James Webb Space Telescope
Scientific Endeavour to Look into the Infinite
The vast array of planets and stars spread across the mystical skies has always fascinated the human mind. It has been a constant endeavour of the scientific community to resolve mysteries of the sky above, the planets, the solar system, the galaxy and the universe beyond. The space telescopes have been the most useful instrument in this quest to explore the universe.
Italian physicist Galileo is considered the first to develop a telescope in 1609. His telescope, though with limited ability, could identify moon craters as well as our Milky Way. Modern astronomy began to flourish in the 19th century. In the 20th century, scientists developed very powerful telescopes and specialised instruments that could look deep into the space but there was a limit to these earth-based telescopes. As we have learnt in our science classes, light rays, as well as electromagnetic waves are subject to attenuation, bending and refraction when they pass through the atmosphere. The ground-based telescopes, however powerful, are thus subject to atmospheric distortions and tricks! Therefore, astronomers began dreaming of having a telescope in space. The space telescope was first conceptualised by astronomer Lyman Spitzer in 1940 and it was not long before the science community converted this surreal dream into reality.
Although there were several successful attempts to place an observatory in the sky- beginning with ‘Orbiting Astronomical Observatory 2’ (OAO-2) launched in 1968 and SAS X-ray explorer satellite in 1970- the dream of a space based observatory was truly fulfilled on 25 Apr 1990 when Space Shuttle Discovery successfully deployed the ‘Hubble Space Telescope’ in an orbit 547 kms above the earth.
Hubble and Beyond
Deployed 547 km above the earth’s atmosphere, for last 30 years Hubble Space Telescope (HST) has made over a million observations including observations of other planets’ atmosphere, of the birth and death of stars and of galaxies millions of light years away and so on. HST has given us insights into black holes, formation of planets and galaxies and discovery of dark energy. But HST’s instruments work primarily in visual and UV spectrum (primarily in 0.1 to 0.8 micron), which has a limitation in understanding the more distant universe. Astral bodies and stars that are forming remain hidden behind cocoons of dust that absorb visible light whereas infrared rays emitted in these formations can penetrate the dust clouds.
The James Webb Space Telescope (JWST) will observe primarily in the IR band, from .75 microns to a few hundred microns, retaining some capability in the visible spectrum, especially the red to yellow part and thus provide a clearer view of several phenomena.
Another advantage of working in the infra-red spectrum is that the JWST will be able to see the first galaxies or the earlier galaxies than the Hubble. Why? The Einstein’s theory of relativity leads us to understand that expansion of the universe causes galaxies to move away from each other, which also implies that any light emanating therein will also stretch, shifting the light’s wavelength to longer wavelengths. These distant galaxies therefore are very dim or invisible at visible wavelengths but can be picked up at IR wavelength.
The JWST Mission
The Webb was launched on 25 Dec 2021 from Arianespace’s launch complex at Centre Spatial Guyanais (CSG) in French Guiana, South America. This site is located very close to the equator, which is beneficial to the launch as the spin of the earth (linear velocity of the earth being the maximum -1670 kmph, at equator) helps give an additional push.
The launch vehicle was the Ariane 5 rocket, one of the most reliable launch vehicles in the world. Ariane 5 is a multi-stage rocket that has a standard 5m diameter nose cone and uses a combination of solid and liquid propellants. The lower core stage consists of a liquid cryogenic single stage engine that ignites 7 sec before lift off and lasts 9 minutes. It provides about 3000 lb thrust. Next stage comprises two strap on solid propellant boosters providing a colossal 2.6 million pound thrust in just about 2.25 minutes! Lastly, another liquid single stage operates for 15.75 minutes delivering 15000 lb thrust. Thus, the total powered flight lasts 27 minutes.
The pay load-in this case the Webb telescope- is housed with in the nose cone fairing, with in an inner space of 4.57 m diameter and 16.19 m length. As will be discussed later, the Webb telescope in its deployed form is almost the size of a tennis court and too large to fit with in the Ariane 5 nose cone. So, it was segmented into 18 hexagonal pieces on a hinged structure, folded up for launch and unfolded in space : a true engineering marvel.
The Webb separates from the Ariane 5 30 minutes after the launch and the solar array deploys automatically. 12h after the launch, small rocket engines on the Webb itself fire to provide the first trajectory correction manoeuvre. This is followed by two more critical course corrections to achieve a successful orbit. The final placement took about a month.
The Webb has been placed in an orbit that follows a special path around the second Lagrange point L2. This (L2) is located about 1.5 million kms from the earth, in direction opposite to the Sun. The Lagrange point is such that the gravity of sun and earth balances the centripetal force required for the observatory to move with them. Further, Webb’s orbit allows it to stay on the (earth’s) night side, in synchronous orbit around the Sun. The Webb’s own orbit around the L2 is a halo orbit.
As explained earlier, JWST is an infra-red observatory and therefore it must be protected from bright/hot sources that may overwhelm the faint IR signal the Webb tries to read. In the L2 orbit described above, the Webb’s instruments will always stay away from the sun (night side) and its sun shield will continuously face the sun, blocking its view from the observatory’s optics. Thus, the position facilitates continuous generation of solar power via sun facing solar array and unobstructed view of the space in the opposite direction.
The Webb telescope or observatory has three main components: a) the optics and instruments, b) the sunshield and c) the bus.
Mirrors. The mirror is concave, 21.5 feet in diameter and made up of 18 hexagonal segments. This is the largest mirror so far to have been launched into space. The mirror has to be large so that it can collect and gather enough of (faint) IR signals. All segments combine to give a near circular shape to the mirror overall, which helps focus the light. Each segment is made moveable so that they can align together to work as a single optic. The secondary mirror is a smaller convex mirror of 2.4 feet diameter. It is supported by a 25 feet long arms extending from the primary mirror. Mirrors are made from Beryllium and are coated with very thin layer of gold, optimising them for reflecting IR energy.
Another requirement is that the mirror should be cold enough to keep the unwanted IR light being observed. The cooling is achieved by the heat shield.
Heat Shield. The heat or sun shield is in the shape of a kite and nearly the size of a tennis court. The shield has five layers, of different thickness. Layers are separated
from each other to prevent transfer of heat.The layer membranes are made of Kapton, a high heat resistance material and coated with aluminium. Each layer is thus cooler than the one below and altogether the sunshield can reduce exposure from the sun from over 200 KW to a fraction of a Watt. So, layer that faces the sun could get as hot as 230 F and the cold side as cold as -390 F.
Instruments. Webb’s instruments-that are considered the heart of the observatory- are contained in the Integrated Science Instrument Module (ISIM), located on the cold side of the platform. The four main instruments are as follows:
- Near Infrared Camera (NIRCam). NIRCam is Webb’s primary imager. It provides both, high resolution spectrography and imaging in the 0.6 to 5 micron wavelength. At this wave length, the space dust becomes transparent. It’s also equipped with coronagraphs that help capture characteristics of planets orbiting near stars.
- Near Infrared Spectrograph (NIRSpec). This too works in 0.6 to 5 microns range and is designed to observe 100 objects simultaneously. This is made possible by an array of micro-shutters (each approximately as wide as human hair) controlled individually to observe a portion of the sky.
- Near Infrared Slit-less Spectrograph (NIRISS). Provides near IR imaging and spectroscopic capabilities. Equipped with an aperture mask, it has the unique ability to capture images of bright objects.
- Mid Infrared Instrument (MIRI). MIRI too is a combination of camera, coronagraphs and spectrographs but operates in the range of 5 to 28 microns. Being operational in mid IR, MIRI is valuable in the study of distant galaxies, new stars and other faintly visible objects.
The JWST reached its final destination and home at the second Lagrange point L2 on 24 Jan 2022.These 30 days of journey were also spent on carefully unfolding its sunshield, mirrors and other vital parts. Nearing the L2, JWST carried out one of the three critical manoeuvres based on the onboard thrusters (correction burn MCC2) in order to accurately get into the final position.
The Webb will begin scientific observations once its commissioning process that further comprises completion of the unfolding process, cooling down to its cryogenic temperatures (about -380 Degree F), aligning and calibrating all its mirrors is completed. Each of its 18 segments need to be aligned within a fraction of near IR wavelength. This process will take about six months. While the astronomical community waits for the first JWST images to arrive, I am sure the result will be worth the wait and the hard work of over thirty years.
-Shaivya Gupta 11B