James Webb Space Telescope: How It Sees the Infrared Universe

The James Webb Space Telescope (JWST) is a large infrared observatory that NASA built with the European Space Agency and the Canadian Space Agency. It launched on December 25, 2021, and sits about 1.5 million kilometers from Earth. Its main purpose is to observe light that Hubble cannot see well, including the faint, stretched light from the earliest galaxies.

Why Infrared Astronomy Needs a Cold Telescope

Many of the most distant objects in the universe emit most of their light in the infrared. The expansion of the universe stretches light from distant sources to longer wavelengths, a process called redshift. Light that started as visible or ultraviolet light from a young galaxy arrives at Earth as infrared light.

Infrared observations have a major problem: a warm telescope glows in the same wavelengths it is trying to detect. Heat from the mirror, the structure, and the instruments would drown out faint signals. For this reason JWST must operate at very low temperatures, with most instruments cooled to about 40 kelvin, roughly minus 233 degrees Celsius, and the MIRI instrument cooled to about 7 kelvin.

The Mirror: Eighteen Gold-Coated Segments

JWST's primary mirror is 6.5 meters across. That is more than twice the diameter of Hubble's 2.4-meter mirror. Because the mirror could not fit inside a rocket in one piece, it is built from 18 hexagonal segments that fold for launch and unfold in space. Each segment is made of beryllium, a light metal that keeps its shape when it cools, and each is coated with a thin layer of gold. The gold coating reflects infrared light efficiently.

Each segment has actuators that move it in fine steps. After launch, engineers aligned the segments so that they work together as a single mirror. This alignment process took several months and was one of the most complex parts of the commissioning phase.

The Sunshield and the Cold Side of Space

The sunshield is the feature that makes the cold operating environment possible. It consists of five thin layers of a high-performance plastic film, each coated with metal. The layers are spaced apart so that heat cannot pass easily from the sun-facing side to the cold side. The sunshield is roughly the size of a tennis court.

The sun-facing side of the sunshield can reach about 110 degrees Celsius, while the telescope side remains near 40 kelvin. The large temperature gap is the reason the layers work. Each layer reduces the heat that reaches the layer beneath it, and the cold side stays cold enough for the infrared detectors to work.

Launch, Orbit, and Commissioning

JWST launched on December 25, 2021, from Kourou, French Guiana, on an Ariane 5 rocket. Unlike Hubble, it does not orbit Earth. It circles the Sun at a point called the second Sun-Earth Lagrange point (L2), about 1.5 million kilometers beyond Earth. At L2, the Sun, Earth, and telescope maintain a stable relative position, so the sunshield can keep the telescope in shade.

Deployment unfolded the sunshield, the mirror, and the instruments over roughly two weeks. After arrival at L2, the team completed mirror alignment and instrument checks. The first full-color science images were released on July 12, 2022, including a deep field view of a galaxy cluster that showed thousands of galaxies in one frame.

Propellant and Station-Keeping

L2 is not a fixed point in space. It is an unstable balance point, so the telescope follows a looping path called a halo orbit around it. Small thruster firings keep the orbit on course. The telescope carries enough propellant for a design life of about 20 years, which is longer than the mission's basic science requirement of 5 to 10 years. Fuel is the main limit on how long the observatory can stay at L2 once its other systems are no longer needed.

Operations and Public Data

The Space Telescope Science Institute in Baltimore runs JWST science operations. Astronomers propose observing programs through a competitive process, and each program has a proprietary period during which its data is kept private. After that period, the data is released to the public through archives, which lets researchers outside the original team analyze the same observations.

Hubble and JWST Compared

The table below compares the two telescopes using rounded values from standard references.

Feature	Hubble	JWST
Primary mirror diameter	2.4 m	6.5 m
Collecting area	about 4.5 m2	about 25 m2
Wavelength range	0.1 to 2.5 micrometers	0.6 to 28 micrometers
Typical operating temperature	warm, near room	about 40 K for key instruments
Location	low Earth orbit	Sun-Earth L2
Mass	about 11,000 kg	about 6,500 kg

Instruments: Four Eyes for Different Jobs

JWST carries four main science instruments. The Near-Infrared Camera (NIRCam) produces wide images in near-infrared light and also serves as the primary wavefront sensor for mirror alignment. The Near-Infrared Spectrograph (NIRSpec) splits light into its component wavelengths, which reveals chemical composition and temperature. The Mid-Infrared Instrument (MIRI) covers longer wavelengths and needs the coldest temperatures. The Near-Infrared Imager and Slitless Spectrograph (NIRISS) supports both imaging and wide-field spectroscopy.

Each instrument answers a different question. Imaging shows where objects are and how they look. Spectroscopy reveals what they are made of, how fast they move, and how far away they are.

Early Galaxies and Exoplanet Atmospheres

One of JWST's major goals is to find galaxies that formed in the first few hundred million years after the Big Bang. Their light has been stretched by redshift, so the infrared capability is essential. Early results identified galaxies that appear to be more luminous and more numerous than many models predicted, which has prompted scientists to revisit how galaxies formed.

Galaxy ages depend on redshift. Astronomers measure how far a spectral feature has shifted toward longer wavelengths, then convert that shift into a distance and a cosmic time. Because the most distant galaxies are seen as they were when the universe was young, JWST acts as a time machine. Its deep field surveys combine imaging with spectra, which confirms distances and avoids mistakes from photometry alone.

JWST also studies planets orbiting other stars. When a planet passes in front of its star, a small amount of starlight passes through the planet's atmosphere. Spectroscopy can detect molecules such as carbon dioxide, water vapor, and methane from that filtered light. In 2022, JWST reported carbon dioxide in the atmosphere of the gas giant WASP-39b, an early demonstration of this technique.

Key takeaways

JWST's 6.5-meter mirror, gold coating, and five-layer sunshield work together to make infrared observations possible. The cold environment at L2 keeps the instruments quiet enough to detect very faint light.

Its science spans two main themes: the first galaxies and the atmospheres of planets beyond the solar system. Both depend on infrared light that the older telescopes could not collect efficiently.

Because the telescope is designed to operate for many years, its results will continue to build as more observing programs complete. The mission shows how a single well-designed instrument can change more than one field of astronomy.