Meteoroid hits main mirror on James Webb Space Telescope The James Webb Space Telescope has barely had a chance to get to work, and it's already taken a micrometeoroid to its sensitive primary mirror. The NASA-built space observatory reached its final destination, the L2 orbit, a million miles away from Earth, at the end of January. In a statement, NASA said the impact happened …
Given where it is and the likelihood of this recurring in the lifetime of the platform the options are rather limited.
1. Is there enough strength in the mirror & coating to avoid damage?
2. If there is an impact & damage occurs, how much can you sustain before the capabilities are degraded?
3. How much adjustment do you have to correct mirror alignment if an impact duffs up more than specs on the coating?
One assumes that this was all calculated into the design but being rather pessimistic and the huge cost over runs on the project, one wonders.......
It will be really sad and somewhat ironic if JWST is outlived by Hubble. There is a planned life of 10 years with an expected life of 20, I just hope that continued exposure to events like this does not screw that up. Bluntly, if it is anything to do with hardware that cannot be corrected by software or post image processing then it is stuffed.
>1. Is there enough strength in the mirror & coating to avoid damage?
For a rock doing 10km/s? Faster than an explosive shaped charge, like the ones you use to make holes in tanks? No
>2. If there is an impact & damage occurs, how much can you sustain before the capabilities are degraded?
If you just punch a hole through a solar panel or sunshield - minor effect.
Even a ding out of a mirror isn't a huge problem. The mirrors are very stiff so you tend to make a small chip rather than bending it.
>3. How much adjustment do you have to correct mirror alignment if an impact duffs up more than specs on the coating?
You can't realign to fix a chip smaller than a mirror segment. Potentially you could lose an entire segment if a rock totally smashed it, or hit it hard enough to bend the support. Still better than losing an entire mirror
Even a ding out of a mirror isn't a huge problem.
The 2.7m Harlan J Smith telescope at the McDonald Observatory in Texas has several bullet holes in it (well, it is in Texas). They slightly reduce light gathering power - by about 1% - but otherwise have no significant effect.
https://en.wikipedia.org/wiki/Harlan_J._Smith_Telescope
I heard about the McDonald shooting back in the 1970's. I heard at the time it was not the only one. Although the only one that hit anything shiny. At least one of the bullet holes in one particular dome was an exit hole. An extra martial affair was involved. Observational astronomy was a much more exciting profession back then.
Much more interesting were the dropped wrenches stories. Usually while trying to attach some instrument in the primary focus cage. The bigger wrenches left some interesting dents when they landed. And at least one of the smaller dropped wrenches was only recovered the next time the primary of one particular large telescope was taken out for surface cleaning / re coating.
Young un's. With their massive CCD's senor arrays and remote observation sessions from the other side of the world. You're not a real astronomer unless you've almost passed out from high altitude hypoxia or got mild frost bite at 4am in the middle of winter while hanging over a 60 foot drop into inky blackness.
"You're not a real astronomer unless you've almost passed out from high altitude hypoxia or got mild frost bite at 4am in the middle of winter while hanging over a 60 foot drop into inky blackness."
Manu Kea? A walk across from one of the Keck telescopes to the other winded me. Good training if you want to know what it feels like to get old.
Response to altitude depends very much on the person. And age. Most people can work OK at aircraft cabin pressure altitude (6K to 8K) feet but usually anything over 10K feet is were the problems start. So Mauna Kea must be fun at 13K. I cannot imagine how it is at the higher altitude observatories in Atacama. 18K feet?
Me? I get a splitting headache once I go over 6K feet and even in my 20's could barely function at 10K feet. One of the downsides of growing up on the coast. At sea level. Even more annoying when the people who grew up and lived at 5K feet plus altitudes are running around full of energy wondering why you are gasping for breath and about to keel over.
"Much more interesting were the dropped wrenches stories."
When I was an apprentice (more years ago than I care to think about), using wrenches whilst hanging at odd angles 20-30 metres in the air up a towers came with a requirement to have them attached to a "piece of string" so that they didn't damage anything (or ANYONE) if dropped
Even a 10mm bolt can kill someone from that height, let alone something bigger and I've seen a dropped nut punch neatly through a car windscreen far below on the ground and embed itself deeply into the instrument cluster (which gets expensive, fast)
"Given where it is and the likelihood of this recurring in the lifetime of the platform the options are rather limited."
Where it is is in space. The probability of its being struck by matter and all sorts of energetic radiation is 100%. The primary problem is radiation from the sun which would overheat Webb. That is why it is at L2 (in the Earth's shadow) and why its only shield it has is the sun shield.
For matter, its a question of how fast it's traveling and how massive it is. For radiation (which is always traveling at the speed of light) it's a question of how energetic it is. For both its a question of where it hits.
This one didn't do any critical damage. Think of it as a little bit of damage in your retina (which, by the by, can be caused by cosmic rays). But, just as your brain can correct for that, the Webb can correct for this.
> That is why it is at L2 (in the Earth's shadow) and why its only shield it has is the sun shield.
It's not in Earth's shadow, otherwise the solar panels on it wouldn't work.
It's at the L2 so that the 2 objects most likely to heat it up - the sun and reflected heat from the Earth (probably from Earth's moon as well considering how delicate the instruments are) - are all in the same direction, so that a sunshield on only one side of JWST can block both sources at the same time. If it was placed closer to Earth, it'd need more shielding covering more sides to be able to cover sources coming in from multiple directions.
> Reflected heat from the earth and moon would be insufficient to have any impact on the telescope - they could only heat it to their own average surface temperature.
The average surface temperature of the Earth - at a guess - is about 20C.
The entire telescope on the 'cold' side of the sunshield needs to be no higher than 50K, -227C. One of the instruments, MIRI, must not exceed 6K (-267C).
Are you really saying that the reflected heat from the Earth would be insufficient to have any impact on the telescope? I mean, maybe you could explain that to the people who spent $10billion on a telescope and decided to put it at the Earth-Sun L2? From NASA's Webb Orbit
... What is special about this orbit is that it lets the telescope stay in line with the Earth as it moves around the Sun. This allows the satellite's large sunshield to protect the telescope from the light and heat of the Sun and Earth (and Moon)....
To have the sunshield be effective protection (it gives the telescope the equivalent of SPF one million sunscreen) against the light and heat of the Sun/Earth/Moon, these bodies all have to be located in the same direction.
...
Am I only person to feel a little disappointed by this? They say they knew it was a risk and tried to mitigate by material choice, but given it will have some impact to the mission before we've even taken a picture it would suggest a type of shield or alternative protection really was a requirement. Pleased they can minimise the impact, but given how early we are in the planned mission it's not looking super promising we'll will get close to 10 years planned, let alone a possible extension.
Apparently, it's been hit five times (including this one) since it went up, but this is the first one on the mirror I believe (not 100% certain about that bit).
It's a matter of trade offs. If you put shielding up, you reduce the lift to your sensors, meaning you need a bigger mirror to get the same resolution, meaning a bigger shield, Mass goes up, costs go up and pretty quickly that's your mission cancelled.
They would have done the maths, calculated the odds of getting hit by debris, based on all of the current experience with things in similar orbits and built in some margin.
Yes there will be a very large number of people cursing right now, rechecking calculations, and all that stuff. But Quite frankly, if you're going to push the boundaries of science you have to take some chances.
Given the speed and energies they're talking about, I really doubt that a shield would have been particularly effective. I'm sure that this was less of an impact, and more of a through-and-through, with the meteoroid passing straight through the mirror.
I'm wondering whether the L2 point will actually be busier than open space. After all, if it's a point of gravitational neutrality, then will debris have gathered since the formation of the Earth and Moon.
If there is more debris, maybe the life of this telescope will be short.
>I'm wondering whether the L2 point will actually be busier than open space
It's not a stable minima so it doesn't collect debris.
It's a lot better than low earth orbit like Hubble, there sin't a lot of space debris 1.5M km away
Possibly there is a higher risk being outside the moon's orbit but it's likely this bit was just a random smaller version of your average dinosaur destroyer.
>with the meteoroid passing straight through the mirror.
These ultra high speed collisions generally result in the projectile vaporising.
At these speeds and energies the projectile has no structural strength at all, even a tungsten bullet is basically a water drop. That's why the field is called Warhead "Hydro"dynamics
with the meteoroid passing straight through the mirror
Actually, it has dented/deformed the mirror panel according to something I read only a few hours ago, but cannot now find. It mentioned that micrometeoroids (grain of sand size) traveling at 10km/s are something the telescope was tested to be able to withstand over its planned lifetime, but bigger ones with speeds up to near 100km/s were more of a problem.
The previous four strikes were within expected limits, but this one wasn't and was somewhat bigger.
I'm wondering whether the L2 point will actually be busier than open space. After all, if it's a point of gravitational neutrality, then will debris have gathered since the formation of the Earth and Moon.
If dust had gathered at L2 its velocity relative to the JWST would be negligible.
What kind of shielding material would you recommend to protect the mirror segments that is:
a) Strong enough to mitigate impacts without transferring damage to the rest of the 'scope, and...
b) Transparent to IR wavelengths, so the shield protecting the mirror doesn't make the telescope useless, and...
c) As others have pointed out, light enough to be able to get the damn thing off the ground.
?
I, for one, am disappointed our new over-lords are dust. Surely we are better than dust. Flash (saviour of the universe) have a new dust brush, could that not have been fitted?
I was also disappointed by the ISS cam app on my phone, I get to see my home town from above every now and then - except I don't because it is always covered by cloud. Always.
Why does it always rain on me? Is it because I lied when I was seventeen?
My smart sister used to steal my bulk-bought C-60s to record the BBC R4 series, Hitchhikers. She had no musical taste, Dave Edmonds, so I was curious what was so important for her to record. Bit of a life changer, eh? Loved the books, loved the TV series, hated the movie of course. I loved every Douglas Adams book and hated every movie.
Actually, I'm not sure if he changed my life. I tend not to panic.
Aye, well, I said she was my smarter sister, and I didn't just mean smarter than the other one.
JG Ballard wrote a book, The Kindness of Women, a title I found patronising. More than half the smartest people I've known have been women, and a few of them have been more than unkind. I guess I should write a book before my eyes give out.
Here's my big idea though: Women - we should start paying them the same. Maybe even a wee bit more to make up fr the groping and such that we just can't stop.
_You_ hated the movie because it was so much worse than the original radio series or the books. On the other hand, I have a friend who thinks HHGTTG is brilliant _despite_ only ever have seen the movie.
DNA had so much of genius, that even when you filter out 90% of it, the result is still brilliant!
We've had satellites floating above the earth for 70 years now and many of them have been able to see meteoroids of this size impacting our atmosphere. As a result we have a pretty good idea of the amount of shit and how big it is flying around out there. This one is a bit of a surprise I believe but its not evidence the mission will be shorter than expected.
Having said that and looking to Ukraine we might not be here to receive the data.
No, he means we have satellites capable of being hit by millimetre sized specs of dust. So we should have some experience with things which we put in space getting hit.
As for the speed, 10-12km/s is normal., but 100km/s has to be rarer, surely? cuz that's a pretty elliptic orbit if it's swinging by Earth rather than slamming into the atmosphere.
Def, you downvoted Tom, didn't you. You could have just corrected him politely. If you downvote anyone here then you go to bed with a heavy heart, but if you upvote three people you go to bed loved and covered in dogs.
We're not the enemy, we're Team El Reg. Unless someone is egregious then polite conversation. Put the downvotes down.
What sort of shield would you suggest to protect against material moving faster than the stuff coming out of a shaped explosive charge (as used for making holes in tanks)?
The mitigation is to make sure (as best as possible) that the result is a mirror with a small hole, rather than a mirror with a bent surface.
Such a shield would have to be both transparent at all wavelengths the telescope operates at, manufactured to as high a tolerance as the mirror is (no point in using cheap crap UV filter in front of your expensive Leica lens for instance), and strong enough to not simply fail and spit vapourised shield meteoroid as well as fragments of both at the mirror. Also, after it has done all that magic, it must not end up covered in opaque marks, or deformed in any way.
Probably (certainly) better just to let the object hit the mirror.
That's a good analogy.
Your phone would have been lab tested on vibration and measured drop machines, and found to be able to withstand them for a considerable period of time. Thus, in normal usage, it would last for years.
Putting said phone under a jackhammer, or dropping it out of the third floor window on to concrete, and it would likely fail rather more quickly. Because those things were outside test parameters.
Although not of the same magnitude by a long way, Webb has experienced that. Instead of dust-sized particles traveling at up to 100km/s, it met one that was ten times larger than anticipated. It hasn't destroyed it, but it has caused more significant damage - but which can still be corrected.
I'm put in mind of
"Holly once forced the boys from the Dwarf to evacuate Red Dwarf in the shuttles when she spotted five Black Holes on the scannerscope. After piloting the ship past them, she realised that they were merely five specks of grit on the scannerscope. ("Marooned", Series III)"
'"As a result of this impact, a specialized team of engineers has been formed to look at ways to mitigate the effects of further micrometeoroid hits of this scale," NASA said.'
This is the most expensive telescope ever built, and *now* they are setting up a specialised team to think about something they knew would be coming?
Is it possible that these space rocks ( you can't have a rock without the ROC) are a communist plot? An attack by the ROC, to turn the Bond-inspired reflectors into a weapon to destroy seaplanes an Mi6 agents, seems to be afoot. Is Benedict available to get to the bottom of this, we seem to be caught between Bonds....
Did someone mention Bond?
Given they do not know the exact time of impact ('between May 23 and 25') is presumable not direct detection of event but perhaps discovery that mirror segment is now not aligned suddenly or is slightly deformed and needs adjusted.
How about for added protection during setup, something like some sort of removable skin/shell/bubble/eyelid that will go into a protective position while the telescope is setting up, then get out of the way when the telescope is ready. And also be able to get quickly get back into/out of protective position if danger is detected/gone, like a blink.
You are aware we are talking about a Micrometeoroid less than 1mm in size travelling at greater than 10km/s?
There is NO technology we possess as a species, which could spot such an event in advance.
Nor do we have the technology to cover a 6.5m diameter satellite dish fast enough to prevent impact of something travelling that speed. And definitely not from a material that could take such an impact without causing even greater problems for the telescope behind (look up spalling in space if your not sure what I'm getting at there).
Sorry, it's a nice idea, but it really is not feasible in the slightest.
Let us assume your shell can be deployed in 1 second (using no more than solar power and not completely destroying the alignment of the telescope due to vibration, so this is pure fantasy). These objects are travelling at upwards of 10km/s and perhaps are 1mm across. So you must detect it 10km out. You will need a telescope to do this, which we assume will use visible light, say 500nm wavelength.
So Rayleigh criterion: we need to be able to resolve this thing. theta = (1/1000m)/10000m = 1E-7. lambda = 500E-9m. D = 1.22 lambda / theta which is approx 5m. So to see this thing you will need a telescope with a 5m mirror working in visible light.
JWST main mirror is 6.5m so this telescope is smaller ... a bit. Oh but you must cover the whole sky with this: you will need many of them pointing in all directions. Let us say 6?
But of course you will need multiple observations to estimate trajectory. And spotting it 10km out is hopelessly later than you must really spot it, perhaps you really need 100-1000km away. So say 100km now D = 50m. Perhaps you could work in UV make it a bit less? Say 25m dishes, 6 of them.
Think of the kind of missile detection and tracking systems warships have, but the objects you wish to track are travelling at mach 30-100 or more, are the size of rice grains or smaller and have no convenient hot exhaust. And they can come from any direction in the full sphere. And the system must run from solar panels, must run cold to not disturb the telescope, must emit no vibration and have no real moving parts (momentum, angular momentum) and can not emit any kind of exhaust or anything which might land on the mirror or optics.
Yes this is not even slightly possible
Ok, so maybe the 'blink' idea isn't remotely feasible.
Even so, some misc related thoughts:
It could be preemptive. Like hey it looks like there could be something coming, erroring on the side of caution blink.
Did you know you can still see with your eyes shut? When your eyes are shut you can still tell when the lights are on or off.
Lazers
-
There's still the 'shell' during set up idea.
"And the system must run from solar panels, must run cold to not disturb the telescope, must emit no vibration and have no real moving parts (momentum, angular momentum) and can not emit any kind of exhaust or anything which might land on the mirror or optics."
This doesn't have to be attached to the telescope. It could be like one of those plastic Easter eggs shells that holds a toy inside that's not attached to it. Throw away the shell when the toy is ready to be taken out.
Or perhaps it could turn into something like a turtle shell, giving the telescope a place to hide for protection if something major was spotted coming it's way. Although, would moving out of the way be better? How often does something like this occur?
Answer is simple: if it is survivable you can neither see it in time nor predict it, and by far your best chance is to allow it to pass through your structure so it does not deposit its kinetic energy in it (a 1g object travelling at 20km/s (seems to be average speed for micrometeoroids) has KE equivalent to about 47g of TNT which is about half a hand-grenade I think).
If it is not survivable then perhaps you can see it in time but ... it's not survivable: a 1kg object at 20km/s has KE equivalent to about 47kg of TNT, and you are not flying a shield which can defend against an impact like that). Even in this case your best hope, by far, would be to hope it just makes a hole but probably any such impact would destroy the telescope.
You could certainly fly several telescopes for the cost of defending one and that would be by far the best approach.
JWST is orbiting the sun while the entire solar system is also orbiting the Milky Way galaxy; so it seems reasonable to assume the majority of such impacts will stem from the direction of movement through space. So why not launch a separate shield specifically designed to cover that risk? Simple!
The Register 
Biting the hand that feeds IT
Copyright. All rights reserved © 1998–2024
