Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

James Webb is fully deployed

nasa.gov

Post reply on HN
Print view View original

341–350 of 522 posts

Re: James Webb is fully deployed

#341

Post by sailingparrot »

ThinkBeat wrote: post #195

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).

> How long will it be breaking? (If at all, or of it is not already doing it).

All* of it's acceleration was given by the rocket at launch, it has been constantly "braking" since then as it's being pulled by earth's gravity, and it will reach L2 at same time it reaches a speed of 0.

* Not actually all in truth, since for margin of safety reason, the Ariane rocket purposefully imparted a lower than necessary speed, and the more precise thrusters on the JWST will be used to top it off with 3 different short burns (2 of which already happened), always staying just short of the required speed. The idea being that you want to make sure to be always missing a little bit of speed and readjust as needed, but never too much, as that is not recoverable, and the telescope would just drift away for ever.

Re: James Webb is fully deployed

#342

Post by terramex »

dtgriscom wrote: post #328
terramex wrote: post #170

Earlier quoted context omitted.

> 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,…

Great info: citation? More details?

Here is a paper detailing metal reaction wheels failure modes going into detail on how charge was built up and solar influence on this process: https://www.esmats.eu/esmatspapers/pastpapers/pdfs/2017/bial...

I cannot find primary source for JWST using ceramic bearings right now, but I know they are manufactured by Rockwell Collins Deutchland and have good track record: https://jwst.nasa.gov/content/forScientists/faqScientists.ht...

TELDIX (today owned by Rockwell Collins Deutchland) invented use of ceramic bearing in reaction wheels for spacecraft use in 1978: https://patents.google.com/patent/DE3027209A1/en

so it would make sense that JWST uses ceramic bearings. Other manufacturers (ITHACO) have switched to ceramic too.

Re: James Webb is fully deployed

#343

Post by mikepurvis »

sandworm101 wrote: post #299
jazzyjackson wrote: post #263

Earlier quoted context omitted.

I’m sure if we ever have sufficient technology to build a city on the moon, the first museum exhibit will be a captured JWST

No. The museum will be built atop its first exibit: tranquility base. See Futurama s01e02.

The book Artemis proposes this also.

Re: James Webb is fully deployed

#344

Post by WalterBright »

rcxdude wrote: post #180
WalterBright wrote: post #141

Earlier quoted context omitted.

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.

It would have lasted a lot longer if its reaction wheels hadn't failed. That was the main failure which shortened its remaining operation.

You'd need at least 3 wheels to orient the craft. I bet that 4 wheels would be able to survive the failure of one.

Re: James Webb is fully deployed

#345

Post by dev_tty01 »

JohnBooty wrote: post #208
daveslash wrote: post #204

Earlier quoted context omitted.

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?

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 spac…

Yes, I get all of that, but they still missed an excellent PR opportunity. NASA really could have built some engagement showing images of the deployment process, even if the images wouldn't have direct scientific benefit. Good PR leads to improved funding which leads to more science.

Re: James Webb is fully deployed

#346

Post by freeflight »

bichiliad wrote: post #250
JohnBooty wrote: post #208

Earlier quoted context omitted.

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 spac…

One thing they mentioned that kinda sends it home to me is that the telescope is aimed at the dark and away from the sun, and the other side is especially reflective[0]. In other words, one side is too dark to see, and the other is too bright to see. [0]: https://twitter.com/NASAWebb/status/1479161991252131841

> the telescope is aimed at the dark and away from the sun

To be more specific; It's "parked" in an orbit [0] that constantly puts Earth between the sun and James Webb.

So it's not only aimed away from the sun, Earth is also acting as a planetary scale sunscreen for it.

[0] https://en.wikipedia.org/wiki/Lagrange_point

Re: James Webb is fully deployed

#347

Post by mikepurvis »

behnamoh wrote: post #290
frosted-flakes wrote: post #230

Earlier quoted context omitted.

L2 is 1.5 million km away. For perspective, the moon is 384 000 km away. That's nearly 4x the distance. It took Apollo astronauts 3 days to reach the moon.

So it will take almost 2 weeks to get to JSWT, and two weeks to come back.

Presumably such a mission would be unmanned and therefore a one-way trip.

Re: James Webb is fully deployed

#348

Post by terramex »

WalterBright wrote: post #320
terramex wrote: post #160

Earlier quoted context omitted.

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 f…

> as gimballed antenna can only rotate so far That's a fixable design problem. Or you can rotate the telescope. > stray light reflected from Earth would limit its field of view What? The earth and the moon are nearby anyway. What about that reflected light? And heck, what about the sun limiting its field of view?

> That's a fixable design problem.

You are right, it is fixable. It was fixed by adding active station keeping to the telescope.

> Or you can rotate the telescope.

There are limits on its rotation with respect to the Sun, dark side must be kept away from sunlight at all times. It can rotate 5 degrees "pitch down" toward the Sun and 45 degrees "pitch up". Gimballed antenna has enough authority so that it can communicate with Earth at whatever valid rotation telescope is so that science operations are not interrupted for transmitting data.

> The earth and the moon are nearby anyway. What about that reflected light? And heck, what about the sun limiting its field of view?

That light is reflected back by sunshield as all 3 bodies are behind it. Sun is limiting field of view but area of exclusion changes as telescope orbits and it can image every point in the sky at least every 6 months and 39% of the sky at any given moment.

Re: James Webb is fully deployed

#349

Post by JohnBooty »

dev_tty01 wrote: post #345
JohnBooty wrote: post #208

Earlier quoted context omitted.

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 spac…

Yes, I get all of that, but they still missed an excellent PR opportunity. NASA really could have built some engagement showing images of the deployment process, even if the images wouldn't have direct scientific benefit. Good PR leads to improved funding which leads to more science.

I just want to make sure I understand.

Your position is that they should have added significant complexity to a maneuver that was already the most complex of its kind. A maneuver with $10 billion and two decades of work on the line. In order to make the first 30 days of a 10+ year mission marginally more entertaining.

You propose this while fully aware that the "entertainment" in question consists of things unfolding in extremely slow motion, in total darkness.

I mean, okay.

(Thought experiment: when you think about the Hubble, do you think about the day they shoved it out of the shuttle's cargo bay, or do you think about those groundbreaking images it captured during its multidecade mission?)

Re: James Webb is fully deployed

#350

Post by willis936 »

freeflight wrote: post #346
bichiliad wrote: post #250

Earlier quoted context omitted.

One thing they mentioned that kinda sends it home to me is that the telescope is aimed at the dark and away from the sun, and the other side is especially reflective[0]. In other words, one side is too dark to see, and the other is too bright to see. [0]: https://twitter.com/NASAWebb/status/1479161991252131841

> the telescope is aimed at the dark and away from the sun To be more specific; It's "parked" in an orbit [0] that constantly puts Earth between the sun and James Webb. So it's not only aimed away from the sun, Earth is also acting as a planetary scale sunscreen for it. [0] https://en.wikipedia.org/wiki/Lagrange_point

Actually JWST orbits around L2 at appreciable altitude. The unique aspect of the orbit is that it is never in the shadow of the Earth or moon. It guarantees consistent solar power and thermal load.

https://jwst-docs.stsci.edu/jwst-observatory-characteristics...

Post reply on HN