The James Webb Space Telescope

A telescope built for infrared

Infrared is light with wavelengths a little longer than red, invisible to our eyes but felt as warmth. Webb was designed mainly for this light for three reasons.

First, the universe is expanding, and that stretches light from the most distant galaxies toward longer wavelengths, a shift astronomers call redshift. Light that left the first galaxies as ultraviolet or visible arrives today as infrared. Second, infrared passes through dust clouds that hide newborn stars. Third, the gases in planet atmospheres leave fingerprints in the infrared.

Mirror and sunshield

The main mirror is about 21 feet across, too big to fit inside any rocket in one piece. It is made of 18 hexagonal segments of lightweight beryllium coated with a thin layer of gold, which reflects infrared very well. The mirror folded up for launch and opened in space.

An infrared telescope must be cold, or its own warmth glows and drowns out faint targets. Webb's sunshield has five layers of very thin film, each about as large as a tennis court. Facing the Sun, the hot side can reach more than 200 °F, while the shaded side stays below about −390 °F. One instrument is cooled further by a small refrigerator.

Launch and the trip to L2

Webb launched on December 25, 2021, from French Guiana on a European Ariane 5 rocket. Over the next weeks it unfolded its sunshield and mirror in a long series of steps, any one of which could have ended the mission. All of them worked.

It then settled into an orbit around the second Lagrange point, or L2, about 930,000 miles from Earth in the direction away from the Sun. At L2 the pull of the Sun and Earth combine so that a spacecraft keeps pace with Earth. From there, the Sun, Earth and Moon all sit on the same side, so a single shield can block them all. The launch was so accurate that Webb saved fuel, and NASA expects enough for about 20 years of operation.

What Webb has found

Its first full-color images were released in July 2022. Since then, Webb has spotted galaxies that existed only about 300 million years after the Big Bang. Many are brighter and more developed than models predicted, which has kept theorists busy.

Webb has measured carbon dioxide, water vapor and other molecules in the atmospheres of planets around other stars. It has imaged dust disks where planets form, tracked the life cycle of stars in nebulas, and found early galaxies with surprisingly large black holes.

It also studies our own neighborhood, capturing the rings of Neptune and Uranus, weather on Jupiter and the faint glow of distant comets.

Webb and Hubble together

Webb is often called Hubble's successor, but the two work as partners. Hubble is strongest in ultraviolet and visible light; Webb is strongest in the infrared. Combining their images shows both the glowing gas and the hidden stars inside the same nebula.

One big difference: Hubble orbits close to Earth and could be repaired by astronauts. Webb is far too distant for any servicing visit, so it was designed and tested to work without help.

Can you see Webb?

Not with the naked eye. At about 930,000 miles away, roughly four times the distance to the Moon, it is far too faint. Skilled amateurs with large telescopes and cameras have occasionally recorded it as a tiny moving dot.

The best way to follow Webb is through the images NASA, ESA and the Canadian Space Agency release, usually with notes on what each picture shows.