November 25, 2020

The James Webb Space Telescope

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OLYMPUS DIGITAL CAMERAThe James Webb Space Telescope (sometimes called JWST) is a large, infrared-optimized space telescope. The project is working to a 2018 launch date. Webb will find the first galaxies that formed in the early Universe, connecting the Big Bang to our own Milky Way Galaxy. Webb will peer through dusty clouds to see stars forming planetary systems, connecting the Milky Way to our own Solar System. Webb’s instruments will be designed to work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range.

Webb will have a large mirror, 6.5 meters (21.3 feet) in diameter and a sunshield the size of a tennis court. Both the mirror and sunshade won’t fit onto a rocket fully open, so both will fold up and open once Webb is in outer space. Webb will reside in an orbit about 1.5 million km (1 million miles) from the Earth.

The James Webb Space Telescope was named after the NASA Administrator who crafted the Apollo program,James_Webb_NASA_med and who was a staunch supporter of space science.

The Webb will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System.

Webb was formerly known as the “Next Generation Space Telescope” (NGST); it was renamed in Sept. 2002 after a former NASA administrator, James Webb.

Webb is an international collaboration between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center is managing the development effort. The main industrial partner is Northrop Grumman; the Space Telescope Science Institute will operate Webb after launch.

Several innovative technologies have been developed for Webb. These include a folding, segmented primary mirror, adjusted to shape after launch; ultra-lightweight beryllium optics; detectors able to record extremely weak signals, microshutters that enable programmable object selection for the spectrograph; and a cryocooler for cooling the mid-IR detectors to 7K.

In July 2008 NASA confirmed the Webb project to proceed into its implementation phase, and the project conducted a major mission review in March 2010. Here is a summary of the current mission status.

There will be four science instruments on Webb: the Near InfraRed Camera (NIRCam), the Near InfraRed Spectrograph (NIRSpec), the Mid-InfraRed Instrument (MIRI), and the Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS). Webb’s instruments will be designed to work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range. It will be sensitive to light from 0.6 to 28 micrometers in wavelength.

Webb has four main science themes: The End of the Dark Ages: First Light and Reionization, The Assembly of Galaxies, The Birth of Stars and Protoplanetary Systems, and Planetary Systems and the Origins of Life.

About James Webb

The man whose name NASA has chosen to bestow upon the successor to the Hubble Space Telescope is most commonly linked to the Apollo moon program, not to science.

Yet, many believe that James E. Webb, who ran the fledgling space agency from February 1961 to October 1968, did more for science than perhaps any other government official and that it is only fitting that the Next Generation Space Telescope would be named after him.

A Balanced Program

Webb’s record of support for space science would support those views. Although President John Kennedy had committed the nation to landing a man on the moon before the end of the decade, Webb believed that the space program was more than a political race. He believed that NASA had to strike a balance between human space flight and science because such a combination would serve as a catalyst for strengthening the nation’s universities and aerospace industry.

As part of an oral history project sponsored by the LBJ Library in Austin, Texas, Webb recalled his conversations with Kennedy and Vice President Lyndon Johnson. He was quoted as saying in one transcript, “And so far as I’m concerned, I’m not going to run a program that’s just a one-shot program. If you want me to be the administrator, it’s going to be a balanced program that does the job for the country…”

Webb’s vision of a balanced program resulted in a decade of space science research that remains unparalleled today. During his tenure, NASA invested in the development of robotic spacecraft, which explored the lunar environment so that astronauts could do so later, and it sent scientific probes to Mars and Venus, giving Americans their first-ever view of the strange landscape of outer space. As early as 1965, Webb also had written that a major space telescope, then known as the Large Space Telescope, should become a major NASA effort.

By the time Webb retired just a few months before the first moon landing in July 1969, NASA had launched more than 75 space science missions to study the stars and galaxies, our own Sun and the as-yet unknown environment of space above the Earth’s atmosphere. Missions such as the Orbiting Solar Observatory and the Explorer series of astronomical satellites built the foundation for the most successful period of astronomical discovery in history, which continues today.

Webb supported science behind the scenes, as well. Shortly after assuming the job vacated by Keith Glennan, Webb chose to continue the same basic organization that his predecessor had adopted for the selection of science programs. However, he enhanced the role of scientists in key ways. He gave them greater control in the selection process of science missions and he created the NASA University Program, which established grants for space research, funded the construction of new laboratories at universities and provided fellowships for graduate students. The program also encouraged university presidents and vice presidents to actively participate in NASA’s Space Science Program and to publicly support all of NASA’s programs.

A Notable Record

This record of accomplishment is perhaps more notable given Webb’s initial reluctance to accept the job. An experienced manager, attorney and businessman, the North Carolina native had served as Director of the Bureau of the Budget and as Undersecretary of State in the Truman administration. Webb also served as president and vice president of several private firms and served on the board of directors of the McDonnell Aircraft Company. He was not, however, a scientist or engineer-something he noted when President Kennedy asked him to consider the job as NASA Administrator.

He told an interviewer that, “I felt that I had made the pattern of my life, and I was not really the best person for this anyway. It seemed to me someone who knew more about rocketry, about space, would be a better person.” Kennedy did not see it that way. With his keen political savvy and exceptional managerial skills, Webb was perfect for the job, the President believed. He made it clear to Webb that the NASA Administrator’s job was a policy job. He needed someone who could handle the large issues of national and international policies.

The scientific community was equally anxious about Webb. The scientists at NASA Headquarters had wanted someone with a keen interest in space science and a desire to bolster the involvement of universities in the space program. Within a few months, Webb proved where he stood.

A Fitting Honor

At the height of the Apollo program, NASA had 35,000 employees and more than 400,000 contractors in thousands of companies and universities across the U.S. Under Webb’s direction, the agency undertook one of the most impressive projects in history-landing a man on the moon before the end of the decade.

As NASA Administrator Sean O’Keefe said when he announced the new name for the next generation space telescope, “It is fitting that Hubble’s successor be named in honor of James Webb. Thanks to his efforts, we got our first glimpses at the dramatic landscape of outer space. He took our nation on its first voyages of exploration, turning our imagination into reality. Indeed, he laid the foundations at NASA for one of the most successful periods of astronomical discovery. As a result, we’re rewriting the textbooks today with the help of the Hubble Space Telescope , the Chandra X-ray Observatory , and the James Webb Telescope.”

The Observatory

The Observatory is the space-based portion of the James Webb Space Telescope system and is comprised of three elements, the Integrated Science Instrument Module (ISIM), the Optical Telescope Element (OTE), which includes the mirrors and backplane, and the Spacecraft Element, which includes the spacecraft bus and the sunshield.

The OTE is the eye of the Observatory. 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 sunshield subsystem 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 Attitude Control Subsystem, the Communication Subsystem, the Command and Data Handling Subsystem, the Propulsion Subsystem, and the Thermal Control Subsystem.

The momentum flap balances the solar pressure on the sunshield, like a trim flap in sailing. It’s not adjustable on orbit, but it is while it’s on the ground.

About Webb’s Orbit

The James Webb Space Telescope will observe primarily the infrared light from faint and very distant objects. But all objects, including telescopes, also emit infrared light. To avoid swamping the very faint astronomical signals with radiation from the telescope, the telescope and its instruments must be very cold. Therefore, Webb has a large shield that blocks the light from the Sun, Earth, and Moon, which otherwise would heat up the telescope, and interfere with the observations. To have this work, Webb must be in an orbit where all three of these objects are in about the same direction. The answer is to put Webb in an orbit around the L2 point.

The L2 orbit is an elliptical orbit about the semi-stable second Lagrange point . It is one of the five solutions by the mathematician Joseph-Louis Lagrange in the 18th century to the three-body problem. Lagrange was searching for a stable configuration in which three bodies could orbit each other yet stay in the same position relative to each other. He found five such solutions, and they are called the five Lagrange points in honor of their discoverer.

In three of the solutions found by Lagrange, the bodies are in line (L1, L2, and L3); in the other two, the bodies are at the points of equilateral triangles (L4 and L5). The five Lagrangian points for the Sun-Earth system are shown in the diagram below. An object placed at any one of these 5 points will stay in place relative to the other two.

In the case of Webb, the 3 bodies involved are the Sun, the Earth and the Webb. Normally, an object circling the Sun further out than the Earth would take more than one year to complete its orbit. However, the balance of gravitational pull at the L2 point means that Webb will keep up with the Earth as it goes around the Sun. The gravitational forces of the Sun and the Earth can nearly hold a spacecraft at this point, so that it takes relatively little rocket thrust to keep the spacecraft in orbit around L2.

Other infrared missions have selected an L2 orbit, like WMAP and H2L2.

About Webb’s Launch

The Webb will be launched from Arianespace’s ELA-3 launch complex at European Spaceport located near Kourou, French Guiana.

The Launch Segment has 3 primary components:

1. Launch Vehicle: an Ariane 5 ECA with the cryogenic upper stage. It will be provided in the single launch configuration, with a long payload fairing providing a maximum 4.57 meter static diameter and useable length of 16.19 meters.

2. Payload Adapter, comprising the Cone 3936 plus ACU 2624 lower cylinder and clamp-band, which provides the separating mechanical and electrical interface between the Webb Observatory and the Launch Vehicle.

3. Launch campaign preparation and launch campaign.

The European Space Agency (ESA) will provide the launch vehicle and the payload adapter to the Webb Mission. The launch campaign preparation and launch campaign is the mutual responsibility of NASA, ESA, NGST, and Arianespace.

How does the Webb Contrast with Hubble?

Webb often gets called the replacement for Hubble, but we prefer to call it a successor. After all, Webb is the scientific successor to Hubble; its science goals were motivated by results from Hubble. Hubble’s science pushed us to look to longer wavelengths to “go beyond” what Hubble has already done. In particular, more distant objects are more highly redshifted, and their light is pushed from the UV and optical into the near-infrared. Thus observations of these distant objects (like the first galaxies formed in the Universe, for example) requires an infrared telescope.

This is the other reason that Webb is not a replacement for Hubble is that its capabilities are not identical. Webb will primarily look at the Universe in the infrared, while Hubble studies it primarily at optical and ultraviolet wavelengths (though it has some infrared capability). Webb also has a much bigger mirror than Hubble. This larger light collecting area means that Webb can peer farther back into time than Hubble is capable of doing. Hubble is in a very close orbit around the earth, while Webb will be 1.5 million kilometers (km) away at the second Lagrange (L2) point.

For much more on this story and graphics go to:

http://www.jwst.nasa.gov/index.html

 

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