December 25 is an exciting day for science, more so for astronomers. Over a quarter of a century in the making, Nasa will finally launch the James Webb Space Telescope (JWST) on Saturday — provided everything goes according to plan — putting mankind one step closer to studying every phase of the universe's history.

The JWST, which includes contributions from thousands of scientists and engineers from 14 countries and 29 US states, will become the most powerful space telescope upon its launch.

While scientists count down the minutes till the rocket carrying it blasts off to space, here's all you need to know about why telescopes are launched and what makes the JWST so special:

Why are space telescopes launched?

In ancient times, we had to rely on our eyes to collect cosmic light, but with the advent of telescopes, we found a larger and efficient tool to do this job for us. For instance, the Keck Observatory in Hawaii, operated by the California Institute of Technology, can collect 600,000 times more photons (particles that constitute light) than our eyes. Compared to only three galaxies and a few thousand stars that can be seen with the human eye, Keck can observe billions of stars and galaxies.

However, the telescopes we build on Earth have certain limitations that cannot be overcome. These limitations stem from the fact that a blanket of atmosphere containing a mixture of gases shrouds the Earth that disperses and absorbs photons of light. The atmosphere only allows the transmission of light in visible and radio frequencies and to explore the universe in broader frequency ranges like gamma rays, x-rays, ultraviolet (UV), infrared, and microwaves, one needs to go above the Earth’s atmosphere.

The purpose of launching telescopes into space is to collect the light before it gets distorted or absorbed by Earth’s atmosphere. To date, more than a hundred space observatories have been launched to investigate the universe across the full length of the electromagnetic spectrum. These advanced instruments helped us move from “twinkle, twinkle little star, How I wonder what you are” to “we know exactly what you are”.

Most powerful space telescope

With a mirror diameter of over 20 feet across, it will be the most powerful space telescope ever built. This telescope was envisaged as a successor to the Hubble Space Telescope (HST) that was launched in 1990. The total cost of the instrument and its five years of operation after its launch is summed to be a mammoth $11 billion.

JWST will be launched using the European Space Agency’s rocket Ariane 5, costing additionally €700 million on the part of the Europeans. Unlike HST which orbits the Earth roughly at an altitude of 540 km, JWST will orbit the sun at a distance roughly one million kms away from us. At this location, called L2, JWST would be very stable in a cold and dark environment essential for its successful operations.

Before discussing the design and scientific objectives of the JWST, it is worth recapitulating the success story of its predecessor — the Hubble Space Telescope — which is the current premier space telescope. During its lifespan of a little over 30 years, this iconic instrument has captured mesmerising images of the universe. Scientific discoveries based on HST data have led to an unprecedented wealth of knowledge about the constituents of the universe, its beginning, past, and future evolution. By observing distant galaxies, HST provided compelling evidence that the universe was created about 13.8bn years ago in a Big Bang and that it is expanding at an ever-increasing rate since then.

With its famous Hubble Deep Filed image, HST revealed that galaxies are spread like grains of sands shining already when the universe was less than a billion years old. Zooming into the center of the giant galaxy M87, it observed gas swirling around at extremely high speeds with the only plausible explanation pointing towards the presence of a gigantic supermassive black hole — approximately 10bn times more massive than our sun. HST not only provided wide-ranging details of planets in our solar system but also led to the discoveries of planets orbiting other stars.

Most daunting task for Nasa

HST collects light mainly in the visible part of the electromagnetic spectrum — a very similar part of the light that our eyes are sensitive to. However, JWST will operate in the infrared wavelengths, the immediate part after the visible spectrum.

Its 22-foot-diameter mirror will be built using 18 small hexagonal mirrors made of beryllium and coated with gold, a very thin layer of about 1000 atoms totalling 50 grams for the entire mirror coating.

The JWST will be kept below -200 degrees Celsius by a large sun shield that protects the mirrors and scientific instruments from sunlight to minimise thermal noise since it operates in the infrared, which are low-temperature radiations.

JWST will carry four instruments on-board to analyse light captured by its giant mirror through imaging and spectroscopy — splitting the light into its constituent colours or wavelengths.

Fully assembled with all its instruments in an operational configuration, the JWST is too large to fit into the rocket housing. Hence, it will be launched in a folded origami-like arrangement and this brings with it the most daunting task Nasa has ever faced — unfurling the telescope mirrors and its giant solar shield in a fully automated way, millions of kilometers away with no repair mission possible.

There are 344 single-point-of-failure operations to be performed from launch till final assembly and any one of those can cause an irreparable loss in the telescope working to its designed potential. That is why the JWST has faced several years of delays to ensure that these tasks are executed in space with impeccable accuracy.

The main scientific goal of JWST is to look deeper in space and probe the universe in its infancy roughly 13bn years in the past. Since its formation until now, the universe has been in a constant state of expansion, which has led to the stretching of light out of the visible range and down into the infrared. For this reason, to see the first stars and galaxies, we need a powerful infrared telescope, which is exactly what the JWST is!

The second major goal of JWST is the discovery of exoplanets — planets outside the solar system. It will also investigate the potential for life in those worlds by studying their atmospheres. The minimum operation period of the mission is five years, however, it is expected to serve for much longer than that. All of mankind should cherish the occasion because as Professor Abdus Salam said, "Scientific thought is the common and shared heritage of mankind."

Unfortunately, here in Pakistan, our contributions in space explorations have been close to non-existent.

In the foreseeable future, chances of making some noticeable progress in this domain are quite slim. Before we can think of launching or joining some space exploration missions, a research-grade telescope (with a mirror diameter of a few meters) should be built at some suitable location.

The academic and research fraternity of public and private universities across Pakistan should have access to use this facility. Such a facility will provide researchers an excellent opportunity to carry out exciting research at the forefront of astronomy and astrophysics like the discovery of extrasolar planets, monitoring of near-earth asteroids, exotic binary stars, and supermassive black holes.

Equipped with the required skills in this domain, Pakistani researchers can then establish linkages with the international scientific community. Pakistani students will get a chance to work in the advanced facilities of Europe, US, and China. Later, on returning, they can contribute to the discipline immensely.

The author is a research fellow at the Vanderbilt University, USA, and an associate professor at the Space Science Institute of Space Technology, Islamabad.

Header photo: Nasa workers are lifted alongside the James Webb Space Telescope Mirror during its media reveal at Nasa’s Goddard Space Flight Centre at Greenbelt, Maryland, US. — Reuters/File