James Webb, Halley's Comet may be set for cosmic dust-up The James Webb Space Telescope is predicted to pass through Halley's Comet's debris trail next year, meaning that particles could further endanger its sensitive primary mirror. JWST's mirror is exposed to the vacuum of space, and while that means it produces images with far more clarity than Hubble, it also has nothing to …
"Webb gets around Hubble's primary limitation of having a limited observational wavelength [by] It doesn't lock its mirror in a tube"
I'd have thought that it was more a case of being a big lump to get into space as it is without the added complication of a tube. Even more significantly the tube itself would have to be protected from solar heating, otherwise its IR radiation would dazzle the instruments.
"it sits far enough from the Sun that it can offset heating that limits wavelengths"
It only sits about 1 million miles further out from the Sun than the Earth, which itself about 93 million miles away from the Sun. So the heating difference based on distance from the Sun alone is about 2%.
The no tube is mostly a function of mass. A tube for a 6m mirror is going to be massive
It's fairly easy to get a telescope tube in space to radiate down to a pretty cold temperature - enough that it has no background at JWSTwavelengths
The distant orbit is mostly so that the Earth doesn't fill the sky and limit your range of observations, it also reduces the heat load from the Earth
"It's fairly easy to get a telescope tube in space to radiate down to a pretty cold temperature - enough that it has no background at JWSTwavelengths"
Only if you shelter it from the sun. That would very likely require a larger shield, requiring larger supports, more mechanish to fold out. Both tube and larger suhshield means more volume. More volume means a bigger nose cone. Still more mass. More fuel to get them into orbit. More mass yet.
Nope you can simply wrap it in layers of reflective foil. You get a vacuum between them for free, think of a Russian doll of 100 thermos flasks.
I saw one proposal for a passively cooled IR telescope that reckoned it could get down to 40K with just shields
JWST sheild is big to keep the whole mirror in shadow of the Sun and Earth
Sorry that was a bit short and rude.
The large self supporting steered shield on JWST is a big bit of engineering. But a thermal shield around a tube is just wrapping it with 20+ layers of Aluminium coated mylar each 0.05mm thick. Like a super lightweight version of the silver blankets at the end of marathon.
Roughly each layer halves the heat load, so 10-20 layers in space is awesome insulation
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> that's if a grain of sand doesn't turn Webb into a $10 billion lesson in protecting sensitive equipment.
Presumably back in 1993 when this telescope was being planned the powers that be assumed it would have been launched, done all its science and gone <phut!> before Halley's dust cloud became bothersome. But with all the delays, that kinda didn't happen.
Still, if they start planning a replacement now, it should be ready by 2050.
Halley's comet (and others) leave a trail of particles along their comets' orbits. We know where these trails are and in which direction most of the bits are traveling. Perhaps it's just a matter of pausing observations and turning the JWST mirror edge-on to the trajectories as it crosses them.
Just be glad the telescope's not in LEO. You get gravitation pulling more natural particles in towards you and space debris too.
IIRC, the impact hazard from space debris starts getting noticeably higher than natural particles at around 0.1mm size (estimated size you can protect an astronaut against), but common knowledge back when I was in the lab was that you'd get about 1 astronaut-killer impact per astronaut-year of space-walk, so with JWST being 25m2, I'm not surprised it's got a few noticeable craters. (as well as hundreds of less-noticeable ones).
As for protecting the mirror.... that's a tough one. If it were my mirror and you gave me the choice of 'protecting' it with 1mm of Aluminium/plastic/whatever or exposing it directly to 0.1-3 mm particles at 66km/s (Orionid meteor shower), then I'd almost certainly choose to not have the inadequate 'shield', and just hold the mirror edge-on to the stream, so as many of them miss as possible.
Each impact on any shield is going to make a shower of ejecta (inside and outside, if the shield isn't thick enough), as the particle blows itself and the target to plasma and makes a pretty crater. Impact ejecta like that can cause a massive amount of damage to surrounding surfaces, far more than the initial impactor would.
To properly protect it, you either need a few tens of km of atmosphere or multiple layers of stuff to break it up, spread it out, break up and slow down the high-speed ejecta, spread that out, etc. How big was that telescope again?
(Fun viewing: https://hvit.jsc.nasa.gov/impact-videos/, but remember children, this is lab simulation of some slow orbital collisions, certainly not head-on, and let alone the impact speed of any natural meteor streams.)
Thank you for that, it led to some interesting reads:
https://en.wikipedia.org/wiki/ValuJet_Airlines_Flight_592
https://www.theatlantic.com/magazine/archive/1998/03/the-lessons-of-valujet-592/306534/
Wikipedia says simplistically that the canisters were expired but labelled "empty". The second goes into much more detail:
According to ValuJet work card No. 0069, which was supplied to investigators, the second step of the seven-step removal process was If generator has not been expended, install shipping cap on firing pin.
This required a gang of hard-pressed mechanics to draw a verbal distinction between canisters that were "expired," meaning most of the ones they were removing, and canisters that were not "expended," meaning many of the same ones, loaded and ready to fire, on which they were expected to put nonexistent caps. Also involved were canisters that were expired and expended, and others that were not expired but were expended. And then, of course, there was the set of new replacement canisters, which were both unexpended and unexpired.
but a certain MSP partly named after mountain in Japan having won a certain contract for Tier 2 local support here in Canadaland, bid so low (So they could have both T1 & T2) wanted its support techs to travel to site by bus\c-train with parts & tools.
One of my former colleagues was given $15K a year extra just to cover the Southern Alberta Area with his car* as there wasn't any public transport that would reach some of the Governmental offices & meeting SLA.. I keep getting recruiters on the phone telling me I would be ideal for a role & would I be interested, the rate is laughable & my response is usually along the line.
This wouldn't be for Fu******* & GOA would it?
Errr yes
& the rate is still laughable I presume?
Errr yes
Please call me back when the rate is worthy of me.
*That was it no further reimbursement
The biggest danger to the Hubble is light and that is what the tube was designed to block.
The biggest danger to the JWST is heat and that is what the heat shield was designed to block.
The Hubble was designed to be serviceable and this design was needed as the initial Hubble configuration didn't work when it was deployed in space. The JWST was designed (just like planetary missions) to be unserviceable by other spacecraft but reprogrammable from ground control.
And, FYI, the next Hubble sized telescope, the Nancy Grace Roman Space Telescope, is under construction and scheduled for launch in 2027. It is currently planned to orbit at L2. Longer term, the Large Infrared/Optical/Ultraviolet (IR/O/UV) Space Telescope is in the study stage with a planned launch in the early 2040s.
I can't afford pints for the hundreds of boffins working on these projects so I'll drink one myself in their honor.
One of the space boffins using the JWST for his work last weekend (Bluedot festival - fun times) he was mentioning about it being a rare thing that luck plays a huge role in how long it lasted, but that saving the fuel will help massively since it means they can move it more to cope with damage.
He also mentioned that he half expects it to be demolished by a passing Tesla... Not entirely sure if he was joking.
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I remember reading somewhere about Lagrange points being "self cleaning" i.e. although it doesn't take much fuel to stay there, nothing can stay there without spending that bit of fuel, because the orbital dynamics result in stuff drifting away from the point, which makes them good places to put spacecraft and not expect rocks to be hanging around to complicate your mission. So... that doesn't apply to comet dust? How do we get a refund?
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