"We'll make 400 years' worth of discoveries in a decade"

In early October 2021, a cargo ship steered to starboard, leaving the Atlantic Ocean off the east coast of South America and entering the muddy waters of the Korou river. It was the final phase of the voyage and no effort had been spared to protect the prized item on board. It was housed inside a specially designed case to keep it safe from the pitch and roll of the waves. The river had been dredged to ensure the ship didn't get stuck in the shallows. Even the exact date of the voyage had been kept secret, to avoid the attention of pirates.

The precious cargo was the James Webb Space Telescope, perhaps the most hotly anticipated scientific instrument ever. Known as the JWST, the telescope has been more than 25 years on the making and its launch has been delayed countless times. But it has now completed its voyage to the launch site in French Guiana and, if all goes smoothly, will finally leave Earth in late December.

"I still haven't wrapped my head around it", says Torsten Böker, deputy project scientist at the European Space Agency (ESA). "It seams a little bit unreal".

Unreal not only because it has often looked like the telescope might never take off, but also because this device is designed to be a time machine that will help us see back to the enigmatic era of the universe's first stars, which we know precious little about. Unreal too, because it will reveal the atmospheres of potentially habitable planets orbiting stars more clearly than ever before. It is no exaggeration to say that this telescope, with its gigantic gold-plated mirror, will transform our view of the universe and our place in it.

How did we get here?

The JWST's story begins around Christmas 1995. For 10 December days, the Hubble Space Telescope stared at a patch of featureless sky that could be covered by a pinhead held at arm's length. As far as ground based telescopes were concerned, this region of the sky was empty. But some astronomers suspected that a closer look was warranted.

What emerged, now known as the Hubble Deep Field Image, showed that this patch of space is crammed with 3000 galaxies, each about 4 billion times fainter than the human eye can see. Among them were the oldest galaxies we had ever viewed.

Light may be fast, but it still takes a lot of time to reach us when travelling across the universe. Because of this, we know that the further objects are from us, the older the light from them is. But how to tell the age of any given star or galaxy? Fortunately, a quirk of starlight can help. Because the universe has been expanding since the big bang, light travelling long distances gets stretched out as it goes. This changes its wavelength, pushing it from the visible spectrum further into the infrared. This "redshift" was extreme for some of the galaxies Hubble has discovered, showing that they were more than 10 billion years old. The takes us a long way back towards the big bang 13.8 billion years ago.

Astronomers hadn't expected that galaxies this ancient would be detectable, especially not in such numbers. Appetites whetted, they hatched a plan to get a better look at the universe in its flush of youth. In early 1996, a group of stargazers convened to kick-start work on what was then called the Next Generation Space Telescope. That became the JWST, which is now a joint project between NASA, NSA and the Canadian Space Agency.

"The chemistry of exoplanet atmospheres will show up clearly in the light the telescope detects"

How does it work?

The James Webb Space Telescope (JWST) will be the largest, most powerful telescope ever launched into space. After it blasts off, it will travel 1.5 million kilometres from Earth to a spot called Lagrange Point 2 (LP2) [?]. Here, the gravitational pulls of our planet and the sun cancel out and the telescope will orbit this point, remaining in an almost static position with respect to us and the sun.

The telescope is designed to pick up the tiniest specks of infrared radiation, or heat, from ancient stars and galaxies. Orbiting so far from Earth, the sun and moon makes it easier to avoid heat coming from those bodies. Nonetheless, the JWST will need a huge heat shield [?], which will be unfurled on its journey through space [?].

What will it see?

Once the telescope is unfolded, it will begin taking test images and adjusting the alignment of its primary mirrors. After 6 months, it should be ready to start science observations. It will be able to see further back in time than we have ever managed before [?].