Science Facts Forum: JAMES WEBB SPACE TELESCOPE

On 25^th December 2021 Ariane 5 rocket carried a new space telescope that will replace Hubble. After nearly 25 years the idea which was once known as Next Generation Space Telescope (NGST) has finally set off. NGST is the name given for the successor of Hubble space telescope during 1996. Which was later changed to James Webb space telescope. After a 25-year wait the launch of the James Webb Space Telescope (JWST) is imminent, and the headline event of the year for the Astronomy community.

JWST will take us back in time to when the Universe was less than 1 billion years old, enabling us to form deep colour images to measure the shapes, masses and star-formation rates of the very first galaxies, and split their light into spectra to measure their chemical composition. It will allow us to peer through the dust of nearby stellar nurseries to see fledgling stars and embryonic planets in formation, and to search for the signatures of life on other worlds.

JWST is a remarkable feat of engineering, one of the most complex instruments ever built, and will demonstrate our capacity to operate new Space technologies at more than 1.5 million km from the Earth.

Australia will play a critical role in tracking the Ariane 5 launch vehicle as it enters space, and later the routine downloading of the science data eight hours a day from NASA’s deep-space tracking station at Tidbinbilla, ACT. “The facility, built by NASA, ESA (European Space Agency) and the Canadian Space Agency, is open to use by all astronomers, regardless of nationality, and is destined to transform our understanding of the Universe over the coming decade and beyond”, says Research Professor Simon Driver.

The James Webb Space Telescope (JWST) is a space telescope designed primarily to conduct infrared astronomy. The most powerful telescope ever launched into space, its greatly improved infrared resolution and sensitivity will allow it to view objects too old, distant, and faint for the Hubble Space Telescope. This is expected to enable a broad range of investigations across the fields of astronomy and cosmology, such as observations of first stars and the formation of the first galaxies, and detailed atmospheric characterization of potentially habitable exoplanets. JWST was launched December 25, 2021 on an ESA Ariane 5 rocket from Kourou, French Guiana, and as of April 2022 is undergoing testing and alignment. Once operational, expected in May 2022, JWST is intended to succeed the Hubble as NASA’s flagship mission in astrophysics.

The U.S. National Aeronautics and Space Administration (NASA) led JWST’s development in collaboration with the European Space Agency (ESA) and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center (GSFC) in Maryland managed telescope development, the Space Telescope Science Institute in Baltimore operates JWST, and the prime contractor was Northrop Grumman. The telescope is named after James E. Webb, who was the administrator of NASA from 1961 to 1968 during the Mercury, Gemini, and Apollo programs.

The Observatory

The Observatory is the space-based portion of the James Webb Space Telescope system. It is comprised of the Optical Telescope Element (OTE), the Integrated Science Instrument Module (ISIM), the sunshield and the spacecraft bus.

The OTE is the eye of the Observatory. It consists of the mirrors and the backplane. The OTE gathers the light coming from space and provides it to the science instruments located in the ISIM. The backplane is like the “spine” of Webb. It supports the mirrors. The ISIM contains Webb’s cameras and instruments. It integrates four major instruments and numerous subsystems into one payload. The sunshield separates the observatory into a warm sun-facing side (spacecraft bus) and a cold anti-sun side (OTE and ISIM). The sunshield keeps the heat of the Sun, Earth, and spacecraft bus electronics away from the OTE and ISIM so that these pieces of the Observatory can be kept very cold (The operating temperature has to be kept under 50 K or -370 deg F). The spacecraft bus provides the support functions for the operation of the Observatory. The bus houses the six major subsystems needed to operate the spacecraft: the Electrical Power Subsystem, the Altitude Control Subsystem, the Communication Subsystem, the Command and Data Handling Subsystem, the Propulsion Subsystem, and the Thermal Control Subsystem.

The Predecessor

The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versatile, renowned both as a vital research tool and as a public relations boon for astronomy. The Hubble telescope is named after astronomer Edwin Hubble and is one of NASA’s Great Observatories.

Hubble features a 2.4 m (7 ft 10 in) mirror, and its five main instruments observe in the ultraviolet, visible, and near-infrared regions of the electromagnetic spectrum. Hubble’s orbit outside the distortion of atmosphere of Earth allows it to capture extremely high-resolution images with substantially lower background light than ground-based telescopes. It has recorded some of the most detailed visible light images, allowing a deep view into space. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe.

Space telescopes were proposed as early as 1923, and the Hubble telescope was funded and built in the 1970s by the United States space agency NASA with contributions from the European Space Agency. Its intended launch was 1983, but the project was beset by technical delays, budget problems, and the 1986 Challenger disaster. Hubble was finally launched in 1990, but its main mirror had been ground incorrectly, resulting in spherical aberration that compromised the telescope’s capabilities. The optics were corrected to their intended quality by a servicing mission in 1993.

Hubble is the only telescope designed to be maintained in space by astronauts. Five Space Shuttle missions have repaired, upgraded, and replaced systems on the telescope, including all five of the main instruments. The fifth mission was initially cancelled on safety grounds following the Columbia disaster (2003), but after NASA administrator Michael D. Griffin approved it, it was completed in 2009. The telescope completed 30 years of operation in April 2020 and is predicted to last until 2030–2040.

Hubble forms the visible light component of NASA’s Great Observatories program, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope (which covers the infrared bands). The mid-IR to-visible band successor to the Hubble telescope is the James Webb Space Telescope (JWST), which was launched on December 25, 2021

In 1923, Hermann Oberth — considered a father of modern rocketry, along with Robert H. Goddard and Konstantin Tsiolkovsky — published Die Rakete zu den Planetenräumen (“The Rocket into Planetary Space”), which mentioned how a telescope could be propelled into Earth orbit by a rocket. The history of the Hubble Space Telescope can be traced back as far as 1946, to astronomer Lyman Spitzer’s paper entitled “Astronomical advantages of an extraterrestrial observatory”. In it, he discussed the two main advantages that a space based observatory would have over ground-based telescopes. First, the angular resolution (the smallest separation at which objects can be clearly distinguished) would be limited only by diffraction, rather than by the turbulence in the atmosphere, which causes stars to twinkle, known to astronomers as seeing. At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.05 arcsec for an optical telescope with a mirror 2.5 m (8 ft 2 in) in diameter. Second, a space-based telescope could observe infrared and ultraviolet light, which are strongly absorbed by the atmosphere of Earth. Hubble has helped resolve some long-standing problems in astronomy, while also raising new questions. Some results have required new theories to explain them.

Age of the universe

Among its primary mission targets was to measure distances to Cepheid variable stars more accurately than ever before, and thus constrain the value of the Hubble constant, the measure of the rate at which the universe is expanding, which is also related to its age. Before the launch of HST, estimates of the Hubble constant typically had errors of up to 50%, but Hubble measurements of Cepheid variables in the Virgo Cluster and other distant galaxy clusters provided a measured value with an accuracy of ±10%, which is consistent with other more accurate measurements made since Hubble’s launch using other techniques. The estimated age is now about 13.7 billion years, but before the Hubble Telescope, scientists predicted an age ranging from 10 to 20 billion years.

Expansion of the universe

While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future. Astronomers from the High-z Supernova Search Team and the Supernova Cosmology Project used ground-based telescopes and HST to observe distant supernovae and uncovered evidence that, far from decelerating under the influence of gravity, the expansion of the universe may in fact be accelerating. Three members of these two groups have subsequently been awarded Nobel Prizes for their discovery. The cause of this acceleration remains poorly understood; the term.used for the currently-unknown cause is dark energy, signifying that it is dark (unable to be directly seen and detected) to our current scientific instruments

Supernova reappearance

On December 11, 2015, Hubble captured an image of the first-ever predicted reappearance of a supernova, dubbed “Refsdal”, which was calculated using different mass models of a galaxy cluster whose gravity is warping the supernova’s light. The supernova was previously seen in November 2014 behind galaxy cluster MACS J1149.5+2223 as part of Hubble’s Frontier Fields program. Astronomers spotted four separate images of the supernova in an arrangement known as an Einstein Cross. The light from the cluster has taken about five billion years to reach Earth, though the supernova exploded some 10 billion years ago. Based on early lens models, a fifth image was predicted to reappear by the end of 2015. The detection of Refsdal’s reappearance in December 2015 served as a unique opportunity for astronomers to test their models of how mass, especially dark matter, is distributed within this galaxy cluster.

In March 2019, observations from Hubble and data from the European Space Agency’s Gaia space observatory were combined to determine that the Milky Way Galaxy weighs approximately 1.5 trillion solar units within a radius of 129,000 light years.

Milky Way Galaxy

Other discoveries

Other discoveries made with Hubble data include proto-planetary disks (proplyds) in the Orion Nebula; evidence for the presence of extrasolar planets around Sun-like stars; and the optical counterparts of the still-mysterious gamma-ray bursts. MACS 2129-1 is an early universe so-called ‘dead’ disk galaxy that lies approximately 10 billion light-years away from Earth. In 2022 Hubble detected the light of the farthest individual star ever seen to date. The star, named temporarily Earendel, existed within the first billion years after the big bang. It will be observed by NASA’s James Webb Space Telescope to confirm Earendel is indeed a star Impact on astronomy. Hubble is still working to capture image of universe below shows recent work of Hubble.

Mounded, luminous clouds of gas and dust glow in this Hubble image of a Herbig-Haro object known as HH 45. Herbig-Haro objects are a rarely seen type of nebula that occurs when hot gas ejected by a new born star collides with the gas and dust around it at hundreds of miles per second, creating bright shock waves. In this image, blue indicates ionized oxygen (O II) and purple shows ionized magnesium (Mg II). Researchers were particularly interested in these elements because they can be used to identify shocks and ionization fronts.

Herbig-Haro object is located in the nebula NGC 1977, which itself is part of a complex of three nebulae called The Running Man. NGC 1977 – like its companions NGC 1975 and NGC 1973 – is a reflection nebula, which means that it doesn’t emit light on its own, but reflects light from nearby stars, like a streetlight illuminating fog. Hubble observed this region to look for stellar jets and planet-forming disks around young stars, and examine how their environment affects the evolution of such disks.

Difference in lens

The lenses of James Webb and Hubble are based on different theory. In Hubble optically, the HST is a Cassegrain reflector of Ritchey–Chrétien design, as are most large professional telescopes. This design, with two hyperbolic mirrors, is known for good imaging performance over a wide field of view, with the disadvantage that the mirrors have shapes that are hard to fabricate and test. The mirror and optical systems of the telescope determine the final performance, and they were designed to exacting specifications. Optical telescopes typically have mirrors polished to an accuracy of about a tenth of the wavelength of visible light, but the Space Telescope was to be used for observations from the visible through the ultraviolet (shorter wavelengths) and was specified to be diffraction limited to take full advantage of the space environment. Therefore, its mirror needed to be polished to an accuracy of 10 nanometres, or about 1/65 of the wavelength of red light. On the long wavelength end, the OTA was not designed with optimum IR performance in mind—for example, the mirrors are kept at stable (and warm, about 15 °C) temperatures by heaters. This limits Hubble’s performance as an infrared telescope. Within weeks of the launch of the telescope, the returned images indicated a serious problem with the optical system. Although the first images appeared to be sharper than those of ground-based telescopes, Hubble failed to achieve a final sharp focus and the best image quality obtained was drastically lower than expected. Images of point sources spread out over a radius of more than one arc second, instead of having a point spread function (PSF) concentrated within a circle 0.1 arc seconds (485 nrad) in diameter, as had been specified in the design criteria.

Analysis of the flawed images revealed that the primary mirror had been polished to the wrong shape. Although it was believed to be one of the most precisely figured optical mirrors ever made, smooth to about 10 nanometres, the outer perimeter was too flat by about 2200 nanometres (about 1⁄450 mm or 1⁄11000 inch). This difference was catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edge of a mirror focuses on a different point from the light reflecting off its centre.

The effect of the mirror flaw on scientific observations depended on the particular observation—the core of the aberrated PSF was sharp enough to permit high-resolution observations of bright objects, and spectroscopy of point sources was affected only through a sensitivity loss. However, the loss of light to the large, out-of-focus halo severely reduced the usefulness of the telescope for faint objects or high-contrast imaging. This meant nearly all the cosmological programs were essentially impossible, since they required observation of exceptionally faint objects. This led politicians to question NASA’s competence, scientists to rue the cost which could have gone to more productive endeavors, and comedians to make jokes about NASA and the telescope. In the 1991 comedy The Naked Gun 2½: The Smell of Fear, in a scene where historical disasters are displayed, Hubble is pictured with RMS Titanic and LZ 129 Hindenburg. Nonetheless, during the first three years of the Hubble mission, before the optical corrections, the telescope still carried out a large number of productive observations of less demanding targets. The error was well characterized and stable, enabling astronomers to partially compensate for the defective mirror by using sophisticated image processing techniques such as deconvolution. JWST’s primary mirror is a 6.5 m (21 ft)-diameter gold-coated beryllium reflector with a collecting area of 25.4 m2 (273 sq ft). If it were built as a single large mirror, this would have been too large for existing launch vehicles. The mirror is therefore composed of 18 hexagonal segments (Guido Horn d’Arturo’s multi-mirror telescope), which unfolded after the telescope was launched. Image plane wavefront sensing through phase retrieval is used to position the mirror segments in the correct location using very precise micromotors. Subsequent to this initial configuration, they only need occasional updates every few days to retain optimal focus. This is unlike terrestrial telescopes, for example the Keck telescopes, which continually adjust their mirror segments using active optics to overcome the effects of gravitational and wind loading.

The Webb telescope will use 132 small motors (called actuators) to position and occasionally adjust the optics as there are few environmental disturbances of a telescope in space. Each of the 18 primary mirror segments is controlled by 6 positional actuators with a further ROC (radius of curvature) actuator at the center to adjust curvature (7 actuators per segment), for a total of 126 primary mirror actuators, and another 6 actuators for the secondary mirror, giving a total of 132. The actuators can position the mirror with 10 nanometre (10 millionths of a millimetre) accuracy.

The actuators are critical in maintaining the alignment of the telescope’s mirrors, and are designed and manufactured by Ball Aerospace & Technologies. Each of the 132 actuators are driven by a single stepper motor, providing both fine and coarse adjustmentst. The actuators provide a coarse step size of 58 nanometers for larger adjustments, and a fine adjustment step size of 7 nanometres.

JWST’s optical design is a three-mirror anastigmat, which makes use of curved secondary and tertiary mirrors to deliver images that are free from optical aberrations over a wide field. The secondary mirror is 0.74 m (2.4 ft) diameter. In addition, there is a fine steering mirror which can adjust its position many times per second to provide image stabilization. The primary mirror segments are hollowed at the rear in a honeycomb pattern, to reduce weight.

Ball Aerospace & Technologies is the principal optical subcontractor for the JWST project, led by prime contractor Northrop Grumman Aerospace Systems, under a contract from the NASA Goddard Space Flight Center, in Greenbelt, Maryland. The mirrors, plus flight spares, were fabricated and polished by Ball Aerospace & Technologies based on beryllium segment blanks manufactured by several companies including Axis’s, Brush Wellman, and Tinsley Laboratories.

Orbit’s of James Webb and Hubble

The Earth is 150 million km from the Sun and the moon orbits the earth at a distance of approximately 384,500 km. The Hubble Space Telescope orbits around the Earth at an altitude of ~570 km above it. Webb will not actually orbit the Earth - instead it will sit at the Earth-Sun L2 Lagrange point, 1.5 million km away! Webb will orbit the sun 1.5 million kilometers (1 million miles) away from the Earth at what is called the second Lagrange point or L2. (Note that these graphics are not to scale.) Because Hubble is in Earth orbit, it was able to be launched into space by the space shuttle. Webb will be launched on an Ariane 5 rocket and because it won't be in Earth orbit, it is not designed to be serviced by the space shuttle.

At the L2 point Webb's solar shield will block the light from the Sun, Earth, and Moon. This will help Webb stay cool, which is very important for an infrared telescope. As the Earth orbits the Sun, Webb will orbit with it - but stay fixed in the same spot with relation to the Earth and the Sun, as shown in the diagram to the left. Actually, satellites orbit around the L2 point, as you can see in the diagram - they don't stay completely motionless at a fixed spot.

Webb will orbit the sun 1.5 million kilometers (1 million miles) away from the Earth at what is called the second Lagrange point or L2. (Note that these graphics are not to scale.)

Lagrange Points.

Mission

Hubble had a mission to discover the vast universe. Humanity has taken first step to explore the universe using Hubble now there might be a leap in this because of James Webber. On the day of launch the James Webb Twitter account tweeted with # unfold the universe. This shows the ambition and resolution to uncover the universe. Because of the time it takes light to travel, the farther away an object is, the farther back in time we are looking.

This illustration compares various telescopes and how far back they are able to see.

Essentially, Hubble can see the equivalent of "toddler galaxies" and Webb Telescope will be able to see "baby galaxies". One reason Webb will be able to see the first galaxies is because it is an infrared telescope. The universe (and thus the galaxies in it) is expanding. When we talk about the most distant objects, Einstein's General Relativity actually comes into play. It tells us that the expansion of the universe means it is the space between objects that actually stretches, causing objects (galaxies) to move away from each other. Furthermore, any light in that space will also stretch, shifting that light's wavelength to longer wavelengths. This can make distant objects very dim (or invisible) at visible wavelengths of light, because that light reaches us as infrared light. Infrared telescopes, like Webb, are ideal for observing these early galaxies NASA has announced that it will unveil the first science-quality images from the James Webb Space Telescope on July 12 which will show us the potential of James Webb and the future path astrophysics will take.