It is the most expensive and complex astronomical instrument ever built. At a cost of nearly $10 billion, it could see the first light in the universe and reveal the secrets of stellar evolution.
The launch date was originally expected to be between 2007 and 2011, but was delayed to 2014 due to funding and technical problems, and then to 2018, 2019 and the end of 2021. For many years, the legend of it has been spread on the “rivers and lakes”.
It is the James Webb Space Telescope (JWST). The Webb Telescope is an international collaboration between NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA), and is the work of thousands of engineers and hundreds of scientists from more than 300 universities, research institutions, and related companies.
Basic information on the Webb Telescope
Total payload mass: approximately 6,200 kg, including scientific instruments, in-orbit consumables, etc
Primary mirror diameter: about 6.5 meters
Aperture area of primary mirror: 25 square meters
Main mirror material: made of 18 sub-mirrors, all beryllium mirrors
Total mass of primary mirror: 705 kg (mass of single beryllium mirror: 20.1 kg, including support structure: 39.84 kg)
Focal length: 131.4 m
Working band: 0.6 ~ 28.5 micron
Shade size: 21.197 m x 14.162 m
Orbit: Lagrange L2, 1.5 million km from Earth
Operating temperature: less than 50K (about -223℃)
Duration: expected 5 ~ 10 years
Working schematic of webb telescope
Its history began 32 years ago…
From conception to launch, the Webb telescope has taken decades and cost ballooned from hundreds of millions of dollars to nearly $10 billion.
In 1989, the Space Telescope Institute (STScL) held a seminar to discuss a successor to the then-unlaunched Hubble Space Telescope (HST). Participants agreed that the NEXT Generation Space Telescope (NGST), HST’s successor, should be able to observe the mid-infrared region of the spectrum. At this time, NGST was not called the Webb Telescope.
In 1990, the “Decade Survey” report from the National Academies of Sciences, Engineering, and Medicine suggested that NGST should be a refrigerated infrared space telescope with a 6-meter aperture, whereas the STScL symposium had suggested that it should be a 10-meter aperture. In 1995, due to budget problems, the size of the NGST primary mirror was set at 4 meters. In 1996, the calibre of the NGST was increased to 8 meters (the CALIBRe of the HST is 2.4 meters). To fit it into a launch vehicle, NASA proposed a collapsible primary mirror structure and announced that NGST would be launched between 2007 and 2011, with the cost of the project rising from $1 billion to $3.5 billion.
In 2002, NASA chose to cooperate with TRW Based on the investigation. On August 14 of that year, NGST was named the James Webb Space Telescope, in honor of NASA’s second administrator and chief architect of the Apollo program.
The Webb telescope changed not only its name but also its “look” many times over the decades, until the 21st century, when it became anything like its current “look.”
In 2004, researchers began building parts for the Webb telescope. By this time, webb had reached $6.5 billion, taking up NASA’s entire budget and leading to the cancellation or delay of many other NASA projects. The WFIRST telescope, for example, was proposed in 2003 to search for planets that might be more hospitable to humans.
In 1990, the layout of the NGST, discussed by the Spatial Ultraviolet Panel in the Ten-year Survey, was completely different from the present configuration
In 1996, NASA sketched the NGST with a foldable 8-meter primary mirror
The final design of JWST
In 2010, the Webb telescope received mission-critical approval, meaning it will be capable of completing all of the science missions it has set out to do. But in 2011, the Webb project was almost cut in half. Congress wanted to cancel the project because of its cost, but strong support from the scientific community persuaded congress to compromise and set an $8bn funding red line for Webb, while delaying the launch until 2018. It has since been postponed to 2019 and 2021, and has already crossed the $8 billion red line.
In 2012, the primary mirror and support structure were manufactured, at which point the final diameter of the primary mirror was determined to be 6.5 meters. In 2014, we started manufacturing hoods, bunkers, gyroscopes, solar panels, etc. A year later, webb’s 18 mirrors, along with their support rods, were attached to the back panel.
In 2016, all the pieces for the Webb telescope began to be assembled.
Webb telescope launch
Webb will take about 27 days to reach the Sun-Earth Lagrange L2 after launch.
First hour: 26-minute lift off from the French Guiana cosmodrome on an Ariane 5 carrier rocket. When the two-stage engines shut down, the Webb telescope will separate from Ariane 5 and, in a matter of minutes, be powered by a solar array that will soon be airborne and ready for flight.
Day 1: The first orbit correction will be made using its own small rocket engine, along with high-gain antennas for data transmission. Webb has two types of rocket boosters; One, called a secondary Combustion Enhanced thruster (SCAT), uses hydrazine and nitrous oxide as fuel and oxidant for orbit correction; The other booster, called the single-component rocket engine (MRE-1), uses only hydrazine as fuel.
Week 1: The focus is on the second orbit correction, followed by the unfurling of the sunshade, and the unfurling of the rest of the telescope in between.
Month 1: The focus is on entering L2 orbit and setting up the telescope, including expanding and cooling the mirror body and turning on the scientific instruments. The cooling time is controlled by electric heating device to prevent equipment failure caused by sudden cooling, so that the residual water inside can be vaporized and escape, and prevent the mirror from freezing.
Next 5 months: at the end of the first month, L2 will be in its optimum position and 5 months will be used to correct confocal optical devices and calibrate scientific instruments.
At the end of the sixth month, the instrument calibration and adjustment are completed, and the real scientific observation begins.
It takes about 5 months for the JWST to unfold, cool down and finally calibrate
In 2019, all scientific equipment will be integrated.
In early 2021, the telescope passed tests and is ready for launch in late 2021.
At the end of 2021, the Webb Telescope, which was originally scheduled to launch on December 22, was pushed back to December 25 due to bad weather.
A folded wing of the JWST and its appearance in the Ariane 5 launch vehicle
The five Lagrange points of the Sun-Earth system, the JWST operates with its back to the sun
So late, is it “obsolete”
Despite repeated failures, expectations are high for Webb because it remains highly advanced and carries innovative equipment.
First look at its spliced primary mirror. The 6.5-meter primary mirror is designed to be partially folded so that it can be tucked into a rocket. The sub mirror is designed as hexagonal, which not only ensures the filling efficiency but also has high symmetry. Once in orbit, the primary mirror redeploys, engineers use an infrared camera to take pictures of known stars and a computer algorithm to confocal the primary mirror by comparing the images of each of the 18 sub-mirrors.
In order to ensure the reflectivity of weak infrared light, the mirror needs to be coated with a uniform thickness of gold film (0.1 micron), and cooled to a very low temperature, to prevent the mirror body’s own absorption of light and its own infrared radiation drowning out the target light information.
Second, look at the workplace. In the 18th century, Lagrange solved the “three-body problem,” in which three objects rotate around each other while remaining in relative position. This problem finally has five solutions, namely l1-L5 Lagrange points as shown in the left figure. At these five points, the gravitational force provided by the two massive objects is exactly the centripetal force required to sustain the circular motion of the third object. The Webb telescope will operate near L2, requiring only a small amount of propulsion to stay in orbit.
Third, the visor, the secret weapon that keeps the telescope extremely cold, is the visor. It consists of five layers of polyimide thin film material called “Kapton”, each coated with aluminum, and the first and second layers near the sun are also coated with doped silicon, which can remain stable in the range of -269℃ to 400℃. The hood is seamlessly assembled using a hot-spot bonding (TBS) process that prevents the tear from extending to other areas if a tear occurs. The shade cools the telescope so well that the temperature on the sun-facing side of the Webb telescope reaches 110 ° C, and in the fifth layer it reaches a low of -237 ° C! In addition, to balance the optical pressure and maintain webb’s attitude, there are mechanical considerations in the design of the visor.
JWST’s five-layer hood and seamless stitching using hot spot bonding (TBS) process
(a) NIRCam (b) NIRSpec (c) MIRI (D) FGS/NIRISS
NIRSpec microshutter arrays, each about the width of a human hair, open and close using magnetic fields
JWST will see the first galaxies to appear in the universe
The distance of the observation target causes the red shift of the spectral line, and the characteristic spectral line can be used to measure the red shift
Four key scientific instruments
The Webb Telescope is equipped with four key scientific instruments: near-Infrared Camera (NIRCam), Near-infrared Spectrometer (NIRSpec), Mid-Infrared Instrument (MIRI), Precision Guidance Sensor/Near-infrared Imager, and Seamless Spectrometer (FGS/NIRISS).
The Webb telescope’s main imaging device, the NIR camera, is built by the University of Arizona and Lockheed Martin. Its operating band is 0.6 ~ 5 micron. Its array of 10 mercury and Cadmium telluride (HgCdTe) probes will be used to discover stars and galaxies in their formative years, observe the number of stars in galaxies, and observe young stars in Milky Way and Kuiper Belt objects
The NIR spectrometer was provided by ESA and covered a band of 0.6 ~ 5 microns. Infrared spectroscopy can not only “see” physical information such as the temperature of the object, but also analyze the chemical composition of the target. Because of the weak light, Webb telescopes need to “stare” at a target for hundreds of hours to accumulate light energy. Its microshutter array allows it to observe 100 objects simultaneously for increased efficiency, making it the first scientific instrument in space with such extraordinary capabilities.
The mid-infrared instrument, which functions as a camera and spectrometer, is a collaboration between research institutions in Europe and the United States and operates in a band of 5 to 28 microns. Its camera module is extremely sensitive, detects faint light and provides wide field images; The spectrometer module is equipped with an array of three arsenic-doped silicon (Si: As) detectors to provide medium resolution spectral and observation information in a small field.
The NIR imager and the seamless spectrometer are manufactured by CSA and have different operating modes for different wavelength ranges. Precision guidance sensors will be combined with star sensors to ensure webb can point precisely at the target for long periods of time, gathering enough light energy to produce high-quality images.
What would it see in the sky
As the universe expanded, the space between objects was stretched, which caused light from the first stars in the universe 13.6 billion years ago to arrive in the solar system to undergo a strong “redshift,” meaning the wavelength of light became longer relative to the time it came out. As might be expected, the ultraviolet and visible light emitted by the earliest stars in the universe has shifted to near-infrared and mid-infrared light. Infrared telescopes in space can spot them.
If the spectral line red shift wavelength is twice the original wavelength, it is called one time red shift; If the spectral line redshift wavelength is 3 times the original wavelength, it is called double redshift. Webb can observe light at 15 times redshift (ultraviolet light has changed to mid-infrared light), making it powerful enough to restore the universe to where it was when the first stars were born!
In addition, The Webb telescope can help us study the formation of galaxies. At the beginning of the 20th century, the universe was thought of as an island of the Milky Way, just a scattering of faint flecks beyond it. In 1923, Edwin Hubble, an American astronomer, observed a star in Andromeda with regular changes in brightness and darkness and determined that it was a separate galaxy far from the Milky Way. The discovery, like Copernicus’s heliocentric theory, brought a new understanding of the vastness of the universe.
The shapes that galaxies take on are thought to be the result of galaxies interfering with and merging with each other nearly a billion years ago. The Hubble Space Telescope has photographed the fuzzy shapes of some galaxies and mapped out patterns. As its successor, webb will be able to see more clearly a wide variety of galaxies.
Another important type of Webb mission is to study the atmospheres of exoplanets and look for other planets where life might exist. Through spectroscopy, we can infer information about the target’s color, seasonal changes, vegetation, and weather (if any).
The Webb telescope will also use spectroscopy to understand the chemical composition of objects in the solar system by comparing the unique spectral lines of different materials, and to gain a more detailed understanding of our home planet.
The Webb telescope has now successfully entered L2 operating point, and is currently working on sub-mirror focusing and co-phasing. We look forward to all successful work in the future.
The James Webb Space Telescope is a masterpiece in the history of human astronomy. The successor to the Hubble Space Telescope, which has extremely high detection capability in infrared wavelengths, hopes it will tell us about the deeper secrets of the universe: the dust enveloped star systems, the atmospheres of other exoplanets and the first light from the very first galaxies in the universe.