The Most Powerful Telescope in History
The James Webb promises to transform our view of the Universe - if only it works.
The world has been here before. In 1990, after years of delays, millions of dollars of funding and decades of planning, Hubble was finally ready for launch. It promised to give an unprecedented view of the universe, to reveal for the first time details of distant stars and galaxies, to unlock secrets of profound cosmic significance.
It would do all this with the most finely ground mirror ever produced; its surface milled to within nanometers of perfection. Polishing it took years, relying on the latest in computer technology to achieve the necessary accuracy. It was, everyone agreed, a technological marvel.
And yet, soon after reaching orbit, things stated to go wrong. Hubble struggled through its first months in orbit. Problems cropped up with communications links, with control systems, and with temperature fluctuations as the telescope repeatedly crossed between night and day.
All those issues paled in significance, however, compared to the flawed images the telescope was producing. Instead of the crystal sharp visions scientists had expected, they saw only blurred lights, as though the telescope was peering through a thick fog.
The mirror, praised for its precision, turned out to be fractionally too flat, an error that prevented it from focusing properly. It was an embarrassing mistake for NASA, who were suddenly left with a billion dollar telescope that could barely see. Many thought Hubble would never recover; that even if engineers could cobble together a repair, its reputation was irretrievably trashed.
Fortunately NASA did find a fix. Four years after that disastrous first attempt, a team of astronauts visited and repaired Hubble. Ever since the telescope has worked almost without fault, sending back a steady stream of stunning photographs. Far from being trashed, Hubble is today celebrated around the world: recognised as a wonder of the twenty-first century.
And yet, sometime over the next decade – unless it can be repaired once more – it will cease functioning. Onboard control systems will inevitably fail, and without the space shuttle to boost its altitude, the irresistible grasp of gravity will slowly pull it back to Earth.
Its replacement, in a sense, is the James Webb Space Telescope. Though it cannot fully match the power of Hubble – it is focused on a different kind of light – in terms of technological innovation and ability to spy cosmic secrets the new telescope is a more than worthy successor.
Unlike Hubble, however, the James Webb cannot be fixed. Should anything go wrong, should any hidden flaw appear after launch, then NASA and astronomers will face certain embarrassment and disappointment. The James Webb promises to be a one shot opportunity: a chance to reshape our view of the cosmos; or a failure of historic magnitude.
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Partly that’s because the James Webb will fly incredibly far from Earth: hovering roughly one million miles away. A special point lies there, a Lagrange point, a place where the gravity of the Sun and Earth perfectly balance against the orbital acceleration of a spacecraft. The result is a stable gravitational region, around which the James Webb – and other telescopes – can orbit.
It means, however, that a repair is all but impossible. No astronaut has ever travelled so far from Earth, and though we do have spacecraft and rockets that theoretically could, the journey would be a long one.
Worse, the James Webb – unlike Hubble – has not been designed for human visits. There are no holds for astronauts, no service panels. NASA thinks that it might – just about – be possible to one day send a robotic refuelling mission, but the plans for manual repairs are a blank page, literally.
To understand the Lagrange point, first picture a straight line running between the Sun and the Earth; a line that moves with the Earth as it revolves around the Sun. Then extend that line outwards, so that you find yourself a million miles further from the Sun. The special point lies here – and since it must always lie on the line connecting the Earth and Sun, a spacecraft orbiting it will shadow the Earth in its motion around our star.
That makes the Lagrange point a unique and useful place for a space telescope. Its gravitational stability reduces the fuel needed to keep it in orbit, and the constant relative position of the Sun and Earth makes it much easier to manage communications. Importantly for the James Webb, it also simplifies keeping the spacecraft cold.
Two of the hottest and brightest things in the sky, at least from the perspective of a space telescope, are the Sun and Earth. At the Lagrange point both are aligned, both appearing in the same small part of the sky. The James Webb, then, can easily block them; deflecting their heat and light with a shield. That, ultimately, allows mission planners to maintain extremely cold temperatures: just forty degrees above absolute zero.
Astronomers long ago realised the stars come in many different colours. Sirius - the Dog Star - glows a brilliant white, Capella a golden yellow, Betelgeuse a forbidding red. For millennia these colours were seen and noted, but only in the nineteenth century did science understand this colour came from the heat of the star.
Those that burn hottest - reaching temperatures of ten thousand Celsius or more - glow blue and white. Cooler stars, like the Sun, burn yellow and orange. The coldest stars - those close to death or far smaller than our Sun - shine red in the sky.
This colour comes from the frequency of the light emitted by the star. Hotter stars emit more energetic light: physically understood as waves with a higher frequency. To our eyes, those higher frequencies, somewhat counterintuitively, look bluer, while lower frequencies (i.e., colder) look redder. It turns out, however, that not only stars give out light.
Indeed, anything that has a temperature - that’s to say, pretty much everything - emits energy in the form of light rays. The colder the object, the lower the frequency - and below a certain frequency our eyes can no longer see the light. Indeed, the everyday objects of our world, which rarely reach temperatures above a few hundred degrees, emit no light that we can see.
They do, however, shine in infrared - a colour somewhere below red, utterly invisible to our eyes. It is this light that the James Webb telescope has been designed to look for. It will, therefore, view a cosmos rarely seen: revealing the heat of objects that shine only faintly in the visible colours.
That means, effectively, that the James Webb will see a colder universe, filled with objects glowing at far lower temperatures. That will include cold stars - the red and brown dwarves that litter the galaxy - as well as distant planets and clouds of interstellar gas, dispersed across the vastness of space.
It will also be able to see an older universe. The universe is expanding, and as it does it stretches lightwaves. Ancient light waves, emitted from stars billions of years ago, have thus been stretched so much that they no longer appear as visible light. These waves can today only be seen with an infrared telescope.
The choice to focus on infrared alone may seem a little odd - after all, Hubble can see several types of light, stretching from near infrared all the way to the ultraviolet. But by specialising in one type of light, one that has never been observed so intensely, astronomers hope to peer deeper through time and space than ever before.
Historically, this light has been hard to see. One reason is simple: telescopes themselves tend to emit infrared light and thus drown out the very signal they are looking for. Only by making them very cold, like the James Webb, can this problem be avoided.
Even if you can do this, however, another problem soon crops up. The sky itself shines in infrared frequencies, even at night, and therefore obscures the stars and planets that might otherwise be visible. The effect is rather like that of the blue skies we see during the day, when only the brightest objects - the Sun and Moon - can be seen.
A true infrared telescope, then, must be placed in space. And indeed, starting in the 1980s, Europe and America sent a handful of telescopes into orbit. The most powerful - the Spitzer Space Telescope - ran for almost two decades, finally shutting down at the start of 2020. Just one infrared space telescope remains in operation - WISE - though it, too, is running with limited abilities.
The James Webb telescope, however, will put all of those predecessors in the shade. Thanks to an array of advanced technologies and a twenty-foot wide gold-plated mirror, the telescope will be at least a thousand times more powerful than the Spitzer. While it may target many of the same targets, it will give us a far sharper view, revealing details never seen before.
One of the most exciting discoveries of the Spitzer Space Telescope was the Trappist-1 system: a solar system of seven rocky, Earth-like planets orbiting a faint, nearby star. Because it was so faint, astronomers realised that infrared gave us a better way to watch it – and thus directed the Spitzer to spend hundreds of hours observing it.
The results proved not just the presence of seven small planets – but also revealed in detail their size, masses and densities. The James Webb, however, should go further: spying details of the planets’ atmospheres and perhaps even picking out details of clouds swirling around them.
That, then, shows one of the most exciting possibilities of the James Webb. Astronomers believe it will be able to detect planets around cooler – and therefore more habitable – stars. It should be able to look at them in unprecedented detail, and – for some of the closer ones – directly photograph them, giving us, perhaps, a first glimpse of alien worlds beyond our Solar System.
It isn’t just exoplanets, however, that astronomers are planning to find. The James Webb will be commanded to peer across vast distances, looking back billions of years into the past. It should be able to see a mysterious early period known as the Epoch of Reionisation: a time when the first stars ignited and light started to flood the cosmos.
Little is so far known about this time – partly because the light from those first stars has stretched on its long journey across the universe to us. It has stretched so much, indeed, that it no longer appears as visible light, instead transforming into the infrared. No telescope has ever had the power to pick out this faint red glimmer, but the James Webb should - finally - be able to.
That, astronomers hope, will shed light, literally, on the formation of the first stars and galaxies. Just what those early stars looked like has long been mysterious. Some think that hypergiant stars, far bigger than anything possible today, formed – and then, collapsing under their own weight, vanished into massive black holes.
Others wonder how the first galaxies came into being, how the earliest stars began to cluster, and how dark matter played a role in the whole thing. The universe today appears highly structured, with galaxies grouped into clusters, clusters forming superclusters and those - at the very top - forming walls and filaments stretching vast distances.
How did all this come into being? Astronomers have plenty of ideas and models that seemingly tell a plausible story. But is this the whole truth? Or are we missing something, some crucial detail that could rewrite our entire view of cosmic history? The James Webb should - we hope - tell us.
The James Webb is spending its last days on Earth in French Guiana in South America. It is located at the Centre Spatial Guyanais, a spaceport, and on December 24 - or thereabouts - an Ariane 5 rocket will blast skywards. For eight minutes its engines will fire, accelerating to thousands of miles per hour.
Thirty minutes later - now high above the Earth - the rocket will open its nose cone, and release the James Webb Telescope into space. Moments later the solar panels will deploy, and the telescope will begin to charge its batteries.
Operators will then begin a sequence of carefully planned checks, verifying each and every component of the telescope. Antennas will be released, solar panels monitored, propulsion units verified. Two and a half days later the telescope will fly beyond the Moon, two hundred thousand miles distant.
At this point, NASA will begin the most delicate stage of the operation. The sunshields will deploy, cooling the telescope and then - carefully, carefully - the giant mirror will unfold. The whole process will take three days to complete, while astronomers hold their breath, fearful of the slightest snag or rip.
If all goes well, operators will then start bringing instruments online, preparing the telescope for the hard scientific work that will follow. By the end of February, the James Webb will be in its final orbit - and astronomers will almost be ready to take the first photographs, focusing and adjusting the mirrors.
The real operations, however, will start in summer next year, after which scientists can expect a steady stream of data. Some of its first targets have already been announced - and astronomers are fiercely competing to fill the remaining years of operational use.
In roughly a decade, however, the James Webb will begin to run low on fuel. Controllers will find it ever harder to maintain its orbit around the Lagrange point, and gradually it will drift further and further away. After that - unless we somehow find a way to refuel it - the telescope will be lost, a marvel of engineering forever floating far from home.
For more about the James Webb Space Telescope, check out the project website at the Space Telescope Science Institute.
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