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James Webb Telescope and L2 Orbit Question


exchemist
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I'm a bit unclear on how earth/moon would cast shadows large enough, at that distance, to much restrict the visible area.  Especially if the Webb is following a system that's in orbit around the sun, which is shifting the star field for it anyway.   Wouldn't the sunshield itself also impede the view in that direction, and again the star field would shift throughout the year such that no area would be blocked for long? 

Also: how does an "orbit" around a libration point use less fuel than nudges at the point itself?  I'm sure it does, but there's something counterintuitive about it.  

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33 minutes ago, TheVat said:

Also: how does an "orbit" around a libration point use less fuel than nudges at the point itself?  I'm sure it does, but there's something counterintuitive about it.  

I'm trying to understand it myself so this may not be correct but the descriptions I've seen make it sound like this to me. When the moon orbits the earth it is because the 'force' of gravity is in the center of the orbit. Similar to how a ball connected to a string will swing around in a circle. With a satellite around a Lagrange point, it is more similar to the way a roulette ball stays in a circle around the center of a roulette wheel since the wall around the roulette wheel contains the ball. It doesn't take much force to keep a ball circling a roulette wheel; the wall does most of the work and you just just need to add small pulse of force occasionally. Keeping a satellite exactly at the Lagrange point is compared to keeping a ball balanced on top of your head; it constantly wants to drift one way or the other.

 

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Thanks, Zapatos.  I found the roulette wheel analogy quite helpful.

 

Just curious about the downvote.  It would be greatly helpful if someone could alert me to which part of the post was problematic, and how I could remedy that?  The first portion was responding to an article posted here earlier which did state there was an advantage in avoiding earth/moon shadows.  I was just seeking a bit of clarification, and was not trying to push any opinion.  A bit mystified by the minus one.  Please clarify?  

Since I don't give downvotes here, I would prefer some directness when someone casts one.   This situation is actually why I don't downvote.  Without an explanation, it can be perplexing and thus  counterproductive.

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3 minutes ago, TheVat said:

Thanks, Zapatos.  I found the roulette wheel analogy quite helpful.

 

Just curious about the downvote.  It would be greatly helpful if someone could alert me to which part of the post was problematic, and how I could remedy that?  The first portion was responding to an article posted here earlier which did state there was an advantage in avoiding earth/moon shadows.  I was just seeking a bit of clarification, and was not trying to push any opinion.  A bit mystified by the minus one.  Please clarify?  

Hmm. I took away the down vote as I couldn't see how it could be justified. I'm guessing the reader of your post somehow misinterpreted.

 

On 1/1/2022 at 4:34 PM, TheVat said:

I'm a bit unclear on how earth/moon would cast shadows large enough, at that distance, to much restrict the visible area.  Especially if the Webb is following a system that's in orbit around the sun, which is shifting the star field for it anyway.   Wouldn't the sunshield itself also impede the view in that direction, and again the star field would shift throughout the year such that no area would be blocked for long? 

I believe Webb only looks outward, away from the sun. The cameras on Webb would not only be impacted by the sun, but also by the earth and moon. Choosing L2 allows the sunshield to block out all of those from the camera. An article I found talked about the fact that only planets outside the orbit of earth could be observed by Webb.

Quote

"With JWST, we can't look inwards towards the sun, but we will be able to look outwards," said McCaughrean. "We can look at planets like Mars, Jupiter, Saturn, Uranus and Neptune but also into the Kuiper Belt."

https://www.space.com/james-webb-space-telescope-science-overview

Given the above, the only thing I can think of that the shadow of the earth would do to the satellite is have some impact on the solar panels.

Since an orbit is around L2 is more stable than Webb being in a fixed spot, perhaps there is no significance to Webb being out of earth's shadow, and was only mentioned as a coincidence.

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On 1/1/2022 at 5:34 PM, TheVat said:

I'm a bit unclear on how earth/moon would cast shadows large enough, at that distance, to much restrict the visible area.  Especially if the Webb is following a system that's in orbit around the sun, which is shifting the star field for it anyway.   Wouldn't the sunshield itself also impede the view in that direction, and again the star field would shift throughout the year such that no area would be blocked for long? 

If you mean the point about passing through shadow, it may be an issue of temperature fluctuations from doing so, and that you’d want to avoid that, so you don’t have to deal with returning to equilibrium 

 

On 1/1/2022 at 5:34 PM, TheVat said:

Also: how does an "orbit" around a libration point use less fuel than nudges at the point itself?  I'm sure it does, but there's something counterintuitive about it.  

Perhaps the angular momentum of being in such an orbit is an advantage.

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Why are the scientists so sure that the JWST can never be serviced again?  Maybe in 10 years from now, as the Web is doing a great job, and we don't want to lose it.  They already spent $10 Billion on it.  Could they not plan a mission to service the Web and thus extend its' life indefinitely?

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25 minutes ago, Airbrush said:

Why are the scientists so sure that the JWST can never be serviced again?  Maybe in 10 years from now, as the Web is doing a great job, and we don't want to lose it.  They already spent $10 Billion on it.  Could they not plan a mission to service the Web and thus extend its' life indefinitely?

Webb, like most satellites, was not designed to be serviced. Hubble on the other hand was designed to be serviced.

One of the reasons it was not designed to be serviced is that they would have to first design a spacecraft to be used for that function.

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3 hours ago, zapatos said:

Webb, like most satellites, was not designed to be serviced. Hubble on the other hand was designed to be serviced.

One of the reasons it was not designed to be serviced is that they would have to first design a spacecraft to be used for that function.

 I read somewhere that certain elements, like the power unit, are replaceable, in the event that such technology may be available in a few decades. I think it's planned service  life is 40 years iirc.

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On 1/1/2022 at 11:28 PM, zapatos said:

I'm trying to understand it myself so this may not be correct but the descriptions I've seen make it sound like this to me. When the moon orbits the earth it is because the 'force' of gravity is in the center of the orbit. Similar to how a ball connected to a string will swing around in a circle. With a satellite around a Lagrange point, it is more similar to the way a roulette ball stays in a circle around the center of a roulette wheel since the wall around the roulette wheel contains the ball. It doesn't take much force to keep a ball circling a roulette wheel; the wall does most of the work and you just just need to add small pulse of force occasionally. Keeping a satellite exactly at the Lagrange point is compared to keeping a ball balanced on top of your head; it constantly wants to drift one way or the other.

 

But you still have the same problem with an orbit around L2, as I understand it. There is a still a tendency for the object to drift either inward towards the Earth/Sun or outward away from them, i.e. to drift perpendicular to the plane of the object's orbit. 

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On 1/7/2022 at 11:47 PM, StringJunky said:

 I read somewhere that certain elements, like the power unit, are replaceable, in the event that such technology may be available in a few decades. I think it's planned service  life is 40 years iirc.

They designed the JWST to be capable of being refueled robotically, just in case anyone gets around to financing such a mission in 10 or 15 years.  If the JWST performs outstanding, maybe that performance will change some minds, and they would extend its' life thru a refueling mission.

"...modest efforts being made to make JWST “serviceable” like Hubble, according to Scott Willoughby, JWST’s program manager at Northrop Grumman Aerospace Systems in Redondo Beach, California. The aerospace firm is NASA’s prime contractor to develop and integrate JWST, and has been tasked with provisioning for a “launch vehicle interface ring” on the telescope that could be “grasped by something,” whether astronaut or remotely operated robot, Willoughby says.  If a spacecraft were sent out to L2 to dock with JWST, it could then attempt repairs—or, if the observatory is well-functioning, simply top off its fuel tank to extend its life."

refueling - Is it possible to refuel the James Webb Space Telescope? - Space Exploration Stack Exchange

Edited by Airbrush
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21 hours ago, Airbrush said:

They designed the JWST to be capable of being refueled robotically, just in case anyone gets around to financing such a mission in 10 or 15 years.  If the JWST performs outstanding, maybe that performance will change some minds, and they would extend its' life thru a refueling mission.

"...modest efforts being made to make JWST “serviceable” like Hubble, according to Scott Willoughby, JWST’s program manager at Northrop Grumman Aerospace Systems in Redondo Beach, California. The aerospace firm is NASA’s prime contractor to develop and integrate JWST, and has been tasked with provisioning for a “launch vehicle interface ring” on the telescope that could be “grasped by something,” whether astronaut or remotely operated robot, Willoughby says.  If a spacecraft were sent out to L2 to dock with JWST, it could then attempt repairs—or, if the observatory is well-functioning, simply top off its fuel tank to extend its life."

refueling - Is it possible to refuel the James Webb Space Telescope? - Space Exploration Stack Exchange

I read that JWST has got enough fuel to last 20 years, but there is another question: it's said that JWST is threatened by being hit by space debris. Does anybody know any details of it?
https://www.space.com/james-webb-space-telescope-fuel-20-years

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4 minutes ago, Kevin_Hall said:

it's said that JWST is threatened by being hit by space debris

The GAIA mission encountered more micrometeoroids than expected.

“the spacecraft is being peppered by far more micrometeoroids – tiny specks of space dust – than had been anticipated”
https://www.newscientist.com/article/dn25925-galaxy-mappers-first-discovery-surprise-space-debris/

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51 minutes ago, swansont said:

The GAIA mission encountered more micrometeoroids than expected.

“the spacecraft is being peppered by far more micrometeoroids – tiny specks of space dust – than had been anticipated”
https://www.newscientist.com/article/dn25925-galaxy-mappers-first-discovery-surprise-space-debris/

Quote

The strikes shouldn’t put Gaia in danger, although mission scientists will have to watch closely as the spacecraft begins its scientific observations.

Read more: https://www.newscientist.com/article/dn25925-galaxy-mappers-first-discovery-surprise-space-debris/#ixzz7HnJS7TAc

 

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My question is about the "power" of JWST compared to Hubble.  I've heard from a couple of scientists that JWST is 100 times as powerful as Hubble.  What exactly does "100 times as powerful" mean?  My guess is that it means if you took a picture of the same region using Hubble and JWST, the JWST will have 10 times as many pixels along the horizontal, and 10 times as many along the vertical, and 10 x 10 = 100.  Is that correct?

On 1/12/2022 at 12:38 PM, swansont said:

The GAIA mission encountered more micrometeoroids than expected.

“the spacecraft is being peppered by far more micrometeoroids – tiny specks of space dust – than had been anticipated”
https://www.newscientist.com/article/dn25925-galaxy-mappers-first-discovery-surprise-space-debris/

Interesting to note that the GAIA team reported this to the JWST team way back in 2014, but it has never been mentioned in recent JWST news to my knowledge.  Has anyone heard of any steps taken by the JWST team to avoid some of these collisions with micrometeors?  They obviously decided to not use any kind of shield but rather totally exposed the mirrors to space.

Edited by Airbrush
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2 hours ago, Airbrush said:

Interesting to note that the GAIA team reported this to the JWST team way back in 2014, but it has never been mentioned in recent JWST news to my knowledge.  Has anyone heard of any steps taken by the JWST team to avoid some of these collisions with micrometeors?  They obviously decided to not use any kind of shield but rather totally exposed the mirrors to space.

https://www.nasa.gov/feature/goddard/2017/how-hardy-is-webb-a-qa-about-the-toughness-of-nasa-s-webb-telescope

We know Webb will get struck by micrometeoroids during its lifetime, and we have taken that into account in its design and construction. We sized Webb’s main mirror so that even after years of little impacts it will still have the reflective surface area and quality necessary to do the science. We even did tests on the ground that emulated micrometeoroid impacts to demonstrate what will happen to the mirrors in space.

Similarly, part of the reason the sunshield has five layers is so it can tolerate more than the number of expected small holes, and even some tears, and still work as it should. 

Also, almost all of Webb’s sensitive components (besides the mirrors and sunshield) are protected behind “micrometeoroid armor.” When micrometeoroids do strike, most are so small that they totally disintegrate upon impact, even when they hit something thin like thermal blankets or a sunshield membrane. Critical wires and electronics are shielded behind even more robust metal “armor” or inside metal boxes

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"James Webb" will rotate around the Sun in synchronism with the Earth at a distance of 1.5 million kilometers in the opposite direction from the star. That is, any equipment breakdowns can only be eliminated remotely; it will not work to bring the telescope closer to low-Earth orbit. Scientists tried to take into account all the various breakdowns and calculate all the options for events, making the device ultra-reliable. Therefore, the launch was postponed several times.

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What boggles my mind about JWST is that it is located four times the distance to the moon.  That seems very, very far away. 😀

Once again I ask does anyone know what it means to say JWST is 100 times as powerful as Hubble"?

Edited by Airbrush
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4 hours ago, Airbrush said:

 Once again I ask does anyone know what it means to say JWST is 100 times as powerful as Hubble"?

AFAICT it means it will be able to observe stars that are 1/100 as bright

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On 1/22/2022 at 11:27 PM, swansont said:

AFAICT it means it will be able to observe stars that are 1/100 as bright

Does that mean that if we could actually see a certain number of stars in a given patch of our galaxy, or a nearby galaxy, the JWST would be able to see 100 times as many stars?  Also will it be able to detect 100 times as many distant galaxies on the edge of the observable universe?

On 1/23/2022 at 12:47 AM, beecee said:

And further back in time.

Yes, a little further back in time, not 100 times as far back.

I thought "JWST is 100 times as powerful as Hubble" meant that a JWST photo will have 10 times as many pixels along the horizontal and 10 times as many along the vertical (10x10=100), compared to a Hubble photo of the same region of space.  But as said above, 100 times as powerful means able to detect light that is 1/100 as bright as Hubble is able to detect.  If the JWST was focused on Pluto, will it be able to resolve 100 times as much detail as the Hubble could?

Edited by Airbrush
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6 minutes ago, Airbrush said:

Yes, a little further back in time, not 100 times as far back.

I thought "JWST is 100 times as powerful as Hubble" meant that a JWST photo will have 10 times as many pixels along the horizontal and 10 times as many along the vertical (10x10=100), compared to a Hubble photo of the same region of space.

Have seen article saying it will see back as far as 13 billion years and the formation of our first stars.

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3 hours ago, Airbrush said:

Does that mean that if we could actually see a certain number of stars in a given patch of our galaxy, or a nearby galaxy, the JWST would be able to see 100 times as many stars?  Also will it be able to detect 100 times as many distant galaxies on the edge of the observable universe?

No. If Hubble could see every star is galaxy 'x' for example, Webb will see the exact same number of stars in that galaxy. However, if Hubble could not detect a given star because it was half as bright as Hubble was able to detect, Webb will be able to detect it.

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13 hours ago, Airbrush said:

Does that mean that if we could actually see a certain number of stars in a given patch of our galaxy, or a nearby galaxy, the JWST would be able to see 100 times as many stars?  Also will it be able to detect 100 times as many distant galaxies on the edge of the observable universe?

No. The number could be higher or lower, depending on how the intrinsic brightness and distance vary.

13 hours ago, Airbrush said:

Yes, a little further back in time, not 100 times as far back.

I thought "JWST is 100 times as powerful as Hubble" meant that a JWST photo will have 10 times as many pixels along the horizontal and 10 times as many along the vertical (10x10=100), compared to a Hubble photo of the same region of space.  But as said above, 100 times as powerful means able to detect light that is 1/100 as bright as Hubble is able to detect.  If the JWST was focused on Pluto, will it be able to resolve 100 times as much detail as the Hubble could?

Webb has a mirror that has ~7x larger area, so you gather more light, and it’s optimized for a different wavelength range. Both would factor into seeing stars the Hubble can’t detect.

(edit: Why are infrared observations important to astronomy? Stars and planets that are just forming lie hidden behind cocoons of dust that absorb visible light. (The same is true for the very center of our galaxy.) However, infrared light emitted by these regions can penetrate this dusty shroud and reveal what is inside.

https://www.jwst.nasa.gov/content/about/comparisonWebbVsHubble.html )

 

I think the resolving power of Hubble and Webb are similar. More pixels isn’t necessarily better, since you get less light per pixel, but if they have better signal/noise this would factor into being able to detect faint stars.

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Given the advances in technology I would assume much of the difference in 'power' between the two telescopes has to do with differences in the quality of the mirrors, computer software, sensors, reduction of dust and gas in front of the telescope, handling of diffraction, and the many other things I'm blithely unaware of regarding optics.

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1 hour ago, zapatos said:

Given the advances in technology I would assume much of the difference in 'power' between the two telescopes has to do with differences in the quality of the mirrors, computer software, sensors, reduction of dust and gas in front of the telescope, handling of diffraction, and the many other things I'm blithely unaware of regarding optics.

It's all cumulative, but I don't think mirror quality, per se, gets you all that much. Or other optics advancement. How much advancement in mirror surface quality has been made in the span we're discussing? It's not like you're going to get a factor of even two in how reflective mirrors are as compared to a few decades earlier.

So these factors could be in play, but I don't think they're the prime movers.

 

 

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