Mostly Everything about James Webb Space Telescope
Its mission, instrumentation and launch.
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Sean
Everything about your next Space Telescope
The James Web Space Telescope (JWST) was purpose built to look for the earliest stars and the first galaxies that bound them. This mission is not something we can achieve from Earth, or even low Earth orbit. To see the oldest stars, the JWST must be stationed a literal million miles away from Earth. This fact presented a challenge: Unlike the Hubble Space Telescope, which closely orbits the Earth, we cannot repair a broken JWST. Everything must be done right the first time.
The technical challenges that persistently delayed - and almost canceled - the JWST have now provided tremendous opportunities as the observatory approaches its operating location.
NASA, ESA and CSA have built and shipped a landmark observatory into space that will close a gaping hole in our understanding of the universe. Almost everything it see will be new to humanity, and many astrophysics models will finally be tested by observation.
In what follows we will review the science that motivates the JWST’s mission, the technology that was purpose built to study it and recount the drama that unfolded as humanity’s latest space observatory when from design to launch.
The Scientific Mission
The universe is expanding.
We are in an expanding pile of cosmic debris of the explosion we’ve come to call the Big Bang, and - inexplicably - that expansion is accelerating.
The expansion of the universe means that the light from the earliest stars and galaxies grows dimmer every day. That light is too dim to see from Earth, and as the universe has expanded, the color of that light has also changed.
Just as the sonic pitch of a firetruck’s siren drops as it zooms past you, the frequency of light emitted by distant objects drops as cosmic expansion drives them ever further away from us. It’s a version of the Doppler effect specific to Special Relativity, and is a direct result of the fact that the speed of light - while fast - is finite. The light waves themselves are being stretched as the universe expands. It’s too subtle an effect to notice around Earth, but it builds up over the vast distances between galaxies.
The visible light emitted from a distant star - once it finally reaches us - has been “redshifted” to a lower frequency of light. For very distant stars, that frequency has dropped below the visible band and into the infrared. The further the star, the bigger the drop in frequency.
Here’s the thing about an expanding universe: the stars that are really, really far away also tend to be really, really old. It takes a long time for the light of distant things to shine upon us, and our telescopes.
For the human astronomer looking to learn more about the ancient history of our universe, the most distant stars serve as the earliest records of our own reality.
These are the stars which formed after the Big Bang cooled enough to allow for the formation of lightest elements. Science suggests they should be out there, but we’ve never seen them.
One of the goals of the James Webb Space Telescope is to observe these ancient stars. The challenge this goal presents is one of noise. Thermal noise. The infrared frequencies of light in which we hope to observe those ancient stars are the same infrared frequencies of light that we normally call heat.
The Telescope
That something as delicate and precise as the JWST was put in a rocket and blasted into space is outrageous. For all its buearuacy, NASA’s latest project was defiantly bold in scope.
But it’s also delicate by design.
The JWST measures light from the visible red to the middle infrared. Infrared light is the same thermal radiation normally associated with heat. To gain signal above the thermal noise of its own components, the operating temperature of the telescope as a whole needs to be 50K, about -369 degrees F. Otherwise, for example, the mirror itself would thermally radiate infrared light in the same frequency.
A telescope cannot be made that cold on Earth.
Even in space, the telescope needs to be shielded from the sun to achieve that level of cold. And it needs to be still. Very still. The servomotors that move individual parts of the telescope can possibly generate enough heat - and therefore infrared light - to swamp the instrumentation.
The Location
The Hubble Space Telescope was placed in low Earth orbit. Any space telescope is a monumental feat in and of itself, but the Hubble could be - and was - repaired in the middle of its mission. The same will not be true for the JWST. It’s far too warm in low Earth orbit.
The JWST will be positioned a million miles out in space. It will be orbiting the sun - together with the earth - at a special point where the forces from the sun and the earth’s gravity cancel each other out.
There are five such special points or “Lagrange points”, and the JWST is headed for L2. This is the point along the line between the sun and the Earth on the opposite side of the Earth from the sun.
As the Earth orbits the sun, so too does the L2 point. Therefore, the telescope will be brought along for the ride. It’s always in the same proximity to us, about five light seconds away.
The downside to the L2 point is that its not completely stable. Like a marble on a horse’s saddle, it can rest flat, but it could easily slide off. Nevertheless, its still the best place to sit. Unlike a horse’s saddle, the L2 point has a very shallow instability because orbiting bodies move very slowly. The “decay” of the orbit is likewise very slow.
To compensate for this instability, the JWST has small thrusters on board capable of boosting it back into orbit around the L2 point at prescribed intervals. These same thrusters will also guide it’s trajectory from the Earth to the L2 point. With 42 gallons of hydrazine (N2H4) fuel - and 21 gallons of N2O4 as an oxidizer - the JWST will have more than enough to withstand its 5 year mission.
Because JWST has thrusters onboard, it will be able to choose its orbits selectively to best aim at the viewing targets. One of the main orbital considerations will be avoiding the shadows of the Earth and the moon, so that the onboard solar panels do not get disrupted.
Other spacecraft-borne experiments have used the L2 point, including the famous WMAP cosmology experiment.
The Mirrors
The JWST uses three mirrors to collect light for its primary instrumentation - that is, its cameras. The main mirror is 21 feet in diameter and made from 18, hexagonal, gold-plated beryllium mirrors.
Why Beryllium? It’s light, strong and metallic, and importantly, these properties hold up well even in the extreme cold.
Why the gold film? It best reflects the red to infrared light spectrum given the telescope’s set up.
Why the hexagons? The mirror had to fold up to fit inside the rocket, which means it couldn’t be a solid piece. Also, it means that they fit together cleanly, and only three axes per mirror where required to fine-tune is position. Any adjustment will increase the temperature of the telescope’s components, so minimizing the number of moving parts - and when they have to move - is critical to the design.
The light from the main mirror is reflected into a smaller collecting mirror which is reflected again by a smaller, third mirror directly into the cameras.
The Cameras
There are four main instruments on the JWST. Following modern tradition, each of which is referred to by a silly - if boring - acronym.
NIRCam: This is the primary observing Camera in the Near InfraRed. The majority of the images we’ll get to see will come from the NIRCam. It was designed to detect light from 0.6 to 5 micrometers in wavelength with 40 MP resolution. NIRCam is equipped with a coronagraph - a device that allows it to selectively block out particularly bright objects in the foreground. It was developed by the University of Arizona.
NIRSpec: This camera is capable of measuring distinct wavelengths of light across the near infrared. Spectroscopy allows astronomers to understand what kinds of elements are present in the stars they’re observing. This is crucial because the earliest stars should have only the lightest elements formed just after the Big Bang. NIRSpec has the capacity to measure one hundred distinct objects simultaneously, using a hundred, micrometer scale camera shutters that can open or close independently. This entire microshutter technology was developed specifically for this JWST. NASA and ESA developed NIRSpec together with a number of contractors across the US and Europe.
MIRI: The Mid InfraRed Instrument is a camera designed to capture light from 5 to 28 micrometers in wavelength. This range sufferers particularly from thermal noise. As a result, MIRI includes its own liquid helium “crycooler”, which reduces the temperature of the instrumentation down to 7K. The same coronagraph used by NIRCam can be used by MIRI. MIRI was developed both by NASA and a group of European countries.
FGS/NIRISS: These are actually two, independent devices packaged together. The Fine Guidance System is a camera that helps point the telescope in the right direction as required for observations. The Near InfraRed Imager and Slitless Spectrometer is, literally that. Aside from the importance for redundancy in a mission like the JWST, NIRISS’ lack of light blocking slits affords better performance in low-resolution / wide field of view situations with a low-density of objects. This scenario is expected for observing the farthest, dimmest stars. Both instruments were developed by the Canadian Space Agency.
A project like JWST has an enormous number of moving parts, and the details behind the scene are manifold. Testing the entire telescope at cryogenic temperatures alone took years to complete. After decades in development, it’s little wonder the entire Astronomical and Astrophysical adjacent community collectively held their breath as the telescope was mounted on a rocket and blasted into space this past Christmas morning.
The Launch
The launch of the JWST was originally planned for November, but the telescope was “dropped”. Essentially, a clamp holding part of the telescope failed, and caused a host of unexpected vibrations. Given the sensitivity of all the components onboard - and no room for error - the drop required quite a bit of testing to verify it was in good shape for launch. Fortunately, all was made well.
The telescope was finally launched on Christmas Day from the Guiana Space Port in South America, aboard an Ariane 5 ESA rocket, known as Ariane flight VA256.
The fuel used by the rocket was precisely tuned to give JWST slightly less velocity than it needed to reach its orbit around L2. This afforded a little wiggle room in the mission. Once in space, the telescope could not turn around. If it overshot its orbit, turning around would render the sun shield useless. Hence, the JWST relied on its own internal thrusters to complete the journey to the L2 point.
Three days after the JWST left the final stage of the rocket, it began deploying each of the five layers of sunshield. Like a tarp the size of a tennis court, each layer hard to be first expanded, and then tightened to specification. The entire process took about a week, all while the telescope moved towards the L2 point.
By January 5th, the shield was deployed and locked. The telescope then began to assemble itself. The secondary mirror and its boom arms unfolded from around the telescope and locked into place. Three days later, the two folded sides of the primary mirror unfolded (see also this test video), culminating in an emotionally grinding two day period while further adjustments to the mirrors position were made and latched in place.
After a scheduled course correction, the individual segments of the primary mirror were moved into their deployment position, sticking out from the telescope by a little over a centimeter. Freed from their launch position, the telescope now awaits the final micro to nanometer adjustments to bring the 18-component mirror into a unified, parabolic shape.
Finally, on January 24th, JWST executed a thruster burn to enter its orbit around the L2 point. Months of final adjustments and calibrations await before we’ll get our view of the telescope’s “first light”.
Closing
Seeing images of the earliest stars and galaxies will certainly be inspiring. Testing our models of the earliest moments of the Universe will be more subtle, but a powerful moment in Scientific History. The light from seemingly countless stars and galaxies - and the as yet unseen ancient ones - reminds us of our context. Of just how small we are, but also special, to be able to see it all.
If the universe continues to expand at its current rate, all galaxies beyond our own will rapidly move away from us in time. If the current models are right, we are in a very special momentum in cosmic time where we can see everything from the Big Bang until now. It is good that we are taking advantage of this timing to look from our vantage point.
But there is so much more to the JWST than just Astronomy.
Space science and technology is one the leading edge of development in the United States and the World. Government funding for fundamental research also provides incentive for technology manufacturing and applications development which simply could not happen at this scale. The wild success of GPS satellites, weather radar, and a private space industry have all come out from NASA’s overcoat, as it were.
It’s difficult to overstate the breadth and depth of the technologies developed in the process of building the JWST. The technology developed for aligning the 18 hexagonal mirrors to nanoscale precision has already found applications in the ophthalmology research and applications to LASIK surgery.
The dual perspective to a speck in a vast universe is the realization that we belong to a vast universe with a vast number of companions. The JWST may seem like just another Scientific program for a few astronomers selected to get “telescope time” over the next few years, but our exposure to its technological wake is vast, diffuse, and constantly expanding all around us.
