1inch = thumb length [...]"
My understanding is that it is thumb width. Easy to apply your thumb to a surface and mark the width. For "length" the delimiters would be more difficult to ascertain.
I thought a "yard" is the measurement from tip of a (usually ruler's) nose to outstretched hand.
There is an interesting book on such derivations, unfortunately out on loan from my books at the moment.
Yes, this always makes me laugh. Imperial measures are not decimal.
But what is very nice to see is how young people on TV quiz shows consistently fail to answer imperial questions correctly. Iain Duncan Smith are you listening? Your wet dream to get us all using lbs and ozes, feet and ins, is already lost.
What have the Europeans ever done for us...?
Back in the days before schematic capture and PCB CAD software was affordable, I used to lay out PCBs by hand on 0.2” graph paper - we used to make the originals of the photo-masks 2x size.
Drawn in with 2 colours of felt tip and then, for each side, drafting film taped over the graph paper and the tracks and pads put on with Alfac rub-down transfers.
I watched the live launch (shame about the clouds!) and been watching the tracking/activity page since:
https://www.jwst.nasa.gov/content/webbLaunch/whereIsWebb.html
Bloody interesting. At one stage once leaving Earth orbit it was doing nigh on 1 mile per second! It has been slowing down ever since due to the drag of Earths (and the Sun/moon I suppose) gravity, all calculated so that is at a slower optimum speed to allow a precise velocity/position to get it into L*2 orbit.
The maths is amazing to work that out what with including the Ariane launch and what not...
Yeah, hopefully it'll be moving very slowly by the time it gets there. I was a little surprised to learn that the delicate sun shades where being deployed so early and there's still manoeuvring burns to be done. I'm assuming they will be quite delicate burns! I know they know what they are doing, but still, it feels risky to me :-)
"I was a little surprised to learn that the delicate sun shades where being deployed so early and there's still manoeuvring burns to be done. I'm assuming they will be quite delicate burns! I know they know what they are doing, but still, it feels risky to me :-)"
I don't think there is any risk since with 0 atmosphere, having burns in the folded or unfolded state is the same.
Actually, they paused the unfolding of the shield for a couple of days, vs. baseline, because, I think, this step WAS the high risk step of the mission. You create a breach into one layer and this is all over.
There was a failure in switches to indicate the cover had rolled up - that delayed some things and meant some late work - most likely the delay were mainly a result of that and they didn't want to start the critical with people tired
Isn't quite the same: when things are unfolded the lever arms are longer and hence higher stress on attachments etc. However the acceleration is in fact very small and the sooner you can get it deployed the longer it has to cool so you want to do this as soon as you can so it is as cold as it can be as soon as it can be.
I think it takes about 6 months to get cold. Since smallest mission is I think 5 years (will be longer of course, but that it what they can count as success) then cooling-down period is 10% of entire shortest mission so is critical to start it very early.
You are completely correct I am sure about additional stress, but I think remaining burns are very tiny. Is probably significant that I think both the firs two mid course corrections (1a and 1b) were before sunshield deployment. They are probably the big ones, with 2 being just enough to get it just this side of L2 (but safely this side, do not want ever to be on other side...).
I since checked, and last night the coldest part of the sat was already reading below -200c. I'd imagine the temperature drop will slow down exponentially and those last 30c will take a lot longer.
EDIT. Maybe I was mis-remembering Fahrenheit temps from last night. It's currently at -198c
I also just had another thought. Is there any use being made of this temperature differential? I remember learning about and trying an experiment using copper an iron wire alternately joined at opposite ends and putting one end into the over and the other left out in the cold to generate a low voltage. I'm sure there have been large scale similar units using cold deep water and warmer surface temps too.
Ah, of course. Working from a vague memory of something I learned about when I was about 12, some 50 years ago isn't always a good idea. Of course, and obviously, there would be a transfer of heat to the cold side and of course that would bad. See icon for image of me ---->
Based from this, mirror (which is point c) is currently -157 celcius which is 116K, I think it all needs to be down at about 50K or lower. Radiator (which is point d) is currently cooler but it is the mirror & sensors which matter. Yes you are right of course doing the last bit takes the time.
And by the way not only do they default to stupid unit system used by fat people, they do not even have option to show temperatures in Kelvin which is only sensible unit in this context. I hate it (but yes I know, the fat people paid for it all so we must pander). Now I will go and get drunk and dream of a better world.
"I don't think there is any risk since with 0 atmosphere, having burns in the folded or unfolded state is the same."
No, I think the survivable G force difference between folded and unfolded are probably very different. I doubt, even at 0 atmosphere, the shields and support booms could survive the same G forces they survived at launch time. Folded or unfolded is very different, but as I said, I'm sure they know what they are doing and understand very well the limits of what they can do.
One interesting thing is that it can never slow down except due to gravity. No engines can be on the cold side of it, and the cold side can never see the Sun so it can never turn around. Thus it can only have engines on one side, which can only push away from the Sun. This is one of the reasons why the launch was so fussy: it had to be going slower than it needed to be (faster is doom) but as close to the right velocity as possible to reduce fuel usage. They did this very well indeed so they now have more fuel, longer mission, good.
According to https://scitechdaily.com/due-to-precision-launch-nasa-says-webb-space-telescopes-fuel-likely-to-last-way-more-than-10-years/
The launch was so accurate that the savings on fuel for corrections means that the telescope should have enough to last 10 years before they need to call on the Lagrange 5 service station for a top up* ;o)
As for the 'kite shaped' sun shield, I now have the joyous final song from Mary Poppins running through my head:
'Let's go fly a kite!
Up to the highest height!
Oh let's go, fly a kite!
Uo in the atmosphere,
Up where the sky is clear,
Oh let's go, fly a kite!''
*(Sorry, this is a joke, there is, as yet, no known Lagrange 5 service station able to re-fuel the satellite.)
There is a nice graphic of the Sun - Earth Lagrange points here:
https://en.wikipedia.org/wiki/Lagrange_point
The top right graphic shows a contour plot of the effective gravitational potential for each point.
And you are quite right (I hope) JWST is on its way to L2, not L5 (well, I did say I was joking about the service station at L5).
Is very unlikely that any human mission would be able to refuel JWST (HST has never been refueled as HST does not have fuel). Storable propellants are almost all horrid: JWST uses hydrazine fuel (horrid, poisonous, corrosive, can explode and burn) and dinitrogen tetroxide oxidizer (horrid, poisonous, corrosive, gives of poisonous fumes, oxidizes anything it can oxidize). Hydrazine is also usable as a monopropellant (and some of JWST's engines use it this way), which means it can 'burn' without oxygen if there is suitable catalyst: hydrazine is poisonous, corrosive, liquid (so gets everywhere) and sometimes it just explodes.
People handling these things on Earth is extremely frightening: this is why it took so long to fuel JWST: if you handle these things you do it very, very carefully if you do not like dying, or destroying a $10bn spacecraft:
People handling these things in space is more than extremely frightening: it's stupid. 'Oh dear, a leak has happened and has sprayed hydrazine all over my spacesuit and all over my spacecraft, what do I do?' Well, what you do, probably, is die, because even if the stuff has not eaten your spacesuit you are not coming back through the airlock covered in that.
Only safe way would be a robot.
I haven't been following the feed, just reading the articles as they pop up. I figure if anything goes seriously wrong, the headline will rip the band-aid off the horrible disaster before you read the first line of text. It'd drive me nuts to watch the moments crawl by in real time. A whole lot of nothing much happening for hours on end, but every moment of it critical to the overall success...
A round all around for the boffins who built and deployed the kit.
But we're still finishing the appetizers. The main course is the deployment of the telescope mirrors and I hope for their continued success.
For JWST status, in addition to the WhereIsWebb, check out the JWST blog which records what happened in detail. BTW, per the final tensioning ElReg refers to, per the blog: "At approximately 11:59 am EST, the fifth and final layer of Webb’s sunshield was fully tensioned, marking the completion of sunshield deployment."
https://blogs.nasa.gov/webb/
Now, I have a very basic understanding of refrigeration thermodynamics, but despite a bit of research I'm none the wiser about how various bits of the JWST can be passively cooled down to just a few Kelvin, given the difficulties of radiating heat energy away in a vacuum and not, I assume, using sublimation .
Anyone got a decent explanation of the physics of this seemingly very clever cooling system?
The idea is to keep the cold side shielded and sufficiently isolated so the radiative heat loss is all it takes to drop the temperature to where it's desired. Relying on just radiation to drop to the target temp takes time, but it'll get there eventually if the sun shield and isolation works as planned.
Wackypedia link on topic, which explains the mechanism pretty well: https://en.wikipedia.org/wiki/Heat_transfer#Radiation
On the dark side of the shield it is looking out to the cold -270C (3K) of space, so it can't get colder than that passively.
Even with that cold view there is some heat bleeding from the sun and from all of the spacecraft electronics, etc (as mush of it has to be a lot warmer to work). So getting to -230C or so just by being on the dark side is perfectly reasonable.
This is one reason the sunshield deployment needs to be so early. It does indeed take a long time to cool down by radiation only, so the sooner the shield is in place the sooner that process can start. Which it now has started, so this is good. I think it is expected to take about 6 months to get cold.
I know I'm going to get flamed for this, but please tell me why the powers-that-be couldn't have installed a few GoPro's on the thing to give us all a look at how things were going. I mean, after all, we paid for the damn thing, why can't we watch? I'm not a rocket scientist, but I know for sure and for certain that a picture is worth a thousand sensors, and many times even the gear-heads looking at the data streams have gotten big help from seeing pics and video. The buzz-cuts at NASA don't have the PR sense that Elon has. He's got cameras all over the place. A few extra pounds--at most. Huh?
I'm a big fan of what Space X has achieved, but nothing they've done yet comes close to the JWST for complexity.
"Where would the cameras go?"
"How would they be fitted in to something already folded origami style?"
"How would the actual science (not PR) instruments be shielded from potential interference?"
How many extra points of failure would they add to the 344 already itemised?"
And I'm sure there are plenty of other similar questions that could be asked to show why, for a project that was already WAY behind time and WAAAAAY over budget, adding video cameras for a TV audience was not considered worthwhile.
I concur that even limited video images of the unfolding process would be great for the wider public to watch, even if the team believed they could get all the info they needed via sensors and feedback.
Here is the official answer: https://youtu.be/AxO3Wm9uPq8?t=3546
100% focused on the technical benefits (or not) of cameras for the unfolding the telescope, but ignoring the benefits of an individual or pair of wide angle cameras mounted at the top, looking down...
" installed a few GoPro's on the thing to give us all a look at how things were going."
Do you realise how far away it is now? It was past the Moons orbit pretty sharpish. At time of this post, it's already more than twice the distance of the Moon at ~600,000 miles and still travelling at about 1/3rd mile per second. Yes, second, not hour. You need some pretty decent power on the transmitter, and accurate positioning for that and weight and power are things they really don't want to waste on stuff deemed non-essential. I'm sure they have images and lots of telemetry for the guys doing the work, but almost certainly not enough bandwidth to livestream it for us mere mortals. No doubt there will be timelapse video of a cleaned up feed eventually,
In lieu of cameras (for all the reasons noted here), they did what I think is a pretty decent alternative - an animation system that mirrors, as it were, the telescope so that engineers and the rest of us can visualize what is going on. Some description of it is buried in the 'real time' video of the unfurling process: https://www.youtube.com/watch?v=IBPNi7uGgWM (NOTE there is music for several minutes before the actual content starts, and the discussion of the animation is further along. If you have the time, it's interesting.)
First question is: where do you put them?
If you put them on cold side then a GoPro will radiate horrid infrared and make your expensive telescope worthless. So you can't use a GoPro: you must use an expensive which will run at 50K or less and not radiate any significant power to fuck up the telescope. And the cold side is dark: very dark. So this camera can not use optical wavelengths as there is nothing to see: must use IR to see anything. Now it is not an expensive: it is a very expensive. And also it is a useless.
If you put them on the hot side then you must either make them significantly radiation-hard (so, not a GoPro, but an expensive) or accept they will not last long. And they will now use up power, need cabling, need software, need all sorts of things. And again: where do you put them? There is no special arm or boom by which the telescope can see itself in the way that a Mars rover for instance needs anyway: almost any mounting point on the telescope has a very poor view of almost all the telescope. Perhaps you could put something on momentum flap? But imagine the headlines: 'momentum flap deployment failed because useless camera cable got stuck, telescope now useless'. No, not there.
And if your big hot robust rocket blows up you build another one – sometimes rockets just explode this is how it goes. If JWST fails that is it: there will never be another one. Even if it succeeds may never be another one, but if if fails no-one is going to spend money on something like this ever again. If it fails because of a useless encrustation you glued onto it then you will feel pretty silly. So they do not glue useless crusts onto it.
Looks like NASA is listening to you, they just published a nice blog post:
"As NASA’s James Webb Space Telescope makes its way out to its intended orbit, ground teams monitor its vitals using a comprehensive set of sensors located throughout the entire spacecraft. Mechanical, thermal, and electrical sensors provide a wide array of critical information on the current state and performance of Webb while it is in space.
A system of surveillance cameras to watch deployments was considered for inclusion in Webb’s toolkit of diagnostics and was studied in-depth during Webb’s design phase, but ultimately this was rejected.
“Adding cameras to watch an unprecedently complicated deployment of such a precious spacecraft as Webb sounds like a no-brainer, but in Webb’s case, there’s much more to it than meets the eye,” said Paul Geithner, deputy project manager – technical for the Webb telescope at NASA’s Goddard Space Flight Center. “It’s not as straightforward as adding a doorbell cam or even a rocket cam.”
First of all, Webb is big, undergoes many configuration changes during deployment, and has many specific locations of import to deployment. Monitoring Webb’s deployments with cameras would require either multiple narrow-field cameras, adding significant complexity, or a few wide-field cameras that would yield little in the way of helpful detailed information. Wiring harnesses for cameras would have to cross moving interfaces around the observatory and add more risk of vibrations and heat leaking through, presenting a particular challenge for cameras located on the cold side of Webb.
Then there’s the issue of lighting. Webb is very shiny, so visible cameras on the Sun-facing side would be subject to extreme glare and contrast issues, while ones on the cold, shaded side would need added lighting. Although infrared or thermal-imaging cameras on the cold side could obviate the need for illumination, they would still present the same harnessing disadvantages. Furthermore, cameras on the cold side would have to work at very cold cryogenic temperatures. This would either require ‘ordinary’ cameras to be encapsulated or insulated so they would work in extreme cold, or development of special-purpose cryogenic-compatible cameras just for deployment surveillance.
Notwithstanding these challenges, engineers mocked up and tested some camera schemes on full-scale mockups of Webb hardware. However, they found that deployment surveillance cameras would not add significant information of value for engineering teams commanding the spacecraft from the ground.
“Webb’s built-in sense of ‘touch’ (for example, switches and various mechanical, electrical, and temperature sensors) provides much more useful information than mere surveillance cameras can,” said Geithner. “We instrumented Webb like we do many other one-of-a-kind spacecraft, to provide all the specific information necessary to inform engineers on Earth about the observatory’s health and status during all activities.” Engineers can also correlate years of data from ground testing with telemetry data from flight sensors to insightfully interpret and understand flight sensor data."
L2 is only about 1% further from Sun than Earth's orbit, so plenty of sunlight. And since Earth is very small compared to Sun, even when Earth is directly between spacecraft and Sun (which it seldom is as spacecraft is in fact orbiting L2) it eclipses only fairly small amount of light (is an annular eclipse of Sun really).
Hooray:
https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html
Yes, I know the link has been posted before, but this is, hopefully the start of something amazing about science and discovery about the universe, and that makes me excited and happy.
All the best to the team for the next few months commissioning the instruments and getting everything ready for the first proper observations.
Hooray!
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