Almost everyone might come across this unanswerable question, how did our universe begin and how do star, galaxies form? Now, it’s the time we can expect the answers with the help of the James Webb Space Telescope (JWST). One of the best engineering marvels that humans have ever developed to fulfill their curiosity. In this article, we will see
It was named after James Edwin Webb who served as NASA’s administrator from 1961-1968. He was a person with great vision and helped NASA to develop and set up the best labs facilities. He always believed that NASA has to create “a balance between human space flight and science.” In early 1965 he had written that there is a much need for a major space telescope for space observation, then known as the Large Space Telescope should be NASA’s major effort.
It was started in 1996 as the Next Generation Space Telescope, and it took 25 years to develop this human marvel with the help of technology and lift-off into space on the 25th December 2021.
It was launched from the French Guiana space station by the European Space Agency(ESA), why is it launched from French Guiana? As this space station is located exactly on the equator, which will be helpful for the telescope to get into its desired position.
The JWST will be positioned in Lagrange point L2. Lagrange points are positions in space where objects sent there tend to stay put. At these points, the gravitational pull of two large masses the sun and the earth precisely equals the centripetal force required for a small object to move along with them. These points in space can be used by spacecraft to reduce fuel consumption needed to remain in position. There are five such points in space between the sun and earth.
Why L2 but not any other Lagrange points?
The Lagrange points 1,3,4 and 5 are in between, behind and beside the sun as shown in the picture which are not ideal for positioning the JWST. As JWST is an infrared sensory telescope, it has to keep away from the sunlight to get its job done. So, L2 is the only point out of earth’s orbiting path and it is behind the earth, so it will be the ideal point for JWST to position for observation. This L2 point is 1.5 million kilometers from the earth.
The primary thing that catches everyone’s eye towards the telescope is the shining gold plated hexagonal mirrors, which are very important as they collect the light rays of wavelength ranging from 0.6 microns to 28 microns. These are made out of beryllium which is a lighter element and it is 6 times stiffer than steel which means it will not contract easily as it will be operating in cryogenic temperatures. The beryllium surface is not reflective, so it is plated with gold. Even though gold is not a good reflector of visible light, it is an excellent reflector of the infrared spectrum. To reflect the light the beryllium surface is coated with 0.1 micron thickness of gold plate. There are 18 such hexagonal mirrors of 6 meter diameter in length, making a total area of 25 meter square, which is very large in area and it helps in gathering large amounts of high red shifting light coming from a very far point.
These large hexagonal mirrors are adjustable mirrors relative to the secondary mirrors that are located at the focal point of the large mirror. These adjustments are done with the help of struts, motors that are installed at the backside of each hexagonal mirror.
All the light that is gathered by the primary mirror is reflected to the focal point of the large mirror where a secondary (convex) mirror is placed.
Now it will reflect the light into the instrument that is mounted at the centre of the hexagonal mirror which has a tertiary mirror, which makes it a three mirror anastigmatic telescope.
This tertiary mirror reflects the light again on to a fine steering mirror used for image stabilization, controlled by a guiding system that is locked on to guide steer and it will make the steer in the center of its field of view. This sends signals to the attitude control system for every 64 milliseconds, to make adjustments such that the telescope stays on the target. This entire system is controlled by 6 flywheels that are located inside the spacecraft, which helps in stabilizing the telescope, which results in minimizing the blur of the image.
The heart of the telescope is “Integrated Science Instrument Module(ISIM)” lies at the back of the mirrors, which contains all instruments of the telescope that detects the light from the distant stars, galaxies and the planets revolving around another star.
ISIM has three regions
Region 1 consists of 4 image sensory instruments they are:
Near InfraRed Camera (NIRCam) is Webb’s primary imager that will cover the infrared wavelength range 0.6 to 5 microns. It has coronagraphs which work by blocking the light from the bright object and allowing to view the dimmer object nearby, this helps in determining the characteristics of planets orbiting near a star.
Near InfraRed Spectrograph (NIRSPEC) is a spectrograph that works over a wavelength range of 0.6 – 5 microns. It helps to study/ determine the physical properties like temperature, mass and chemical composition of the object. It is a unique spectroscope that can capture the light(spectra) of 100 celestial objects simultaneously, with a micro-electromechanical system called “ micro shutter array”.
Mid-InfraRed Instrument (MIRI) has both a camera and spectrograph which enable it to see the light in the mid-infrared region covering a wavelength range of 5 – 28 microns. It allows us to see the redshifted light of distant galaxies and newly forming stars. It is operated at a temperature of 7K (-266oC) with a cryocooler system.
Fine Guidance Sensor/ Near Infrared imager and Slitless spectrograph (FGS/NIRISS) is a guider which allows the telescope to point at an object precisely, to get high quality images. Its wavelength has a range of 0.8 – 5 microns.
These four instruments are inside a cryogenic instrument module, which is maintained at a temperature of 39 kelvin (-234oC), which is a very essential cooling effort to avoid the spacecraft’s own heat, that does not interfere with the infrared light detected from distant sources.
Region 2 consists of ISIM Electronics Compartment (IEC), which provides a platform for instrument control electronics.
Region 3 is inside the spacecraft bus which is located beneath the telescope, this region 3 is the ISIM command and data handling subsystem, with integral ISIM flight software and MIRI cryocooler compressor and control electronics.
Till now we have seen the working, processing an image and the extreme cold temperatures of the telescope. It is all happening due to the immune system that is protecting from the extreme sunlight which leads to damaging the components and entire spacecraft, that is sunshield, which is beneath the mirrors always facing towards the sun. JWST will primarily observe the infrared light from distant celestial objects, so it has to be maintained at extremely cold temperature, protecting it from external sources of light and heat from the sun, earth, moon and its own heat. JWST is installed with a 5 layered sunshield which is almost the length of a tennis court.
Why are 5 layers arranged but one thick layer?
The sunlight that is incidenting on the surface of the sun shield will be around 200KW of power, but that much amount of heat damages the entire telescope. So to avoid that 5 layers are arranged such that each layer of the sunshield is cooler than the below layer and vacuum spacing between each layer also acts as a good insulator protecting the telescope. If it is a single thick sheet then it will conduct more heat than at the bottom layer.
This sunshield is made from a very thin high performing plastic material called Kapton. The thickness of the sunshield layer varies, the bottom most layer (i.eLayer 1) is the thickest layer with a thickness of 0.05millimeter, and the remaining four layers are 0.025 millimeter thick.
Kapton is a transparent material which does not block the entire sunlight so each layer is coated with aluminum of thickness around 100 nanometers and layer 1 is coated with doped silicon around 50nm to make sure that coating is electrically conductive. Silicon has high emissivity, which reflects most of the light and heat and coating of the aluminum surface is highly reflective which bounces off the heat through the gaps to keep the top most layer very cool.
The layers are slightly differ in size and shape, Layer 1 which is the bottom most layer facing towards the sun is largest and relatively flat compared with Layer 5 that is just beneath the telescope which is smaller and curved shaped. The layers are closer together at centre and further apart at the edges which helps to redirect the heat from center to the outside of the layers.
Spacecraft bus which is located beneath the sunshield and it is a hub for six major subsystems. They are:
Electric power subsystem which converts sunlight into power that is required to operate the other subsystems and main payload.
Attitude Control Subsystem observes the orientation of the telescope and keeps it in a stable orbit.
Communication system which receives the commands and transmits the data to Operations Control Center on the earth.
Command and Data Handling System a computer that takes the commands from the communication system and directs them to the OCC, it also controls the interaction between the science instruments, solid state recorder and the communication system.
Propulsion System contains fuel tanks and thrusters to maintain the orbit, these are controlled by the Attitude Control system.
Rocket thrusters and Propellants, the telescope has 191 liters of hydrazine and 96 liters of oxidizer dinitrogen tetroxide is stored in the spacecraft, which are used to feed 20 different thrusters around the telescope.
Thermal Control Subsystem helps to maintain the right temperature of the spacecraft bus, beneath the telescope.
The communication with the telescope is easy as it stays relatively in the same position with respect to the earth. We can communicate with it through the Deep Space Network, using three large antennas which are situated in Australia, Spain and California.
Timeline of JWST from launch
It lifts off from the French Guiana Space station on December 25th around 4:30 AM
On the first day, the JWST will be directed to route L2.
On the first week, after maneuvering the trajectory the sun shields will deploy
During the first month, the telescope starts unfolding and the tripods of the secondary mirror are unfolded during this time . As the sun shield was unfolded earlier which helps the telescope to cool down and reach the required temperatures. After this unlock the primary mirror segments and verify their movements. By the end of the first month the instruments are powered up and the telescope will be maneuvered and inserted into the optimum orbit around L2.
During the second, third and fourth months, there will be an initial checkout of the entire systems and alignment of the telescope and try to collect sample images by targeting a single star.
During the fifth and sixth month, checking the calibrations of scientific instruments and trying to track the moving target and get the images of the comets, asteroids etc. To confirm the functions and to showcase the capabilities of JWST.
After six months, the mission will start and it will continue its operations.
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Astrophysics and Space Science Proceedings by Jonathan P Gardner, John C. Mather, Mark Clampin (auth.), Harley A. Thronson, Massimo Stiavelli, Alexander Tielens (eds.)