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| James Webb Space Telescope |
James Webb Space Telescope is going to be the successor to the Hubble Space Telescope. It aims to have a mirror with the combined surface area of 25m², which is roughly five times bigger than Hubble’s. Developments for it began in 1996, with an original launch date of 2007, but this date has continuously been pushed back. But what’s the hold up? What is taking so long for this telescope to get into operation? Why the JWST is taking so long.
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| James Webb Space Telescope |
This telescope features 18 hexagonal segments made of gold-plated beryllium. They combine up to make a 6.5-meter mirror, the biggest that has ever been in space by a long shot. There is a very good reason for having such a big telescope in space, namely that in the vacuum of space, there is no atmosphere to get in the way of observations made by the telescope. The other big reason for having the James Webb Space Telescope in space is that it is an extremely sensitive infrared telescope, and in this way, it is different from Hubble – which is only capable of looking in visible light and ultraviolet. In fact, the James Webb telescope is more like the Spitzer telescope, another space telescope but with a much smaller mirror – only 85cm across. Seeing as any warm object emits infrared radiation, a ground-based telescope would easily have its readings contaminated by nearby objects and the atmosphere. In the vacuum of space however, the JWST is protected from the Sun by this massive sunshield, which means the scientific instruments stay a cool -220°c. Such a big infrared telescope will mean we can look back in time billions of years to just a few hundred million years after the big bang. This will give us an insight into the formation of the universe like never before. The James Webb telescope will also look at individual stars and even attempt to observe exoplanets, specifically to try and see the composition of their atmospheres. They do this by looking at the light spectrum of the planet as its parent star shines through the planet’s atmosphere.
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| James Webb Space Telescope |
What’s been the hold up over all these years? Well, the biggest delays were caused by the design specifications themselves. For instance, the mirrors. There is no launch craft that could fit a 6.5-metre-wide mirror inside, so the mirrors had to be designed in a way that allowed them to be folded back during launch. This adds a massive amount of complexity to the design, as 18 hexagonal mirrors aimed at an object billions of light years away means that they must be aligned correctly to nanometre precision. As a result, not only do the mirrors fold out once launched, but each mirror can be controlled individually to a very fine degree.
The other design challenge with the mirror would have been the weight of it. To use a mirror similar in weight to the one on Hubble would have meant the James Webb’s mirror would be 10 times heavier than it is now, too heavy for a launch craft to get it to its final destination. So, engineers used a ground-breaking design, a beryllium mirror that is light but also strong, and plated with gold for the reflective surface. Incredibly, with this design, each mirror segment only weighs 20kg. But Beryllium is a very difficult metal to polish, and designers needed this mirror to be smooth to within nanometres. This adds a layer of difficulty to the building process. Beryllium also isn’t ideal for reflecting infrared light, but gold is. So the mirror is inserted into a vacuum chamber, and some gold is vaporised into the chamber. The gold in this vapour form fills the chamber and condenses on all the surfaces, including the mirror itself. This gold condensation gives an extremely even finish, something that couldn’t be accomplished through any other method. There is less than 3 grams of gold in total.
One of the other key design specifications of the James Webb Space Telescope was to be able to view hundreds of objects simultaneously. The way they will achieve this is through some innovations invented specifically for James Webb, but this technology will go on to benefit many other sectors like biotechnology, medicine and communication. Specifically, it is an array of micro-shutters that can measure the intensity and spectra of light from many distant individual objects at the same time. While spectroscopic technology isn’t new, the ability to see up to 100 objects at the same time is. This is an example of the data it will collect; each band is an individual shutter’s spectroscopy reading. Each shutter is also amazing in that it is only the width of a few human hairs. More bespoke devices that had to be designed specifically for this telescope were the infrared camera sensors. These are state-of-the-art, the biggest and most sensitive infrared detectors to ever be made. There will be three different detectors, each for different wavelengths in the infrared. They are very advanced in that they don’t just take one sample per pixel, but several, meaning they can reduce noise and sense if a cosmic ray hit the pixel and cancel it out.
Another design issue they had to deal with was excess heat. As I mentioned, infrared telescopes are extremely sensitive to heat, even heat generated by the telescope itself. There is a radiator designed into this side to enable the telescope to radiate any heat it generates itself, as the instruments need to be cold, -220°c cold. One of the instruments onboard JWST, MIRI, requires even colder temperatures, it can only operate at 7° kelvin, or -266°c. This means it needs its own cryocooler, which is basically a pipe filled with cold helium that flows by the instrument, from a pump at the bottom of the spacecraft. Pumps are an issue though because they vibrate, so a super low vibration pump had to be developed. The biggest heat source in our solar system though is the Sun, and to counteract this, engineers designed the sun shield membrane. There are five layers in all, each thinner than the width of a human hair to keep the mirrors cool and protected from solar rays. This membrane means that while the side facing the Sun can almost reach 100c, the instruments on the other side remain at around -220°c.
Again, due to launch limitations, the membrane will start out folded away, and when it reaches space it will begin to pull the membrane delicately out over the course of several days until it is fully stretched out. The membrane is in fact one of the reasons for the most recent delay to the telescope. During the testing of this deployment process, one of the membranes tore, meaning they had to replace it and look into the design to make sure this doesn’t happen in the actual launch. Because this is the big thing with the James Webb Space Telescope, if something goes wrong, there is no way to fix it once its in space. So, they have to ensure they do everything within their power to get it right the first time. And with such a complicated design, there is so much that could go wrong. Just look at this launch process for it to get to its final orbital location, which by the way is the L2 Lagrange point behind Earth and beyond the orbit of the moon. The James Webb space telescope is actually built now, everything is completed, they are just thoroughly testing each and every one of their systems to make sure everything goes smoothly come the launch. Because if this mission is a success, just this one telescope will unravel so many of the mysteries of the universe by itself. Hubble was already a wonder, but this will be a serious step up.