For those of you that don’t know, the James
Webb Space Telescope is going to be the successor to the Hubble Space Telescope. An ambitious
project, 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. From the time of writing
this script, the scheduled launch date is in March 2021.
But what’s the hold up? What is taking so long for this telescope to get into operation?
Well, it’s complicated. Literally. I’m Alex McColgan, and you’re watching Astrum,
and together we will understand why the JWST is taking so long.
First of all, let’s have a quick overview to this magnificent bit of engineering. The
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.
If you look at videos taken by ground-based telescopes, you can see that there is a slight
wobble to the image. This is due to the heat in the atmosphere, much like if you looked
at a road on a hot day. That’s not to mention all the dust and other particles in the atmosphere,
reflecting and refracting light which interferes with telescope observations. Technology is
improving to counteract atmospheric influences on ground-based telescopes, but you just can’t
replace actually being in space. 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.
So, 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.
You might wonder then, why don’t they always use beryllium? Well, it is actually 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. You may look at these mirrors and think oh wow,
how much gold is on them? Well actually, not much at all, less than 3 grams in total. How
did they get such a fine layer of gold on these mirrors? Well the technique they used
is pretty ingenious. 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.
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 ground-breaking 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. It’s crazy. Nothing has been attempted on this scale before,
and I don’t know anything that will match it for a while to come. 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. Thanks for watching! Do you enjoy the science
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Budget issues
Very interesting subject matter but very annoying voice-over... I'm torn between wanting to hear more and not wanting to hear him ever again, lol.