In Depth

James Webb Space Telescope: a new window on the cosmos

If all goes to plan, the JWST will be launched this month

If there are no further delays – of which there have been many – the James Webb Space Telescope (JWST) will be launched on 22 December.

The telescope, which has been jointly developed by Nasa, the European Space Agency and the Canadian Space Agency, weighs seven tons and, once fully assembled in space, will be the size of a tennis court.

It was assembled at the Northrop Grumman factory in California, then sent by ship through the Panama Canal to the launch site – a spaceport in Kourou, French Guiana. The Ariane 5, an ultra-reliable European launch vehicle, will take it on a 26-minute journey into space. The JWST will then continue on its own for 29 days before reaching its destined orbit, a million miles from Earth.

What happens then?

Getting into space is just the first hurdle. After that, it will spend several months slowly getting ready before it can start providing scientific data. It will orbit at a point known as L2, one of five “Lagrange points” where the gravitational pulls of the Earth and the Sun balance each other out, so spacecraft can maintain a stable position relative to both, orbiting without using too much fuel.

Once in position, the spacecraft will have to carefully unfold itself from its flatpack travelling form and into its fully functional final shape. Each instrument will have to be tested and calibrated before it can be properly used.

Every movement during these first few months will be watched nervously by engineers and scientists back on Earth. The JWST is expected to send its first data back in mid-2022, and to operate for at least ten years.

James Webb Space Telescope engineers and scientists at work

James Webb Space Telescope engineers and scientists at work on 2 April 2015

Jim Watson/AFP via Getty Images

What will the JWST be able to see?

The telescope has unprecedented capabilities. It will look about five times further and thus – because light takes time to travel – much further back into history than any observatory before. It will be sensitive enough to “see” all the way back to the first generation of stars that burst forth from the maelstrom of hot gas in the universe 13.5 billion years ago, just 300 million years after the Big Bang.

It will be able to watch the births of ancient stars and galaxies usually obscured by vast clouds of dust: most of the light from these new stars is absorbed by the surrounding dust, and they are therefore hidden to normal optical telescopes.

How is this telescope different?

The JWST was conceived in 1989 as the successor to the Hubble Space Telescope which, at the time, was about to be launched. Whereas Hubble looks at the universe mostly using visible light, the JWST will observe infrared light – electromagnetic radiation invisible to the human eye – allowing it to see further.

Its Optical Telescope Element collects light using a primary mirror that will be the largest ever launched into space: 6.5 metres in diameter (compared to the Hubble’s 2.4m), and consisting of 18 gold-plated hexagons made out of the metal beryllium. The telescope has to stay very cold – stray heat signals would interfere with the infrared detectors.

A model of the JWST

A model of the JWST from 2017

QAI Publishing/Universal Images Group via Getty Images

How does it stay cold?

The optical telescope has to sit atop a shield, made from a five-layered sandwich of metal foils, that will protect the main mirror and its sensitive instruments from the heat and radiation of the Sun. The cold side of the shield, where the instruments will be, will be a very chilly –234°C.

However, on the Sun-facing side of this shield, where the spacecraft’s solar array and propulsion systems will sit, temperatures reach up to 110°C. Nasa says that the sunshield provides the equivalent Sun Protection Factor, or SPF, of more than a million.

What are some of the other instruments on board?

The Near Infrared Camera will generate a stream of epic images of stars, galaxies and planets that will no doubt delight astronomy and space nerds (amateur and professional) around the world.

The Mid-Infrared Instrument will be able to analyse the coldest and dustiest parts of the universe – a crucial part of the mission.

The Near-Infrared Spectrograph has 250,000 shutters to split incoming light into its constituent wavelengths – helping scientists understand which chemical elements the light came from.

Why is that important?

The JWST will be able to take images of nearby exoplanets – planets outside our solar system – and examine the contents of their atmospheres for molecules such as oxygen, water or methane.

For the first time, astronomers will also be able to see weather patterns on these distant worlds. It represents a leap in the exciting field of astrobiology: the hunt for life in the universe.

Why has it been so delayed?

The most recent delay involved the “sudden, unplanned release” of a clamp which set it back several days; every day it remains on the ground costs $1m.

But this is only the latest in a very long series of setbacks. It was originally planned, in the 1990s, to launch around 2007 and to cost $500m. But the complexity of the technology – and later additions such as the Mid-Infrared Instrument – mean that its total cost will be around $10bn.

Ultimately, what’s it all for?

By observing young, distant stars and currently dark regions of the universe, the JWST should shed light on some of the most profound cosmological mysteries, such as the role of dark energy and dark matter – which make up, respectively, 68% and 27% of the stuff in the universe, yet are very poorly understood.

“How did we get here? What is the universe? And how did it come into being?” said David Hunter of the Space Telescope Science Institute at Johns Hopkins University. “With something like the JWST, you actually have a tangible way of finding answers.”


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