James Webb Space Telescope: Our Path to Study the Origins of the Universe

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In the early hours of Christmas day of 2021, NASA launched its flagship telescope, The James Webb Space Telescope, on an Ariane 5 rocket from the Guiana Space Centre, Kourou, French Guiana. The James Webb Space Telescope or more commonly known by its abbreviation JWST is the successor to the highly successful Hubble Space Telescope. However, unlike Hubble which was designed to detect light in infrared, ultraviolet as well as visible light, the JWST is designed to detect light in near-infrared and mid-infrared wavelengths. We are gonna see the importance of this in a later section as well. Now let’s talk about the history of its development from an idea to its deployment.

History

While the talks of Hubble Space Telescope’s successor started in the early 1990s the JWST as an idea was conceived only after 2 years into the new millennium. In September 2002 NASA’s Next Generation Space Telescope (NGST) was renamed the James Webb Space Telescope after the Second administrator of the organisation James Edwin Webb. The idea was to design a larger telescope that functioned in the infrared wavelength and could detect light from the time when the first galaxies of the universe were born, and to do that they had to maintain the temperature of the telescope at a temperature way below the freezing point of water. The idea to achieve this was through radiative cooling.

At the beginning of the year 2003, John C. Mather of the Goddard Space Flight Centre took charge of developing JWST as the project head. Thus began the story of the development of this telescope. The initial plan was to launch JWST in the year 2011 but the analysis of the budget led to a delay of 2 years. The budget review in the year 2005 revealed the total estimated cost to be $4.5 billion and there was an international collaboration between NASA, ESA, and CSA to help manage the cost of this incredible mission.

This story has many different facets that led to a higher cost of nearly $10 billion, better scientific goals, and a delayed launch date by another eight years, which in itself could be the content of the whole article but we could do that some other time, but for now let’s focus on the scientific aspects of this mission.

Why do we need to observe in the infrared?

Infrared Astronomy
Credit: webtelescope.org

As we talked about earlier, the aim of the James Webb Space Telescope was to detect light from the time when the first galaxies of the universe were born. However, this ancient light after traveling for billions of years is stretched into infrared wavelength, technically known as red-shifted, becoming infrared radiation. Therefore for JWST, it was important that it observed light in the infrared wavelength. However, infrared imaging comes with its challenges as it corresponds to heat. Practically, infrared radiation cannot be differentiated from any other heat emanating in space. That is the reason this radiation could be enveloped by other heat sources (such as the heat from Solar radiation) in the spaces for it to be detected by the telescope. Therefore, the telescope has to be maintained at a very low temperature. This also was the reason that JWST had to be a space telescope and not a ground-based one, as numerous sources of heat could affect the infrared imaging of the light as the heat would tend to saturate the IR signals.

Location of the JWST

Webb's Orbit
Credit: webtelescope.org

 As we know there are sources such as the Sun, Earth, and the moon all heat the telescope which is detrimental to its picking up the faint light from the cosmic dawn. So, it is placed in a halo orbit near the Sun-Earth L2 Lagrange point. The Lagrange point L2 is on the opposite side of the Earth from the Sun and is also about 1.5 million kilometers away from the earth’s orbit. The specialty of this point is that all three bodies Sun, Earth, and the Moon, lie on a single line and the telescope can just raise its tennis court-sized sun shield to block them all. Placing the telescope in this arrangement helps maintain the temperature of the telescope at a temperature below 50 kelvin, and allows the telescope to catch the faint glimmers from the earliest of galaxies. The other benefit of placing the telescope at this point is that it revolves around the Sun synchronously to that of the Earth thereby not having to constantly maintain its course with the help of rocket engines because the gravitational pull of the Sun and the Earth provides the centripetal force to maintain its orbit.

Design features of the Telescope and the instrumentation

The telescope consists of 18 foldable hexagonal mirrors assembled to form a large gold-coated mirror with a diameter of 6.5 metres. The reason to make it foldable is that if it was one single mirror of width 6.5 metres then it would not fit any launch vehicles that are currently in use. After the telescope was launched into space the mirrors were unfolded and the optics were adjusted to bring the mirror into optimal focus.

The instrument on the James Webb Space Telescope called the ISIM or the Integrated Science Instrument Module forms the unit that is responsible for rendering the computing, power source, and cooling capacity to the telescope. The four different kinds of imaging instrumentation consist of:

  • The Mid-Infrared Instrument (MIRI)
  • The Near-Infrared Camera (NIRCam)
  • The Near-Infrared Imager and Slitless Spectrograph (NIRISS)
  • The Near-Infrared Spectrograph (NIRSpec)

Now, let’s talk about them in a little bit of detail.

The Mid-Infrared Instrument (MIRI)

(I am really excited to talk about this because one of the two scientists who were in charge of leading this instrumentation is from my University, and he is none other than Professor George Reike.) The MIRI allows the JWST to observe in the range of mid-infrared which is 4.9 to 28.8 microns, consisting of a mid-infrared camera and a spectrometer. The imager helps us obtain images from 5.6 micrometers to 25.5 micrometres. 

THE MIRI Consortium was responsible for building the MIRI, and consisted of scientists from the European Space Agency, the Steward Observatory at the University of Arizona, the Jet Propulsion Laboratory in California, and several other US institutions, and was led by George Rieke (University of Arizona) and Gillian Wright from the UK Astronomy Technology Centre. 

Now let’s talk about some of the Specifics of MIRI:

 The Optical Module consists of the MIRI Imager, spectrometers, and chronographs. The Infrared radiation collected by the telescope, after entering the input optics, then the calibration structure is divided between the moderate-resolution spectrometer and the imager, consisting of the low-resolution spectrometer and a chronograph.

The focal plane module of the MIRI array of three Arsenic doped Silicon IBC detectors is capable of imaging 1024 X 1024 pixels, and also provides shielding and thermal isolation.

Woohoo! JWST's Mid-Infrared Instrument is Fully Operational Again -  Universe Today

Credit: The University of Arizona

The MIRI requires an operational temperature of 6~7 K which is way below the temperature of 40~50K that is maintained for the JWST. Therefore, a Joule Thomson Loop heat exchanger reduces the temperature to 7 K, which in return is precooled by a pulse tube cryocooler to a temperature of 18K.   

The Near-Infrared Camera (NIRCam)

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Credit: NASA Goddard

NIRCam is JWST’s primary imager covering an imaging range from 0.6 to 5 micrometres. This was built under the leadership of Marcia Rieke, by a group of scientists at the University of Arizona and Lockheed Martin’s Advanced Technology Center. Its main purpose is to find the faint twinklings from the first light sources such as the star clusters and the galaxies formed right after the big bang. The NRICam has two imaging modules that employ dichroics for observing two wavelengths at the same time, one is a short wavelength channel of 0.6 to 2.3 μm and the other is a longer wavelength channel of 2.4 to 5 μm. The coronagraphs inside the NIRCam help it to image dimmer objects by blocking the light from a bright object.

The Near-Infrared Imager and Slitless Spectrograph (NIRISS)

The NIRISS functions in a wavelength range of 0.8 to 5 μm. This was developed by the Canadian Space Agency. It will help us detect the first light, exoplanet, its characterisation, and transit, and can be used to observe in four modes. These are Wide Field Slitless Spectroscopy, Single field slitless spectroscopy, Aperture Masking Interferometry, and imaging.

NIRISS components, wavelength range, field of view, observing modes
Credit: webtelescope.org

Credit: Webbtelescope.org, NASA &STScI

The Near Infrared Spectrograph (NIRSpec)

The Near Infrared Spectrograph of James Webb disperses light in the range of 0.6 to 5μm and is used in three observing modes, low-resolution mode, multi-object mode, and lastly long slit spectroscopy. It was built by the European Space Agency and consisted of people from different organisations such as Airbus Defence and Space and even members from the Goddard Space Flight Center, and NASA. The NIRSpec is used to analyse the different properties of the bodies that are under investigation, such as their chemical and physical properties. This helps in understanding the evolution of the universe in terms of its composition and how that development might have occurred and what might be the effects of such a development, finally giving us a peek into the very origins of life-based on these properties. The NIRSpec on the Web is capable of studying one hundred objects simultaneously, which is an achievement in itself when it comes to spectrographs observing more than one object. Now, this is achieved through a micro-shutter mechanism, developed by engineers at the Goddard Space Flight Center, that lets the NRISpec observe a hundred objects simultaneously.

Credit: Nasa

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