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Webb flies Ariane 5: watch the launch live on 25 December

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Re: Webb flies Ariane 5: watch the launch live on 25 December

#171

Post by petschge »

lamontcg wrote: post #160
onphonenow wrote: post #151

Earlier quoted context omitted.

What I don't understand is for $11 billion - can't we do 3 $3B space telescopes that work reasonably well? Spread risk? More science? Let's say a Falcon 9 launch is $90M. Falcon heavy let's say $200M. So you take your 3x $3B. Put $200M/instrument into launch, have $2.8B per telescope leftover. There just seems to be something wrong that it costs THIS much to build a telescope. That said, the Thirty Meter Telescope is…

JWST is driven by the size of the mirror, the need for an IR instrument and the temperature you need to keep the instrument at to do the observations. Spamming a bunch of hubble-sized instruments up into orbit won't accomplish the same thing. And I don't think you can do space VLBI in the optical/IR or it would have been done already (but I don't know why?). It is kind of like asking why 5 Ford Rangers can't replace…

The reason why we are barely getting started into VLBI in the IR on the ground (and nothing that I have heard of yet in space) is that the different apertures need to be stable relative to each other with a precision better than a small fraction of a wavelength. (One tenth, one twelves and one twenties are often used, depending basically on which performance drop relative to the theoretical optimum you are willing to live with.)

For radio astronomy, where we do VLBI everyday, we have to handle waves of wavelength 1 cm and position antennas to a precision better than a millimeter. Not easy when the antennas are scattered across the country, but something we can pull off.

For IR astronomy we are talking wavelength in the range of 1000 nanometers to 30 microns. So at the easiest end of the spectrum you would have to position satellites to a precision better than 3 microns relative to each other, while flying on orbit and being pulled and pushed by tidal forces, gradients in the graviational fields and solar wind pressure (which contains turbulent fluctuations). For it to actually work in near IR you would have to get the positioning right to within 100nm.

For comparison: The mirrors of JWST itself are flat to within about 25nm. And in some sense we ARE doing IR VLBI with JWST since we have separate mirror segments that we all position correctly relatively to each other. But doing so we separate freeflying satellites is something we just aren't capable of yet.

PS: Yes, LISA Pathfinder has demonstrated measurements of spacecraft separation down to a few picometer, so we are slowly getting there.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#173

Post by guerrilla »

alserio wrote: post #159
ultramegachurch wrote: post #69

Earlier quoted context omitted.

I suspect the deployed structure cannot handle the acceleration required for escape velocity. That also may require much more propellant. Then on top of that, we don’t have the capability for humans to service satellites other than the ISS. So this is all a moot point.

What about an unmanned mission to fix it? And maybe to refuel it.

I was watching a documentary about James Webb and they claimed that NASA has plans of using unmanned robots to potentially refuel it but no mention of fixing.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#174

Post by lamontcg »

petschge wrote: post #171
lamontcg wrote: post #160

Earlier quoted context omitted.

JWST is driven by the size of the mirror, the need for an IR instrument and the temperature you need to keep the instrument at to do the observations. Spamming a bunch of hubble-sized instruments up into orbit won't accomplish the same thing. And I don't think you can do space VLBI in the optical/IR or it would have been done already (but I don't know why?). It is kind of like asking why 5 Ford Rangers can't replace…

The reason why we are barely getting started into VLBI in the IR on the ground (and nothing that I have heard of yet in space) is that the different apertures need to be stable relative to each other with a precision better than a small fraction of a wavelength. (One tenth, one twelves and one twenties are often used, depending basically on which performance drop relative to the theoretical optimum you are willing to…

That's kinda what I guessed. So on the ground you can measure the distance between two observatories that are a few hundred meters apart with incredibly high precision and track that over time as the ground under them slowly deforms and moves.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#175

Post by lamontcg »

CamperBob2 wrote: post #165
lamontcg wrote: post #160

Earlier quoted context omitted.

JWST is driven by the size of the mirror, the need for an IR instrument and the temperature you need to keep the instrument at to do the observations. Spamming a bunch of hubble-sized instruments up into orbit won't accomplish the same thing. And I don't think you can do space VLBI in the optical/IR or it would have been done already (but I don't know why?). It is kind of like asking why 5 Ford Rangers can't replace…

Spamming a bunch of hubble-sized instruments up into orbit won't accomplish the same thing. It will, and then some, once we get optical interferometry nailed down. JWST is great, and needed to be done, and I'm glad it's finally getting deployed. But if I had an argument to make against it, it would be, "Let's wait until we know how to do this properly. We're not there yet." That's a weak argument and should almost ne…

> That's a weak argument and should almost never be heeded

Well that's an incredibly strong opinion with almost nothing to back it up.

Maybe it all goes boom tomorrow on launch or the deployment is ultimately completely FUBAR'd and you can feel satisfied with yourself, but I don't buy that your principle is what we should always follow.

Hopefully we start getting data from JWST in a few months and then I invite the scientific community to figure out how to spamcraft optical and IR instruments into LEO and achieve VLBI with them in the future, but we won't have to keep holding our breath for the technical breakthroughs there.

And even as a jobs program and Keynesian stimulus the JWST beats digging holes and filling them up again, even if it all goes boom. Better than building yet another weapons program as well. And the Senate just tacked on an extra $23B to the 2022 Defense Appropriations Act for one year of spending. The $10B that JWST cost over 25 years of the program doesn't really matter at all.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#176

Post by kzrdude »

throw0101a wrote: post #60
dimtion wrote: post #42

I do have a question about JWST that I wasn't able to find a fine answer in other forums. A lot of people and engineers are saying that the JWST is a marvel of engineering, with truly inovative technical solutions and a giant step up compared to Hubble Telescope. And it does seems like so! However, I'm always baffled how everyone seems proud that the telescope has something like 200 SPOF during deployment, and if eve…

> Is JWST as brittle as the news want to make us think? Or are there technical reasons or acceptable failure modes that gives confidence that those steps are not as critical as the news let us people know? There is no room for redundancy in many aspects of the design, unlike, say a Boeing 777 or Airbus A350. How can you have a redundant heat shield, or primary mirror (two parts of which swing)? I'm sure some computer…

Intermeshing rotors are interesting as an alternative to having a tail rotor. They are pretty slow & stable, should feather relatively safely as a failure mode.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#177

Post by sailingparrot »

rrss wrote: post #51
mrtksn wrote: post #41

Earlier quoted context omitted.

They didn’t pick that either, it was supposed to launch earlier but a clamp broke down when attaching the telescope to the rocket sending vibrations all over it and and as a result they had to inspect for damages, further delaying the launch. Then the weather wasn’t good and delayed once more.

They did though. It was delayed, they chose to delay it from October to December 18 to December 22 to Christmas instead of like the first week of January or whatever. It’s not like this thing is launching to Mars and they’d have to wait a couple years if they miss this week - it could launch on 210 days of each year. https://jwst.nasa.gov/content/about/faqs/faq.html#launchWind... I don’t mind at all, I just think it’…

So they should have delayed to next year after the clamp issue? Hundreds of people that were on site from Europe and the US, now go back home. You move the JWST back in a safe storage place, moving it is always risky, as the clamp issue showed.

You now plan a new date in January, knowing Omicron is spreading and countries are starting to close their borders once again, and knowing Kourou had to be closed for like 6 months already once because of COVID, preventing any launch.

Do you want to take the risk of your 11B$ payload maybe getting stuck in a hangar for 6 months? Any issue with the ventilation, de-humidifiers or hundreds of other things could potentially damage it.

Then you bring everyone back on site, and now you have to reinspect everything, rocket and payload and restart the whole process, move JWST back again into the assembly tower, again a risky manoeuver, and as always, many things including weather can make the launch window slides for days. Being stuck in Kourou for weeks is not as fun as it may seems.

So no, it makes 0 sense to move the date around for something non-critical, and I'm pretty sure everyone already on site would rather just get it done now and celebrate christmas with their family a few days late, than add unnecessary stress on JWST and themselves for potentially months.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#178

Post by GekkePrutser »

krisoft wrote: post #153
GekkePrutser wrote: post #116

Earlier quoted context omitted.

Does this have a heat shield? It won't re-enter right? It's far too far away for that.

Not the kind of heat shield you would use to re-enter. The telescope is designed to image very faint sources of infrared light. The problem is that everything (including the telescope itself) glows in infrared. The hotter things are the more infrared they emit. Because of this you want to keep the instrument as cold as possible. (You do this because you don’t want to drown the faint sources by the glow of the telesco…

Thanks for the detailed answer! I forgot it was mainly for infrared but I didn't think of this implication anyway. Very interesting! I hope we'll see this in the coming days.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#179

Post by throw0101a »

twistedpair wrote: post #93
beamatronic wrote: post #80

Earlier quoted context omitted.

The answer to your redundancy question is: Assembly line. Instead of ramping up a project, and building 1 of something, you would plan to do more than one, and you could iterate over time as you learn. SpaceX is doing a good job of this. If 1 Webb telescope is valuable then wouldn't 3 or 5 also be valuable? We have a number of proven space designs at this point: Soyuz, Spirit/Opportunity rovers.

They calculated that. Building a second JWST would have added 10% to the budget, but the budget committee nixed that. YOLO (you only launch once).

> They calculated that. Building a second JWST would have added 10% to the budget, but the budget committee nixed that.

Do you have a citation for that?

Re: Webb flies Ariane 5: watch the launch live on 25 December

#180

Post by rrss »

sailingparrot wrote: post #177
rrss wrote: post #51

Earlier quoted context omitted.

They did though. It was delayed, they chose to delay it from October to December 18 to December 22 to Christmas instead of like the first week of January or whatever. It’s not like this thing is launching to Mars and they’d have to wait a couple years if they miss this week - it could launch on 210 days of each year. https://jwst.nasa.gov/content/about/faqs/faq.html#launchWind... I don’t mind at all, I just think it’…

So they should have delayed to next year after the clamp issue? Hundreds of people that were on site from Europe and the US, now go back home. You move the JWST back in a safe storage place, moving it is always risky, as the clamp issue showed. You now plan a new date in January, knowing Omicron is spreading and countries are starting to close their borders once again, and knowing Kourou had to be closed for like 6 m…

lol

thanks for the treatise on the consequences of a few more days delay, but all I’ve said is that I find it amusing that the launch has ended up on Christmas day. They could have conceivably tried to avoid the holidays, but obviously they are not, and this is fine.

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