For the past 31 years, the Hubble Space Telescope has been an invaluably versatile observation platform for astronomers, but lately it has begun to show its age. The telescope was last serviced in 2009 and had to go into “safe mode” for partial shutdowns several times over the past few years — most recently in October. And while optimistic estimates suggest the Hubble could remain in operation until the end of the decade, NASA, with its ESA and CSA partners, have spent more than a dozen years developing a successor, the James Webb Space. Telescope (JWST). When the Webb launches — currently poised for launch on Christmas Day — it will become humanity’s main eye in the sky for decades to come.
The 7.2-ton JWST will be the largest telescope NASA has ever put into orbit. The 6.5-meter primary mirror array — made up of 18 gold-plated hexagonal segments — is more than twice the size of Hubble’s and nearly 60 times larger in area than the Spitzer Telescope, which retired in 2020. The sunshade it uses to protect its delicate infrared sensors is almost as long as a tennis court, and the telescope device as a whole is three stories high. The 458 gigabits of data collected daily will be routed first through NASA’s Deep Space Network and then sent to the Space Telescope Science Institute in Baltimore, Maryland, which will collect and disseminate that information to the wider astronomy community.
When it reaches its orbital home at the L2 Lagrange point 930,000 miles from Earth, the JWST begins its four-point mission: searching for light from the earliest post-Big Bang stars; studying the formation and development of galaxies, investigating the evolution of stars and planetary systems; and in search of the origin of life.
To do this, the Webb will take a different approach than the Hubble before it. While the Hubble looked at the universe in the visible and ultraviolet spectrum, the JWST will see in infrared, just like the Spitzer did, but with much greater resolution and clarity. Using this infrared is critical to the Webb’s mission, as that wavelength can peer through clouds of interstellar gases and dust to see further obscured objects.
The Webb’s camera suite consists of four separate components: the Mid-Infrared Instrument (MIRI), Near-Infrared Camera (NIRCam), Near-Infrared Spectrograph (NIRSpec), and the Near-Infrared Imager and Slitless Spectrograph/Fine Guidance Sensor (NIRISS). /FGS). These instruments are actually so sensitive that they can detect their own heat radiation when they are in operation. To minimize these infrared emissions, three of the sensors are cooled to negative 388 degrees Fahrenheit (-233 degrees C). The highly sensitive MIRI is cooled even further to -448 degrees F (-266 degrees C) — that’s just 7 degrees Kelvin above absolute zero.
It’s not easy to get the MIRI so cold. After the JWST enters orbit, the telescope will take weeks to slowly cool the sensor to its optimum operating temperature using a helium-based cooling system.
“It’s relatively easy to cool something on Earth to that temperature, usually for scientific or industrial applications,” said JPL cryocooler specialist Konstantin Penanen in a recent NASA blog post. “But those Earth-based systems are very bulky and energy inefficient. For a space observatory, we needed a cooler that is physically compact, very energy efficient and very reliable because we can’t repair it. So those are the challenges we faced, and in that respect I would say that the MIRI cryocooler is definitely leading the way.”
The extra effort MIRI requires will be well worth it, as ground-based infrared telescopes — especially those that operate within the mid-infrared spectrum like MIRI are — are largely hampered by the heat emissions from the devices themselves and the surrounding atmosphere.
“With the other three instruments, Webb detects wavelengths of up to 5 microns. Adding wavelengths down to 28.5 microns with MIRI really expands the scope of science,” George Rieke, an astronomy professor at the University of Arizona, said in a NASA blog earlier this month. “This includes everything from studying protostars. and their surrounding protoplanetary disks, the energy balance of exoplanets, mass loss of evolved stars, circumnuclear tori around the central black holes in active galactic nuclei, and much more.”
Given the very specific low-temperature needs of the JWST, keeping the telescope’s sensor suite out of direct sunlight (and shielded from other light sources such as the Moon and Earth) is critical. To keep those cameras in the shade all the time, NASA engineers created a five-layer sunshade made of aluminum-coated Kapton film to keep them out in the cold, cold dark.
“The shape and design also directs heat to the sides, around the perimeter, between layers,” said James Cooper, JWST’s Sunshield Manager at Goddard Space Flight Center. “The heat generated by the spacecraft bus in the ‘core’ or center is pushed out between the membrane layers so it can’t heat up the optics.”
The kite-shaped sunshade measures 69.5 feet by 46.5 feet by 0.001 inches and is stacked five layers high so that the energy absorbed by the top layer radiates into the space in between, making each successive layer slightly cooler than the one above. . In fact, the temperature difference in the outer (383K or 230 degrees F) and inner layers (36K, about -394 degrees F) is about an order of magnitude.
To gather enough light to see the faintest, most distant stars possible — some as far as 13 billion light-years away — the JWST will rely on its massive 6.5 m primary mirror array. Unlike the Hubble, which used a single 8-foot-wide mirror, the Webb’s mirror is divided into 18 separate segments, each weighing just 46 pounds thanks to their beryllium construction. They are coated with gold to improve their reflection of infrared light and are hexagonal in shape so that when fully mounted in orbit they fit together just enough to act as a single, symmetrical, reflective plane without holes. Their small size also makes them easy to split and fold down to fit within the cramped space of the Ariane 5 rocket that will launch them into orbit.
The role of coordinating these segments to focus on a single spot in a distant galaxy lies with the actuator assembly of the mirrors. Seven small motors sit at the back of each mirror segment (one at each corner and a seventh in the center), allowing precise control over their orientation and curvature. “Aligning the segments of the primary mirror as if they were one large mirror aligns each mirror 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, costing $10 billion and the efforts of more than 10,000 people, the Webb Telescope is finally ready for launch — and hopefully it will take it for real this time. The program has been delayed, after delay, after delay in the launch schedule. NASA abandoned the March 2021 initial date in the wake of the initial COVID-19 outbreak and associated shutdowns — although, to be fair, by January 2020 the GAO had only given the JWST a 12 percent chance of getting off the ground by the end of this year — and set a vague timetable for launch “sometime in 2021.”
NASA later revised that estimate to a firm “sometime in October 2021,” eventually settling for a Halloween launch window, only to postpone it again to late November/early December. Of course, early December quickly turned into late December, specifically the 22nd, which was then reverted back to the current date of December 24. Actually, make that the 25th.
These delays have been caused by the myriad of factors required to prepare an instrument of this size and sensitivity for launch. Upon completion of construction, the JWST had to undergo an extensive series of tests, then be carefully loaded into a shipping container and transported to its launch site in Kourou, French Guiana. Once there, the actual task of preparing, refueling and loading the JWST onto an Ariane 5 rocket took another 55 days.
That timeline was further extended due to an “incident” on Nov. 9 in which “a sudden, unplanned release of a clamp band — which Webb attaches to the launch vehicle adapter — caused a vibration throughout the observatory,” according to NASA. The Webb’s anomaly rating board kicked off an additional round of testing to make sure those vibrations don’t damage other components or throw anything important out of line.
Now that the telescope has been approved, the final preparations are being made. Barring further setbacks, the JWST will launch at 7:20 am ET on Christmas Day (watch live here!) to begin its 30-day, 1.5 million-mile journey from the Lagrange 2, where it just spent two weeks. slowly will unfold its mirrors and sunshade, then begin to explore the depths of the early universe.
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