Do we see anything up there yet?
James Webb is fully deployed
251–260 of 522 posts
Re: James Webb is fully deployed
#252Earlier quoted context omitted.
>344 single points of failure How can a system have more that one single point of failure?
If you're in a car being held above the ground by a chain, every single link is a potential single-point of failure.
The classic counterexample to the chain (where each link is a SPOF) is the braided cable or rope, in which each individual strand shares the load collectively, and loss of some fraction of the total strands won't itself cause failure.
That's not to say that cables or ropes cannot fail, but when they do so, failure of many individual components is required.
A recent example is the failure of support cables for the Arecibo radio observatory in Puerto Rico in 2020. This was the result of a progressive failure played out over months, though at an accelerating rate, until the final catastrophic failure and loss of the instrument as a whole. (This wasn't wholly unforseen and was precipitated by a long period of rather intentional neglect of maintenance.)
Even after the failure of two complete cables, the remaining cables supported the load of Arecibo's instrument platform. On 1 December 2020, strands of one of the remaining cables began breaking, at an accelerating rate, leading to the total failure of that cable, then a second and third, and with it loss of the instrument platform. The final moments were captured on video.
Sequence begins at about 55s here: https://yewtu.be/watch?v=b3AASKr_iHc
Re: James Webb is fully deployed
#253What 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.
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?
> Easy.
Nothing in space is easy.
Re: James Webb is fully deployed
#254Earlier quoted context omitted.
> 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…
Instead of comparing to other telescope projects maybe better to compare to other mega science projects. Or better yet, other government costs. Though of course we'd need to run a cost benefit analysis. The thing is that these numbers are big for us and most companies, but these are government numbers and decades of work. Both factors are important. It's $11bn over 24 years, or 458m/yr. I'm happy to pay an extra $3/y…
> I'm happy to pay an extra $3/yr for this project.
Yes, I think many of us would be happy to. But the reality is that NASA has to fight a pretty ferocious budget battle every year, and often (not always) delay and increase in JWST cost has meant postponing or cancelling other missions.
For example the Nancy Grace Roman Space Telescope, which was deemed the top priority by the decadal survey in 2010 almost got canceled in 2018/2020 because of JWST overruns.
This Nature article is a good read: "The telescope that ate astronomy" [1] (and at the time of the article, JWST cost was "only" at 5B$).
I am very excited about what we will observe with the JWST, but shelling yet a few other billion of dollars out of NASA's tight astronomy budget to get an identical copy, mean we are yet again cancelling or postponing other exciting missions that could help us answer very important questions.
Re: James Webb is fully deployed
#255Something I am wondering about the L2 orbit ... per NASA:
> Webb's orbit [~ around L2] ... is actually similar in size to the Moon's orbit around the Earth! This orbit (which takes Webb about 6 months to complete once) keeps the telescope out of the shadows of both the Earth and Moon. Unlike Hubble, which goes in and out of Earth shadow every 90 minutes, Webb will have an unimpeded view that will allow science operations 24/7.
https://jwst.nasa.gov/content/about/orbit.html
Wouldn't remaining in Earth's shadow result in less interference, with Earth providing an extra sunshield? And "an unimpeded view" of what? I can't believe those cold, sensitive optics, with that carefully engineered sunshield, will point anywhere but away from the Sun.
Re: James Webb is fully deployed
#256The picture is a visualisation, but does the telescope have a camera pointing at itself?
As others have noted, no.
Ground-based observations of the JWST are possible. They're rather more in the style of Seurat than Hirothropologie, and rather minimalist-pointilist at that.
https://www.space.com/james-webb-space-telescope-video-image...
Re: James Webb is fully deployed
#257Earlier quoted context omitted.
Since L2 is an unstable equilibrium, won’t there also need to be routine adjustments throughout the mission?
Yes. Those adjustments, as well as unloading of the reaction wheels used to position JWST, and the fuel burns reqiured for them, are the principle determinants of the JWAT's lifespan, estimated at 10 years. Nasa have lowballed such estimates for numerous missions in the past (Hubble and numerous Mars lander and rover missions come to mind), and it's possible JWST will exceed the planned lifetime, potentially by a wid…
Thought the same. The launch and everything went down so perfectly I bet it gets more than 10 years.
Re: James Webb is fully deployed
#258Re: James Webb is fully deployed
#259Earlier quoted context omitted.
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
Since L2 is an unstable equilibrium, won’t there also need to be routine adjustments throughout the mission?
Does anybody know exactly how unstable L2 is?
I've seen lots of people talk about how some Lagrange points are stable and others are unstable, but I've never been able to find a source for how much this matters.
How long would it take for Webb to move so far off L2 that we can't communicate with it anymore, or for it to be at risk of being completely thrown out of orbit to the point that it would impact earth or the sun?
Re: James Webb is fully deployed
#260Earlier quoted context omitted.
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?
No L2 is on the further side of the moon. Maybe when we have active bases on the moon or Mars would a rendezvous with L2 be feasible but not before. Hubble is in low earth orbit and far more accessible.
Not exactly. Actually it's on the opposite side of the Earth relative to the Sun. So the 3 are always on one line but it has nothing to do with the position of the Moon. (Maybe you thought it was at the L2 of Earth-Moon system?)