Over the next few weeks, we will be exploring different aspects of the James Webb Space Telescope in anticipation for its upcoming launch later this month. This is part two of our series on this incredible mission.
When you think of a telescope, you probably imagine a tube with mirrors or lenses resting on top of a tripod, and for the most part, you would be correct. To some extent, this generalization also applies to research observatories. For instance, if you take a look under the domed observatories on Mauna Kea, you’ll find mirrors that resemble those on amateur telescopes, just on a much larger scale. Even the orbiting Hubble Space Telescope shares similarities with the “standard” design of a telescope; place an oversized tripod under it, and it’ll be nearly indistinguishable from the average Newtonian reflector. However, the upcoming James Webb Space Telescope deviates from this trend drastically, and for good reason: it’s not just a telescope. No, the JWST is much more than that because it may very well be the closest thing that we have to a time machine.
If you ask ten different people to identify the weirdest component of the JWST, you’ll likely get ten different answers (assuming people can even identify this beast as a telescope to begin with!). From the honeycomb-like main mirror to the tennis court-sized sunshield, there’s just so much to this telescope that is not just unusual, but exotic. However, as you might expect on a mission that has been in development for nearly three decades, every single design choice on the JWST has an explanation, and for more than a few of these choices, the explanations are rather unusual.
When you think of a telescope, you probably imagine a tube with mirrors or lenses resting on top of a tripod, and for the most part, you would be correct. To some extent, this generalization also applies to research observatories. For instance, if you take a look under the domed observatories on Mauna Kea, you’ll find mirrors that resemble those on amateur telescopes, just on a much larger scale. Even the orbiting Hubble Space Telescope shares similarities with the “standard” design of a telescope; place an oversized tripod under it, and it’ll be nearly indistinguishable from the average Newtonian reflector. However, the upcoming James Webb Space Telescope deviates from this trend drastically, and for good reason: it’s not just a telescope. No, the JWST is much more than that because it may very well be the closest thing that we have to a time machine.
If you ask ten different people to identify the weirdest component of the JWST, you’ll likely get ten different answers (assuming people can even identify this beast as a telescope to begin with!). From the honeycomb-like main mirror to the tennis court-sized sunshield, there’s just so much to this telescope that is not just unusual, but exotic. However, as you might expect on a mission that has been in development for nearly three decades, every single design choice on the JWST has an explanation, and for more than a few of these choices, the explanations are rather unusual.
Before we can begin to explore the rationale behind the JWST’s unique design, we should first identify its primary components. To begin with, there’s the massive primary mirror which is made of 18 identical hexagonal segments. All the instruments are housed behind this mirror, so that’s where the science really happens. Then, there’s the 0.74-meter wide secondary mirror which gets more impressive when you consider that it is suspended from tubes that are over seven meters in length. And of course, there’s the massive five-layer sunshield which is made of materials so exotic that you have probably never heard of them.
The JWST’s primary mirror is easily its most iconic component just because there has never been anything like it before. In terms of size, the 6.5-meter wide primary mirror easily dwarfs all other space telescopes, and in this case, mirror size makes all the difference. This is because the larger the mirror, the more light the telescope can gather, so it can glimpse farther into the universe than ever before and thus go even further back in time. However, not only would a single, 6.5-meter wide mirror be extraordinarily difficult to manufacture, it would also be impossible to launch to space due to size constraints on rockets.
This is where the hexagons come into play. The JWST’s 18 individual hexagonal mirrors are a much more manageable 1.3 meters wide, so they can fold up for launch and deploy in space to produce a much larger mirror than could ever be launched in a single piece.
But why hexagons? Why not circles or squares or triangles? Well, there are two main factors to consider here. Firstly, hexagons, unlike circles, can assemble together without leaving any gaps between adjacent panels. Secondly, hexagons can also be put together to form a roughly circular shape, which is something that just can’t be done with squares or triangles. The overall circular shape is important because it ensures that images are both bright and focused.
Another interesting feature of the hexagonal mirrors is their colour. Most mirrors that we use are silver, but that’s because we see in the visible light portion of the electromagnetic spectrum. The JWST, on the other hand, is designed primarily for infrared wavelengths which reflect off of gold much better than any other surface.
However, the gold on the JWST’s primary mirror is just a thin surface coating on top of a beryllium base. Beryllium was chosen because it is strong, yet also light. In fact, each hexagonal segment only weighs about 20 kilograms, so compared to the Hubble telescope, The JWST’s mirrors are only one-tenth the mass per unit area! This is important because weight is a significant limiting factor on existing rockets.
Designers didn’t just stop at the mirrors when looking for ways to cut mass. For instance, you would expect the booms that support the secondary mirror to be hefty, but they are actually incredibly light because they are made of hollow composite tubes that are just 1 millimetre in thickness. As a result of all of these design considerations, the JWST is nearly half the mass of the Hubble telescope even though its mirror is more than 6 times as large!
Arguably just as important as the mirrors, the JWST’s five-layer sunshield is also a remarkable piece of innovation the likes of which the world has never seen. To understand why the sunshield is necessary, you can think of backyard astronomy. When we step out to see the stars from a city, there isn’t much visible because of light pollution, but in rural areas, the view is significantly better. This idea of “pollution” and interference also applies to the JWST, just not in the form of regular light. Instead, since JWST will observe at infrared wavelengths, it will need to avoid infrared radiation coming from the Earth, Sun, and telescope itself. This is part of the reason why the JWST will orbit much further from the Earth than the Hubble telescope. However, distance alone cannot solve this issue of infrared interference because radiation from the Sun is present everywhere in the solar system. To combat this, the JWST will use a sunshield to effectively separate its scientific instruments and mirrors from direct sunlight so that they are kept at cryogenic temperatures.
Why five separate layers? Why not a single layer? It turns out that this design feature also has a purpose. By having multiple layers that are separated by the vacuum of space, less heat will be conducted through the sunshield compared to just a single layer. The layers themselves are made of a special material called Kapton which was selected specifically for its high heat resistance. An aluminum coating on top of the Kapton further enhances infrared reflectivity, and some of the layers have yet another coating of silicon, which is very efficient at remitting absorbed thermal energy. All of this means that the JWST’s mirrors won’t just be the largest in space, but they will also be some of the coldest.
The sheer number of new technologies that had to be invented for this one project to be successful is astonishing, but it all goes to show the strength of human innovation. Every single design choice on this incredible telescope was carefully thought out, and the result of this is that we are now closer than ever to having a functional time machine that could show us how the universe was born. So, with such an incredible design, it’s safe to say that the JWST is truly a technological marvel, a testament to human ingenuity.
...to be continued
Sources & Further Reading
JWST's sunshield
JWST's primary mirror
JWST's seconday mirror
Cover image credit: NASA