The Webb Space Telescope, which cost tens of billions of dollars and took 25 years to build, has finally been launched! How is it different from the Hubble?

The Webb Space Telescope is finally launched! It cost tens of billions of dollars and was planned to be launched into space 14 years ago. Why did it take so long? How is it different from the Hubble?

At 20:20 Beijing time on December 25, 2021, after several years of delay, the "successor" of the Hubble Space Telescope, the James Webb Space Telescope, was launched from the Kourou Space Center in French Guiana.

It is reported that the Webb Space Telescope cost about 10 billion US dollars, is six times larger than the Hubble Telescope, and is expected to observe the universe 13.5 billion years ago.

▲Diagrams of the instruments in the Webb telescope's integrated scientific instrument module and related displays of the launch and operation process (Photo source: Beijing Science and Technology News)

Twenty-five years of polishing a mirror

▲Five Lagrange points (green points) of the Earth (blue point)-Sun (yellow point) two-body system

Since 1996, NASA has been looking for a plan to build this new space telescope for 25 years. As one of the most complex projects in NASA's history, the Webb telescope has huge risks. The "Webb Space Telescope" will be placed at the second Lagrange point of the sun-earth, 1.5 million kilometers away from the earth (the average distance between the earth and the moon is 380,000 kilometers). Because it is too far away from the earth to send astronauts for maintenance, its design and manufacturing must be perfect, otherwise it will fail! Therefore, if any unknown problems are found during the test, the launch will be postponed.

The “trinity” of optical components

The optical system component of the Webb telescope is the most important payload. It has the characteristics of large equivalent aperture, high surface reflectivity, excellent extremely cold environment and good focusing accuracy, showing innovation in technology and engineering. The optical system component is a "three-mirror system", including the primary mirror (first level), the secondary mirror (second level) and the third mirror - the optical subsystem. The complete optical system component is like a radio telescope.

▲Working principle of optical components

The primary mirror is the largest and heaviest, requiring a large support structure. It is difficult to grind it into a single mirror surface, and it cannot be installed in the existing launch vehicle fairing. Therefore, the primary mirror surface is composed of 18 independent hexagonal lenses. The hexagonal lenses are easy to seamlessly splice together to form a nearly circular primary mirror surface. The equivalent diameter of each lens is 1.32 meters. After the splicing is completed, the total polished area of ​​the primary mirror surface is 26.3 square meters. Excluding the obstruction of the secondary mirror and the supporting pillar, the effective collection area is 25.4 square meters, which is far larger than the primary mirror of the Hubble Telescope.

In order to make the lenses of the primary mirror focus accurately, in addition to maintaining extremely high precision during processing, 6 micro-drive motors are installed on the back of each lens, and there is another motor in the center of the primary mirror to adjust the curvature. In order to align the lenses of the primary mirror to form a single large mirror, each lens is aligned to one ten-thousandth of the thickness of a hair. Together with the adjustment actions of other instruments, the entire telescope is equipped with 132 micro-motors.

▲The back structure of the lens

After many processes, the beryllium lens finally weighs 20 kilograms, which is only one-tenth of the mass per unit area of ​​the Hubble telescope's primary mirror. Including motors and other components, the total mass of the lens is only 40 kilograms, and the total mass of the primary mirror is about 720 kilograms.

The third-stage mirror is a finely adjustable rear-view mirror located in the black nose cone protruding from the center of the primary mirror, also known as the rear optical subsystem. The light captured by the primary mirror is reflected and focused on the secondary mirror, which reflects the light to the third-stage rear mirror, then to the finely adjustable rear mirror, and finally focuses on the scientific instruments arranged behind the primary mirror. This part is equipped with starlight analysis equipment or cameras, which makes the telescope's field of view very wide.

As the successor to the Hubble telescope, it will provide improved infrared resolution and sensitivity based on the Hubble telescope, with a wider infrared spectrum and 100 times the observation capability of the Hubble telescope, which is a considerable improvement for astronomical observations.

More powerful and more "cool"

▲Left: "Webb" telescope Right: "Hubble" telescope (Source: Baidu Encyclopedia)

Compared with the Hubble Space Telescope, the James Webb is almost completely different from the Hubble Telescope in appearance. The Webb Telescope does not have a tube. The tube of the telescope is mainly used to prevent stray light from affecting imaging and observation. The larger the telescope, the larger and heavier the tube required. The Webb Telescope has eliminated the tube, greatly reducing the weight of the launch, but in order to block the influence of stray light, it is equipped with a huge sunshade.

▲The mirror of the Webb telescope. (Image source: Internet)

▲A five-layer sunshade is being deployed in the Northrop Grumman Group's factory. It is worth mentioning that in March 2018, a layer of the sunshade broke during testing, which directly delayed the progress by at least six months. (Photo source: Internet)

The use of sunshades enables the Webb telescope to drop to extremely low temperatures. The Hubble telescope usually maintains a working temperature of 15 degrees Celsius and observes only visible light and near-ultraviolet light. However, the Webb telescope is designed to reduce its own temperature to below -223 degrees Celsius, because once its own temperature exceeds -223 degrees Celsius, its own infrared radiation will cover up the faint light quanta from the depths of the universe. In order to observe near-infrared light, the Webb telescope is equipped with five layers of sunshades, each as large as a tennis court. The first layer of the sunshade faces directly towards the sun and is only 0.05 mm thick. The other four layers are 0.025 mm, and the fifth layer is mainly used to prevent defects, micrometeor impacts, etc.

In terms of area and shape, the first layer is the largest and relatively flat; the fifth layer is the smallest and most curved. The gaps between the layers provide additional insulation, with the smallest spacing between the layers at the center and the largest spacing at the edges, which can guide heat from the center to the outside and finally dissipate it into the space. Each layer of the sunshade is coated with about 100 nanometers of aluminum, and the highly reflective aluminum surface can reflect the remaining energy from the gaps at the edges of the sunshade layer.

The two hottest layers (the first and second layers) facing the sun also have a silicon coating doped about 50 nanometers thick to reflect heat back into space and improve its optical performance and service life in the space environment. The sunshade can reflect excess light and heat, so low-energy, planets, dust disks and other previously unobserved celestial bodies can be observed. However, with current rocket launch technology, it is still impractical to fully unfold the Webb telescope for launch, so the Webb telescope's sunshade and lenses will be folded during launch and unfolded according to prescribed steps after launch.

“Four major components” ensure smooth operation

The Integrated Science Instrument Module is a whole that provides power, computing resources, cooling capabilities and structural stability for the Webb telescope. It is made of bonded graphite epoxy composite material and attached to the bottom of the Webb telescope structure. It has four scientific instruments and a guidance camera.

▲Integrated scientific instrument module

The near-infrared camera is an infrared imager that covers a spectral range from the edge of visible light (0.6 microns) to the near-infrared (5 microns) band. It can observe the most distant celestial bodies to date, detect the light of the first stars and galaxies, and assist in the alignment of the telescope's field of view.

The near-infrared spectrometer will also perform spectral analysis in the corresponding wavelength range, which can detect the temperature, mass and chemical composition of celestial bodies, and simultaneously capture the spectra of 200 celestial bodies for spectral mapping.

The mid-infrared instrument will be used to measure the mid- and long-infrared band range of 5-27 microns, including a mid-infrared camera and an imaging spectrometer. It can be used to observe low-temperature, distant celestial bodies. Mid-infrared light can penetrate the cold dust that is nurturing stars and reveal the impact of large stars and black holes on the space around them.

The Fine Guidance Sensor, Near Infrared Imager and Seamless Spectrometer, used to stabilize the telescope's line of sight during scientific observations, are two instruments with completely different purposes that are referred to as one assembly or unit only because they are physically installed together.

It can detect the temperature, mass and chemical composition of celestial bodies, analyze the atmospheric composition of exoplanets, and perform high-precision targeting. The near-infrared spectrometer and mid-infrared instrument use starlight-blocking coronagraphs and can also be used to observe faint targets such as exoplanets and circumstellar disks very close to bright stars.

Dark matter may not be "hidden" anymore

(Image source: Baidu Encyclopedia)

Webb may help solve the mystery of dark matter. Dark matter is a mysterious, invisible form of matter that makes up a large portion of the known matter in the universe and has up to six times the mass of visible matter. So far, dark matter has eluded direct detection. Although Webb cannot "see" dark matter directly, scientists think it could find these masses when taking pictures of distant galaxies and determine if there is "missing" or unobservable mass that could be dark matter. Webb is particularly well suited to making these kinds of measurements because its imaging resolution is so high that it can detect very small disturbances. In addition, Webb is designed to see deep into the universe, far beyond our previous range, and thus further back in time. These deep-space observations will play an important role in studying the early universe and galaxies, as well as how dark matter evolved.

Although the launch process was full of twists and turns, with the efforts of many parties, we finally got our official launch. We hope that after the successful launch and operation of the Webb telescope, it will be of great help to human cosmic research and observation. Let us also look forward to Webb's free travel in space!

Compiled by New Media Editor Duan Dawei

(Content compiled from the Institute of Geology and Geophysics, Chinese Academy of Sciences, Science and Technology Daily, Science and Technology Herald, Sina Technology)

Produced by: Science Central Kitchen

Produced by: Beijing Science and Technology News | Beijing Science and Technology Media

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