[HN Gopher] James Webb is fully deployed
___________________________________________________________________
James Webb is fully deployed
Author : Sami_Lehtinen
Score : 584 points
Date : 2022-01-08 18:19 UTC (4 hours ago)
(HTM) web link (www.nasa.gov)
(TXT) w3m dump (www.nasa.gov)
| sam-2727 wrote:
| Press release with more details: https://www.nasa.gov/press-
| release/nasa-s-webb-telescope-rea...
| dang wrote:
| Ok, we've changed to that from
| https://twitter.com/NASAWebb/status/1479880178021060609 above.
| Thanks!
| WalterBright wrote:
| What puzzles me is the maneuvering fuel. When that fuel runs out,
| the telescope can no longer orient itself. This ended the life of
| the Kepler telescope.
|
| But aren't there other ways to orient in space?
|
| 1. use pressure from the solar wind
|
| 2. have 3 electric motors on 3 axis. Wouldn't spinning those
| motors rotate the craft? Electric power to do it would come from
| solar panels, giving it plenty of fuel.
| rcxdude wrote:
| As far as I can tell it only really needs the fuel to maintain
| an L2 orbit, which is important because if it's too far away we
| can't really communicate with it effectively (i.e. actually
| download much of the data it's generating). For orientation it
| uses reaction wheels as you mention, and then there's a general
| plan to desaturate these momentum wheels by managing the
| average orientation of the telescope (it's effectively like an
| inverted pendulum: the solar wind will push it further away
| from having its back to the sun), but this might intefere with
| some observations so they may burn some fuel to maintain
| orientation in certain circumstances.
|
| https://space.stackexchange.com/questions/35399/how-will-jws...
|
| Kepler used a similar strategy (though I don't know what its
| desaturation strategy was): it only ran out of fuel very
| quickly after its reaction wheels failed.
| ak217 wrote:
| JWST, like many other spacecraft, has reaction wheels to orient
| itself. The reason Kepler ran out of fuel when it did was that
| it was expending more maneuvering fuel for attitude control
| because two of its four reaction wheels failed. Hubble and the
| ISS also had similar failures.
|
| We're getting better all the time at building more reliable
| components (including reaction wheels and cryocoolers) though.
| Until a few years ago, the life of something like JWST would be
| limited by the amount of liquid helium on board to cool the
| components. Modern cryocooler technology (aka a space grade
| refrigerator) is good enough to cool it indefinitely. Solid
| state cryocoolers, previously unachievable, are now apparently
| available for some applications (important not only for
| reliability but also to reduce vibrations).
|
| Reaction wheels can be used for attitude control but they still
| have to be unloaded by thrusters after maneuvering for a while.
| You're right that you could use a rudder (probably two rudders
| would be required for 3d attitude control) and have to have a
| balanced solar wind profile (JWST does actually have a solar
| wind balancing flap, but I don't think it's adjustable like a
| rudder). But solar wind won't act fast enough if you want to
| quickly change attitude for observations. And you can't use
| reaction wheels for stationkeeping. It very much matters where
| the telescope is, since if it drifts too far away from Earth it
| will be much harder to send high bandwidth data, and if it's
| too close to Earth, Moon etc. it will have no way to orient
| without heating up or blinding itself with the IR sunlight
| reflected by them.
| terramex wrote:
| > The reason Kepler ran out of fuel when it did was that it
| was expending more maneuvering fuel for attitude control
| because two of its four reaction wheels failed. Hubble and
| the ISS also had similar failures.
|
| And to calm down anyone afraid of JWST sharing the same fate
| - construction of reaction wheels have been changed some time
| ago to make them significantly more reliable. The source of
| issues on Hubble, Kepler, FUSE, Hayabusa, Dawn and TIMED was
| electrical arcing between metal parts of reaction wheels.
| Static charge was building up like when you rub a ballon
| against your head. That charge caused arcing that in turn
| caused metal pitting and increased friction leading to
| failures. That failure mode was understood only in late 2007,
| when Kepler was already fully build and ready for launch.
|
| JWSt uses new generation ceramic bearing in its reaction
| wheels, they have been used in spacecrafts since 2010 with
| great performance.
| oakwhiz wrote:
| IIRC the thrusters are also used for station-keeping since
| practical Lagrange orbits are unstable.
| WalterBright wrote:
| Being a telescope, what matters is where it is pointing, not
| where it is.
| terramex wrote:
| It matters a lot, L2 is unstable point so if it ventures
| beyond L2 there would be no way to bring it back and it
| would enter a heliocentric orbit. Communication between
| Earth and telescope would become impossible after some time
| as gimballed antenna can only rotate so far and stray light
| reflected from Earth would limit its field of view.
|
| Due to that JWST will always be on 'close side of L2' and
| technically in slow freefall back to Earth and boosted up
| periodically, but always a bit short of passing to the
| other side.
| [deleted]
| kortilla wrote:
| Reaction wheels let you orient but they don't allow you to
| maintain an orbit.
| voldacar wrote:
| not sure about 1, but it already orients itself using reaction
| wheels, which are basically what you are describing with 2.
|
| The issue is that its position at the lagrange point L2 is an
| unstable equilibrium, which requires occasional adjustment
| using thrusters. In terms of gravitational potential energy,
| its position in space is a saddle point, not a local minimum.
| irdc wrote:
| They've already mentioned having more than the of amount of
| manoeuvring fuel (or, as we should really be calling it,
| delta-v) they had planned to have left at this point. Space
| craft have a limited life time anyway (CCD's wear out,
| semiconductors are subject to electron migration, solar panels
| degrade), so having a limited amount of fuel to stay at Sol-
| Earth L2 is just part of the whole lifetime equation.
| WalterBright wrote:
| Right, but NOVA said that running out of fuel ended Kepler.
| irdc wrote:
| It worked for 9 years and 7 months, whilst being planned to
| operate for 3,5 years, so beyond it's expected lifetime
| then.
| WalterBright wrote:
| And it would have worked for longer if it didn't depend
| on the fuel!
|
| BTW, I doubt there's enough experience with space
| hardware to accurately predict it's life, especially
| since each machine is a one-off.
| rcxdude wrote:
| It would have lasted a lot longer if its reaction wheels
| hadn't failed. That was the main failure which shortened
| its remaining operation.
| terramex wrote:
| Pressure from solar wind will constantly put some torque onto
| telescope as its center of pressure is offset from center of
| mass. This torque will be counteracted by reaction wheels but
| they have maximum rotation speed and need to be unloaded using
| thrusters periodically.
| WalterBright wrote:
| A rudder would work to counter any torque from the solar
| wind, and can be used to orient it.
| ThinkBeat wrote:
| They should have made a port for refueling it.
|
| An internal fuel tank and a high-capacity tank externally
|
| Once the fuel is getting low, launch a refuel. when the new
| supply gets closed, eject the previous tank, dock the new one.
|
| Easy.
|
| When they managed to du an in air refuel of a SR71 in 1970s .
| Surely we can dock a fuel tank in 2022?
| brink wrote:
| I've never been one for being hyped about telescopes, but I am
| particularly excited about the images that this one will come up
| with.
| microjim wrote:
| Great news! I'm happy about this. Just curious - had this mission
| critically fail, how likely would we be to simply try again? Who
| needs to approve the cost for that? Would it be politically
| important to succeed currently?
| PaulKeeble wrote:
| Hopefully it inserts into L2 orbit successfully and starts
| doing science. If the results are interesting I can well see a
| future mission to try and refuel it to extend its life past its
| projected 10+ years.
| randmeerkat wrote:
| https://www.cnn.com/2021/12/30/world/james-webb-space-
| telesc...
|
| "It also used less propellant than planned due to the
| precision of the telescope's launch aboard the Ariane 5
| rocket, so "the observatory should have enough propellant to
| allow support of science operations in orbit for
| significantly more than a 10-year science lifetime,"
| according to NASA."
| ttul wrote:
| I don't think there was a way to fix it remotely. It would just
| do minor science missions with whichever instruments actually
| worked.
| emptyfile wrote:
| Amazing feat of engineering. NASA can be proud of achieving
| something that no other space agency could do. It took courage to
| keep shovelling money and effort into this project and some
| amazing science to get it deployed into space.
| davesque wrote:
| I agree with the sentiment, but I think some people might take
| issue with the suggestion that this was entirely NASA's
| achievement.
| [deleted]
| Crash0v3rid3 wrote:
| Who else was involved?
| davesque wrote:
| ESA and CSA to name a couple. Probably also lots of other
| uncredited, small cross-institutional interactions. Not to
| mention all the details of the launch. I was partly trying
| to speculate as to why the comment appeared to be getting
| down-voted. But I guess it's also my opinion that it's
| kinda weird to claim that no one else could have done it.
| Who else are we talking about other than earthbound humans?
| coldcode wrote:
| I wonder when it can start observing.
| fotta wrote:
| It'll take a few weeks after L2 insertion to cool the
| components and another 5 months to calibrate the instruments.
| dylan604 wrote:
| I'm hoping that the 5 months of calibration time is one of
| those where the engineers were asked "how long", so they
| thought about it and then padded the shit out of it, then the
| PMs took that number and padded it yet again. After all that,
| it only takes a few weeks. It'll be the first milestone JWST
| would meet early, so not too likely. We've only waited
| decades, so a few more months isn't that bad. At least it is
| off the ground now!
| terramex wrote:
| Similar question was just asked during press conference and
| the answer was "telescope is cooling a bit faster than we
| simulated but it is not a significant difference, it might
| be ready one or two days ahead of current estimation".
|
| Current timeline is for getting all 4 cameras to work, but
| I think only one needs cryogenic cooling, other 3 could
| work with just sunshield. In another answer one of the
| engineers said that first images will be released once
| telescope is fully operational (aka all 4 instruments
| cooled down).
| max-ibel wrote:
| Surely, it can take pics during this time with suboptimal
| cooling.
|
| Probably hard to resist the temptation:)
|
| There might be good engineering reasons to experiment
| early, like testing comms systems etc
| terramex wrote:
| It will surely take plenty of engineering and calibration
| photos during that time. I don't think they will be
| published instantly because telescope operators want to
| make a 'wow' effect with first photos from JWST.
|
| Here is first technical image from Hubble, before its
| mirror's flaw was discovered: https://stsci-
| opo.org/STScI-01EVTBEME0059ASGX8X3DFWZMX.jpg
| taftster wrote:
| I can tell, you're an engineer. Of some variety.
| [deleted]
| asteroidbelt wrote:
| No mention of ESA again, looks like it is solely NASA project,
| which it is not.
|
| Shame.
| spixy wrote:
| To be fair ESA contributed only with ~10% of total budget, and
| I guess much of that was the rocket.
| kitd wrote:
| I was under the impression that ESA's role was to get it into
| orbit, and is thus largely done. Is that wrong?
| asteroidbelt wrote:
| From Wikipedia:
|
| > NIRSpec (Near InfraRed Spectrograph) will also perform
| spectroscopy over the same wavelength range. It was built by
| the European Space Agency at ESTEC in Noordwijk, Netherlands.
|
| and more.
| elefanten wrote:
| For those not up on the project, what are some of the earliest
| novel types of information we expect to receive from it?
| ducktective wrote:
| Word on corners of the interwebz has it that this is gonna
| prove the existence of aliens (or at least gov. are gonna frame
| JW as the indicator of alien life)
| api wrote:
| It could find a candidate biosphere. If we see a lot of free
| oxygen or other super reactive things like fluorine that
| would be very suggestive. It's not proof but we don't know
| many other processes that can maintain a high percentage of
| oxidizers in an atmosphere.
|
| If we saw that the next step would probably be a telescope
| designed specifically to observe that target. There are some
| thoughts about using a telescope out near Pluto that could
| use the sun as a gigantic gravitational lens to photograph an
| exoplanet and get very detailed spectroscopic information.
|
| If we have a planet nine that is a primordial black hole that
| would be an absolute killer gravitational lens.
| Iv wrote:
| "Meet hot methane-breathing life forms in your neighborhood!"
| binarymax wrote:
| Being able to see how galaxies evolved - you can see a galaxy
| 200M light years after the Big Bang and compare it to Hubble's
| 500M years after the Big Bang.
|
| Also spectra will be available to understand physical and
| chemical compositions at early times.
| micromacrofoot wrote:
| One of the more interesting things that we'll get is data about
| what's in the atmosphere of exoplanets
|
| https://jwst.nasa.gov/content/science/origins.html
|
| Note that the next 5 months or so will consist of mirror
| cooling and calibration, so nothing until summer.
| MegaDeKay wrote:
| Would you happen to know if they can make intermediate
| observations while this is in process e.g. observe a
| relatively nearby object before the telescope is fully cooled
| and aligned? I'm wondering if they would be able to prove out
| the optics early on - this is no slam dunk as the Hubble
| proved.
| q1w2 wrote:
| This will only work for a handful of very nearby exoplanets,
| unfortunately.
| sp332 wrote:
| This one was really interesting to me because this kind of
| observation didn't exist when JWST was being specced out. We
| didn't know there were that many exoplanets around that we'd
| want to look at.
| rich_sasha wrote:
| Huge. This is the first large telescope observing in far
| infrared. Some things:
|
| - very early galaxies (so far that they are redshifted to far
| IR). Hence the "looking into the start of the Universe" talk).
| We know they are there, and that they are unusual and super-
| interesting, but just can't see them.
|
| I think this is high on the agenda so I'm guessing some PR
| shots of ancient galaxies are due.
|
| - cold objects nearby; brown dwarves, rogue planets etc. Maybe
| planets around nearby stars.
|
| - I haven't seen this discussed, but maybe: Kuiper Belt
| objects, maybe looking for Planet X etc.
| micromacrofoot wrote:
| Even earlier than galaxies, we'll be able to see some of the
| first stars that existed before galaxies formed.
|
| IIRC the telescope will be able to see "back" into 98% of the
| observable universe.
| gitgrump wrote:
| Yup! If the universe's age were a single year, we're
| looking back to January 6th. Truly remarkable. :)
| sul_tasto wrote:
| what prevents us from seeing back any further?
| baq wrote:
| Redshift and mirror diameter. There is very little old
| light, so to see further, you have to collect more light
| to have a chance of catching those ancient photons. Also,
| due to expansion of space, the older the photon, the less
| energy it has, so you have to look deeper and deeper into
| infrared.
| EMM_386 wrote:
| > what prevents us from seeing back any further?
|
| The other comment mentioning darkness is wrong.
|
| In the very early universe, it was extremely bright and
| hot. It was only after 100,000 years or so the universe
| cooled down enough to become transparent.
| code_biologist wrote:
| Disclaimer: not a cosmologist. We _can_ see back further
| with other instruments, just there 's less to see.
|
| The universe's history: https://en.wikipedia.org/wiki/Chr
| onology_of_the_universe#/me...
|
| - We can see the cosmic microwave background (CMB), the
| earliest photons after the big bang still observable.
| This is ~14 minutes into Jan 1 if the whole age of the
| universe is a year. Satellites like the WMAP have done a
| great job of that.
|
| - The dark ages that follow had few photon sources.
|
| - JWST will be observing the earliest stars following
| that era.
|
| There's a proposed radio telescope (that would have to be
| on the far side of the moon!) to observe neutral hydrogen
| photon emissions from the dark ages:
| https://en.wikipedia.org/wiki/Dark_Ages_Radio_Explorer
| joeyh wrote:
| There were maybe 100 million years after the big bang
| with no star formation. Darkness. Here's a page about
| that era as it relates to Webb:
| https://webb.nasa.gov/content/science/firstLight.html
| [deleted]
| fotta wrote:
| I'm excited to see what we learn about the earliest days of the
| universe since it can observe highly red-shifted objects.
| smoe wrote:
| It will take another 6 months before JWST will go into
| operation it seems. This page has a list of approved programs.
|
| https://www.stsci.edu/jwst/science-execution/approved-progra...
| anarazel wrote:
| Apparently it takes about 100 days for the instruments to
| cool down far enough.
|
| https://planet4589.org/space/misc/webb/time.html
| jazzyjackson wrote:
| "Images of the first galaxies, the first black holes" iirc from
| Smarter Everyday's interview with John Mather, sorry don't have
| a time code for you
|
| https://youtu.be/4P8fKd0IVOs
| mabbo wrote:
| The full list of the first set of scientific missions is
| available here: https://www.stsci.edu/jwst/science-
| execution/approved-progra...
|
| The overall idea is that JWST can see very faint objects in the
| infrared spectrum. The analogy I've heard is that it could pick
| out a bedroom nightlight on the moon from earth.
| WalterBright wrote:
| Since 90% of the cost is probably in R+D of the telescope, one
| could build and deploy another for another 10%. Why isn't this
| done? Why is every space telescope completely unique?
| Voloskaya wrote:
| Because it's a wrong assumption. Significant cost come from the
| assembly and testing of all the components itself, that you
| would have to redo entirely.
|
| Also if you were to build just 2 or 3 of them, you can't expect
| any economy of scale.
|
| On top of that, the operational cost of JWST is expected to be
| around 1B$ for it's lifetime, you could expect that to be
| similar for every single replica you have.
|
| And finally, you can only put one per rocket, and just the
| rocket is about 200M$ dollar, and you need to add all the cost
| of shipping the telescope to Guyana, that's not cheap.
|
| So overall, while a second replica would not cost you another
| 10B, it would probably cost in the order of 3-5B$, that's a lot
| of billions for a telescope with exactly identical capabilities
| to another one. It would still be useful, as astronomers are
| going to have to compete for time on the JWST and not everyone
| will be served, but the benefits of a second one would be
| marginal compared to the benefits of the first one. So the
| price/benefit ratio might actually be worse on a new copy.
|
| Meanwhile, there is a myriad of other very cool NASA projects
| that would greatly benefit from 3-5B$ instead and do things
| that we haven't done so far.
| BurningFrog wrote:
| > _the benefits of a second one would be marginal compared to
| the benefits of the first one._
|
| This can't be true.
|
| The benefit of the JWST is the observations it can make Two
| JWSTs can make twice as many observations as one, so it
| provides twice the benefit.
|
| In some sense there is a diminishing return in that the most
| important observations will be attempted first, and and over
| time the average observation will be less and less important.
| But surely there is many decades of pent up very important
| research!
| Voloskaya wrote:
| JWST was built to see in IR to help us get answers to a set
| of important questions. The best known one being trying to
| elucidate the question around the rate of expansion of the
| universe. We hope to get some answer by looking at objects
| much further away in space and time with the JWST and
| getting new estimates for the expansion rate. Something we
| cannot do today.
|
| You don't need two telescopes to do that, you just need one
| with a specific set of IR capabilities.
|
| Sure, having twice the imaging power is better, but it's
| definitely far from doubling the benefits. The lifetime of
| the JWST is expected to be 10+ years, that's a lot of data
| that will come to us already, and everything the astronomer
| community deems important will have time on the telescope.
|
| Just like we only needed 1 LHC to confirm the existence of
| the Higgs boson and the robustness of the standard model.
| Building two of those would have been a massive waste of
| money, it was much better to build one, run experiments,
| assess the results and then use the money that was saved by
| building a single one to build new tools with new
| capabilities to answer the new questions.
|
| Of course the reality of government budgeting is a little
| bit more complicated than my rosy picture but the point
| stands.
| thiagotomei wrote:
| A small nitpick: yes, we only needed 1 LHC to confirm the
| existence of the Higgs boson, BUT we made sure to have
| two experiments (ATLAS and CMS) looking for it. As far as
| I know, every modern high energy physics accelerator has
| had two (or more) sister experiments to cross-check each
| others!
|
| Of course, it is not a perfect analogy, since the two
| experiments are not replicas. They try to address the
| same physics cases, but they were designed, built and are
| operated in a completely independent way.
| WalterBright wrote:
| I remember a NOVA on one of the Mars landers. Nobody had ever
| landed anything on Mars with a parachute before, so the
| developers had a massive problem. They built this humongous
| facility to test parachutes. One design after another failed,
| for several months. They finally found a design that worked.
|
| I don't know what building a second chute would cost, but I
| bet it would be less than one thousandth of the cost of #1.
| WalterBright wrote:
| Assuming your figures are correct, you'll get double the data
| for another 30% cost.
|
| If you wait a month before launching #2, if problems appear
| in #1 (like the telescope mirror was ground improperly) it
| can be fixed in #2.
|
| The operational cost will not double. The same ground
| facility, equipment, and staff can manage both.
|
| If you're buying two identical launches, you can get a
| quantity discount.
|
| > the benefits of a second one would be marginal compared to
| the benefits of the first one
|
| And yet I read many glowing accounts about how much extra
| value came from extending the Hubble's lifetime.
|
| I do have some experience with this. I worked for 3 years on
| the design of the 757. Thousands of engineers spending maybe
| 5 years on it. None of that has to be repeated. In a machine
| shop, most of the cost is in the setup. Making two adds
| little cost. I had a job assembling electronics to help pay
| for college. The first board would take 2 hours to build. The
| next one, half that. The fourth, 20 minutes.
|
| I'm sure there's plenty of software on board that machine. On
| HN we all know how expensive making software is. Making a
| copy costs nothing.
| lisper wrote:
| > double the data
|
| Yes, but the data has diminishing returns as well.
|
| The real value in a second unit is having a backup in the
| event of a total failure of the first. But that's a hard
| sell to the taxpayers who fund these things.
| WalterBright wrote:
| > Yes, but the data has diminishing returns as well.
|
| I find that hard to believe considering there's literally
| a _universe_ of things to look at. We find surprising
| things everywhere we look.
| Beldin wrote:
| $3-5B would go a long way to a new Einstein telescope
| (ie. gravitational waves detector). It would likey allow
| for significant sensitivity increases in the existing
| ones. It could propel the National Ignition Facility or
| ITER forward. It could be used to build a new Arecibo. It
| could add an enormous amount of low-frequency telescopes
| - or other scopes for very large integrated scope arrays.
|
| I'd much rather broaden our view than double it in one
| narrow band. (Or advance fusion research.)
| lisper wrote:
| That's not true. The potential for surprises gets smaller
| and smaller the more data you collect. The Webb will
| produce surprises at first because it is able to make new
| _kinds_ of observations that were not possible before,
| but after a while that new data will be used to improve
| our models and subsequent observations in the same regime
| will (almost certainly) be less surprising as a result.
| WalterBright wrote:
| > The potential for surprises gets smaller and smaller
| the more data you collect.
|
| Mathematically, you are quite correct.
|
| But do you really believe that with _one_ little ole '
| telescope pointed at the freakin' _universe_ you 're
| going to reach a significant point of diminishing
| returns?
|
| It's like saying if you invent the first microscope, and
| discover bacteria, why bother with another one?
| caconym_ wrote:
| It's not quite the same thing, but as an analogy: should
| we have built two CERNs to "find" the Higgs boson faster?
| Or was it better just to build one, expecting that it had
| a good chance of finding the particle and other new
| physics, and then put future funding toward
| different/more advanced and capable instruments that will
| give us a window into even higher energies/entirely new
| observation spaces, rather than just accelerating our
| search of the same spaces by low integer multiples?
|
| My understanding is that the JWST opens up new
| observation spaces, specifically very distant and highly
| redshifted objects that Hubble couldn't capture. So we
| should have a lot of data on a new class of objects in
| fairly short order, and thanks to the cosmological
| principle we can expect to see similar distributions of
| the same objects and phenomena no matter what direction
| we look in. As we gather more data, we will converge on
| an understanding of these new spaces, and eventually the
| error bars will shrink to the point where further
| observation is generally not giving us much new
| information.
|
| Is it better to build more JWSTs, to accelerate that
| convergence by low integer multiples and similarly
| increase the chance that we'll happen to point one at
| something truly new and "surprising"? Or should we spend
| our money on bigger and more capable instruments that we
| know will give us access to entirely new observation
| spaces that are completely out of reach of the JWST and
| other extant instruments?
|
| I don't think it would be such an obvious question, if we
| weren't constantly getting better and better at designing
| and launching large and complex instruments. As things
| stand, we can let JWST and its ilk blaze the trail, then
| follow up with cheaper instruments building on lessons
| learned and our general technological and economic
| progress.
| BobbyJo wrote:
| Double the data may not be worth 30-50% more money to NASA.
| Going from no data of type X to 1 data of type X is worth a
| lot more than going from 1 to 2, and so the leap from 1 to
| 2 might just not be worth it, even if it is 50% cheaper
| than the leap from 0 to 1.
| mlyle wrote:
| > Assuming your figures are correct, you'll get double the
| data for another 30% cost.
|
| I think 30% is probably a reasonable guess, based on past
| programs where people have flown 2. You get to reuse design
| and some fixturing. You get to share some operational
| costs. But you're not at unit counts where you benefit from
| mass production techniques and a whole lot of verification
| and qualification work are still effectively one-offs for
| each one.
|
| (You save a whole bunch of costs related to making a
| repeatable program that can turn out hundreds of an item,
| but more has to be validated/verified for each unit).
|
| The thing is-- what's the marginal value of the additional
| data (and of the higher priority data arriving earlier)?
| Would you rather have 2 James Webbs for $13B, or 1 James
| Webb & some other $3B mission?
|
| (Or, at the outset/original decision making: do you aim for
| 2 somewhat simpler telescopes or 1 really awesome telescope
| with the block of money you're given?)
| ramesh31 wrote:
| >I do have some experience with this. I worked for 3 years
| on the design of the 757. Thousands of engineers spending
| maybe 5 years on it.
|
| Just wanted to say that's really awesome. The 757 is by far
| my favorite Boeing jet of all time. They are so overpowered
| it feels like taking off in a fighter jet. Delta still
| flies them from LAX to HNL, and it's always so much better
| than cramming into a 737.
| WalterBright wrote:
| I am always pleased to be boarding a 757! I was sad when
| the front page of the Seattle Times showed one being cut
| up for scrap.
| nabla9 wrote:
| >you'll get double the data for another 30% cost.
|
| Sending just another similar telescope that provides more
| same typed data (wavelength, angular resolution) is
| probably not worth it. Spending the same sum for different
| type of telescope would be better use of the money.
|
| Hubble, Hershel and Webb were are made for different
| wavelengths, they are complementary.
|
| The Extremely Large Telescope (ELT) is ready around 2027
| and it will be the next revolution. 0.005 arc-seconds
| compared to 0.1 arc-seconds of JWST. (978 m2 vs 25.4 m2
| collecting area)
| bumby wrote:
| > _Making two adds little cost._
|
| I think you are overextending your experience in a
| production environment here.
|
| I've worked in both, and the type of builds in these
| aerospace applications still have huge costs in subsequent
| runs. Hell, even rebuilding an existing component can be
| prohibitively expensive.
|
| Much of the GSE was likely existing so that's probably a
| non-issue.
| WalterBright wrote:
| > huge costs in subsequent runs
|
| Often that's because they have to rebuild and reset the
| tooling. Within the run, though, the incremental cost
| should be minimal.
|
| The huge costs may be huge, but not huge compared to the
| cost of the prototype.
| bumby wrote:
| IMO your logic fits with large production runs.
|
| In space applications costs can be exaggerated compared
| to actual production environments because the risks
| aren't mitigated by something like the FAA, meaning they
| are often mitigated by some downstream process. Besides,
| a lot of the the designs already include critical spares,
| so there's more than a single run, even in a one-off
| design
| WalterBright wrote:
| So build a copy out of the spares! If they're there,
| they're already paid for.
| monocasa wrote:
| I think it's more that this argument can be continued ad
| infinitum. Yes the JWST had significant cost overruns, but
| in the design phase these types of projects necessarily are
| looked at as how to best utilize a fixed size grant of
| money. At the end of the day the decision got made to make
| a better single telescope than to make two simpler
| telescopes.
|
| I wouldn't be surprised though if we start to see clever
| design proposals coming down the pipeline, like several
| cheaper telescopes, swarm designs for giant radio telescope
| arrays, and even amateur designed and operated space
| telescopes. Remember that the JWST started it's design
| phase back in 1996, and the economics of space launches
| have changed considerably since then.
| WalterBright wrote:
| I remember having manuals printed in the 1980s. The first
| manual cost $1000 to print, the second one $1. This was
| with camera-ready copy. Never mind all the cost of writing
| the manual, proofing it, and formatting it.
|
| When I worked at Boeing, the first forging of a part cost
| $250,000. The next, just a handful of dollars.
|
| At Boeing, the first airplane gets a ton of testing, as the
| _design_ is being tested. Airplanes #2 and on only get
| tested to verify it was built according to the design, at a
| tiny fraction of the cost of testing #1.
| mlyle wrote:
| You still don't choose _forging_ as a process to make
| quantity 2 (unless you absolutely need forged parts for
| strength). Instead, you 're milling two parts from
| billet.
|
| Milling 2 parts from billet is cheaper per unit than
| milling 1 (some shared setup and programming costs), but
| it's the same amount of raw material and basically the
| same amount of operator time.
| WalterBright wrote:
| You're right that a one-off forging is so expensive that
| a hogout will be used instead. My point was how expensive
| one-offs can be compared to multiple ones, and I used
| extreme examples as illustrative.
|
| Even in custom machine work, the cost is in the setup. A
| machinist can make two identical cuts on two parts for
| not much more cost than one cut on one part.
| hypertele-Xii wrote:
| Doubling the data will also double the processing
| requirements for that data. The observatory is just the
| remote part of this operation. The data doesn't do squat
| unless stored, analyzed and interpreted by expensive
| professionals on expensive equipment.
|
| It's like saying if you're gonna build a chip fab, why not
| build two while you're at it? Well because the _building_
| isn 't the operation.
| ianai wrote:
| Agree. A replica wouldn't have to be sent to L2 for that
| matter. Pretty sure a lot of science could be done with
| even a slow trajectory leaving the solar system, if
| orbiting something else is infeasible.
| ascar wrote:
| As I understood the engineering of the JWST requires it
| to be at L2 so it's in permanent shadow of the sun and
| can keep the very low temperatures necessary for good
| infrared image quality.
| nitrogen wrote:
| It has solar panels, so it can't be in shadow. IIUC the
| advantage of earth-sun L2 is it's away from the light and
| shadow of the earth and moon, while being reachable by
| radio year round.
| ascar wrote:
| I didn't check for sources again earlier, but this NASA
| article [1] explains it.
|
| Yea shadow was the wrong word and doesn't actually apply
| because of the relative size of the sun and earth.
| However the L2 point is by definition in a straight line
| Sun -> Earth -> L2, so if the sun would be a single point
| light source that would block the sun and place the JWST
| in the permanent shadow thrown by the earth. The point is
| actually that the sunscreen is always facing earth and
| sun at the same time to block/reflect a maximum of heat.
|
| [1] https://jwst.nasa.gov/content/about/orbit.html
| godelski wrote:
| Honestly that doesn't sound like much money. Especially since
| these projects aren't solely funded by one country, even if
| one foots most of the bill. Also, $1bn over it's lifetime is
| really cheap even just for the US alone.
| Voloskaya wrote:
| > Honestly that doesn't sound like much money
|
| Well by comparison, here is the cost of two of the most
| impactful recent-ish (post 2000) space telescopes the US
| has launched:
|
| - Spitzer: 700M$, JWST being it's successor. This telescope
| allowed us to detect an exoplanet through light for the
| first time, refine our understanding of the shape of the
| milky way, find candidate objects to be further observed by
| JWST and many more contributions.
|
| - Kepler: 600M$, this is the telescope that allowed us to
| understand that planets were not rare at all, detecting
| more than 2500.
|
| So imagine what you can do with 3 to 5B$. Certainly more
| interesting things than just doubling your data gathering
| rate of a single telescope.
| bumby wrote:
| This question comes up in every one of these threads and this
| is the correct response.
|
| If you look at the line items on a build, you might see
| something like a $200 bolt. It's not that there was $198 of
| R&D going into the design of the bolt, it's that quality
| management drives the cost. Chain-of-custody, bonding,
| material testing, witnessing etc. are all part of that effort
| and they don't scale like a design spec does.
| WalterBright wrote:
| If it's a one-off bolt with a custom design, it will likely
| cost thousands of dollars.
| bumby wrote:
| The GPs point was that subsequent builds would make it no
| longer a one-off design. My point is that there are other
| substantial cost drivers that break their assumptions. My
| analogy of a $200 bolt was not meant for a custom design,
| but the point stands regardless.
|
| (I've worked in a custom machine shop for aerospace, and
| depending on the tolerances, the actual build is
| typically not thousands until you factor in all the
| aspects in my previous post)
| henrydark wrote:
| Reminds me of the line in Contact: "why build one when you can
| have two at twice the price?"
| WalterBright wrote:
| Contact was written by a scientist who had no experience
| whatsoever in building things.
| yongjik wrote:
| That doesn't sound right...
|
| > Carl Sagan played a leading role in the American space
| program since its inception. He was a consultant and
| adviser to NASA beginning in the 1950s, he briefed the
| Apollo astronauts before their flights to the Moon, and was
| an experimenter on the Mariner, Viking, Voyager, and
| Galileo expeditions to the planets. He helped solve the
| mysteries of the high temperature of Venus (a massive
| greenhouse effect), the seasonal changes on Mars (windblown
| dust) and the reddish haze of Titan (complex organic
| molecules).
|
| > For his work, Dr. Sagan received the NASA Medals for
| Exceptional Scientific Achievement and for Distinguished
| Public Service twice, as well as the NASA Apollo
| Achievement Award.
|
| https://www.planetary.org/profiles/carl-sagan
| WalterBright wrote:
| I've met Sagan (he came by our dorm for conversation and
| dinner with the students). He was a great man. He was
| wonderful to talk to. He has made great contributions to
| science. He deserves all the credit and accolades you
| mentioned.
|
| But I doubt he ever set foot in a machine shop. Making
| things is an entirely different skill.
| guerrilla wrote:
| I wish there were a FAQ. This comes up every single time
| there's a thread about Webb on HN. Search previous posts.
| You'll see a hundred answers to your question, some of them
| well researched.
| Server6 wrote:
| Why would we need two? I would think the next one 20+ years
| from now would hopefully be exponentially better.
| BurningFrog wrote:
| To get twice the science done, of course.
| ajuc wrote:
| In 10 years we can send a bigger, simpler, cheaper, one-
| piece, non-foldable telescope in Starship for a fraction of
| the cost. The biggest saving wouldn't even be the launch
| costs - it would be the simple design allowed by relaxing the
| volume and mass constraints.
| wolverine876 wrote:
| If only most of the cost was folding it (and if only Elon
| Musk's solutions somehow changed all the problems to fit
| them).
| alimov wrote:
| So that more researchers have access. Afaik access is shared
| and researchers have to make a proposal and get it approved
| before use, which to me suggests some kind of queue
| mrtnmcc wrote:
| Queueing researchers should be fine given the universe will
| be around for a while.
| mendigou wrote:
| Next time you have to wait for your code to compile we'll
| say "should be fine, computers will be around for a
| while".
| bokchoi wrote:
| Redundancy would be good reason.
| loonster wrote:
| The Hubble telescope wasn't unique. It was simply the first one
| that was pointed away from earth.
|
| For the JWST, is there even room for a 2nd telescope at L2?
| [deleted]
| baq wrote:
| Space is huge.
| varajelle wrote:
| But L2 is just one point
| CamperBob2 wrote:
| That's an abstraction. In reality, what we call "L2" is
| an orbit around the L2 point. No shortage of room.
| [deleted]
| ajuc wrote:
| There's room for a lot of them, it will be in orbit around
| L2, not exactly in L2.
| lazide wrote:
| When it takes so long to build, it's already obsolete by the
| time it gets launched.
|
| If you made a new one every year, it wouldn't be so bad of
| course, but a decade+? Oof.
|
| It was so bad they kept redesigning it mid way through to
| upgrade things as new discoveries were made, which caused even
| more delays.
| WalterBright wrote:
| > redesigning it mid way through
|
| And that doesn't have to be redone when making a copy.
| lazide wrote:
| Except it will because no one wants an exact copy, because
| if you do it at the same time that's twice the rework, and
| if you do it later, you're building something you know is
| obsolete. No one can resist throwing in some upgrades.
| macintux wrote:
| I believe in this case it's not 90%.
|
| Much of the cost here was (on the ground) assembly and testing,
| given the intricate nature of the setup.
| WalterBright wrote:
| What about all the costs in developing the test procedures?
| Obtaining the test equipment. Designing and building the test
| rigs. Designing, coding, and debugging the test software.
| Training people on how to do the tests. Endless committee
| meetings on if the tests are accurate and complete. Failure
| analysis. I bet they're enormous.
|
| Running the same tests again on another part would be at very
| little incremental cost.
| scottlawson wrote:
| Part of the reason is that each telescope has different
| scientific goals. For example, Hubble is a visible wavelength
| telescope and we learned a lot from the data it collected and
| continues to collect. JWST is an infrared telescope designed to
| see wavelengths that Hubble cannot see, and has a different set
| of science goals.
| [deleted]
| BurningFrog wrote:
| I know nothing specific about the JWST, but here is a theory:
|
| NASA is a government agency, which makes what it does
| ultimately political.
|
| In a political "economy", the return on investments is mainly
| PR. Politicians aim to get voter sympathy in return for
| investing $B in NASA.
|
| And voters won't much care if you put up 3 JWSTs or 1.
| wolverine876 wrote:
| > 90% of the cost is probably in R+D of the telescope
|
| While I might have guessed that, IIRC someone at NASA said that
| most of the cost is parts, assembly, and testing of a massive,
| highly sensitive, highly unusual custom build. Many (most?)
| parts are custom made, and even finding vendors to make them
| again would be difficult - wasn't the manufacturing completed
| several years ago? Again, IIRC, they said a second one might
| even cost more.
| mdavidn wrote:
| Projects to widen freeways face a similar diminishing return.
| The most value arises from the new capability. Each lane
| thereafter does not yield linearly increasing returns.
| [deleted]
| bmitc wrote:
| I don't think that's necessarily the case. For advanced R&D
| projects like this, it's super difficult to capture all the
| processes and knowledge the first time around required to build
| it a second time. I think it would probably be cheaper and
| faster, but not by such a large amount. Also, given the
| timeframe of this project, I would estimate that many of the
| parts may be difficult to obtain again.
|
| There's also the fact that for projects like this, so much is
| learned along the way that you probably wouldn't even want to
| build it the same way again, having found better, more
| efficient, cheaper, etc. ways of doing things.
| halfdaft wrote:
| After reading / watching all the info on this page [1], I find
| it very hard to imagine even the tiniest reduction in cost in
| going from 19 insanely complex mirrors to 38, then there's all
| the insanely complex instruments. It might even be the case
| that the super specialised and expensive machines that were
| built to construct the telescope itself wouldn't be able to
| produce 2 full telescopes without being upgraded / refurbished,
| hence costing a lot more. Everything about this ambitious
| project is probably once-off for a very good reason.
|
| [1]
| https://www.jwst.nasa.gov/content/observatory/ote/mirrors/in...
| brohoolio wrote:
| I was wondering that myself. How much would a second Webb cost?
| Or three? Or a fleet of smaller ones.
|
| I'm assuming all of this was looked at, just curious what the
| answers to these questions.
|
| Kudos to the team! Huge accomplishment.
| chrsig wrote:
| My guesses:
|
| - due to the time between the start of the project and the end,
| enough time has passed where there are sufficiently new
| advances in science/tech/robotics/etc to open up new
| possibilities
|
| - a second one will probably still cost >10% of the original
|
| - a second one wont yield enough benefit to be worth it
|
| like...the JWST isn't anything like hubble, and can do things
| that hubble cannot. So it's not like a fleet of hubbles would
| equal one JWST or something.
| _Microft wrote:
| Why spend money on JWST 2 when you could as well spend it on
| ... _LUVOIR_? ^^
|
| https://en.wikipedia.org/wiki/Large_Ultraviolet_Optical_Infr...
| moffkalast wrote:
| _taps temple_ Why go big when you can go gigantic.
| joering2 wrote:
| JWST versus LUVOIR-A... just wow! Imagine difference in
| quality of pictures!
|
| https://en.wikipedia.org/wiki/Large_Ultraviolet_Optical_Infr.
| ..
| spaetzleesser wrote:
| The technology is pretty outdated already at launch time. I
| think they take the lessons from this one into the next
| telescope.
|
| There is also the question of the part if the spectrum they are
| looking at. The JWST is for infrared so I assume the next one
| will be for different frequencies.
| [deleted]
| irrational wrote:
| > Next up for Webb? Five months of alignment and calibration
| before we start getting images
|
| I thought the L2 insertion burn was the next thing. Has that
| already happened?
| kurthr wrote:
| From the Where_Is_Webb site, insertion doesn't happen for
| another 2 weeks. Like coasting up a hill half the time to L2
| has elapsed but 75% of the way there (50% of the way in the
| first few days!).
|
| The next thing looks like aligning the mirror segments. See the
| status bellow. I think there were already two relatively early
| correction burns.
|
| https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html?un...
| Laremere wrote:
| It has not happened yet. See:
| https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html
|
| However, the alignment and calibration starts soon, before the
| l2 insertion burn. It will just continue for months afterwords,
| too.
| 541 wrote:
| For some context around what makes this deployment so remarkable,
| watch this[0] video that talks about the engineering/building
| aspects of the James Webb [0] https://youtu.be/aICaAEXDJQQ
| ArmandGrillet wrote:
| Thanks for the link. I am interested in knowing more about the
| organization behind that project: how many people took care of
| the deployment, how are they organized, how has quality control
| been done.
|
| https://www.latimes.com/business/story/2021-12-25/james-webb...
| gives some info but not an in-depth view of how things work at
| the NASA.
| jonahbenton wrote:
| Thank you, this is great, have been looking unsuccessfully for
| technical content like this.
| davesque wrote:
| This feels perhaps like a silly comment, but I have this
| intuition that the data collected by JWST could prove to be some
| of the most important ever collected.
| gitgrump wrote:
| I keep telling people that, no matter what, we're going to
| learn something cool about the universe. I'm so excited to see
| these images. I mean, imagine humans 10,000 years ago, just
| surviving, maybe figuring out agriculture, and thinking about
| their place in the world. They looked up at the stars in
| wonder. Now, we've progressed to the point where we can polish
| gold down to the nanometer, and we're sending a giant hunk of
| origami circuits out to L2 to squint back to nearly the
| beginning of time as part of our eternal quest for answers.
|
| Your comment is not silly.
| davesque wrote:
| I suppose I meant it felt silly in the sense that it would
| probably come across as seeming vague. And I may as well say
| a few more words. If I had anything specific in mind, it was
| the possibility that the JWST could find evidence of
| compounds in the atmospheres of distant planets that made it
| seem likely that there is life on those planets. Even if that
| doesn't happen, the fact that it feels like it's even on the
| table is amazing.
|
| I'm also thinking back to when the Hubble came online and
| they started releasing the deep field images. And there was
| this moment where we all realized, "Wow, those things up in
| the sky that we all casually assumed were stars...many of
| them are actually galaxies. And all those black spaces in
| between are full of...more galaxies." Maybe astronomers
| already knew this; I don't know. But the average person
| didn't and it was hard to deny once we started seeing those
| images.
|
| Not sure if it's justified, but I expect similar kinds of
| moments when the JWST starts collecting its first images.
| iammjm wrote:
| Awesome news. How much longer untill it gets to its destination
| and sends data and observations?
| neversaydie wrote:
| Another while to go - 2 weeks left to its destination, but
| several months (~5?) of setup and calibration once it gets
| there.
| moonbug wrote:
| https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html?un...
| rwmj wrote:
| The picture is a visualisation, but does the telescope have a
| camera pointing at itself?
| dang wrote:
| (This comment was posted in response to
| https://twitter.com/NASA/status/1479837936430596097, so that's
| the picture)
| PaulKeeble wrote:
| Alas no its too dark on the cold side and far too bright on the
| hot side, no camera can work in these extremes. Everything we
| know is based on telemetry and they set up a 3D model based on
| the telemetry so that we could see the live state.
| asplake wrote:
| No. https://www.space.com/amp/james-webb-space-telescope-no-
| came...
| albertopv wrote:
| I think it's one of the greatest engineering results ever, a 6
| meter infrared telescope 1 million km away, just wow!
| maxdo wrote:
| Curious did they do a sample data with current temp? Since the
| deployment is complete. I know it will be not as good as at L2,
| but still.
| pfdietz wrote:
| So now we just have to worry about the spacecraft ever losing
| attitude control, since exposing the telescope and its
| instruments to sunlight now will permanently damage them.
| tannhaeuser wrote:
| Can't wait for the first science results from JWST. Any
| predictions?
| thanatos519 wrote:
| "We apologise for the inconvenience." written in the deep IR
| background.
| dgrin91 wrote:
| One of the big numbers that been thrown around is that JWST has
| 344 single points of failure in its mission. Now that deployment
| has been completed, is there somewhere that lists how many of
| those points we have passed?
| bmitc wrote:
| Can anyone elaborate on these single points of failures? For
| example, are they _actually_ single points of failure? Or is it
| a bit of exaggeration for marketing purposes (a bit of under
| promise, over deliver)?
| terramex wrote:
| There is a bit of exaggeration, for example non-explosive
| actuators that needed to be released for sunshield to deploy
| have two redundant electric circuits for their deployment but
| are considered 'single points of failure' as compound part.
| Propulsion system is also considered SPoF but there is some
| redundancy built in, like there are two independent sets of
| thrusters feeded from one fuel tank.
|
| Some other parts don't and cannot have redundancy due to
| design.
| ProAm wrote:
| They are single points of failure.
| bmitc wrote:
| That's not much of an elaboration. Further, I am doubtful,
| because something always goes wrong. So if a project that
| has seen decades of delays and billions of dollars of
| budget overruns has suddenly invented engineering and
| processes that yields zero failures, count me surprised.
|
| I just wish there was more elaboration of things they are
| able to accommodate for as things inevitably pop up and not
| this hyper focus on a number of supposed single point
| failures.
| FiberBundle wrote:
| Not sure about the points of failure, but according to [1] only
| four deployment stages remain. So the large majority of point
| of failure should have been passed already.
|
| [1]
| https://jwst.nasa.gov/content/webbLaunch/deploymentExplorer....
| ducktective wrote:
| >344 single points of failure
|
| How can a system have more that one single point of failure?
| lordnacho wrote:
| I think SPF means a construction where there's no redundancy
| provided? Though it does raise the question of how this is
| counted: table with 4 legs, any of them breaks, table is
| still standing. Table with three legs, any of them breaks,
| table is broken.
| _Microft wrote:
| _Single point of failure_ is a term for non-redundant,
| critical parts of a system:
|
| https://en.m.wikipedia.org/wiki/Single_point_of_failure
| ISL wrote:
| If you're in a car being held above the ground by a chain,
| every single link is a potential single-point of failure.
| ctdonath wrote:
| Contrast a chain with double links, where if any link fails
| another is entirely capable of holding the load.
| sorokod wrote:
| A system may be implemted as a set of cooperating
| microservices each one being a (single) point if failure of
| the entire system.
| dylan604 wrote:
| Like many Node dependencies
| jacquesm wrote:
| What a fantastic comment. In a visual and immediate way to
| explain such a complex concept in one sentence.
| systemvoltage wrote:
| But the chain itself can be considered as one component and
| it becomes a single point of failure. Or if we go in the
| opposite direction, then we can breakdown links and say
| "Every single grain boundary is a single point of failure".
| ISL wrote:
| Partially-agreed on the former, but not the latter.
|
| So long as the chain is not loaded near its tensile
| limit, then the grain boundaries that support the load
| within each link do so in parallel and are therefore
| redundant.
|
| The selection of the allegory of the chain was
| intentional -- each link must be properly formed, or the
| entire chain will fail. If it breaks, it is surely
| correct to say, "the chain broke", but in truth, it was
| actually link-86.
|
| For JWST, the remarkable/audacious thing is that many
| links in the chain from launch to observation are
| potential single-point failures. Furthermore, many of
| them haven't ever been tested independently in space...
| ever. It is a hell of a triumph that JWST has gotten this
| far already.
|
| If even a small fraction of the instrumentation works at
| this point, we are going to learn a _ton_ about the
| universe, simply due to JWST 's position, collecting-
| area, and mirror-diameter.
| systemvoltage wrote:
| You're right. I think I was searching for a better
| analogy. Abstractly: Sub-system SPOF conditions can be
| bundled up as a single System SPOF condition by
| multiplying the probabilities.
|
| Also, I don't want to underplay JWST's success or its
| challenges. But, when saying 300+ SPOF conditions, one
| has to specify at what abstraction level. Otherwise, it
| can be misleading.
| earleybird wrote:
| Each link in the chain is itself
| manufactured/created/constructed. If the weld is bad, the
| link may fail. "every grain boundary" is not formed in
| the same way each link is. The analogy is solid -
| actually, it's very good in that it conveys clearly and
| succinctly the concept the parent was asking about
| without introducing extraneous concepts. Very much in the
| flavour of EWD: "The purpose of abstraction is not to be
| vague, but to create a new semantic level in which one
| can be absolutely precise."
| lovecg wrote:
| Another example is a small single engine airplane. The
| engine can be considered a single point of failure but if
| you zoom in it's a complex system with many redundant
| components (multiple cylinders, multiple spark plugs per
| cylinder, etc.)
| jes wrote:
| In a real chain, only one link is actually the weakest, and
| it is at that link where the chain will break, in the
| hypothetical you offer.
| capableweb wrote:
| Which is true, but also not what's being discussed, which
| is "single point of failures", not "weakest link".
| kortilla wrote:
| That's not correct, someone with a bolt cutter can get
| any link in the chain and it causes collapse.
|
| Single point of failure doesn't mean "weakest link". It
| means if this one piece ("single point") fails, the whole
| system will fail.
| ctdonath wrote:
| But you don't know which, and others may fail first due
| to various adverse events (say, random micrometeoroid
| strike). If any link fails, the whole chain fails.
|
| Contrast a second chain, or double linked chain, so if
| any link fails the load is not lost.
| adrianmonk wrote:
| This assumes the environment is completely predictable.
|
| If you know the strength of every link in the chain with
| perfect accuracy, and you know that the only potential
| cause of failure is too much weight being placed on the
| chain, then the only link that can fail is the weakest[1]
| one because no other failure can happen before that one.
|
| But really you need to design for the idea that various
| things might happen. Someone else gave the example of a
| person choosing a link to cut with bolt cutters. The
| person's choice is what's not predictable in that
| example.
|
| ---
|
| [1] And if you assume it's not possible to have two links
| that are exactly as strong as each other.
| binarymax wrote:
| The sequential deployment process had many possible
| individual points of failure along the way - if any step went
| wrong then the full deployment failed.
| ducktective wrote:
| well yes, but wouldn't we refer to them simply by "points
| of failure" not "single"?
| jbay808 wrote:
| I think it's single in the sense that _only_ a single
| failure in any of those points would render the entire
| project a failure, not that there 's only one point that
| can possibly fail.
|
| Contrast, say, a single-engine jet plane with a twin-
| engine jet plane that can still make it to the airport
| safely with the remaining engine, should one engine fail
| mid-flight.
| furyofantares wrote:
| "single point of failure" is a phrase that refers to any
| subsystem which the whole system is dependent on.
| bryanlarsen wrote:
| Most failure points on Webb have redundancies. So their
| failure doesn't brick the telescope, both the part and
| its redundancy have to fail before Webb is bricked. The
| single points brick Webb with only a single failure.
| jacquesm wrote:
| Because a point of failure could have multiple
| redundancies, and in this case they did not, every one of
| those points was implemented in a non-redundant way.
| [deleted]
| [deleted]
| mabbo wrote:
| I think linguistically the term can be hard to parse.
|
| If any one of 344 pieces were to fail during deployment, then
| all of the deployment has failed and the entire $10B was a
| loss. Consider the engine in your car- how many single pieces
| of it could fail before the entire engine can't work? The
| difference with Webb is that most of those points were single
| actions that had to work once.
|
| And we're now at a stage where most of them _did not fail_.
| burtonator wrote:
| Are these 'points of failure' ? not single points of failure?
| I assume some of these have redundancy but could fail.
| WalterBright wrote:
| Airliners are designed so that no single failure can bring it
| down.
| funklute wrote:
| That's not entirely true. Any reasonably complex system
| will almost certainly have single points of failure,
| including airliners. Those single points of failures might
| be very unlikely, but they are still there. E.g. what if
| the front fell off?
| WalterBright wrote:
| It is entirely true. Source: I worked on 757 flight
| controls design for 3 years.
|
| > what if the front fell off?
|
| All structural components are redundant and have
| redundant attachments. The wing spars are doubled, for
| example.
| [deleted]
| marcosdumay wrote:
| AFAIK no single mirror segment failure would destroy the entire
| telescope, so the only one still on the future is the L2
| insertion burn.
|
| As always, it's useful to post this link:
|
| https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html
| terramex wrote:
| > Mike Menzel: 49 of the 344 single point failures remain and
| will remain throughout the mission. They are the same types of
| things on every mission, like propulsion. 15 are related to the
| instruments.
|
| https://twitter.com/SpcPlcyOnline/status/1479900221131964421
| jl6 wrote:
| More great news. Congratulations to all who have worked on the
| project.
|
| I understand the reasons for not putting a camera on or near the
| JWST, but I'm still a little sad that we'll probably never get to
| see the thing in situ in all its operational glory.
|
| Maybe one day when it finally expires, we can launch a "sample
| return" mission to tow it back.
| daveslash wrote:
| RE>> _" I understand the reasons for not putting a camera on or
| near the JWST"_
|
| What are the reasons? I'm sorry ~ I'm ignorant on the topic,
| but I'd love to learn more. Why/what are the reasons?
| fjarlq wrote:
| NASA explains why in this twitter thread:
|
| https://twitter.com/NASAWebb/status/1479161843595759618
| JohnBooty wrote:
| Great blog post from NASA themselves explaining it:
| https://blogs.nasa.gov/webb/2022/01/06/why-doesnt-webb-
| have-...
|
| the post is really not very long and I would suggest getting
| it straight from the horse's mouth, but for those not willing
| to devote a click here are arguably the most relevant bits!
|
| "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."
| surfsvammel wrote:
| Won't it be pretty close to earth in its final position at L2?
| If so, is there no chance that there will be future missions to
| do service to it? If so, maybe we can get a photo of if then?
| verelo wrote:
| Family guy has got to me, i keep thinking of James Woods high.
| ThinkBeat wrote:
| It says it will arrive in 14 days or so at L2.
|
| Right now, it is moving 0.3933km/s which according to Google is
| 1415.88km/h. That is pretty fast.
|
| What speed does it need to be at for insertion and steady state?
| How long will it be breaking? (If at all, or of it is not already
| doing it).
| baq wrote:
| About 0. It's climbing a hill and has just enough energy to get
| to the top. A small burn then will be all it takes to put it in
| an orbit around L2.
| lovecg wrote:
| It also needs to get as close as possible to the top of the
| hill without going over. L2 is not a stable point: you're
| either falling back to Earth or drifting away. If it goes
| over it can never get back as it can only fire in one
| direction - away from Earth (to protect the instruments from
| the Sun). So it's a continuous balancing act where it falls
| downhill towards Earth a bit, then jumps up without going
| over the edge, rinse and repeat.
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