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.
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