When the James Webb Space Telescope launches on Dec. 25, it will begin a multiyear mission to help humankind catch a glimpse of the origins of the universe.

The telescope, equipped with the most sophisticated array of stargazing equipment ever assembled, will be able to peer into the farthest reaches of space, some 13.8 billion light-years away. The data it collects will allow earthbound scientists to better understand the formation of stars and galaxies just after the big bang.

“When the lights turned on in the universe, that’s what Webb is trying to see,” said Paul Geithner, a NASA project manager who worked on the telescope.

Webb is designed to capture more distant light than any of its predecessors, aiming to examine the first stars in the universe.

Telescopes’ range of sight

Older objects

Modern universe

First galaxies formed

First stars

Particles formed

Big bang

13.8 billion years

after big bang

1 billion years

100 million

years

100 seconds

Hubble (1990-present)

Spitzer (2003-2020)

Herschel (2009-2013)

Webb

Telescopes’ range of sight

Older

objects

Modern

universe

Spitzer

(2003

-2020)

13.8 billion

years after

big bang

Herschel

(2009

-2013)

First

galaxies

formed

1 billion

years

Hubble

(1990

-present)

First

stars

Webb

100 million

years

Particles

formed

100 seconds

Big

bang

But the region of space scientists are trying to observe is so far away that only faint traces of light and heat are detectable from Earth.

To overcome this, the $10-billion telescope needs to be able to see through various interstellar objects that would otherwise obstruct its view. Here’s how it works.

Seeing through space and time

The key to Webb’s deep-space vision is its ability to sense infrared light.

As the universe expands, the light waves from the earliest stars and galaxies stretch out. By the time those waves reach us, they’re too long to be seen as visible light. Instead, they appear as infrared — a form of invisible energy we can detect only as heat.

Infrared waves readily pass through clouds made of gas and dust. By taking in infrared light, the telescope can essentially see through objects that would otherwise block its view, much the way X-rays are used to create images of the structures inside a human body.

Past telescopes have been able to capture infrared light, but only in a more limited range of wavelengths. Webb will fill a large gap by detecting light waves from the earliest stars and galaxies in the universe.

Webb will see what other telescopes could not

Longer wavelength

UV

Visible

Infrared

Microwave

Hubble

Webb

Spitzer

Herschel

Gap in infrared

coverage

Webb will see what other

telescopes could not

UV

Hubble

Longer

wavelength

Webb

Visible

Gap in

infrared

coverage

Spitzer

Infrared

Herschel

Microwave

Orbiting a million miles away

The heat of the sun can easily interfere with the Webb’s sensitive infrared detectors. That’s why it’s equipped with a shield to reflect the heat.

The Webb must also be positioned at a “Goldilocks” location for telescopes, almost 1 million miles from Earth — a spot that, gravitationally speaking, is not too far and not too close.

L2 is one of five points where the gravitational pull of the sun and Earth is balanced enough to allow a small object — such as a satellite — to remain in a fixed orbit.

“It’s easy to get there and easy to orbit there,” Geithner said. L2 “orbits along with the Earth, so ... we’ll have access to any point in the sky.”

A super-size mirror

Thanks to Webb, scientists will be able to produce images at a higher resolution than was possible with any other infrared telescope before it. The key is Webb’s large mirror, whose 18 hexagonal pieces will help it absorb as much infrared light as possible.

Because infrared light has a wavelength 10 times as wide as visible light, the Webb needed a significantly larger mirror than the Hubble. The result was a gold-plated mirror that is 2.7 times larger than its predecessor.

Telescope mirrors by diameter

4.3 ft

Spitzer

3 ft

Hubble

7.9 ft

Herschel

11.5 ft

Webb

21.3 ft

Telescope mirrors by diameter

Spitzer

3 ft

Hubble

7.9 ft

Herschel

11.5 ft

4.3 ft

Webb

21.3 ft

Though older telescopes such as Herschel and Spitzer have used infrared sensors, they weren’t able to deliver the same quality of images with their smaller mirrors.

Folding it up for launch

The Webb’s massive mirror and delicate sun shield means its six-month journey to L2 will be a complex operation.

Once launched, making any physical repairs to the telescope will be impossible. So each step in its deployment must play out perfectly for the mission to succeed.

What happens next?

The observatory is expected to begin sending science data back to Earth about six months after deployment.

The Webb’s initial mission to explore the moments after the big bang will last five to 10 years, though if all goes well it could be extended.

Geithner expects the telescope’s contribution to science — and our curiosity — to be profound.

“Some of [Webb’s] greatest discoveries are likely to be answers to questions that no one has yet asked,” he said.