James Webb Space Telescope

The most awaited space mission of the 21^st century is finally happening.

Overview

The James Webb Telescope is a successor of Hubble space telescope. It is the biggest and powerful space telescope. It is 100 times more powerful than Hubble Space Telescope. JWST is manufactured by Northrop Grumman Ball Aerospace and Technologies. For the launch, rocket Ariane 5 ECA is used. Ariane 5 is a two stages heavy lift vehicle. It is launched on 25^th December 2021 at 7:20 am ET (12:20 UTC) (5:50 pm IST) from Kourou, French Guiana, on the northeastern coast of south America. It will place in orbit at L2 point which is 1.5 million km from earth.

Cost

The James Webb Space Telescope mission cost is around $10 Billion.

Mission Duration

The mission Duration is planned for next 10 years. But scientist believe that JWST will survive more than that.

Mission

Everything we see in the cosmos is past. James Webb Space Telescope has some major goals.

• To search for light from the first stars and galaxies that formed in the universe after the big bang.
• To study the formation and evolution of galaxies.
• To understand the formation of stars and first stages of planetary system.
• To study the physical and chemical properties of planetary system and investigate the potential for life in those system.

Scientific Instruments

James Webb Space Telescope has four major instruments with him.

Near infra-red camera

Near infra-red cam or NIRCAM is Webb’s primary imager. It has infra-red wavelength range of 0.6 to 5 Microns. NIRCAM is equipped with coronagraphs. NIRCAM will detect light from early universe, stars and galaxies. The NIRCAM has ten mercury-cadmium-telluride detector arrays. It provides the vision correction.

Near infra-red spectrograph

Near infra-red spectrographs is also termed as NIRSpec. It is used to spread light from object into Spectrum. The analysis of this spectrum can tell scientist and astronomers about its temperature, physical properties, chemical composition. The NIRSpec have wavelength of 0.6 to 5 Microns. The NIRSpec will be the first spectrograph in space that has multi object capability. NIRSpec can observe 100 objects simultaneously.

Mid infra-red instrument

It is also term as MIRI. The universe is relatively unexplored at mid infra-red wavelength. Since, everything at room temperature emits mid infra-red wavelength. MIRI will operate between wavelength of 5 to 28 Microns. This wavelength includes everything from studying protostar and their surrounding protoplanetary disk, the energy balance of exoplanets, mass loss from evolved stars, circumnuclear tori around the central black holes in active galactic nuclei etc. The detectors present on this instrument is able to see the redshift light of distant galaxies. The MIRI have both camera and spectrograph, which will allow to captured light in mid infra-red region of electromagnetic spectrum. The operating temperature of MIRI is 7k. This temperature is cannot attend by thermal management subsystem. Therefore, JWST carries ‘cryocooler’ that is dedicated to colling MIRI’s detectors.

Fine guidance sensor/ Near infra-red imager and Slit-less spectrograph

FGS/NIRISS have operating wavelength of 0.8 to 5.0 Microns. FGS is a guider which helps to point the telescope and also it allows telescope to point precisely, so that it can obtain high quality images. FGS/NIRISS will be work to investigate first light detection, exoplanet detection and characterization, and exoplanet transit spectroscopy.

Coronagraph

There will be two opaque disks used to block the bright light of stars to detect fainter light of planets, stars, galaxies and other objects. Coronagraph will allow astronomers to take pictures of faint objects around bright object.

Spectrographs

All the instruments on Webb have spectrographs. These spectrographs will spread light into a rainbow-like spectrum. Due to this, scientist and astronomers can measure brightness of each individual wavelength. 

Micro-shutter Array

The micro-shutter Array is only placed on NIRSpec. The grid of 248,000 little doors can be opened and closed to transmit or block light to capture spectra of 100 individual object. NIRSpec micro-shutter cells is approximately as wide as human hair. They have lids, which opens and close when a magnetic field is applied. Each cell is controlled individually.

Aperture mask

This component is only present on NIRISS. It is metal plate with seven hexagonal holes that is placed in front of detectors. This will help to separate light of bright objects that are close together in space.

James Webb Mirror

The JWST primary mirror is made up of 18 hexagonal mirror segments. These mirrors are made up of gold-plated beryllium. The mirror is 6.5 meter in size. The gold-coated beryllium reflector has a collecting area over seven times larger than Hubble Telescope. It has collecting area of 25.4 m.sq. In those 18 segment mirrors there are 126 small motors are present. These motors will use to adjust the optics if there are any environmental disturbance in space. Image plane wavefront sensing through phase retrieval will be used for positioning the mirror.

Webb’s Sunshield

JWST have a sunshield on his back. The telescope will work in infra-red wavelength. To work all instruments flawlessly, the temperature of its instruments needs to be maintained below 50k. If the temperature increases more than that, then the telescope itself would overwhelm its instruments. Therefore, to block large amount of light and heat from sun, earth and moon. Also, its position near L2 keeps all three bodies on the same side of the telescope at all the time. This will keep its mirror and instruments below 50k. The large sunshield is made up of silicon and aluminum coated Kapton. The sunshield is made up of 5 thin layers. These layers are thin as human hair. The sunshield is made up of 5 layers. Each successive layer is cooler than the one below. The heat radiates out from between the layers and vacuum between the layers is a very good insulator. Therefore, temperature is maintain for telescope to perform on its best potential.

JWST journey

After the launch, the real challenges begin for JWST. On his journey towards L2, the telescope will be going through lot of tough task and deployments. These deployments are not automatic, it is man controlled. Therefore, according to NASA, the timing, order, location and duration might change.  JWST default journey is as follows.

Hubble vs Webb Comparison

How faint can Webb see?

JWST is designed to discover light from the first star formation after Big-bang. To see these faint objects, telescope must be able to detect objects that are ten billion times faint as the faintest stars visible without a telescope. This is 10 to 100 times fainter than Hubble can see.

Why Webb is optimized for near and mid infra-red light?

The goal of Webb is to detect and study first star and galaxy after the big-bang. Therefore, we have to look deep into space to look back in time. We all know that universe is expanding. So, the farther we look in space, the faster objects are moving away from us, it is also known as Redshift. Redshift means, the light emitted as UV or visible light is shifted more towards red wavelength of electromagnetic spectrum, into the near and mid infra-red part of spectrum for high redshift. Therefore, to study the earliest universe, we have to observe infra-red light.  

Why Lagrange 2 for JWST orbit?

The Sun-Earth system have total 5 Lagrange point. The scientist decided to placed JWST at L2 point due to following. At L2, the telescope is close enough to communicate with earth. From this point, the telescope will able to put the sun, earth and moon behind the heat shield. It is position in the space where the gravitational pull of two large masses precisely equals centripetal force required for a small object to move with them. These points in space can used by spacecraft to reduce fuel consumption needed to remain in position.

Is it NASA’s $10 billion gamble?

$10 billion is a huge budget. It is NASA’s 1/3^rd of total budget of space mission. Sending JWST in space is initial part but real challenges for scientist and astronomers are begins after that. The telescope is too large enough to fit in rocket. Therefore, it has to transported into space as folded origami style. Unfurling is a complex task. The most daunting deployment Nasa has ever attempt. Also, the sunshield is folded like an accordion. It has very thin membranes and will be guided by complex mechanism involving 400 pulleys and 1312 feet of cable. The 107 pins holding the sunshield together has to be released in sequence. This process will take 3 days to complete. After that, optical components will unfold and locked into the place. During this process, even any single pin or pully is not open systematically, then it will cost Nasa $10 billion. Fingers are crossed.

Webb is 100 times more sensitive than Hubble and observing in infra-red wavelength will open up new window to the universe. This mission will give new direction to the space and astronomy.