For the past 31 years, the Hubble Space Telescope has been an invaluable all-round observation platform for astronomers, but it has recently begun to show its age. The last time it was serviced in 2009, the telescope had to enter the “safe mode” of partial shutdown several times – the last time, this October. And while optimistic estimates suggest that Hubble could remain in operation for the rest of the decade, NASA, with its ESA and CSA partners, has already spent more than a decade developing a successor, the James Webb Space Telescope (JWST). When Webb is launched – which is currently scheduled to take off on Christmas – it will take on the role of the greatest eye of humanity in the sky for decades to come.
The 7.2-ton JWST will be the largest telescope NASA has ever put into orbit. Its primary array of 6.5-meter mirrors – consisting of 18 gilded hexagonal segments – is more than twice the size of Hubble’s and nearly 60 times larger in area than the Spitzer Telescope, which withdrew in 2020. The sun shield he uses to protect his delicate infrared sensors are almost as long as a tennis court, and the telescopic apparatus as a whole is three stories high. The 458 gigabits of data collected daily will first be passed through NASA’s Deep Space Network and then forwarded to the Baltimore Space Telescope Science Institute, Maryland, which will collect and distribute that information to the wider astronomical community.
When it reaches its orbital home at L2 Lagrange at 930,000 miles from Earth, JWST will begin its mission at four points: seeking light from the earliest stars after the Big Bang; studying the formation and development of galaxies, examining the evolution of stars and planetary systems; and seeking the source of life.
To do so, Web will have a different approach than Hubble before him. While Hubble observed the universe in the visible and ultraviolet spectrum, JWST will see in the infrared, just like Spitzer once did, but with far greater resolution and clarity. Using this infrared radiation is crucial to Web’s mission because that wavelength can peek through clouds of interstellar gases and dust to see otherwise obscured objects behind them.
Webb’s package of cameras consists of four individual components: a medium infrared instrument (MIRI), a near-infrared camera (NIRCam), a near-infrared spectrograph (NIRSpec), and a near-infrared imaging device and a slot-free spectrograph / fine guidance sensor (NIRISS / FGS). These instruments are actually so sensitive that they can detect their own thermal radiation while working. To minimize these infrared emissions, the three sensors were cooled to negative 388 degrees Fahrenheit (-233 degrees C). Particularly sensitive MIRI cools even more to -448 degrees F (-266 degrees C) – that’s just 7 degrees Kelvin above absolute zero.
Getting MIRI so cold is not an easy feat. Once JWST enters orbit, the telescope will spend weeks slowly cooling the sensor to its optimum operating temperature using a helium-based cooling system.
“It’s relatively easy to cool something to that temperature on Earth, typical of scientific or industrial applications,” JPL cryocooling expert Konstantin Penanen said in a recent NASA blog post. “But these Earth-based systems are very cumbersome and energy inefficient. For the space observatory, we need a cooler that is physically compact, highly energy efficient, and must be highly reliable because we cannot go out and fix it. So, these are the challenges we have faced, and in that respect, I would say that MIRI cryocooler is definitely at its peak. ”
The extra effort required by MIRI will be worth it because terrestrial infrared telescopes – especially those operating in the mid-infrared spectrum such as MIRI, are greatly hampered by the emission of heat from the devices themselves and the surrounding atmosphere.
“With the other three instruments, the Web observes wavelengths up to 5 microns. “Adding wavelengths to 28.5 microns with MIRI really increases the scope of science,” said George Rike, a professor of astronomy at the University of Arizona, earlier this month on the NASA blog. “This includes everything from studying protostars and their surrounding protoplanetary disks, the energy balance of exoplanets, the loss of mass from evolved stars, the circumnuclear manure around central black holes in active galactic nuclei, and more.”
Given the very specific needs of JWST for low temperature, keeping the telescope sensor package away from direct sunlight (and blocked from other light sources such as the Moon and Earth) is crucial. To ensure these cameras are permanently shaded, NASA engineers have made a five-layer sunscreen made of a film-coated film-coated film to keep them in cool, cool darkness.
“Shape and design also direct heat from the sides, around the perimeter, between layers,” said James Cooper, JWST’s Sunshield manager at the Goddard Space Flight Center. “The heat generated by the spacecraft’s bus in the ‘core’, or in the center, is expelled between the layers of the membrane so that it cannot heat the optics.”
Dimensions 69.5 feet x 46.5 feet x 0.001 inches, the dragon-shaped sun visor is stacked five layers in height so that the energy absorbed by the top layer radiates into the space between them, making each subsequent layer slightly cooler than the one above it. . In fact, the temperature difference at the end (383K, or 230 degrees F) and innermost layer (36K, about -394 degrees F) is about the order of magnitude.
To gather enough light to see the faintest, farthest possible stars – some distant and 13 billion light-years away – JWST will rely on its massive array of 6.5-meter primary mirrors. Unlike Hubble, which used a single 2.4 m wide mirror, Web’s mirror is divided into 18 individual segments, each weighing just 46 pounds thanks to their beryllium construction. They are coated with gold to enhance their reflection of infrared light and are hexagonal in shape so that, when fully assembled in orbit, they will bond tightly enough to act as a single, symmetrical reflective plane with no gaps. Their small size also allows them to be easily disassembled and assembled to fit into the narrow frames of the Ariane 5 rocket that will be launched into orbit.
The role of coordinating these segments to focus on a single point in a distant galaxy belongs to the set of mirror actuators. Seven small engines are located on the back of each mirror segment (one in each corner and the seventh in the middle), allowing precise control of their orientation and curvature. “Aligning the primary mirror segments as if they were one large mirror means that each mirror is aligned to 1 / 10,000 the thickness of a human hair,” said Webb Optical Telescope Element Manager Lee Feinberg.
After more than 20 years of development and delays, which cost $ 10 billion and involve the efforts of more than 10,000 people, the Webb Telescope is finally ready to launch – and hopefully this time will really take time. The program had a delay, after a delay, after a delay in the startup schedule. NASA dropped the start date in March 2021 after the initial COVID-19 epidemic and related blockades – although, to be fair, GAO gave JWST only a 12 percent chance of being launched by the end of this year in January 2020 – and set a vague schedule “sometime in 2021.” for its launch.
NASA later revised that estimate to a firm “sometime in October 2021,” eventually deciding to launch it for Halloween, only to postpone it again until late November / early December. Of course, the beginning of December quickly became the end of December, more precisely the 22nd, which was then returned to the current date of December 24. In fact, let it be 25.
These delays are caused by a myriad of factors that contribute to the preparation of an instrument of this size and sensitivity for launch. Upon completion of construction, JWST had to pass an exhaustive battery of tests and then be lightly loaded into a shipping container and transported to the launch site in Kourou, French Guiana. Once there, the actual task of preparing, refueling and loading the JWST on the Ariane 5 rocket took another 55 days.
That time frame was further extended due to the “incident” on November 9, in which “the sudden, unplanned release of the clamp – which fastens Webb to the launcher adapter – caused vibration in the entire observatory,” according to NASA. Webb’s anomaly review board launched an additional round of testing to ensure that these vibrations do not damage other components or erupt something important from alignment.
Now that the telescope is rated OK, final preparations are underway. Excluding more downtime, JWST will launch a 7:20 ET on Christmas (watch live here!) To begin its 30-day, 1.5 million-mile journey from Lagrange 2 where it will spend two net two weeks slowly developing its mirrors and a sun shield, then start exploring the depths of the early universe.
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