James Webb Space Telescope was designed with a life of 15 years, it is the only space telescope designed to be maintained in space by astronauts
On December 25, 2021, James Webb Space Telescope (JWST) lifted atop the Ariane 5 rocket from Kourou, Europe’s Spaceport in French Guiana. It is touted to be undoubtedly the most important astronomy payload to be sent to space ever.
On 24 April 1990, NASA’s satellite- Hubble Space Telescope was launched into low Earth orbit (547 kms above the Earth) aboard the Space Shuttle Discovery. It was designed as a general purpose versatile observatory, a vital research tool and a tool for astronomers/astrophysicists. Hubble hosts a 2.4 m mirror. Its five main instruments enables it to observe in the UV, visible, and near-IR regions of the electromagnetic spectrum. Hubble’s orbit lies outside the earth’s atmosphere, hence, enables it to capture extremely high-resolution images with substantially lower background light than ground-based telescopes.
IMAGES DATA BY HUBBLE
It has recorded some of the most detailed visible light images, allowing a deep view into space. This has helped to pin down the age of the universe, which the stars suggests is some 13-14 billion years. Hubble has also captured images of many ancient galaxies, in their various stages of evolution. It lets astronomers look back into the process of young and developing universe and the first atmospheric studies of planets beyond our solar system, exploring their compositions and building data that could aid the search for extraterrestrial life.
Hubble was designed with a life of 15 years, it is the only space telescope designed to be maintained in space by astronauts. So far five Space Shuttle missions have repaired, upgraded, and replaced systems on the telescope, including all the five of the main instruments. However, Hubble is nearing the end of its life. On June 13, 2021, Hubble’s payload computer, which controls and coordinates the observatory’s onboard science instruments, suddenly stopped functioning. The same could be remotely repaired. It is observed that computer is having various technical issues. It is now predicted to last another 10 years.
|1||D0||• Launch from Kourou, French Guyana|
|2||D0+ 31 mins||• Solar Array Deployment|
|3||D0+ 2 Hrs||• First Contact with Ground Station (Malindi, Kenya)|
• Sunshield pellet deployment
|4||D1 – D2.5||• Early Deployment steps|
|5||D4||• Major Structural Deployment|
• Deployable Tower Assembly
|6||D3-D8||• Sunshield Deployment|
|7||D10||• Secondary Mirror|
|8||D12-D13||• Primary Mirror|
|9||D15-D26||• Mirror Segments Deployment|
|10||D29||• L2 Insertion|
|11||D30||• Firing of Thrusters|
|12||2-4 Months||• Testing Instruments & Aligning the optics|
|13||5-6 Months||• Telescope Calibration|
|14||End- June 2022||• First Images|
Planning for Hubble’s successor commenced in 1996. It is a 10 billion dollars collaborative endeavour by 10,000 scientists of 14 different nationalities, from NASA, in partnership with European Space Agency (ESA), and the Canadian Space Agency (CSA). The telescope is named James Webb Space Telescope (JWST), after the NASA administrator from 1961 to 1968, who played a pivotal role in success of the Apollo programme.
JWST is NASA’s state of the art mission in Astrophysics, with an objective to observe the most distant objects in the universe, beyond the reach of existing ground based or spaceborne telescopes. It is an IR band telescope, expected to unravel the mystery of universe and detect stars in the early Universe approximately 280 million years older than stars HST now detects.
JAMES WEBB SPACE TELESCOPE-A HUMAN ENGINEERING MARVEL
The combination of various technologies required to make the James Webb telescope possible are unique and comprise of highly precise machined sub-systems viz. the launch vehicle, the image processing, the electromechanical systems, the cooling systems, the mirror, and an absolutely unique design of sunshield to provide temperature protection to the telescope.
THE LAUNCH VEHICLE
Two stage Ariane 5 rocket was chosen for this launch due to its powerful weight lifting capability and reliable safety record. This was Ariane 5’s 107th successful launch out of 112 so far. It is capable of carrying a payload of upto 10,500 kg to a Geostationary Transfer Orbit (GTO) or upto 21,000 kg to Low Earh Orbit (LEO).
DESTINATION FOR JWST
The James Webb Telescope will not be in orbit around Earth like Hubble. It is meant to look deep into the universe to provide the astronomers a peek into 13.5 billion years away. Its destination is 15,00,000 kms away from Earth, at Lagrange Point 2.
Lagrange points are points in space where satellites, can stay in the same position relative to the gravitational bodies that they are traveling with. This happens at a point where the gravitational pull from two bodies equals the centripetal force required for the object to move with the gravitational bodies. They are like parking spots in space that allow satellites to sit in a relatively stable position with minimal use of fuel.
There are five Lagrange Points between the Sun and Earth. Points L3 to L5 are more academic and do not serve much useful purpose for a mission of this type. L1 lies between the Sun and Earth, extremely useful for Sun observation satellites, however, JWST is an IR telescope and needs to avoid the light from the Sun. JWST’s parking slot is Lagrange Point 2, where the telescope will have its back to the Sun.
Telescope is designed to look away from the Sun. In order to operate efficiently, the dark side of the telescope needs to operate at minus 233°C. If the heat from the Sun is not blocked, the temperature would be 83°C, close to boiling temperature. To block this heat, JWST has been innovatively designed with a massive shield on it’s back, like a tortoise, of the size (21 m by 14 m), comparable to a tennis court, opening and deploying with precision.
Sunshield is made of material called Kapton, a type of high performance sturdy plastic, chosen to fabricate five thin layers of varying thickness. The layer closest to the sun is the thickest, just 0.05mm, while the remaining layers are just 0.025mm thick. Each layer has a 100 nanometer aluminum coating, for its reflective property, thereby preventing heat transfer through radiation.
Further, all layers are fabricated to have precisely calibrated vacuum spaces between each layer, thereby reducing heat transfer through conduction or convection. Also, each of the layers are angled relative to each other to ensure the reflected radiation between each layer is funneled outwards to space. Layer 1 and 2 are also coated with a special high emissivity silicon coating of 50 nanometers, helping it emits a lot of the energy it absorbs out as thermal radiation.
Next biggest challenge has been to precisely and accurately design a complicated yet reliable system for correctly unfolding in space. It had to be folded and stowed in the fairing of Ariane rocket before launch to ensure that it unfolds correctly at a scheduled time, on the way to its destination.
JWST is a highly accurate electromechanical system, with origami at play in its best form. There are over 300 single points of failure in this unfolding sequence, with 107 pins to be released in sequence, orchestrated with the system of pulleys, motors, cables, bearings and springs to begin unfurling the sunshield into its precise complete shape. This process will take three days, and once complete the optical components will unfold and lock into place. For strengthening, to prevent a tear from even micrometeorites, a 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.
|Primary Mirror Size||21.3 feet (6.5 meters) across|
|Mirror Shape||The mirror is comprised of 18 gold-plated hexagonal deployable segments|
|Sunshield||Webb’s five-layer deployable sunshield is the size of a tennis court|
|Instruments||Webb has four science instruments: Near-Infrared Camera (NIRCam), Near-Infrared Spectrograph (NIRSpec), Mid-Infrared Instrument (MIRI), and Near-Infrared Imager and Slitless Spectrograph (NIRISS) with the Fine Guidance Sensor (FGS)|
|Wavelengths||Visible, Near Infrared, Mid Infrared (0.6-28.5 micrometers)|
|Travel Distance||1 million miles (1.5 million kms) from Earth|
|Location in Space||Orbiting the Sun around the second Lagrange point (L2)|
THE COOLING SYSTEM
This passive cooling system, as described above helps in ensuring the dark side of the telescope is shielded from the Sun’s heat and keeps it’s heat-sensitive detecting IR-instruments at 40 Kelvin, i.e. about -233° C. But parts of the telescope, specifically the mid-IR detection instrument, needs to be at 7 Kelvin, just 7° off the absolute minimum, and for this they need active cooling. It is achieved through an innovatively designed cryocooler, developed at the cost of 150 million dollars.
Vibration can cause massive blurs in the images, that means eliminating moving parts wherever possible, if unavoidable, design has deployed parts through precise machining and movement to balance weights as they move.
THE MIRROR DESIGN
JWST is primarily an IR instrument, its coverage extends down to 600 nm wavelength light. Its mirror has a collecting area of about 25 m2, 5.5 times larger than Hubble’s 4.5 m2 circular glass mirror, yet it is 62 per cent lighter than Hubble’s massive solid glass mirror. JWST mirror is made of 18 hexagonal segments of Beryllium, 6.5 metres in diameter, thinly coated with 0.1 micron layer of gold, amounting to a meager 48.2 gms of gold.
Hubble was designed to be serviced throughout its lifetime, having modular equipment bays for older equipment to be removed and replaced. But JWST is beyond the range of any space vehicles capable of carrying humans to service or repair it. Thus, JWST is designed to adjust it’s focus by itself. The rear side of the 18 beryllium mirrors are equipped with electromechanical mechanism that can adjust their curvature to adjust the focal point of the mirrors, and all of it being orchestrated from the ground control stations. 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. This too has six motors to adjust its position through motors and control systems in steps according to wavelengths of light, by increments 1/10000th the size of a human hair.
The engineers of the James Webb telescope performed this calibration test on earth in a massive vacuum chamber, cooled to the same temperature that the telescope will operate at.
The telescope also has thrusters for larger position maintenance. 191 litre of hydrazine and 95.5 litres of oxidizer di-nitrogen tetroxide stored inside the spacecraft bus to feed 20 different rocket thrusters scattered around the telescope. Of these 16 engines will be fed with hydrazine, a mono-propellant reaction where the hydrazine is passed over a catalyst, causing a highly exothermic reaction; and the remaining four motors are for orbital and positional control, needing more power, will be fed with hydrazine and dinitrogen tetroxide, to react hypergolically, meaning they ignite on contact with each other.
JWST PRESENT STATUS
The JWST has had a perfect launch, flawless separation, deployment of solar panels and deployment of Tower Assembly for the sunshield. Webb is currently at its observing spot, Lagrange point 2 (L2), nearly 1 million miles (1.6 million km). By the beginning of March 2022, the James Webb Space Telescope team had completed the third of seven planned steps to align the 18 segments of Webb’s big mirror and had started working on the fourth stage of the long process to align the mirrors. After alignment and calibration, the world would be able to receive first few images by end June 2022.
Samir Dhurde, Scientific Officer at the Inter-University Centre for Astronomy and Astrophysics (IUAAC), Pune said, “Like all astrophysicists and astronomers, we eagerly look forward to the images that would start appearing soon. The cosmic information is expected to be novel and exciting right from the start. It will help with broad investigations across the various fields of astronomy and cosmology. Indian scientists would also utilise the JWST, as was done in case with HST, on as required basis.”
He further brought out, “Equally interesting would be the study of solar observations. The UV light from the Sun is required to be studied in detail and the next Indian space astronomy mission, Aditya L-1, is planned for the purpose. It will be equipped with indigenously developed instruments and will be positioned at the first Lagrangian Point, about 1.5 million kilometers from the Earth, to provide continuous observation of Sun. The data would help us better understand Solar and space weather processes and maybe even solve unique problems of solar physics.”
Satellite Aditya L-1 is planned to be inserted in an orbit around the Lagrangian Point 1, which is 15,00,000 kms from the Earth. The satellite will carry six payloads and provide observations of Sun’s Corona.
In a new year message, Indian Space Research Organisation (ISRO) Chairman K. Sivan has brought out that ISRO will execute many missions this year and amongst them, Aditya L-1 is also planned to be launched.