James Webb Space Telescope or JWST was launched mainly by NASA and ESA on the 25th of December, 2021. Countless companies, universities and organizations from all over the world have collaborated to develop JWST.
Thousands of scientists and engineers from 14 countries were required to design, develop, launch and operate Webb. The name “James Webb” was chosen as a memory and tribute to James E. Webb who ran NASA from 1961 to 1969.
But first, what is a Telescope?
A telescope is an optical instrument that uses a lens or mirror to make a distant object appear nearer. There are mainly 2 types of telescopes depending on if they use lenses or mirrors. They are refractors and reflector telescopes. First, let’s look at refractor telescopes.
Refractor Telescopes
Refractor telescopes use lenses. It has a primary and secondary(eyepiece) lenses. In a refractor telescope, the light first enters the optical tube, passes through the primary lens, and then reaches the secondary lens(or the eyepiece lens). This lens is placed on a focus ring. When the ring is turned, the eyepiece lens can be moved forward or backward for accurate focusing.
Now let’s look at reflector telescopes.
Reflector Telescopes
Reflector telescopes use mirrors, instead of lenses, and JWST is one among them. A reflector telescope uses two mirrors and an eyepiece lens. All telescopes have to use a lens for their eyepiece.
The diagram above shows the inner structure of a reflector telescope. Here, the light first enters the optical tube, and then reaches the primary mirror. The primary mirror is a one-sided concave mirror. Since it is concave, all the light will get reflected toward the center of the optical tube. At the end of the optical tube is the secondary mirror. The secondary mirror is much smaller. It is a flat mirror that is tilted 45 degrees so that it reflects light at 90 degrees. Then, that light reaches the eyepiece lens which is again placed on a focus ring that can be moved.
The Launch of JWST
The JWST was launched on the 25th of December 2021 using the Ariane 5 rocket. The Ariane 5 is developed by Arianespace for ESA(European Space Agency).
You might be wondering why they used this specific rocket, right? Well, the Ariane 5 rocket has been in operation since 1996 that is, for 26 years as of 2022. In its 26 years, it has done a total of 111 launches and 106 of them were successful. This means that it has a success rate of about 95.5%, which is one of the highest success rates for any launch vehicle. Ariane 5 was chosen specifically for this very reason. And, because of its high success rate, Ariane 5 was able to launch JWST successfully.
Components of JWST
The image below shows the different parts of JWST. They are the Primary mirror, Secondary mirror, Tertiary mirror, Fine Steering mirror, Backplane, Integrated Science Instrument Module (ISIM Module), Sun shield, star tracker and spacecraft bus.
First, let’s talk about Mirrors. All of JWST’s mirrors are made of beryllium and coated with gold. They chose beryllium because of its strength and lightweight. JWST’s primary mirror has a diameter of 6.5 meters. It is a concave mirror so all the light that hits the mirror in the corners will also be reflected and focused toward the center. This primary mirror is not a single mirror, instead, it is a combination of 18 hexagonal mirrors. Such mirrors are called segmented mirrors. In JWST we use this type of mirror because of two reasons. First, such large single mirrors are very hard to make. Second, such a large single mirror may not be able to fit inside the nose cone of the rocket, so having a segmented mirror means that it can be folded inside the telescope and can be unfolded after it reaches space (like origami). Additionally, a very large single mirror, when damaged, may not be able to observe at all but if you are using a segmented mirror, even if one of the segments are damaged, it can still observe using the other segments.
The primary mirrors use Cryogenic Nano Actuators(CNA). An actuator is a device that uses a motor to move a piston very accurately. CNA has an operating range of 20mm and a position accuracy of 10 nanometers. Behind each segment of the primary mirror, there are 7 actuators to calibrate the mirrors very accurately. The primary mirror has a total of (7×18)144 actuators for very precise calibrations. JWST’s secondary mirror is a 74cm(0.74m) single convex circular mirror. The secondary mirror has 6 CNAs to calibrate the mirror. The secondary mirror is small enough that it need not be folded hence made as a single mirror.
Now, let’s look at the second component of JWST, the Backplane. The backplane is JWST’s “spine”. The backplane is the structure on which almost all of the components of JWST including its mirror and the ISIM module are mounted and supported. The backplane is made out of lightweight graphite composite materials. Lightweight graphite materials are used because they are expected to move (shrink) only by 32 nanometres when cooled down to -240 degrees Celsius. Metals shrink when they are cooled and they expand when heated. The specialty of the material used in the backplane of JWST is that it shrinks by a very small amount (32 nanometres). And, because of this shrinking property, the backplane has to be made 32 nanometres larger than the required size. So that when it reaches the space and cools down, it will shrink by ~32 nanometres which will bring it to the correct required size.
The composite parts of JWST are connected using high-precision metallic fittings made of invar and titanium. Invar is a nickel steel alloy. It is popular because it has only very small changes due to thermal shrinking. The backplane carries ~2400 kg of instruments and it can also be folded into three using origami to fit into the nose cone of the launch vehicle (Ariane 5).
The next component of JWST is the ISIM Module. The ISIM Module is the brain of JWST. ISIM stands for Integrated Science Instrument Module. All the scientific instruments are present in this module. It is located behind the backplane. The main components of the ISIM module are the NIRCam, NIRSpec, MIRI and the FGS/NIRISS.
The NIRCam stands for Near Infrared Camera. NIRCam is a camera that observes in the near-infrared wavelength. Near-infrared means the part of infrared that is close to visible light or the short wavelength infrared. NIRCam observes at a wavelength of 0.6 microns(600nm) to 5 microns(5000nm). NIRCam is one of the most sensitive infrared cameras made to observe the universe.
The NIRSpec stands for Near Infrared Spectrograph. NIRSpec takes spectrographs of stars and atmospheres of extraterrestrial planets. It has the same wavelength as that of the NIRCam which is 0.6 microns(600nm) to 50 microns(5000nm).
Purpose of JWST
JWST has 2 main goals. The first is to observe distant galaxies(up to ~13.5 billion light years away). Since the observable universe is ~13.8 billion years old and if the galaxy is 13.5 billion ly away, then it means that the light took 13.5 billion years to reach us. So we can see the first galaxies which were formed after the big bang. The next goal is to observe the exoplanets using the Transit method.
Observing faraway galaxies
As mentioned earlier, observing distant galaxies is one of the most important goals of JWST. But why are faraway galaxies important to us? Light travels at ~300,000,000 m/s. The speed of light is very fast but it is not that fast in terms of the scale of the universe. One light year (ly) is defined as the distance traveled by light in one year. The nearest star to us is Proxima Centauri which is ~4.2 ly away. It means that light takes 4.2 years to reach us even from Proxima Centauri. But it also means that we are observing the light emitted by Proxima Centauri 4.2 years ago. So, basically, we are looking at the past.
Our observable universe is estimated to be ~13.8 billion years old. So, for example, if Webb can observe galaxies that are 13.5 billion light years away, it means that we are observing the light which was emitted by the galaxy 13.5 billion years ago. Therefore, we can observe the first galaxies which were formed billions of years ago and get a lot of information regarding the early universe and the Big Bang. But, it’s not as easy as it sounds. We all know that our universe is expanding and that the farther the galaxies are, the higher the velocity at which they are moving away from us. So after a certain distance, the space expands(the galaxies are moving away) faster than the speed of light. When this happens, the light from those galaxies will never reach us because the expansion of space-time will outrun the speed of light. This is why we can only see a small portion of our universe which we call the Observable Universe. Another problem is when spacetime gets stretched, electromagnetic waves will also get stretched. This is known as Cosmological Redshift.
Though the far-away galaxies might be emitting light in mostly UV, Visible, Infrared range etc., due to cosmological redshift, the UV light emitted by the source might have been shifted to Visible or Infrared range, likewise Visible light might have been shifted to Infrared, Microwave or Radiowaves and so on.
Observing Exoplanets
The second most important goal of JWST is to observe exoplanets. So, you might be wondering how can JWST observe exoplanets. Because they are much smaller than stars and they are much fainter. JWST cannot directly “see” exoplanets but it can know if there is an exoplanet in the area of sky where JWST is observing. This is done by using the transit method.
Transit Method
In this method, JWST observes many different stars for a particular period of time. Almost every star has a planetary system around it that has at least one planet in it. Sometimes, the orbit of the planet of one of the stars observed by JWST is tilted at the right angles so that, when observed from JWST the planet passes in front of the star causing a transit. When this happens, JWST will observe a very small decrease in the brightness/luminosity of the star.
The transit light curve of WASP-96 b exoplanet is shown in the above image. Here, we can see that the luminosity/brightness of the star only decreased by about 1.5% because the planet is small when compared to the star. To make sure that it’s a planet in the star’s planetary system which is transiting the star, we keep observing the star for a while. Then, if it’s an exoplanet, the star’s brightness will decrease again; then again, after the same time interval. After this, we can be sure that it’s an exoplanet. JWST can also use its NIRSpec or Near Infrared Spectrograph to take spectra of the exoplanet. But, you might be wondering how to take the spectra of the exoplanet if the star is behind it(it doesn’t reflect any of the star’s light).
Wavelength Observed by JWST
JWST’s primary goal is to observe very faraway galaxies. But due to redshift, it has to observe at longer wavelengths.
So, JWST observes from 600 to 28,500nm, which is from very long wavelength visible light to short, mid and a small part of long wavelength infrared. But you might be wondering, why can’t JWST observe at wavelength longer than that? To observe longer wavelengths, we need a larger mirror. For example, if it needs to observe 10cm wavelength radio waves, it will need at least a 4-meter mirror to have an average angular resolution. Angular resolution can be defined as the ability to differentiate two light sources which are very close to each other. The higher the angular resolution, the closer together the dots can be. If you have a very low angular resolution, that means that you will observe the two light sources as one because they are very close to each other. JWST can observe much farther stars using radio waves but to observe in that wavelength with a good angular resolution it needs a much larger mirror. So, it uses infrared. Using infrared, JWST can still observe very far away stars and because of infrared’s short wavelength (compared to radiowaves), it can observe at a much higher angular resolution as well.
Why Gold?
As mentioned earlier, JWST’s mirrors are coated with gold. So, you might be wondering why it is coated with gold specifically. One’s first guess might be that the gold has some special reflective properties in the infrared wavelength. Let’s look at a wavelength-reflectance graph of some metals including gold.
In this figure, the x-axis is the wavelength which is shown in micrometers (the wavelength of infrared in µm is 0.7 µm to 100 µm) and the y-axis is the reflectance of the metal which is shown as a percentage. So now, let’s look at the reflectance of silver(the blue line). Here, we can see that silver at ~0.8 µm(infrared) has a reflectance of ~95% and it keeps increasing till 1.2 µm. Next is Aluminium(the green line). Here, we can see that Aluminium at ~0.8 µm has a reflectance of ~85% which is a bit lower compared to silver. Next is Copper(red line). Copper’s reflectance at ~0.8 µm is very similar to Silver, with its reflectance being ~95%. Now finally, let us look at gold. Gold’s reflectance at ~0.8 µm is ~95% which is very similar to the reflectance of Copper and Silver. So why didn’t they choose Silver or copper to coat JWST’s mirror? The true reason is oxidization. Metals like Copper and Silver react with oxygen and get oxidized. Oxidization can cause the metal to lose most of its reflective properties. Yet, you might be wondering why should we be afraid of the mirrors getting oxidized because there is no oxygen in space. The mirrors will not oxidize in space but sometimes the mirrors might have to stay on earth for a few weeks or months after development and assembly till launch. During this time, there is a small chance that the mirrors can get oxidized. This is why gold is used to coat the mirrors of JWST.
Why a Space Telescope?
The total cost for developing and launching JWST is about 798 billion INR. A large amount of it is spent to send the telescope to space. The largest current ground-based radio telescope is FAST(Five-hundred-meter Aperture Spherical Telescope) which has a mirror diameter of 500 metres. The development cost of FAST was 13.5 billion INR which is much lesser compared to the development cost of JWST. With the development cost of JWST, we could build a much larger ground-based telescope on Earth. So, why do we need to send telescopes to space? Well, the reason is our Earth’s atmosphere.
This figure is a wavelength and atmospheric opacity graph. Here in this figure, the x-axis is the wavelength of the light and the y-axis is the atmospheric opacity shown in percentage. Atmospheric opacity is the amount of light that is blocked by the earth’s atmosphere.
100% atmospheric opacity means that all the light(100% of the light) is blocked by the Earth’s atmosphere. The wavelength of the light in this graph increases from left to right on the x-axis. So, first, let’s look at the short wavelength light. Short wavelength light usually refers to all the light which has a wavelength that is shorter than that of visible light. So, here all of the short wavelength light like Gamma rays, X-rays and UV rays are blocked by ~100%, so these wavelengths cannot be observed from ground-based telescopes. Now let’s look at visible light. Visible light is blocked by ~10-15%, these wavelengths can still be observed from Earth but they are still blocked by a small amount. Now let’s look at Infrared. Infrared is blocked by ~100%, so it cannot be observed by ground-based observatories. Next is short wavelength radio waves. Short-wavelength radio waves which have a wavelength of 1 mm to 10 metres are not blocked at all. So, we can observe these short-wavelength radio waves can be observed with very good angular resolution from ground-based. The reason it hasn’t measured microwaves is that microwaves are actually a part of the radio wave spectrum.
Microwaves are the short wavelength part of radiowaves which has a wavelength between 1mm and 1 metre. The Cosmic Microwave Background Radiation or CMB was observed using radiowaves(specifically the microwave part of radiowaves). CMB is very faint background radiation that can be observed from ground-based observatories in microwaves. It is very faint radiation that was emitted during the Big Bang. The age of the observable universe (~13.82 billion years ± 50 million years). Now let’s look at long-wavelength radio waves. Long wavelength radio waves have a wavelength from 10 metres to even 10 thousand kilometres. Long-wavelength radio waves are blocked by ~100%. So long-wavelength radio waves cannot be observed from ground-based observatories. Here we can see that Infrared(wavelength observed by JWST) cannot be observed from ground-based observatories. So we have to send JWST to space.
Orbit & Trajectory of JWST
Now, let’s talk about the orbit/position of JWST. JWST is currently orbiting around the second Lagrange point(also known as L2). So, you might be wondering what is a Lagrange point.
There are 5 Lagrange points in the Sun, Earth, and Moon system. Lagrange points are very special points in space where the gravitational forces of two or more bodies and the centripetal force due to rotation cancel out. So, basically, at a Lagrange point, there is very little gravitational pull which means that if we send a spacecraft to one of these points, it requires very less force(thrust) to stay at that particular point. In the Sun, Earth and Moon system, the main forces acting here are the Sun’s, Earth’s and Moon’s gravity and the centrifugal force(because this is a rotating frame of reference).
Lagrange Point2(L2)
L2 or Lagrange point 2 is located behind the Earth when viewed from the Sun. L2 also moves as the Earth goes around its orbit around the Sun. The orbital period of L2 is the same as that of the Earth. A spacecraft can ‘hover’ at this point because, at this point, there is an equilibrium between the Sun’s and Earth’s gravitational pull and the Centrifugal force due to the rotating frame of reference. The gravitational pull of the Sun and the Earth will pull the spacecraft inwards(towards the sun) and the centrifugal force will pull it outwards which will balance out. Even at L2, it’s not a perfect equilibrium, so there will be some small forces acting in different directions.
But there are still some problems with L2. JWST requires solar energy to power scientific instruments. L2 is located directly behind the Earth as viewed from the Sun. This means that L2 is in the shadow of the Earth. So, no sunlight will reach the solar panels of JWST which is a major problem. So, JWST doesn’t actually stay at L2. Instead, it orbits around L2, which is a thermally stable orbit and allows sunlight to reach JWST’s solar panels. JWST always faces away from the Sun such that the Sun shield can reflect most of the heat and light. to the Sun because if otherwise, due to the heat, JWST will not be able to function. Another problem with L2 is the distance from the Earth. Because JWST is 1.5 million kilometers away from the Earth, this means we cannot repair it after it reaches its orbit around L2.
JWST has made many observations of Galaxies, Comets, Exoplanets, Nebulae and is yet to unravel other exciting observations.
Want to know more about Telescopes in general? Checkout the article Telescopes and how they view it across the spectrum
Author
