The James Webb Space Telescope Explained

On December 25, 2021,

an Ariane 5 rocket lifted off from French Guiana carrying the largest and most complex space telescope launched up to that time.

The James Webb Space Telescope, a partnership between NASA, the European Space Agency and the Canadian Space Agency,

had been in development since the 1990s and cost about 10 billion dollars.

Webb was designed to see the first galaxies that formed after the Big Bang,

peer through dust clouds where stars are born and study the atmospheres of planets around other stars.

Here is how it works, why it looks so different from other telescopes and what it showed the world in its first years.

Why Infrared?

Webb observes mostly infrared light, which has longer wavelengths than the red light your eyes can see.

There are three big reasons for that choice.

First, the universe is expanding, and that expansion stretches the light from very distant galaxies to longer wavelengths, a shift astronomers call redshift.

Light that left the earliest galaxies as ultraviolet or visible light arrives at Earth as infrared. To see those galaxies at all,

you need an infrared telescope.

Second, infrared light slips through clouds of dust that block visible light, revealing newborn stars and planet-forming disks hidden inside.

Third, relatively cool objects, such as planets, brown dwarfs and cold gas, glow most brightly in infrared.

A Giant Golden Mirror

Webb's main mirror is 6.5 meters (21 feet) across, giving it about six times the light-collecting area of Hubble's mirror.

A single mirror that large would never fit inside a rocket,

so it was built from 18 hexagonal segments that folded up for launch and opened like a flower in space.

The segments are made of beryllium, a light metal that holds its shape well at very low temperatures,

and are coated with an extremely thin layer of gold, which reflects infrared light exceptionally well.

Tiny motors behind each segment adjust its position in steps far smaller than the width of a hair,

so the 18 pieces can work together as one precise mirror. Aligning them took months after launch.

Behind the mirror sit four science instruments: a near-infrared camera,

a near-infrared spectrograph that can study up to 100 objects at once,

a mid-infrared instrument built by European and American teams,

and a Canadian-built instrument paired with the sensor that keeps the telescope locked on its targets.

The observatory is named after James E. Webb, who led NASA through most of the 1960s as the agency prepared for the Apollo Moon landings.

The Sunshield and the Big Chill

Because warm objects glow in infrared, a warm telescope would be blinded by its own heat.

Webb's solution is a five-layer sunshield about the size of a tennis court.

Each layer is a sheet of coated plastic film thinner than a human hair, separated by gaps that let heat radiate away into space.

The sunshield keeps the Sun, Earth and Moon always on one side.

On the shaded side, the mirror and most instruments cool passively to below about minus 223 degrees Celsius (minus 370 degrees Fahrenheit).

One instrument, which observes longer mid-infrared wavelengths, needs to be colder still,

so a special cooler takes it down to just a few degrees above absolute zero.

A Million Miles From Home

Webb's destination was the second Lagrange point,

a gravitational balance point about 1.5 million kilometers (roughly a million miles) from Earth,

on the side facing away from the Sun.

From there, the telescope orbits the Sun in step with Earth, keeping its sunshield pointed toward all three bright, warm bodies at once.

Getting there required one of the most complicated unfolding sequences in spaceflight history.

Over about two weeks after launch, the telescope deployed its sunshield, tensioned each layer,

swung out its secondary mirror and unfolded the side wings of its main mirror.

Hundreds of mechanisms had to work perfectly, and they did.

Webb reached its orbit about a month after launch and spent the next several months cooling down, aligning its mirrors and testing its instruments.

The launch was so accurate that the telescope was left with more maneuvering propellant than planned,

a welcome bonus for a spacecraft that must fire small thrusters periodically to hold its position.

Unlike Hubble, Webb is far beyond the reach of astronauts, so it was designed with no expectation of repair visits.

Webb and Hubble Compared

Webb is often called Hubble's successor, but the two telescopes were built for different jobs:

What Webb Revealed in Its First Years

Webb's first full-color images were released in July 2022, and they lived up to the anticipation. Some early highlights include:

• A deep field in about half a day: Its first deep image, built from roughly 12 hours of exposures, showed thousands of galaxies behind a massive galaxy cluster, whose gravity magnified even more distant galaxies behind it.
• Glimpses of the early universe: Webb found galaxies seen as they were only a few hundred million years after the Big Bang, and more bright galaxies at those early times than many models had predicted.
• Chemistry of alien worlds: In 2022, Webb made the first clear detection of carbon dioxide in the atmosphere of a planet orbiting another star, the hot gas giant WASP-39b.
• Stellar nurseries in new detail: Its views of the Carina Nebula and the Pillars of Creation revealed young stars and jets hidden from visible-light telescopes.
• Fresh looks at familiar planets: Webb captured Neptune's faint rings more clearly than they had been seen in decades, along with auroras and haze on Jupiter.

Frequently Asked Questions

Is Webb a replacement for Hubble?

Not exactly. Hubble sees mainly ultraviolet and visible light, while Webb specializes in infrared. Astronomers have used them together,

with each revealing things the other cannot, much like viewing the same scene with two different kinds of cameras.

Why are Webb's pictures in color if it sees infrared?

Infrared light is invisible to human eyes, so image specialists translate it into visible colors.

Shorter infrared wavelengths are usually shown as blue and longer ones as red, preserving the order of the real light.

The colors are not what your eyes would see, but they carry real scientific information.

Can Webb see planets around other stars?

In some cases, yes.

Using special masks that block a star's glare, Webb can photograph large, young planets far from their stars.

More often, it studies planets indirectly, analyzing starlight that filters through a planet's atmosphere as it passes in front of its star.

Seeing the Unseen

The James Webb Space Telescope combines a folding gold mirror,

a sunshield that keeps its optics hundreds of degrees below freezing and a remote orbit a million miles from Earth.

Together, they let it see light that has traveled for billions of years and the faint chemical fingerprints of distant worlds.

Its first years of images and discoveries have already reshaped questions about how the first galaxies formed.