James Webb Space Telescope: The Revolutionary Infrared Observatory

Webb did not replace Hubble. It sees a different kind of light, with a much bigger mirror.

1. What is Webb, and who built it?

The James Webb Space Telescope is a large space telescope that works in infrared light. People call it JWST or simply Webb. NASA leads the mission with the European and Canadian space agencies.

It launched on 25 December 2021. An Ariane 5 rocket lifted it from Europe’s Spaceport near Kourou in French Guiana. The European Space Agency supplied the launch.

The name honours James E. Webb. He ran NASA from February 1961 to October 1968. He pushed for a balanced programme that included space science.

2. Why does Webb look in infrared light?

Infrared is light just beyond the red end of the rainbow. Our eyes cannot see it. Webb sees from 0.6 to 28.5 microns. A micron is one millionth of a metre.

Hubble was built mainly for shorter waves, from 0.1 to 2.5 microns. NASA’s Spitzer was another infrared telescope. It scanned the sky for more than 16 years and ended in January 2020.

Infrared matters for two reasons.

• Redshift: the universe is expanding. Light from far galaxies is stretched toward longer, redder waves. Their light reaches us mostly as infrared.
• Dust: infrared passes through dust better than visible light. Webb can look inside clouds where stars and planets form.

3. How does Webb's mirror beat Hubble's?

A telescope’s mirror collects light. A bigger mirror collects more of it.

• Webb: 6.5 metres across, made of 18 hexagonal segments.
• Hubble: one mirror, 2.4 metres across.
• Result: Webb has more than six times Hubble’s light-collecting area.

Each segment is made of beryllium. Beryllium is strong, light and holds its shape across a range of temperatures. A thin coat of gold covers each segment. The gold is 100 nanometres thick and reflects infrared light well.

The mirror folds up to fit inside the rocket. The extra area lets Webb see objects up to 100 times fainter than Hubble can.

Warm things glow in infrared. Webb’s own heat would drown out faint signals. So it must be far from Earth and very cold.

4. Where does Webb orbit, and why?

Webb does not circle the Earth like Hubble. It orbits the Sun at a spot called the second Lagrange point, or L2. L2 lies about 1.5 million km from Earth.

At L2, Webb stays in line with Earth as both move around the Sun. One sunshield can then block the Sun, Earth and Moon together.

The trip took about 30 days. Webb loops around L2 in an orbit that takes about six months. No servicing mission is planned, because Webb is far beyond the Moon. Hubble was built to be repaired by astronauts.

5. How does the sunshield keep Webb cold?

The sunshield has five layers. Each layer is about as thin as plastic wrap. The layers do not touch each other. It measures about 21 metres by 14 metres.

The hot side faces the Sun. It can reach 358 kelvins, which is 85 degrees Celsius. The cold side holds the science instruments. It stays near 40 kelvins, which is minus 233 degrees Celsius.

For comparison, the coldest temperature ever recorded on Earth was minus 89 degrees Celsius, at Vostok station in Antarctica.

One instrument needs even more cooling. A cooler brings the detectors of MIRI down to 7 kelvins.

6. What do the four instruments do?

Webb has four science instruments. All sit behind the main mirror.

• NIRCam: the main near-infrared camera. It sees light from 0.6 to 5 microns.
• NIRSpec: splits light into spectra. It can study more than 100 objects at once.
• MIRI: the mid-infrared instrument, from 5 to 28 microns. It studies cool dust disks and very distant galaxies.
• FGS/NIRISS: the fine guidance sensor and a slitless spectrograph.

NASA groups Webb’s work into four themes. Its first images showed how well they work.

7. What are Webb's four science themes?

NASA lists four themes.

• Early Universe: the era called Cosmic Dawn, from about 50 million to one billion years after the big bang. Webb can look back over 13.5 billion years to see the first galaxies.
• Galaxies Over Time: how galaxies grow and change, including black holes at their centres.
• Star Lifecycle: star birth, planet formation and star death.
• Other Worlds: the atmospheres of planets around other stars, and objects in our own solar system.

8. What did Webb's first images show?

NASA released Webb’s first full-colour images on 12 July 2022. The first deep field showed a galaxy cluster named SMACS 0723. It holds thousands of galaxies in a tiny patch of sky. The patch is about the size of a grain of sand held at arm’s length.

The cluster’s gravity acts as a lens. It magnifies galaxies far behind it. The image combined 12.5 hours of exposure. Hubble’s deepest fields took weeks.

Webb keeps setting records. Some findings are firm. Others are still hints.

9. Which galaxy holds the distance record?

In May 2024, NASA reported JADES-GS-z14-0. Its redshift is 14.32. We see it as it was less than 300 million years after the big bang.

A higher redshift means a farther and earlier galaxy. In June 2025, a galaxy named MoM-z14 took the record. Its redshift is 14.44, about 280 million years after the big bang.

So always date a “most distant galaxy” claim. The record can change.

10. What did Webb find on K2-18 b?

K2-18 b is a planet 8.6 times as massive as Earth. It orbits a cool star in the habitable zone, 120 light-years away.

In September 2023, NASA reported methane and carbon dioxide in its air. Some studies suggest it could be a Hycean world. That means a hydrogen-rich atmosphere over a water ocean.

Webb also gave a possible sign of dimethyl sulfide. On Earth, only life makes this gas. NASA called the result a possible detection, not proof.

11. What did Webb find on 55 Cancri e and a pulsar planet?

Two findings show why Webb is unique.

• 55 Cancri e (May 2024): a hot rocky planet, 41 light-years away. Webb data hint at an atmosphere rich in carbon dioxide or carbon monoxide. NASA called it the best evidence yet for a rocky-planet atmosphere outside our solar system.
• PSR J2322-2650b (December 2025): a Jupiter-mass planet that orbits a pulsar. Its air seems to be helium and carbon. Soot clouds may float there. How it formed is a mystery.

A pulsar is a fast-spinning neutron star.

The 2022 Mains question had three parts. The third asked about benefits for people.

12. How does Webb help people on Earth?

Webb’s benefits come in three kinds.

• Knowledge: Webb studies every stage of cosmic history. This includes the first glowing stars and the makeup of other worlds.
• Technology: a method used to measure Webb’s mirrors now helps eye surgery. NASA says it improved vision for millions through LASIK.
• Teamwork: NASA, ESA and the Canadian Space Agency share the mission.

13. How did UPSC Mains 2022 ask about Webb?

The question came in General Studies Paper III. It carried 15 marks and a 250-word limit.

The question has three parts. Part one asks for features. Part two asks for goals. Part three asks for benefits.

14. How should you answer in 250 words?

Give each part of the question its own short block.

• Opening: launched 25 December 2021; infrared observatory at L2.
• Features: 6.5 m gold-coated mirror; 0.6 to 28.5 micron range; sunshield; 40 K cold side.
• Goals: early universe, galaxies, star life cycle, other worlds.
• Benefits: new knowledge, spin-off technology, shared teamwork.
• Closing: one line on records such as the most distant galaxy.