For the past 31 years, the Hubble Space Telescope has been an invaluable versatile observation platform for astronomers, but recently it’s showing its age. The telescope was last serviced in 2009 and had to go into partial shutdown “Safety mode“Several times in the last few years – last this October. And while optimistic estimates suggest the Hubble could stay operational by the end of the decade, NASA, with its partners ESA and CSA, has already spent more than a dozen years developing a successor, the James Webb Space Telescope (JWST ). When the Webb launches – currently slated to launch on Christmas Day – it will function as the preeminent eye of mankind in the sky for decades to come.
The 7.2-ton JWST will be the largest telescope NASA has ever put into orbit. Its 6.5-meter main mirror arrangement – consisting of 18 gold-plated hexagonal segments – is more than twice as large as that of the Hubble and almost 60 times larger in area than the Spitzer telescopewho retired in 2020. The sun protection it uses to protect its sensitive infrared sensors is almost as long as a tennis court, and the telescope equipment is three stories high. The 458 gigabit data collected daily is first routed through NASA’s Deep Space Network and then transmitted to the Space Telescope Science Institute in Baltimore, Maryland, which compiles this information and shares it with the wider astronomy community.
When it reaches its orbital home at the L2 Lagrange point 930,000 miles from Earth, the JWST will begin its four-point mission: to find light from the earliest stars after the Big Bang; Study of the formation and evolution of galaxies, investigation of the evolution of stars and planetary systems; and in search of the origins of life.
To do this, the Webb will take a different approach than the Hubble did before. While the Hubble viewed the universe in the visible and ultraviolet spectrum, the JWST will see in the infrared, just as the Spitzer did earlier, but with far greater resolution and clarity. The use of this infrared is critical to Webb’s mission as this wavelength can look through clouds of interstellar gases and dust to see otherwise obscured objects behind them.
NASA / Chris Gunn
Webb’s camera suite consists of four individual components: the Mid-Infrared Instrument (MIRI), the Near-Infrared Camera (NIRCam), the Near-Infrared Spectrograph (NIRSpec) and the Near-Infrared Imager and Slitless Spectrograph / Fine Guidance Sensor (NIRISS / FGS). In fact, these instruments are so sensitive that they can detect their own thermal radiation when in use. To minimize these infrared emissions, three of the sensors are cooled to minus 388 degrees Fahrenheit (-233 degrees C). The particularly sensitive MIRI is cooled even further to -448 degrees F (-266 degrees C) – that’s only 7 degrees Kelvin above absolute zero.
Getting the MIRI this cold is not an easy task. After the JWST has made its way into orbit, the telescope will slowly cool the sensor to its optimal operating temperature for weeks using a helium-based cooling system.
“It is relatively easy to cool something down to this temperature on earth, typically for scientific or industrial applications,” said JPL cryocooler specialist Konstantin Penanen in a recent NASA blog post. “But these earth-based systems are very bulky and energy inefficient. For a space observatory, we need a cooler that is physically compact, very energy efficient and has to be very reliable as we cannot repair it. So these are the challenges we have faced, and in that regard I would say that the MIRI cryocooler is certainly up to date. “
The additional effort that MIRI requires will be worthwhile, as ground-based infrared telescopes – especially those that work in the mid-infrared spectrum such as MIRI, are largely hindered by the heat emissions from the devices themselves and the surrounding atmosphere.
“With the other three instruments, Webb observes wavelengths of up to 5 micrometers. Adding wavelengths to 28.5 microns with MIRI really increases the scientific field, ”George Rieke, Professor of Astronomy at the University of Arizona, said on a NASA blog earlier this month. “This includes everything from studying protostars and their surrounding protoplanetary disks, the energy balance of exoplanets, the mass loss of evolved stars, circumnuclear tori around central black holes in active galactic nuclei, and much more.”
Given the JWST’s highly specific low-temperature requirements, it is critical to protect the telescope’s sensor suite from direct sunlight (and other light sources such as the moon and earth). To make sure these cameras are constantly shadowed, NASA engineers built a five-layer sun protection Made of aluminum-coated Kapton film to keep you in the cold, cold darkness.
“The shape and design also dissipate heat on the sides, around the perimeter, between the layers,” said James Cooper, Sunshield Manager of JWST at Goddard Space Flight Center. “The heat generated by the spacecraft bus in the ‘core’ or in the middle is pushed out between the membrane layers so that it cannot heat up the optics.”
The kite-shaped sunshade measures 69.5 feet x 46.5 feet x 0.001 inches and is stacked five layers high so that the energy absorbed by the top layer is radiated into the space between them, making each subsequent layer slightly cooler than the one above lying. In fact, the temperature difference in the outermost (383 K or 230 degrees F) and innermost layers (36 K, about -394 degrees F) is about an order of magnitude.
To collect enough light to see the faintest, most distant stars – some of them up to 13 billion light years away – the JWST will rely on its massive 6.5-meter primary mirror array. Unlike the Hubble, which uses a single eight-foot-wide mirror, the Webb’s mirror is divided into 18 individual segments, each weighing just 46 pounds thanks to their beryllium construction. They are coated in gold to improve reflection of infrared light and are hexagonal in shape so that when fully assembled in orbit, they fit tightly together to act as a single, symmetrical, gapless reflective plane. Also, thanks to their small size, they can be easily divided and folded to fit within the tight confines of the Ariane 5 rocket that they put into orbit.
The task of coordinating these segments so that they focus on a single point in a distant galaxy falls to the actuator assembly of the mirrors. On the back of each mirror segment, there are seven small motors (one on each corner and a seventh in the middle) that allow precise control of their orientation and curvature. “Aligning the main mirror segments like a single large mirror means that each mirror is aligned to 1 / 10,000th the thickness of a human hair.” said Webb Optical Telescope Element Manager, Lee Feinberg.
After more than 20 years of development and delays that cost $ 10 billion and required the efforts of more than 10,000 people, the Webb Telescope is finally ready to go – and hopefully this time it actually takes. The program has seen delays, after delays, after delays in its launch plan. NASA has given up on the original date March 2021 after the first COVID-19 outbreak and associated lockdowns – although GAO had in January 2020, to be fair only given the JWST a 12 percent chance To give the go-ahead at the end of this year – and to set a vague schedule for the introduction “sometime in 2021”.
NASA later revised this estimate to a company “sometime in October 2021” that finally agreed on a Halloween start window, just to delay it again until the end of November / beginning of December. Of course, the beginning of December quickly became the end of December, to be more precise the 22nd, which was then pushed back again to its current date 24th of December. Actually, do this on 25th.
These delays were caused by the myriad of factors that help get an instrument of this size and sensitivity ready to launch. After completion of the construction, the JWST had to go through extensive tests, then carefully loaded into a shipping container and transported to its launch site in Kourou, French Guiana. Once there is the real job of Preparation, refueling and loading of the JWST onto an Ariane 5 rocket lasted another 55 days.
This timeline was extended further due to an “incident” on November 9th, in which “a sudden, unplanned release of a clip – the Webb attached to the launcher adapter – caused vibration throughout the observatory,” according to NASA. the Webb’s Anomaly Review Board has initiated an additional round of testing to ensure that these vibrations do not damage other components or throw anything important out of alignment.
Now that the telescope was rated A-OK, final preparations are in progress. Other setbacks aside, the JWST will start at 7:20 a.m. ET on Christmas Day (watch it live here!) to begin its 30-day, 1.5-million-kilometer journey out of Lagrange 2, where it will spend two weeks the web slowly unfolding its mirrors and sunshades, and then exploring the depths of the early universe begin.
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