The Webb telescope starts observing

The story

Hubble's successor

Since 1990 a telescope called Hubble has been circling the Earth and has given us beautiful photographs of the sky. Hubble looks mostly at visible light, the light that our eyes can see too. But there is much more to discover in another kind of light that we cannot see: infrared, that is, heat. Infrared passes through clouds of dust and gas, and reaches us from the most distant stars and galaxies: their light, travelling for billions of years through expanding space, has been “stretched” until it became infrared.

To see the first galaxies, born a few hundred million years after the Big Bang, you would need a large infrared telescope in space. In 1996 a group of American astronomers proposed building one. The project was called the Next Generation Space Telescope, later renamed after James Webb, the head of NASA at the time of the Apollo missions. It took twenty-five years of work, delays and very high costs, about ten billion dollars, and the collaboration of three agencies: NASA, the European Space Agency ESA and the Canadian one.

A golden mirror

The Webb telescope has a main mirror 6.5 metres across, about three times that of Hubble. A mirror that wide would not fit in the rocket, so it was divided into eighteen hexagonal segments, made of beryllium, a light and strong metal, and covered with a very thin layer of gold, which reflects infrared extremely well. They were folded like the panels of an origami, to unfold in space. The sunshield too, as big as a tennis court, was made of five very thin layers, like sheets of plastic, which had to unfold.

The telescope has to be extremely cold, at almost 230 degrees below zero, so that its own heat does not disturb the detection of faint infrared. That is why it does not orbit the Earth, like Hubble, but sits about a million and a half kilometres from us, at a special point called L2, where the Earth, the Sun and the Moon always stay on the same side: the shield keeps them all behind it.

The journey and the unfolding

Webb set off on 25 December 2021 from the European base at Kourou, in French Guiana, on an Ariane 5 rocket. In the following thirty days, as it travelled towards its destination, it had to carry out hundreds of delicate operations: open the shield, stretch the layers, unfold the mirrors. The engineers called them the “344 things that could go wrong”: if even one had failed, the mission would have been over. One after another, they all succeeded. The launch was so precise that the telescope has fuel for far longer than the planned twenty years.

The first images

After months of testing and fine-tuning, on 12 July 2022 NASA, with ESA and Canada, showed the world the first scientific colour photographs. The first, unveiled the day before by the American president Joe Biden, was a “deep field”: a tiny patch of sky, as big as a grain of sand held at arm's length, full of thousands of galaxies, some seen as they were more than thirteen billion years ago. Then came the images of the Carina Nebula, coloured like a landscape of mountains, of five nearby galaxies touching one another, the Southern Ring Nebula and, above all, the spectrum of a planet outside the Solar System, in which the telescope found traces of water vapour.

What it is looking for

Webb studies how the first stars and galaxies were born after the Big Bang, how stars and planets form, and what the atmosphere of planets orbiting other stars is like, to understand whether any of them could be home to life. From the first months it has found galaxies older and brighter than expected, and has revealed details never seen before.