James Webb Space Telescope: How does it work and what will it see? © NASA/Goddard Space Flight Centre

James Webb Space Telescope: How does it work and what will it see?

The tech behind the £7.2bn successor to Hubble, and what it will be used for.

The James Webb Space telescope will image the little-known places in the Milky Way and beyond. Here are just a few of the things it hopes to see and the tech it will use to see them.


JWST: The facts

Full name: James Webb Space Telescope

Size: 21 x 14m (sunshield)

Launch mass: 6,200kg

Cost to build: $10bn

Launch date: 31 October 2021

Expected first images: 2-3 months after launch

Collaborators: NASA, ESA and Canadian Space Agency

Mission duration: 5-10 years

Orbit: 1.5 million km from Earth

The James Webb Space Telescope’s tech


1: Secondary mirror

Reflects light from the primary mirror and focuses it into the Integrated Science Instrument Module (ISIM).

2: Primary mirror

18 hexagonal segments, coated with gold, capture the light from distant celestial objects.

3: Sunshield

The size of a tennis court, it protects the telescope from light sources, such as the Sun.



The Integrated Science Instrument Module produces images from light captured by secondary mirror.

2: Spacecraft bus

Contains most of the steering and control machinery.

3: Star trackers

Small telescopes that observe star patterns to help aim the telescope.

4: High gain antenna

Transmits data back to Earth and receives commands from NASA’s Deep Space Network.

What will the JWST see?

The early Universe

The JWST will be able to look back to around 200 million years after the Big Bang, when the first stars in the Universe formed.

The first stars are thought to have been massive giants made of hydrogen and helium, whose short lives ended in the supernovae that created the heavier elements we detect in younger stars today. To see this period in cosmic history, we need sensitive infrared instruments to detect the faint traces of light that have travelled through space and time to reach us.

Ancient galaxies

The JWST will also look back to the very first galaxies in the Universe to learn more about their evolution and why there’s so much variety in them. Nearly all the spiral and elliptical galaxies that we see today have experienced at least one collision or merger with another local galaxy.

Yet older galaxies look entirely different to their modern counterparts – smaller, clumpier, less structured. Examining galaxies can also inform us of the macrostructure of the Universe and how it’s organised on a large scale.

Dark matter

Dark matter is thought to play an important role in the structure of the Universe, accounting for five times the mass of normal, baryonic matter such as atoms and particles. Considered to be the scaffolding for the Universe, we’re only able to observe dark matter indirectly by measuring how its gravity affects stars and galaxies.

The JWST won’t be able to see dark matter, but it will employ gravitational lensing techniques to study the most distant galaxies and look at their rotation for signs that dark matter is at play.

Exoplanet atmospheres

The JWST will help answer the big question of whether life exists beyond Earth by studying a variety of exoplanets – planets outside our Solar System.

Of particular interest is the TRAPPIST-1 system, where three of its seven planets are in the habitable zone and one may harbour liquid water. The JWST will observe the planet as light from its parent star passes through the planet’s atmosphere, revealing its chemical composition and the gases that are present there.

Our ice giants

While the JWST’s primary science aims lie more in cosmology and star formation, it’ll also take a closer look at a couple of familiar objects – our ice giants, Neptune and Uranus.

The JWST will map their atmospheric temperatures and chemical composition to see how different they are – not only to each other, but also their gas giant cousins, Jupiter and Saturn. The ice giants are at least 30 times further from the Sun than Earth and are the least understood planets in our Solar System.

Pluto and the Kuiper Belt Objects

Dwarf planet Pluto and its fellow Kuiper Belt Objects will also be receiving some observation time.

The JWST is powerful enough to study such icy bodies including comets, which are often-pristine leftovers from our Solar System’s days of planet formation and could hold clues to Earth’s origins. There are no planned missions dedicated to the outer Solar System for years, so new observations and data will play a big part in planning for future planetary missions.