A new powerful ‘eye’ in the sky

Today, we are at the threshold of a great "astronomical revolution"—a revolution that will show the universe in a completely new light. At 18:20 Bangladesh standard time, Nasa is scheduled to launch into orbit the James Webb Space Telescope, the next-generation telescope to "serve as the premier deep space observatory for the next decade." The USD-10-billion, 6,200-kg telescope, referred to simply as Webb, will lift off from a launch site in French Guiana for its celestial home, the L2 Lagrange Point, about 1.5 million kilometres from Earth.

What is L2? It is a point in space—there are four more—determined by the 18th century Italian-French mathematician Joseph-Louis Lagrange at which a satellite, under the gravitational influence of the Earth and Sun, will remain approximately at rest relative to them. The point can, therefore, be used by Webb as a "parking spot" to maintain a stable position, with minimal fuel consumption as it goes around the Sun.

At L2, which is directly behind the Earth as viewed from the Sun, Webb will always be at the same location relative to Earth. As a result, astronomers can have continuous communications with Webb as the Earth rotates. Also, Webb will always see the Sun, Moon and Earth on one side of it, with a clear view of deep space on the opposite side, making it ideal for it to see much farther into the universe.

It will take about a month for Webb to reach its destination and unfurl its mirrors and a tennis-court-size sunshield. The shield will protect the telescope by blocking light and heat from the Sun, Earth and Moon. Scientists will need another five months to align the mirrors and cool down the instruments to their operating temperatures. Approximately six months after the launch, Webb will begin collecting and transmitting data.

Among the pantheon of space telescopes, the Hubble telescope, placed in orbit by Space Shuttle Discovery in 1990, is the most famous observatory in space. At an altitude of about 560km, Hubble orbits the Earth once every 97 minutes, or 15 orbits per day, with an orbital speed of 28,000km per hour. Far above rain clouds, free from light pollution and unencumbered by the distorting effects of the Earth's atmosphere, Hubble can operate round the clock with an unimpeded view of the universe.

Hubble's domain extends from the ultraviolet through the visible and into the near-infrared light. This range has allowed Hubble to deliver stunning images of stars, galaxies and other astronomical objects that have changed our understanding of the universe beyond measure. However, as great as Hubble is, it may have reached its limits, although its importance is not likely to fade any time soon.

Why do we need Webb?

Electromagnetic radiation, ranging from gamma rays to radio waves, is our measuring stick in space. It is the cosmic messenger that carries far more information than any other messenger does. Having said that, many of the objects we want to observe in space are too cold to radiate visible light, or other forms of short wavelength radiation. Instead, they radiate long wavelength infrared light. The reason is that the wavelength of light leaving the stars and galaxies of the primordial universe, initially shortwave and highly energetic, has been shifted to infrared by the Doppler Effect. It is stretching of the short wavelength of light towards larger values, because the sources of light are receding from the observer during the journey to Earth due to the ongoing expansion of the universe. In the jargon of astronomy, it is called "cosmological redshift."

In many ways, Hubble's infrared views are fundamentally limited by its very design. Specifically, the telescope's perch in low-Earth orbit—where it has to contend with not only radiation from the Sun, but with infrared light radiated and reflected from Earth itself—interferes with any attempt to observe infrared light from the cosmos. Hence, the Webb, designed to take us far beyond Hubble's limit. In particular, Webb will observe primarily in the infrared region, which will show us things never before seen by Hubble or any other telescope.

With a 6.5-metre diameter primary mirror shaped like a "golden sunflower" and cryogenic operating temperature—about 225 degrees Celsius below zero—Webb will be the largest and the most powerful space-based telescope—100 times more powerful than Hubble—ever built with unprecedented sensitivity. The dimension of Webb's mirror will translate to a 6.5-time increase in the size of data-collecting area, as opposed to other telescopes.

But why does Webb have to be cooled to extremely low temperatures? As noted above, Webb is designed to detect the faint infrared signals of objects billions of light years away (1 light year = 9.46 trillion kilometres). In order to detect these signals, which can sometimes be felt as heat, the instruments inside Webb has to be kept at very cold temperatures. Otherwise, all Webb will detect is its own infrared radiation.

Webb's mission

The primary mission of Webb is to unlock the enduring mysteries of the universe. To that end, astronomers and cosmologists hope to use the telescope to look back in time over 13.5 billion years, which is closer to the beginning of time, and see some of the earliest galaxies to form in the universe. Hubble cannot see these galaxies because of redshift. Furthermore, Webb will be able to look inside dust clouds where stars are forming today. Besides, Webb will provide insights into the formation of planetary systems—including our own solar system—search for life-supporting exoplanets inside our galaxy—the Milky Way—and look for signs of alien life.

Webb's exceptional infrared imaging power will offer researchers new views of three active supermassive black holes known as quasars, their host galaxies and their neighbourhoods, located more than 13 billion light years away. Moreover, Webb will allow astronomers to observe gravitational distortions, a consequence of Einstein's General Relativity Theory, caused by smaller black holes with mass only 100,000 times the mass of the Sun. Webb will also shed light on how galaxies got supermassive black holes at their centres. Additionally, astronomers hope to use Webb to find the origin of violent bursts of bright flares from the colossal black hole Sagittarius A*, located at the centre of our galaxy.

Cosmic Dark Ages and the Webb

One of the unsolved problems in cosmology is the structure of the universe between the first few minutes and 300,000 years or so after it came into existence 13.7 billion years ago. This period was filled with darkness—both literal and metaphorical. That is why astronomers call this period the Cosmic Dark Ages. We know very little about this period because light could not escape its surroundings through the universe to hit detectors here on Earth. The emergence of the first sources of light, which are stars and galaxies that were formed by gravity, marked the end of the Dark Ages.

The design of Webb provides unique capability to address key questions about this era in cosmic evolution. Most importantly, Webb is expected to provide answers to the following questions: When and how did the Dark Ages end? What is the nature of the first galaxies? How and when did ionisation of the space between the galaxies occur? And what sources caused the ionisation? (Ionisation is the process in which an electrically neutral atom becomes negatively or positively charged by gaining or losing electrons.)

The lifetime of Webb will be dictated by the amount of fuel it will use. Unlike Hubble, which has been operating for nearly 32 years, Webb is expected to operate for at least five years—perhaps, with a bit of luck, up to 10. It is not designed to be refuelled, repaired, or upgraded in any way simply because it will be so far away from Earth. When Webb will run out of fuel, it will no longer be able to maintain its orbit and thus will not be able to point at its targets of interest with the requisite precision. And that will be the end of Webb's mission.

Nevertheless, in its short lifetime, Webb will explore every phase of cosmic history that will help us understand the origin of the universe. It will rewrite the history of the cosmos and reshape humanity's position within it by piercing through the hitherto "dark curtain" of the early universe.

Dr Quamrul Haider is a professor of physics at Fordham University in New York, US.