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Meet the James Webb Space Telescope

How Do You Know? – Episode #33:

Who Was James Webb?

  • NASA’s Chief from 1961-68

    • Webb championed a balance between manned and unmanned exploration

    • Strong supporter of Space Science

    • Wanted NASA to be more than a political tool

      • “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 [between human spaceflight and science] that does the job for the country.”

        • -- James Webb speaking to President John F. Kennedy and Vice President Lyndon Johnson.

        • (From NASA transcript)

  • NASA’s Accomplishments under Webb

    • Robotic exploration of the Moon (Ranger series)

    • First probes to Mars (Mariner series)

    • First probes to Venus (Pioneer series)

    • Initiated plans in 1965 for a space-based observatory

      • Originally called the Large Space Telescope or LST, this later became the Hubble Space Telescope

    • More than 75 space science missions were launched during Webb’s tenure, and many more were launched after his retirement that he had an active role in planning and guiding.


      Basic Facts about JWST:

  • Webb is a 6,200 kg (6.8 ton) telescope assembly

  • The primary mirror is 6.5 meters in aperture (21.3 ft) with 25 m2 of surface area.

    • Compare this to a 300 mm (12-inch) Dobsonian with 0.07 m2 of surface area!

    • Hubble’s mirror is 2.4 meters in aperture (7.9 ft) with 4.5 m2 of surface area

      • Webb has 5.5 times more surface area than Hubble!

  • The focal length of the telescope is 131.4 meters! (131,400 mm!)


    Bling! Those Fabulous Gold Mirrors:

  • Webb’s mirrors are made of beryllium, coated with gold

    • Beryllium is a very light and rigid metal

    • Physically stable structurally (strength), and dimensionally (shape) at cryogenic temperatures

    • Gold is the best reflective material for IR radiation – significantly better than silver or aluminum

  • Hubble’s mirror scaled up to Webb’s size would be too heavy to launch

  • The primary mirror masses 705 kg

    • Each Beryllium mirror weighs 20.1 kg

    • A single mirror assembly weighs 39.5 kg

  • Optical resolution is 0.1 arc seconds

  • The hexagonal mirror shape allows for tiling, space-filling with regular tiles.

    • Circular mirrors would have gaps between them

    • Hexagonal mirrors allow folding

  • Because of the space-filling hex pattern, only 3 different mirror curvatures or prescriptions need to be created (6 tiles each)

    james webb space telescope segments information for amateur astronomers kids and adults

     


    Throw Some Shade!

  • The sun shield or shade is 21 x 14 meters

    • A standard tennis court is 24 x 11 meters

  • The sun shield helps Webb keep its instruments at operating temperatures under 50 Kelvin (-370 F, - 233 C)

  • The sunlit side of the shield will be at 380 K (230 F, 110 C)

  • The total differential between light and dark side: 600 F, 343 C

  • There are 5 layers to the shield

    • Each layer has vacuum insulation

    • The spacing acts like a “light pipe” giving total internal reflection to IR radiation (similar to how fiber optics work)

      • Precise spacing is controlled by a tensioning system

    • The shape, layers, and spacing are all designed to radiate heat outward and away from the telescope

  • The shield is made of Kapton, a polyimide film developed by DuPont in the 1960’s

    • Stable from -269 to +400 C

    • All layers coated with aluminum – similar to a telescope mirror

    • 2 sunside layers also coated with doped silicon to help reflect IR radiation

    • “Rip-stop” seams are thermally bonded to contain damage from meteorite impact.


      Why an Infra-Red Telescope?

  • Longer wavelength light penetrates dust and gas which blocks or scatters shorter wavelengths

  • Olbers’ Paradox: If the universe is infinite, there should be a star in every direction, no matter where you look. If that were true, then why isn’t the sky uniformly bright in all directions?

    • The answer to Heinrich Olbers’ question is dust and gas.

    • Dust and gas block visible light, the farther away an object is, the more dust and gas intervene between it and the observer – obscuring the view.

  • Webb is NASA’s solution to Olber’s paradox. Dust and gas block visible, UV, and other wavelengths, but IR radiation shines through.

  • Because looking out in space is looking back in time – the further away (in time and space) the more dust intervenes.

  • Webb’s IR telescope is able to see through this dust – to see further back in time and space.

    • The Near Infra-red Camera can see from mid-infrared into the red and orange areas of the spectrum.

    • Webb can see out to a redshift of z=20, 180 – 200 million years after the Big Bang when the first stars and galaxies were forming.

Further Reading and Viewing:

These articles will give you more information to explore these topics: From NASA.gov:

Learn about James Webb: https://jwst.nasa.gov/content/about/faqs/whoIsJamesWebb.html

Audio transcript of Webb’s meeting with JFK: https://history.nasa.gov/JFK-Webbconv/index.html

JWST home page at NASA.gov: https://jwst.nasa.gov/

Learn about Webb’s mirrors: https://jwst.nasa.gov/content/observatory/ote/mirrors/index.html

Learn about Webb’s sunshield: https://jwst.nasa.gov/content/observatory/sunshield.html

Learn about Webb’s Infrared Detectors: https://jwst.nasa.gov/content/about/innovations/infrared.html

 

For More Information:

Many of the activities and materials provided free for the How Do You Know? program are based upon Dr. Barth’s award-winning book: Astronomy For Educators. This book is used as a resource in more than 7,250 schools across the United States and in more than 50 countries world wide. The book is published as an Open Educational Resource Text by the University of Arkansas Library Press.

You are welcome to download a free copy! If you would like to help Dr. Barth, please take our 5-minute survey! All responses are anonymous and information is used in STEM education research and in planning upcoming books in this series.

Download Astronomy For Educators for free:

Scholarworks.uark.edu/oer/2


Contact Dr. Barth at: AstronomyForEducators@gmail.com


Astronomy For Educators Survey:

https://www.surveymonkey.com/r/D85KK75


For curriculum guides or further questions, email Dr. Barth at: AstronomyForEducators@gmail.com

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