Can we build a more powerful space telescope?

From the time Galileo first turned his telescope to the heavens in 1609, until the dawn of the space age on October 4, 1957 when the Soviet Union successfully launched Sputnik, the world's first artificial satellite, into space, astronomical telescopes were located on the ground. Even today, the vast majority of observatories are ground-based—and will probably remain so into the future. Nevertheless, Earth's surface is anything but ideal as sites for observatories, except for radio and optical telescopes.

Daylight, weather and light pollution at night are the most obvious problems with observing from the ground. Another problem, somewhat less obvious but equally serious, is atmospheric turbulence. As a result, objects in the sky appear to jiggle around, causing them to twinkle. Twinkling of celestial objects, such as stars, may be beautiful to the naked eye, but it causes problems for astronomers because it blurs images seen through the telescope.

Also, because the universe is expanding, light from most interesting objects in the universe, primarily from the primordial universe—originally short-wavelength, high-energy radiation—has been stretched to the long-wavelength infrared light. (Light is our most reliable messenger in the sky.) At ordinary temperatures, the Earth's surface and even a telescope itself also emit infrared light that will interfere with any attempt to observe these objects in the universe.

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Is the James Webb Space Telescope delivering on its promises?

The ultimate solution to these problems is to put telescopes into space. Until 2021, among the pantheon of space-based telescopes, the Hubble telescope was the most famous observatory in space. Hubble allowed us to venture into regions of the cosmos "where no man has gone before." Notwithstanding, the most powerful space-based telescope today is the James Webb Space Telescope (JWST), launched on December 25, 2021. Clearly, while Hubble has been extraordinarily superb, JWST is incredibly spectacular, taking us close to the beginning of time.

Since it first began sending pictures back home in July 2022 from its location 1.5 million kilometres beyond Earth's orbit, JWST returned a treasure trove of images from all over the cosmos. It has peered deeper in space and farther back in time than any previous telescope could manage, sending back stunning snapshots of cosmic vistas, complete with a gold mine of scientific data that is changing our understanding of the universe.

Although JWST ushered in the golden age of astronomy, scientists are already asking the question: what's next after its 20-year lifetime? Can we build a telescope more powerful than JWST? Because telescopes are portals through which we study the universe, answering this question requires an understanding of what makes a telescope powerful.

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What makes Webb’s first images such a big deal?

Telescopes are essentially giant eyes that can collect far more light than our own eyes, allowing us to see much fainter objects in considerably greater detail. The amount of light a telescope can collect depends on the diameter, and thus the surface area of its mirror. Hence, compared to Hubble's 2.4-metre diameter mirror, the 6.5-metre diameter mirror of JWST has approximately 6.25 times more light collecting area. Therefore, unlike Hubble, JWST can see the overly dim and longer-wavelength infrared light with exceptional clarity.

Besides, the angular resolution, which is the smallest angular separation between two objects that a telescope can discern, is inversely proportional to the diameter of its mirror. This means larger telescopes can have amazing angular resolution. For example, Hubble's angular resolution for visible light is about 0.05 arcsecond, whereas that of JWST is 0.02 arcsecond (1 arcsecond = 1/3,600 degrees.)

Obviously, a telescope more powerful than JWST should have a mirror appreciably larger than 6.5 metres. But the dimension of a space-based telescope's mirror is limited by the size of the spaceship that will transport it to outer space. That is why the JWST's mirror has a sunflower-like hexagon design. The weird design facilitated its large mirror to fold down and fit inside the Ariane 5 rocket that transported it, whose payload area is only 4.5 metres wide.

Despite their enormous light-gathering ability, it is debatable whether telescopes with mirrors larger than JWST will be able to see any deeper into the cosmos

In an article published in the April 2023 issue of the journal Applied Optics, Dr Sebastian Rabien, a scientist at the Max Planck Institute for Extraterrestrial Physics in Germany, reports developing a technology to build massive mirrors that can be easily transported to space. His team of researchers developed a prototype "rollable" telescope-grade mirror using small flexible mirrors with a diameter of 30 cm. While his prototype mirrors are very small, this technology, according to Rabien, can be easily scaled up. As such, he estimates that using his method, we can build flexible mirrors of up to 20 metres in diameter. Such a mirror will be able to capture at least 9.4 times more light than JWST.

Despite their enormous light-gathering ability, it is debatable whether telescopes with mirrors larger than JWST will be able to see any deeper into the cosmos. However, because larger telescopes need less exposure time to gather light, a 20-metre telescope will be able to scan the sky in great detail noticeably faster than JWST. This will enable us to search through the hundreds of thousands of observable exoplanets for alien life in only a few years.

To conclude, considering the fact that the 20-metre mirror, albeit promising, is in its embryonic stage, we have to wait until the viability and scaling of the mirror is proven. After all, it took almost a decade to make JWST's mirror. Yet, telescopes larger than JWST seem to be in the realm of possibility in the next decade or so.

Dr Quamrul Haider is professor emeritus at Fordham University in New York, US.

Views expressed in this article are the author's own. 

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