The James Webb Space Telescope was hit by a micrometeoroid
which damaged one of its mirrors.
The impact occurred sometime between May 22nd and 25th.
Now, if we didn't know any better, that would sound pretty bad.
And the truth is, is that it's not great news, but
it's also the truth that Webb is really doing just fine.
In fact, in a blog post explaining the overall situation, NASA
announced that Webb is still performing better than it needs to for science.
The impact occurred during instrument
commissioning in late May, when the mission team started to notice
a slight degradation in the image quality coming down from the telescope.
Now, NASA hasn't released any images that demonstrate this, so I don't know
if this was a tiny blur or some scattered light or something else entirely.
But however the distortion manifested itself in the data,
the team were able to work backwards to conclude
that it was a micrometeoroid impact on the C3 mirror segment.
Now, micrometeoroids are tiny particles
that are typically about the size of a grain of sand.
However, those cosmic sand grains are moving at tens of kilometers per second.
So even though the sand grains
are very tiny, they're still packing a lot of kinetic energy.
So that means impacts like these are inevitable.
Spacecraft get hit like this all the time.
In fact, how much they get hit depends a great deal on their location.
Low Earth orbit is particularly bad because there's a lot of debris
trapped there.
The Hubble Space Telescope has been pelted by micrometeoroids
pretty much nonstop since it was launched in 1990.
The Sun-Earth L2 environment is considered to be safer overall
because it is gravitationally metastable.
And that means particles easily drift in and out of this region
rather than linger around and build up over time.
So while the chances of an individual impact
are expected to be a little bit lower at L2,
that doesn't mean that impacts are never going to happen.
ESA's Gaia satellite is still out there
getting pelted every now and again since 2013.
So the fact that Webb was hit at least
five times since it launched is really not surprising.
In fact, it's pretty much in line with the predicted frequency of impacts.
But this particular May impact
has gotten quite a bit of extra attention because not only did it strike
one of the mirror segments, but it was also larger than anticipated.
And to make matters more interesting, the impact was beyond
what engineers could have tested on the ground before launch.
Now, to be clear, it's not that larger impacts were not expected to happen, but
rather they were just expected to occur much less often than normal impacts.
But the fact that Webb suffered such a relatively large impact
so early on in its mission raises the question
of whether Webb was just really unlucky this time
or if there is some misunderstanding of the dust environment at L2.
But either way, Webb is going to be more sensitive
to impacts like these because it has the largest mirror
ever flown and that mirror is exposed to open space.
More on that in a moment.
But for now, the first order of business was to correct for the distortions
to the image.
To that end, a team of optical engineers got together
to analyze the effects of the impact and work backward
to determine the exact adjustments that would be necessary
Now, the blog post doesn't say exactly how they went about doing this,
but I suspect they may have used the same phase
retrieval algorithm to determine just how far out of alignment
or how far out of phase the C3
mirror was relative to the other segments.
Now, I talked about how the phasing process works in a previous video,
so feel free to check that out if you want to get caught up on all those details.
But the short version is that each mirror
segment is backed by a hexapod of six actuators.
Together they can adjust the segment's yaw, pitch,
roll, and its position relative to the other mirrors.
A seventh central actuator can even adjust the shape of the segment
by increasing or decreasing its radius of curvature.
So far,
the team have already made at least one set of adjustments to the C3 segment,
and they've managed to cancel out some of the degradation this way.
Now, the blog post indicates that the team were still analyzing
the effects of these adjustments
and they'll probably go back and make some additional corrections
until everything is as close to just exactly perfect as they can get it.
And by the way, I know it would have been really nice to have a camera onboard Webb
to image the mirror segments and evaluate the damage.
But mounting a camera that could operate a cryogenic temperatures
and zoom in to inspect the mirrors at that level of detail
turned out to be far more complex and added too much risk to the mission.
Still, Webb's NIRCam instrument does have the ability
to image the mirrors with its Pupil Imaging Lens.
And we saw this in a selfie image that was released during phasing.
Now, I don't know if they attempted to image the mirrors this way,
but I would be surprised if a selfie would have revealed anything.
And that's because micrometeoroid impacts
are on the order of just a millimeter or so in size,.
A ding that small would probably not even show up in any selfie image.
Still a tiny ding like that would produce a small
but measurable degradation in the overall image quality.
Now, to be clear, this micrometeoroid impact doesn't
really pose any impact of science, but how is that even possible?
And isn't Webb's mission going to have to last longer than 20 years?
How is it going to continue to do that without just getting pummeled to death?
Well, the answer is, yeah, it actually will eventually get pummeled to death,
but that would be long after its mission ended.
And even during its mission, micrometeorites are not the only threat
to the Webb telescope.
It'll continue to get hammered by ultraviolet sunlight
and solar wind particles trying to break down Webb's sunshield.
And cosmic rays that were accelerated in gamma ray bursts and in supernovae
will relentlessly smash through Webb
at a huge fraction of the speed of light.
And that's why Webb was overengineered from the outset
to perform way above the levels needed for science.
In other words, it was built with plenty of performance margin
in its optical, thermal, mechanical and electrical systems.
For example, its thermal shield layers include ripstops
that prevent a single puncture from rendering the entire layer useless.
And it incorporates more shield layers than are strictly necessary
to keep Webb cooled to the required temperatures.
Likewise, the mirrors' image quality is
much higher than is needed for science.
In a worst case scenario, if a mirror segment gets totally smashed
to the point where it can no longer contribute to the science, engineers
can even pivot that mirror segment out of the line of sight
and essentially take away its contribution to the science instruments.
And the instruments, by the way,
are also the most sensitive and delicate components on board Webb.
And that's why they're physically shielded by the surrounding science
instrument module.
Now, as Webb orbits the sun, it will occasionally pass
through the tails of long, extinct comets.
We see their effects all the time here on Earth in the form of meteor showers.
When those events occur, Webb will be turned such
that its backside is facing into the oncoming debris.
And by the way, this is part of the reason why everything was kept to such
insanely high cleanliness standards while Webb was still on the ground.
Engineers knew that Webb would accumulate space dust during those meteor showers,
so starting out as clean as possible helps to maximize its performance
over its mission.
Still, the impacts that Webb has endured
thus far weren't part of any meteor showers.
They were just random grains of dust flying around out there.
So that raises the question, why didn't they just enclose Webb in a giant tube?
I mean, after all, that would have been the best protection for its mirrors.
Well, you've got to remember that Webb is an infrared telescope,
and that means it has to be kept as cold as possible.
The tiny amount of heat making it through the sunshield layers
warms the mirrors ever so slightly.
And that heat needs to be radiated away.
An enclosure would trap that heat and warm up the mirrors over time.
And then there's the extra complexity of just having to figure out
how to fold all of that stuff up and deploy it along with everything else,
which of course means higher risks and higher costs.
In the end, it was more cost effective to just overengineer
Webb, such that it would degrade gracefully over time.
Now, if it's any consolation, we know from lived experience
just how hard it is to shoot up a telescope.
In 1970, a deranged employee at the MacDonald Observatory in Texas
fired a handgun point blank into the primary mirror of the Harlan J.
Smith Telescope.
However, the bullet holes didn't affect the image quality
so much as it just reduced the amount of light it collects.
So instead of a 107-inch mirror
the telescope performs as well as a 106-inch mirror.
Those holes are still in the telescope's mirror to this day.
And yes, it's still being used for science.
Meanwhile, Webb is continuing to perform well above the levels it needs to.
And despite this impact being a little bit larger than expected,
it hasn't affected Webb's operation schedule at all.
The instruments are still
being commissioned, and the Where is Webb website is showing the current status
of all 17 instrument modes as they are each being readied for science.
And if that weren't cool enough, NASA announced that the first full color
images and spectra will be publicly released on Tuesday, July 12th.
Now, we've already seen some images, but remember, these are monochromatic
test images that were used to verify the alignment of Webb's mirrors.
But soon, we're going to witness the power
of this fully operational space telescope.
And no, I have no idea what the images are going to be of.
But that's OK because we're going to go live on that day
on this channel and geek out on those first images together.
And that's partly because of these fantastic people right
here, my patrons who are helping to keep Launch Pad Astronomy going.
And I'd like to welcome Abe Alianz, Daniël, Jacob Harris, Floris Messack,
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