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How the James Webb Space Telescope Will Work

A full-scale model of the James Webb Space Telescope dazzles the city of Austin, Texas, during the 2013 South by Southwest festival.
A full-scale model of the James Webb Space Telescope dazzles the city of Austin, Texas, during the 2013 South by Southwest festival.
Image courtesy NASA/Chris Gunn

Our knowledge of our universe is bounded by the scope of our senses, but our minds know no such limits. When a campfire's glow blinds us to the source of a twig-snap in the wooded darkness, we imagine all sorts of dire prospects. But step out a few paces, set the fire to our backs, and we see more deeply and clearly. Imagination meets information, and we suddenly know what we are dealing with.

If only it were always so simple. But it takes more than a good set of eyes and some distance from city lights to comprehend the cosmos; it requires instruments capable of expanding our senses beyond our evolutionary limits, our atmosphere or even our planetary orbit. Astronomy and cosmology are both compelled and limited by the quality of these instruments. Around 400 years ago, the telescope revealed unsuspected moons, planets and sunspots, sparking a succession of new cosmic theories and better tools to test them, revealing billowing nebulae and congregating stars along the way.

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In the mid-20th century, radio telescopes showed that galaxies, far from static blobs, were in fact active and bursting with energy. Before the Kepler space telescope, we thought exoplanets were rare in the universe; now we suspect they might outnumber stars. Two decades of the Earth-orbiting Hubble Space Telescope helped pierce the veil of time, image stellar nurseries and prove that galaxies collide. Now, the James Webb Space Telescope stands poised to place its back to the sunlight, step away from Earth and make the keen, delicate observations possible only in the cold, dark spaces beyond the moon.

Slated for a 2018 launch date and team-built by 14 countries, 27 states and the District of Columbia, Webb is charged with answering some very ambitious questions. When the mighty telescope launches atop a European Space Agency Ariane 5 ECA rocket, it will mark the start of a new wave of aspiring ground- and space-based instruments, including several new observatories in Hawaii and Chile [sources: Billings; Overbye].

If it survives its launch and its 1-million-mile (1.5-million-kilometer) journey from Earth to its orbit of the second Lagrange (L2) point -- one of five spots in the Earth-sun system where gravity will naturally hold a spacecraft more or less in place -- it will take astronomers closer to the beginning of time than ever, granting glimpses of sights long hypothesized but never seen, from the birth of galaxies to light from the very first stars.

Webb's mission builds upon and expands the work of NASA's Great Observatories, four remarkable space telescopes whose instruments cover the waterfront of electromagnetic spectra. The four overlapping missions enabled scientists to observe the same astronomical objects in the visible, gamma ray, X-ray and infrared spectra.

The school-bus-sized Hubble, which sees primarily in the visible spectrum with some ultraviolet and near-infrared coverage, kicked off the program in 1990 and, with further servicing, should last long enough to hand off the baton to Webb. Appropriately named for Edwin Hubble, the astronomer who discovered many of the occurrences that it was built to investigate, the telescope has since become one of the most productive instruments in scientific history, bringing phenomena like star birth and death, galactic evolution and black holes from theory to observed fact [source: NASA].

Joining the Hubble in the big four are the Compton Gamma Ray Observatory (CGRO), Chandra X-ray Observatory and Spitzer Space Telescope.

  • The CGRO, launched in 1991 and no longer in service, detected high-energy, violent spectacles in the 30 kiloelectron volts (keV) to 30 gigaelectron volts (GeV) spectrum, including the energy-spewing nuclei of active galaxies.
  • Chandra, deployed in 1999 and still going strong, monitors black holes, quasars and high-temperature gases in the X-ray spectrum, and offers vital data about the universe's birth, growth and ultimate fate.
  • Spitzer, which occupies an Earth-trailing orbit, views the sky in thermal infrared (3-180 microns), a bandwidth useful for viewing star births, galactic centers and cool, dim stars, and for detecting molecules in space.

Webb will gaze deeply into the near- and mid-infrared, aided by its position at the L2 point beyond the moon and by its solar shields, which will block intrusive light from the sun, Earth and moon while also efficiently cooling the craft. Scientists hope to observe the very first stars in the universe, the formation and collision of infant galaxies, and the birth of stars and protoplanetary systems -- possibly ones containing the chemical constituents of life.

These first stars could hold the key to understanding the structure of the universe. Theoretically, where and how they formed relates to early patterns of dark matter -- unseen, mysterious matter detectable by the gravity it exerts -- and their life cycles and deaths caused feedbacks that affected the formation of the first galaxies [source: Bromm et al.]. And as supermassive, short-lived stars, estimated at around 30-300 times the mass (and millions of times the brightness) of our sun, these firstborn stars might well have exploded as supernovae then collapsed to form black holes, later swelling and merging into the huge black holes that occupy the centers of most massive galaxies.

Witnessing any of this is a feat beyond any instrument we've built so far. That's about to change, thanks to a package of instruments -- and a spaceship -- built for the job.

NASA engineer Ernie Wright studies Webb's first six flight-ready primary mirror segments as they're prepped to begin final cryogenic testing. The primary mirror will have 18 segments in all.
NASA engineer Ernie Wright studies Webb's first six flight-ready primary mirror segments as they're prepped to begin final cryogenic testing. The primary mirror will have 18 segments in all.

Before you sign on the dotted line, we know you'll want to kick the tires and give the vehicle a quick walk-around. Take your time -- this baby's one of a kind.

Webb looks a bit like a diamond-shaped raft sporting a thick, curved mast and sail -- if the sail were built by giant, beryllium-chewing honeybees. Pointed bottom-toward-the-sun, the "raft" portion consists of five gap-separated layers of Kapton-based heat shield. Each separated by a vacuum-filled gap for effective cooling, they together protect the main reflector and instruments.

Kapton is a very thin (think human hair!) polymer film made by DuPont capable of keeping stable mechanical properties under extremes of heat and vibration, and it's already done time in space. If you were so inclined, you could boil water on one side of the shield and liquefy nitrogen on the other. Oh, and it folds up rather nicely, too, which it'll need to do for launch.

The ship's "keel" consists of a Unitized Pallet Structure, which stores the sunshield during liftoff, and solar cells for power. In the center lies the spacecraft bus, which packs all of the support functions that keep Webb running, including electrical power, attitude control, communications, command and data handling, and thermal control. A high-gain antenna adorns the bus's exterior, as do a set of star trackers that work with the Fine Guidance Sensor (see next section) to keep everything pointed in the right direction. Finally, at one end of the heat shield, and perpendicular to it, lies a momentum trim tab that offsets the pressure that photons exert on the ship, much like a trim flap does on a sailing ship.

On the spaceward side of the shield lies the "sail," Webb's giant mirror, part of an optics suite and instrumentation package. Its 18 hexagonal beryllium sections unfold after launch, then coordinate to act like one whopping primary mirror that stretches 21.3 feet (6.5 meters) across.

Opposite this mirror, held in place by three supports, stands the secondary mirror, which focuses light from the primary mirror onto the aft-optics subsystem, a wedge-shaped box jutting from the main mirror's center. This structure deflects stray light and directs the light from the secondary mirror to the instruments housed within the backplate "mast," which does double duty by also maintaining the segmented main mirror's structure.

Once the ship completes its six-month commissioning period after launch, it will last 5-10 years and, we hope, longer, depending on fuel consumption, but it will orbit too far out for servicing. Actually, Hubble and the International Space Station are the exceptions in that regard, but like Hubble and other general observatories, its missions will then derive from competing, peer-reviewed and ranked proposals, submitted by scientists around the globe. The results will find their way into published studies and data available on the Internet.

Let's take a closer look at the instruments that will make all of those studies possible.

Webb's Near-Infrared Camera hangs out in a clean room at the Lockheed Martin Advanced Technology Center on Feb. 12, 2014. It's safe to say that a working NIRCam will take in some seriously awesome cosmic sights.
Webb's Near-Infrared Camera hangs out in a clean room at the Lockheed Martin Advanced Technology Center on Feb. 12, 2014. It's safe to say that a working NIRCam will take in some seriously awesome cosmic sights.
Image courtesy Lockheed Martin

Although it sees somewhat into the visual range (red and gold light), Webb is fundamentally a large infrared telescope (see sidebar).

Its primary imager, the Near-InfraRed Camera (NIRCam), senses in the 0.6-5.0 micron range (near-infrared). It will detect infrared light from the earliest stars and galaxies being born, take a census of nearby galaxies and spot objects swinging through the Kuiper Belt -- the expanse of icy objects orbiting beyond Neptune that contains Pluto and other dwarf planets. It will also aid with correcting Webb's telescopic vision as needed.

NIRCam comes equipped with a coronagraph, which will enable the camera to observe the wispy halo surrounding bright stars by blocking their blinding light -- an essential tool for spotting exoplanets.

The Near InfraRed Spectrograph (NIRSpec) operates in the same wavelength range as NIRCam. Like other spectrographs, it analyzes the physical characteristics of objects such as stars by splitting light their light into a spectrum, the pattern of which varies according to the target's temperature, mass and chemical makeup.

NIRSpec will study thousands of ancient galaxies with radiation so faint that a single spectrograph will require hundreds of hours to make. To aid in this daunting task, the spectrograph equips a remarkable gadget: a grid of 62,000 individual shutters, each measuring roughly 100 by 200 microns (the width of a few human hairs) and capable of opening and closing to block out the light of brighter stars. Thanks to this microshutter array, NIRSpec will become the first space-based spectrograph capable of observing 100 different objects at a time.

The Fine Guidance Sensor / Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS) is actually two sensors packaged together. The NIRISS incorporates four modes, each associated with a different wavelength range. These vary from slitless spectroscopy, which creates a spectrum via a prism and grating combination called a grism, to aperture-masking interferometry, which uses a mask to create interference patterns that help distinguish exoplanetary light from background star shine [source: STSI].

The FGS is a sensitive, unblinking camera that takes navigational pictures and feeds them to the attitude control system to keep the telescope pointed in the right direction.

The final Webb instrument extends its range beyond near-infrared and into the mid-infrared, handy for picking up redshifted objects, as well as planets, comets, asteroids, starlight-heated dust and protoplanetary disks. Both a camera and a spectrograph, this Mid-InfraRed Instrument (MIRI) covers the widest wavelength range, from 5-28 microns. Its wide-field broadband camera will snap more of the kinds of images that made Hubble famous.

But infrared observation is essential to understanding the universe. Dust and gas can block the visible light of stars in stellar nurseries, but infrared passes through. Moreover, as the universe expands and galaxies move apart, their light "stretches out" and becomes redshifted, sliding toward longer EM wavelengths such as infrared. The farther away the galaxy, the faster it recedes and the more redshifted its light -- hence, the value of a telescope like Webb.

Infrared spectra also can provide a wealth of information on exoplanet atmospheres -- and whether they contain molecular ingredients associated with life. On Earth, we call water vapor, methane and carbon dioxide "greenhouse gases" because they absorb thermal infrared (aka heat). Because this tendency holds true everywhere, scientists can use Webb to detect such substances in the atmospheres of distant worlds by looking for telltale absorption patterns in their spectroscopic readings.

Author's Note: How the James Webb Space Telescope Will Work

It's been said that we spend too much time thinking about the past, but in truth our senses feed us nothing but dated information. Everything we sense already happened, whether a fraction of a split second earlier or millions of years ago, which makes our eyes, to some very small degree, time machines.

But there's so much more information in the sky than we can perceive -- fading Polaroids of the barely recognizable early universe, fading into redshifted images billions of years old, that lie beyond the limited EM window to which our eyes are sensitive. But that's the beauty of being a tool-making species: Our tools can extend our capabilities, our reach and our vision, even to the very birth of the cosmos.

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Sources

  • Billings, Lee. "Space Science: The Telescope That Ate Astronomy." Nature. Vol. 467. Page 1028. Oct. 27, 2010. (Sept. 11, 2014) http://www.nature.com/news/2010/101027/full/4671028a.html
  • Bromm, Volker, et al. "The Formation of the First Stars and Galaxies." Nature. Vol. 459. May 7, 2009. (Sept. 19, 2014) http://sdcc3.ucsd.edu/~ir118/SIO87W13/FirstStars.pdf
  • NASA. "The James Webb Space Telescope." (Sept. 21, 2014) http://www.jwst.nasa.gov/
  • NASA. "A Look at the Numbers as NASA's Hubble Space Telescope Enters its 25th Year." May 12, 2014. (Sept. 18, 2014) http://www.nasa.gov/content/goddard/a-look-at-the-numbers-as-nasas-hubble-space-telescope-enters-its-25th-year/#.VBr4UfldV8E
  • Overbye, Dennis. "More Eyes on the Skies." The New York Times. July 21, 2014. (Sept. 11, 2104) http://www.nytimes.com/2014/07/22/science/space/more-eyes-on-the-skies.html?_r=0
  • Space Telescope Science Institute (STSI). "James Webb Space Telescope FGS - Fine Guidance Sensor." (Sept. 11, 2104) http://www.stsci.edu/jwst/instruments/fgs/
  • Space Telescope Science Institute (STSI). "James Webb Space Telescope Near-InfraRed Imager and Slitless Spectrograph." (Sept. 11, 2104) http://www.stsci.edu/jwst/instruments/niriss
  • Stiavelli, M., et al. "A Strategy to Study First Light with JWST." Space Telescope Science Institute. (Sept. 11, 2104) http://www.stsci.edu/jwst/science/strategy-to-study-First-Light.pdf

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