This episode of Real Engineering is brought
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24th. “I don’t think it’s very useful to speculate
on what god might or might not be able to do, rather we should examine what he actually
does with the universe we live in, all our observations suggest that it operates according
to well defined laws. These laws may have been ordained by god,
but it seems he does not intervene in the universe to break the laws, at least once
he set the universe going” In the cauldron of the early universe no light
could escape the dense opach fog of primordial gas, but as this cosmic soup of atomic particles
began to cool down hydrogen atoms began to form, leading to the universe's first bright,
violent new starts burning through the fog that once blocked all light from escaping
the expanding universe. Some of these early photons have travelled
unhindered through the vast empty expanse of space for 13.5 billion years, and will
reach their destination, here on the man-made detectors of the James Webb telescope. A space odyssey coming to an end because of
the curiosity of humans. The James Webb telescope is going to give
us our first detailed glimpse of this early universe from which we and everything we know
was born. [1] The James Webb telescope is a 10 billion dollar
endeavor. An endeavor that has eaten into NASA’s limited
budget, consuming one quarter of NASA’s entire astronomy budget for years, and in
the early hours of a tenuous launch date of December 24th, this 10 billion dollar gamble
will launch aboard the Ariane 5 rocket, a European heavy lift launch vehicle, from the
European SpacePort in Kourou, French Guiana [2] Astronomers, physicists, and enthusiasts
alike will look on with nervous excitement as this rocket carries the next generation
in human curiosity. This is the insane engineering of the James
Webb Telescope. The combination of technologies required to
make the James Webb telescope possible are unique to this time period in human history. The launch vehicle, the image processing,
the electromechanical systems, the cooling systems, the mirror, and the sun shield. This endeavor is the culmination of not just
the decades of work from the engineers and scientists at NASA, but thousands of years
of work of our ancestors. The materials and engineering required to
peer back 13.5 billion years into the reionization epoch are a punctuation point in human history,
that we, the human race, should be celebrating and watching with bated breath together. The launch will take place here, in French
Guiana, a space port ideally located on the Earth’s equator to give the James Webb Telescope
an extra push towards its final destination. The James Webb Telescope will not be in orbit
around Earth like Hubble, it will be launching to a destination 1.5 million kilometres from
Earth, Lagrange point 2.[3] Lagrange points are special points in space
where small objects, like satellites, can stay more or less in the same position relative
to the gravitational bodies that they are traveling with. This happens because the gravitational pull
from two bodies precisely equals the centripetal force required for the object to move with
the gravitational bodies. Like little parking spots in space that allow
satellites to sit in a relatively stable position while using a minimal amount of fuel to stay
there. There are 5 lagrange points between the Sun
and Earth. L1 lies between the Sun and Earth. It’s extremely useful for Sun observation
satellites. However, the nature of the James Webb telescope's
job wants it to avoid the light from the Sun as much as possible. It is an infrared telescope, infrared is heat,
and the heat emanating from the Sun would completely saturate it’s sensors and make
observing the distant past impossible. So, it will be launching to L2, located about
here. Here the telescope can turn it’s back to
the Sun, Earth and Moon, which will stay in the same position, nicely lined up behind
the telescope thanks to Lagrange point 2’s unique physics. In order to operate correctly the dark side
of the telescope needs to operate at minus 233 degrees celsius (-388 fahrenheit). Without a way to block out the heat from the
Sun and Earth the telescope would be scorched at 83 degrees celsius, nearly hot enough to
boil water. [3] This is a huge amount of heat to block and
to do this the James Webb Telescope will carry a massive shield on it’s back, like a tortoise. “And making such a device, is a very very
tough problem.” That’s Mike Menzel, Missions Systems Engineer
for the James Webb Telescope. “We had to map every heat flow to make sure
that we do not let any leak through from the hot side to the cold side. to make sure that that sunlight, which is
dumping approximately 200,000 watts of power in our direction - we only want less than
a watt of that to get through to the telescope and passively cools the telescope. “ Preventing that heat transfer is, as Mike
said, a very tough problem. Heat can transfer in 3 ways. Conduction where heat is transferred from
atom to atom in direct contact with each other, like heat travelling down a copper pipe. Convection, where heat is transferred from
the physical movement of atoms, and radiation heat is transferred by electromagnetic waves. In the vacuum of space convection isn’t
a concern. So that leaves conduction and radiation as
methods for heat transfer, let's see how the James Webb Telescope is managing these. First, material choice. The sunshield needs to be light, strong, resistant
to degradation from solar radiation ,dimensionally stable across a range of temperatures, and
reflective.[4] That’s a long shopping list of requirements,
and Kapton, a type of high performance plastic, manages to check all the boxes. Each layer of the kapton sunshield is incredibly
thin. Layer 1, the layer closest to the sun is the
thickest at just 0.05 millimeters, while the next 4 layers are just 0.025 millimeters thick. [5] Kapton by itself is actually mostly transparent,
which isn’t a fantastic trait for a sun blocking heat shield. Thankfully, the wonder material that is kapton
can also be easily coated in other materials. Each layer is coated in a 100 nanometer thick
coating of aluminium, giving the sunshield its reflective appearance. This reflective quality helps prevent heat
transfer through radiation, by simply bouncing that radiation back to space, and with gaps
between each layer, the heat that is absorbed can’t easily transfer through conduction
or convection, taking advantage of the highly insulating vacuum of space between each layer. Heat could still transfer between each layer
through radiation. The outermost layer will gain heat and start
glowing with infrared radiation, just as we see through an infrared camera. In order to prevent this the sunshield has
some clever engineering designs. The layers are angled relative to each other
to ensure the reflected radiation between each layer is funneled outwards to space. Ensuring that each layer gradually reduces
the temperature as it gets closer to the critical components in the instrument bay. [6] The layers gradually get smaller in area from
layer 1 through 5, ensuring the mirror only has a direct line of sight with the coldest
layer at all times. Layer 1 itself is also coated in a special
silicon coating 50 nanometers thick, giving it this pink appearance. [7] Silicon was used because it has high emissivity. Simply meaning it emits a lot of the energy
it absorbs out as thermal radiation. Meaning, the material will not hold onto its
heat, which would give it time to conduct through the structure of the spacecraft to
areas we want cool. This high emissivity silicon coating is applied
to layer 1 and 2, the two hottest layers, helping them send their heat back out to space,
away from the spacecraft, as fast as possible. These design choices are what allow the heat
shield to maintain the massive heat differential between the hot and cold side, but blocking
heat is just one challenge. “That’s one of the bigger challenges,
along with just designing a deployment system that does this complicated and necessary unfolding-
reliably and correctly. “ In order to fit into the fairing of the Ariane
5 rocket, the sunshield has to be folded and stowed before launch, leading to some incredibly
complicated mechanics to ensure it unfolds correctly when gametime arrives. “deploying things in space is always difficult. But when you’re deploying a rigid structure
- that’s generally what engineers call deterministic… that’s relatively easy….Membranes and
cables are almost inherently non-deterministic. And if you want to have or illustrate what
that means - try pushing on a string. The string will move. If i ask you to determine the shape that it
will assume you will have a very very hard time doing it. So to control these almost non-deterministic
things takes a great deal of effort, takes a great deal of trial and error. And even after we’re done getting the design
right, the one thing about the sunshield is it’s almost like a parachute or similar
to a parachute. You know the parachute will work, but it’s
also only as good as the very very last time you fold it. And you’re going to find out whether you
folded it correctly or not when you use it. “ The unfolding process will begin a few days
after launch, not too far from Earth. [8]Starting with relatively simple mechanisms
with the solar panels and communications antenna deploying. The truly nerve wrecking process begins on
Day 7, as the satellite is coasting towards L2. There are over 300 single points of failure
in this unfolding sequence. 300 chances for a 10 billion dollar, 25 year
project to end. 107 pins, [9] holding the sunshield together,
have to be released on queue, to allow the system of pulleys, motors, cables, bearings
and springs to begin unfurling the sunshield into its precise complete shape. This process will take 3 days, and once complete
the optical components will unfold and lock into place. Completing the transformation process, but
we are most certainly not in the clear. The likelihood of the tennis court sized sunshield
being struck by micrometeorites is fairly high, and because this a thin layer of plastic
stretched out under tension, a small tear caused by an impact could cause a runaway
tear ripping through the whole sunshield. To prevent this, rip stop seams have been
molded into the sunshield, which will arrest tears and keep them confined to a single portion
of the shield without compromising structural integrity. The film has also been carefully moulded with
corrugations and other shapes to stiffen and shape the shield as needed. [10] This passive cooling system helps tremendously,
ensuring the dark side of the telescope is shielded from the sun's heat keeping it’s
sensitive heat detecting instruments at 40 degree Kelvin, about -233 degrees celsius. But parts of the telescope, specifically the
mid-infrared detection instrument, located here, needs to be even colder to work correctly. It needs to be 7 degrees Kelvin, just 7 degrees
off the absolute minimum temperature of the universe of zero degrees Kelvin, and for this
we need active cooling. [11] The James Webb telescope includes an innovative
cryocooler for this purpose. The challenge in developing this cryocooler
alone was immense, costing 150 million dollars. Getting cold temperatures is just one small
part of the design. Vibration has to be eliminated, as the tiniest
movement at the telescope would cause massive blurs in the image as it attempts to focus
on objects billions of lightyears away. That means eliminating moving parts where
possible, and when that can’t be done incredibly precise machining and movement is needed to
balance weights as they move. The cooler also needs to use a tiny amount
of electricity, as the telescope only has 2000 watts of power provided by it’s solar
array, and it needs to run reliably for years. That means a closed loop cycle, with our refrigerant
being continually reused. I found this explanation of the cryocooler
on NASA’s site. The precooler features a two-cylinder horizontally-opposed
pump and cools helium gas using pulse tubes, which exchange heat with a regenerator acoustically. Okay, horizontally opposed pumps, with carefully
balanced pistons that will cut vibrations as the weights balance each other out, but
the rest of that explanation sounds like it came straight out of a sci-fi novel. A sound wave is just a pressure wave and pressure
and temperature are directly proportional. Higher pressure will cause higher temperature. One way we can take advantage of this is by
creating a standing wave, where the peaks and troughs of the wave are stationary. We can do this in a closed tube where the
resonant frequency of the tube is determined by the tube's length. Here the sound wave will bounce off the closed
end and create a region of compression and high pressure, and therefore high temperature. This alone isn’t terribly useful. The energy and temperature in this system
will stay relatively stable, left on its own, but what if we could extract some of this
heat with each cycle? Then, on each cycle, we could gradually cool
the overall system. To do this, we need a way to pass energy out
of the system, This is done with a stack, a porous material with air gaps that allow
sound to pass through it, which is placed so that it smoothly spans both the hot region
at the end of the tube and the cold region in the centre, like this. [12] A heat exchanger is then placed on either
end of the stack, one for the hot side and one for the cold. The hot heat exchange will conduct its heat
to the centre of the sunshield, where it can radiate out to space, while the cold portion
will conduct its heat, or lack thereof, to a copper plate attached to the back of the
infrared sensors to cool them to 6.2 degrees kelvin. [13] This is an extreme over simplification of
the actual operation of the pulse tube cryocooler. This is just a basic explanation of the physical
phenomenon that allows it to work. The pulse tube cryocooler is quite possibly
the most fascinating part of this spacecraft to me, utilizing a simple physical phenomenon
with extreme precision. Allowing those infrared sensors, located in
the centre of the telescope's beautiful golden mirror to work. The golden mirrors are the most striking part
of the telescope. Made of 18 hexagonal segments 6.5 metres in
diameter.[14] So, what’s the deal with design? It’s unlike any telescope mirror I have
ever seen. The mirror surface itself is beryllium plated
in gold. That’s a unique and expensive material choice. We need the structure of these mirrors to
remain in an extremely precise shape to reflect light as desired. They can’t bend and they can’t warp with
temperature changes, and they also need to be extremely lightweight to reduce launch
costs. Beryllium is a lightweight metal, with an
atomic weight of just 4 it’s much lighter than silica glass, a more traditional mirror
subsurface material, while being far more capable in dealing with the cryogenic temperatures
the mirror will operate in. Keeping its shape and not contracting so much
that it ruins the carefully shaped curves of the mirror. While nowhere near as strong as steel, beryllium
is much stiffer with a young’s modulus of 300 Gigapascals [15]. This means, while the beryllium is easier
to break than steel, it’s harder to deform before it actually breaks. Giving it excellent dimensional stability. On a pound for pound basis, beryllium is 6
times stiffer than steel. [16] Making it the perfect subsurface material
for a mirror. However, it is not reflective, and for that
we turn to gold. Gold is not the best reflector of visible
light, being particularly poor reflecting the lower frequencies of the visible spectrum,
giving it its distinctive golden hue, but critically it is an excellent reflector of
the infrared spectrum, while being very unreactive, ensuring the mirror surface will not tarnish
and lose its shine during its operation. [17]
To reflect that light a very thin coat, just 0.1 micron in thickness, is coated over the
polished beryllium subsurface. Taking just 48.2 grams of gold, about the
same weight as a gold ball. A surprisingly small amount for the huge mirror,
which has a collecting area of about 25 m2, 5.5 times larger than Hubble’s 4.5 metre
squared circular glass mirror. [18] The mirror needs to be massive, and to explain
why, I asked Mike Menzel. So, Mike, why is this mirror so big? “Well I can tell you it collecting, first
we’re looking for uh, stars or stellar objects that are approximately going to be a nanojansky. And to explain what a nanojansky is, its units
of brightness, very very dim. “ Okay gotcha, could you put that in practical
terms? “If I were to put a child’s night light,
that’s about 5 watts, put it on the surface of the moon and look at it from the Earth,
that source would appear to be 20 nanojanskies. So we;re looking for objects that are 1/20th
as bright as that. To do that, you need a big telescope. Picture light as rain coming in, if you want
to collect a lot of rain you take a big wide bucket. Well even at the size of our bucket, 6 meters
across, we’re only collecting about 1 photon per second. 1 particle of light per second. And to put that into perspective, i;ll go
out tonight or any night and look at the brightest star there is in the sky. Your eye is probably collecting 1 million
photons per second from that star. So to see these very dim things, the dimmest
things there are to see in the universe you need a light bucket that’s at least 6 meters
in diameter. “ 1 photon per second really puts things into
perspective. Mike and the rest of the team working on the
James Webb Telescope actually wanted the mirror to be bigger, but the cost of launching a
mirror that size, between the increase in weight and limited space available inside
the Ariane 5 fairing, was not cost effective. They maximized the size with the resources
available, and incredibly, even though the mirrors collecting surface is 5.5 times larger
than hubbles, the James Webb mirror is 62% lighter than Hubble’s massive solid glass
mirror. That is an astounding weight saving, driven
by launch weight requirements to get the telescope to L2. And the mirror is even programmable. When Hubble first began transmitting images
back to earth it became clear that there was something wrong with the telescope's optics. Instead of the crisp awe inspiring we are
familiar with today, the early images came back blurred.[19] The mirror had been ground
down too flat, by a mere 2000 nanometers 1/50th the thickness of a human hair, but that was
enough to cause the light to be focused incorrectly on the telescope's sensors. Replacing the mirror was not an option, but
Hubble was designed to be serviced throughout its lifetime, featuring modular equipment
bays that allowed older equipment to be removed and replaced. In order to correct the issue, corrective
optics were installed into one of these equipment bays, like a giant pair of glasses for the
1.5 billion dollar telescope. James Webb will not be serviceable. It’s simply too far away from earth, beyond
the range of any space vehicles capable of carrying humans to service it. If there was a problem with the mirrors, that
would be game over, but the engineers were not taking any chances this time, and have
engineered a system capable of adjusting it’s focus by itself. Each of the 18 separate mirrors can contort
its shape and adjust its position relative to the secondary mirror located in the main
mirror's focal point. The weight saving isogrid rear side of the
beryllium mirrors are assembled with a system of back plates, struts and motors that can
not only adjust the mirrors rotation, but with the centre motor and these struts, the
mirrors can actually change their curvature to adjust the focal point of the mirrors,
a feature that could have corrected Hubble's issues from Earth. Once fully deployed the telescope will begin
it’s calibration phase, with each mirror adjusting itself until each of the 18 segments
have aligned correctly with the secondary mirror, a 0.74 metre convex mirror, which
itself has 6 motors to adjust its position. These motors and control systems are so precise
that the mirrors can adjust their position in steps on the scale of wavelengths of light,
creeping closer to alignment by increments 1/10000th the size of a human hair. That is an astoundingly accurate electromechanical
system. The engineers of the James Webb telescope
performed this calibration test here on earth with an absolutely massive vacuum chamber
that can be cooled to the same temperature that the telescope will operate at, ensuring
proper focus can be achieved. But the job to get a clear image isn’t done
with primary and secondary mirror alignment. They focus the light onto the cassegrain focus,
which is located inside the aft optics subsystem. This black protrusion in the middle of the
primary mirror, which blocks stray light from entering the aperture. [20] In the darkness there are two more mirrors,
one of them being the fine steering mirror, and this thing is the world's most expensive
image stabilisation tool. It is controlled by the fine guiding system. The fine guiding system is locked onto a guide
star and it’s job is to keep that star in the centre of its field of view. Every 64 milliseconds [21] the fine guiding
system will send signals to the attitude control system to make adjustments to ensure the telescope
stays on target. This attitude control is done with a combination
of 6 reaction wheels, located inside the spacecraft bus, below the heat shield, and with the fine
steering mirror. This mirror will constantly be adjusting itself
to ensure the target of the telescope stays steady on the sensors, minimizing blur. The telescope also has thrusters for larger
position maintenance. [22] 191 litre (42 gallons) of hydrazine and
95.5 litres (21 gallons) of it’s oxidizer dinitrogen tetroxide will be stored Inside
the spacecraft bus that will feed 20 different rocket thrusters scattered around the telescope. There are 8 thruster modules, 2 on each corner
of the spacecraft bus, to aid the reaction wheels in spinning the telescope to point
towards points of interest. These 16 engines will be fed with hydrazine
only, a monopropellant reaction where the hydrazine is passed over a catalyst, causing
a highly exothermic reaction, breaking the hydrazine down into nitrogen, hydrogen and
ammonia. The other 4 motors are for orbital and positional
control, and require more power. They will be fed with both hydrazine and dinitrogen
tetroxide. This fuel and oxidizer mixture react hypergolically
to form nitrogen and water. Hypergolic meaning they do not need an igniter,
they simply ignite on contact with each other. Hydrazine is an excellent choice for a long
lasting mission like this. The hypergolic reaction means the motors can
repeatedly and reliably fire without a point of failure causing issues, like an ignitor
breaking. Hydrazine is also stable for long periods
at room temperature. Allowing it to be stored over the expected
10 year life cycle of the James Webb telescope. Unfortunately that life cycle is limited to
10 years precisely because of the fuel. Between pointing and orbital maintenance,
we will run out of fuel at the same point and we currently has no way of refueling the
telescope, but rumors are, behind the scenes, NASA is looking to develop the technologies
required to refuel the James Webb telescope before it’s demise 10 years from now. Robots capable of refueling spacecraft far
from earth is an exciting concept. The James Webb Telescope could end up teaching
us many more fascinating things, beyond the early stages of the universe. It's my hope as an engineer, after being 25
years on this job, that eventually telescopes, the really really big ones of the future,
will be built in space. Testing James Webb - a telescope that’s
designed to work in space, has been a very difficult thing to do on the ground. And I'm hoping that someday we’ll be building
these things in space, testing them in space, tweaking them in space, and then deploying
them in space. We are on the frontier of a new space age,
and the James Webb telescope is a milestone on our journey towards being a more capable
space faring society. This is just one of many milestones in our
brief time as a species capable of escaping our planet's gravity. From building our incredible global position
network and sending satellites to the far reaches of our solar system to visiting the
moon and building reusable rockets. It’s never been more exciting to be an aerospace
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watch Real Science’s latest video on the insane biology of Orcas.
Just the idea of keeping something at 7K is absolutely insane, it should have been a deal breaker from the start. Instead smart, hard working, determined people figured out a way to do it. Incredible
Excellent video, really enjoyed it!
But a few minor mistakes to point out:
5:22 You say "83 °C" but have written "-83 °C". I assume it should be +83°C.
18:56 Infrared is (unlike you said) lower _frequency_ than ultraviolet and gold reflects lower frequencies well. You're thinking about _wavelength_ .
Why build one when you can build two for twice the price
The whole intro sequence was 10/10.
Visuals, narration, audio.
The graphics is incredible. The attention to detail was almost as good as the heat shield
I really like that this is launching on xmas eve, the first time humans saw the "dark" side of the moon with their own eyes was xmas eve 1968.
If they're able to figure out a way to have robots refuel it within 10 years that will be amazing. Will dramatically change what can be done in space I think.
Great video. Expect for the speakers pronounciation of the sound ".. ar..". I already watched a video on Mars where he did the voiceover. I coudlnt watch it to the end due to his pronounciation of "Mars."
Luckiley in this video here there are not so many words like that.