At the Dawn of Time… plus a few hundred million years
The James Webb Space Telescope (JWST) is an Infra Red (IR) telescope as tall as a three story house, as long as a tennis court, weighing the same as a school bus (6200kg) and designed, built and tested over 32 years using 40 million hours to built it with people from 14 different countries who are part of 3 space agencies (NASA, ESA, and the Canadian Space Agency, CSA).
You may think it’s ‘just another Hubble’, but, whilst the images produced will be equally as stunning, the wavelengths (or colours) of light observed overlap a bit, and they’re both in space, that’s about all they’ve got in common.
The James Webb Space Telescope partly folded up. Credit: NASA/Chris Gunn
Let’s Start at the Beginning
Thought of in a 1989 conference called “Next Generation Space Telescope Workshop” at the Space Telescope Science Institute (STScI) in the USA (with the JWST formally called the “Next Generation Space Telescope or NGST), scientists formally proposed it in 1996. They said it would be an Infra Red Telescope, using redder light than you can see (right so far) with a mirror larger than 4m in diameter (only 2.5 metres out!) with a budget of $500 million (Ah, just a mere $9.16bn out!).
2002 brought the selection of people to make it a reality, and 2004 marked the start of building work. By 2005, ESA’s spaceport in the French territory of French Guiana in north east South America was picked as the launch site. Being free from cyclones and earthquakes, and near the equator to give the reliable Ariane 5 rocket a boost in speed as it leaves the Earth due to Earth’s centripetal force, this was the perfect launch site.
Indeed, everything was going so well… until they had to redesign it in 2005 to pick only the worthiest of instruments to be on board. This led to the original 2007 launch date being pushed back.
An early concept of the JWST, called the Next Generation Space Telescope or NGST.
A True Multitasker
JWST was given a lot of goals to achieve in its mission, selected over the many years of building, as science has progressed and many new questions have come to light.
First off, the early Universe. It will look over 13.5 billion years into the past by looking at distant light and, due to light’s fixed speed, old light. Old enough to reveal the first stars (formed through cooling of molecular hydrogen) and galaxies which formed 300 million years after the Universe’s start in the Big Bang. The light emitted was at such high energy (Ultra Violet or UV, bluer in colour, outside of what we can see) that it’s detectable today as IR radiation, due to redshift (the elongation of light due to spacetime, the fabric of the Universe, is growing in all directions, meaning everything is moving away from everything else). This period of star and galaxy formation is called reionisation, where the first hydrogen atoms clumped to make the first and ‘purest’ stars, called ‘Population III Stars’.
Artist's concept of the first population III stars
Credit: NASA/WMAP Science Team
Then, JWST can look over a galaxy’s lifetime to see how they evolve and how chemical elements distribute themselves in galaxies, before seeing what happens when they combine, like what will happen with the Milky Way and the Andromeda Galaxy in 3 billion years time.
In terms of stars, JWST can look at how and where they first formed, helping to find what determines how many stars form in a location and their masses. Death also comes into play when JWST will observe how they ‘die’ (stopping fusing elements together) , how this ‘death’ impacts the surrounding environment. The black holes that stem from massive stars (over 10 times our Sun’s mass) will then be studied to find out, once and for all, what came first: the black hole in the centre of a galaxy or the galaxy itself.
Another tension it may solve is that of the Hubble Constant: a number showing how fast the Universe is expanding. For an unknown reason, using supernovae (the cataclysmic event occurring after a star’s collapse) gives a higher value than that of the Cosmic Microwave Background Radiation (the CMBR: the remnants of the Universe’s earliest light which remains observable).
The Cosmic Microwave Background Radiation (CMBR)
Credit: NASA/WMAP Science Team
Then there’s planetary systems, specifically looking at both exoplanets (planets outside our Solar System) and how they evolve, as well as if they’re habitable; and our Solar System. Notably, the JWST will observe superior planets (planets outside the area of Earth’s orbit) since JWST won’t need to look at the Sun (a source of IR light). For instance, scientists will use JWST’s IR vision to pier beneath the clouds of Jupiter, Uranus and Neptune to see what’s hidden (since IR light can penetrate clouds, also helpful when looking at early stars and planets, shrouded in gas and dust).
As well as this, other mysterious objects will be imaged. Comets (balls of ice and dust) will have their spectra taken, showing what elements they hold from light they emit, and moons such as Jupiter’s Europa and Saturn’s Enceladus, improving our understanding of their potential habitability.
False colour Cassini image of jets in the southern hemisphere of Enceladus. Credit: NASA
If you thought that wasn’t enough, JWST has many more thousands of objectives, plus that of searching for the unexpected. Within 48 hours, JWST can stop a planned observation and whisk round to observe transient events like supernovae to improve our understand of them.
Mister Gold… Mirror
2011 marked the completion of JWST’s array of mirrors. The primary mirror is the one you’ve probably noticed on all images of JWST: 6.6m diameter, with a collecting area of 25.4 square metres, and made of eighteen 20kg hexagonal segments. As the largest mirror to be sent into space, its back plate is made of beryllium (a rare, very light and strong metal at number 4 on the Periodic Table). This large mirror, 6 time the size of Hubble’s, means for a high resolution leading to detailed images, though you should expect strange lines from stars instead of the normal points of light you’ve seen in Hubble’s images, due to the unusual shape of the mirror
A graphite-epoxy composite structure covers the beryllium, before a layer of gold is placed on top. You may have noticed a theme in science… we love gold, but not just because it’s great on rings. The 700 atom thick gold layer reflects IR light better than the normal silver-coloured coating used for space telescopes.
JWST holds 3 other mirrors. The secondary is on the end of 3 arms and is a convex (bulging) circular mirror, only 0.74m in diameter. The tertiary mirror is within the telescope, almost rectangular in shape, at 0.73x0.52m. Then there’s the Fine Steering Mirror (FSM), which is the same shape as the primary, only much smaller! It is also within the telescope, there to stabilise the image with milli-arcsecond precision (where 1 arcsecond is a 60th of a degree).
JWST's mirrors each individually photographed. "SM" stands for "Secondary Mirror".
Please DON’T Shine Bright Like a Diamond!
From 2013-14 the mirror came together at NASA’s Goddard Space Flight Centre north of Washington DC. Then in 2013, work on the sunshield began.
As long as a tennis court (21.2m by 14.2m) and made of five 0.025-0.05m thick membranes, this shield puts JWST in a 24 hour night. It reduces the light from the Sun from 200KW to less than a Watt, leading to a -233ºC telescope and a 110ºC outer shield: equivalent to a sun cream of SPF 1 million! This means there’s no interference of IR light from the Sun, as well from Earth and the Moon (IR light being responsible for most transfers of thermal energy, or heat, around an environment).
Its extremely thin thickness means ripstops are built into all the sunshields to minimise the impact of micrometeorites and debris, so it doesn’t rip.
JWST's Sunshield. Credit: NASA/Chris Gunn
A Telescope with Many Eyes
During 2013-16, all the scientific instruments onboard were subjected to vibration tests so we were sure they wouldn’t break on the way up to their destination in space.
The ‘black box’ on JWST’s back houses the Integrated Science Instrument Module (ISIM) containing all these important instruments which will take light focused by the telescope and analyse it.
The Near-InfraRed Camera (NIRCam) is a NASA instrument designed with a coronographic imager (a camera with something to block a source of light from shrouding out objects around it) and a wide field slitless spectrograph using grisms: a prism and a diffraction grating which spreads light into a spectrum, used so the camera can be used for spectroscopy and imaging. NASA hopes to observe distant transiting exoplanets, along with the first galaxies and their formation. Y dwarfs (cool stars in the Y part of the OBAFGKMLTY stellar classification), which contain methane, carbon dioxide, water and ammonia (organic, life giving compounds) and exoplanets will also be observed for signs of life and understanding what makes somewhere habitable.
NIRCam in 2013. Credit: NASA/Goddard Space Flight Centre
The Near-InfarRed Spectrograph (NIRSpec) is slightly different. A joint ESA NASA venture, it has a spectrograph with over a quarter of a million individually addressable shutters thinner than a human hair, to observe the spectra of around 100 sources simultaneously. This gives it a large 9 square arcminute (0.15 square degree) Field of View (FOV) to see transiting exoplanets and protoplanets (planets which haven’t yet developed), taking their temperature, mass and chemical compositions using their spectra.
NIRSpec. Credit: Astrium GmbH, CC BY-SA 3.0
CSA’s (Canada’s) contribution to JWST is the Near-Infrared Slitless Spectrograph (NIRISS): the only instrument to contain an aperture mask (an opaque circle improving the contrast of bright objects by dimming them), used to investigate molecules present in the atmospheres of exoplanets, and find their temperature, mass and chemical composition, all using the transit method (where an exoplanet goes in front of its parent star, blocking some light) and spectroscopy to investigate them.
NIRISS. Credit: NASA
The Mid Infra-Red Instrument (MIRI) is another ESA and NASA partnership, headed partly by the UK Astronomy Technology Centre in Edinburgh. Its use of a redder part of the IR spectrum means it can look at star formation, since many molecules have fundamental bands in the mid IR spectrum (being bands which scientists can use to say a molecule is present). It can also look the furthest back in time at galaxies with the highest redshifts (along with colder, and so redder, but closer objects). As well as the spectrograph, it has an integrated field unit, with a camera and spectrograph, capturing and mapping spectra across its field of view.
Its Mid-IR imager means it also must be cooled to 33ºC less than everything else (reducing interference from cooler Mid-IR light from the Sun), cooling it to just 7ºC warmer than Absolute Zero: the temperature where molecules stand still. However, this and the technology behind the imager mean it’s 50 times more sensitive than Spitzer: one of the largest IR telescope in space.
MIRI. Credit: NASA
All Systems Go! Now… no! Now!… I mean, erm… now!!
In 2017 vibration and temperature tests were underway, ensuring nothing would stop working due to the vibration from the rocket on lift off and the dramatic changes in temperature from Earth to near Absolute Zero in space.
Then, in 2018, during the assembly and testing phase, the sunshield tore! Yes, the one with all the protections to stop this from happening tor. So, it took another 2 years and another push back of the launch date to get it ready for launch.
All eyes looked to 2020, but Covid-19 happened and it had to be pushed back to make way for missions like NASA’s Perseverance Rover, which had a specific time window for launch (unlike JWST).
2021 came and, in August, the Ariane 5 rocket to take it to space was grounded due to an issue with the payload fairings (what stores the observatory when getting it to space). Then, in late November, a clamp band (something on the telescope, a bit like an elastic band) released, shaking the telescope, leading to a further delay to wait for it to stop shaking.
Then came December, where unfavourable winds led to the date being moved again to Christmas Day, when we currently expect lift off to occur.
On Christmas Day at 12:20pm, I’ve got just the thing to have on in the background during Christmas lunch! Any time after 12:20pm, Ariane 5 will lift off from French Guiana, taking with it JWST. 2 minutes later, its boosters (the towers attached either side) will separate, before the fairing (containing JWST) splits in two a minute later.
Just 9 minutes after launch, JWST will be freed from the main stage and flown for 18 minutes by a spacecraft below, before that too leaves. Since JWST is battery powered, the first thing to deploy will be its solar panel, just 2 minutes later, before the high gain antenna deploys 2 hours after launch to make contact with Earth.
Ariane 5 on the launch pad with JWST in it. Credit: ESA - S. Corvaja
12 hours after launch, JWST will fire up to make its 29 day journey to the Sun-Earth Lagrangian Point 2 Halo Orbit (L2 for short). This is a point where Earth and the Sun are always directly in front, and where the gravitational pull from both equals the centripetal (turning) force from JWST, decreasing the fuel needed to stay there. At 1.5 million km away from Earth, it’s 4 times further away than the Moon.
Whilst it’s on the voyage, JWST will unfurl its sunshield, before opening out its primary and secondary mirrors, like, as NASA’s Keith Parish put it, a transformer, to become a star destroyer (from Star Wars) with a ray gun on top (accurately put!). That said, there are 344 things that can go wrong, 80% of which are in the deployment, so this is arguably the most tense part.
2 to 6 months after launch, JWST will be calibrated and cooled to its chilly -233ºC, making it ready to take its first images.
JWST's Sunshield opening. Credit: NASA/GIPHY
Let the Science Begin!
6 reaction wheels which store angular momentum, 6 gyroscopes and 3 star trackers help position JWST within arcseconds of its target, pointing at 60% of the sky at any one time, pointing at any one point for anywhere from a few minutes to 14 days.
The science for JWST’s first year (known as ‘Cycle 1’) has already been planned, with the General Observers Programme including 2,200 investigators from 41 different countries using 6,000 hours of time.
There’s also the Director’s Discretionary Early Release Science, taking place in the first few months of JWST’s life, with 13 programmes in place to demonstrate Webbs capabilities to the world.
Then, for anyone luck enough to be affiliated with JWST, there are the Guaranteed Time Observations, where people have been allocated time on it for working on creating the telescope.
The End… already?
Hubble has, so far, worked for 32 years, providing amazing images that we’ve come to take for granted. However, JWST works a bit differently.
When any telescope turns, the Sun exerts a pressure on it, making it tumble. To counteract this, Hubble used magnetic bars to connect to Earth’s magnetic field to counteract this (needing no limited fuel supplies). However, being 1.5 million km from Earth instead of just 570km, JWST can’t do this, so it uses propellent instead. And there’s only enough for 10 years of operations, so there’s no way we’re going to get any extra years out of JWST (since it can’t be refuelled). Eventually, JWST will drift out of orbit in L2 and become dysfunctional, since it won’t keep a steady view of objects.
The next 10 years of science from JWST will certainly be exciting, especially for the first image which will be produced in, hopefully, 6 months time. However, even after its time is up, as with many missions from Apollo to Hubble, the scientific papers will keep coming for years to come as more and more people look at data produced from this ground breaking telescope.
Webb being packed up before its launch on Christmas Day (hopefully!!).
Credit: NASA/Chris Gunn
by George Abraham, ADAS member.
#JWST #NASA #ESA #CSA #Sunshield #Telescope #Exoplanet #Saturn #Jupiter #Europa #Enceladus #Comet #CMBR #Redshift #InfraRed
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Cover Image Credit: NASA