The James Webb Space Telescope, which will launch in October 2018, has a large, segmented mirror 255 inches across. A sunshield protects the telescope from heat so it can study the cosmos in infrared light. Credit: NASA
Remember when the 200-inch Hale Telescope at Mt. Palomar in California was the biggest in the world? It’s now surpassed by at least 18 other scopes, the largest of which is the Gran Telescopio Canarias with a mirror 410 inches in diameter. The bigger the mirror, the greater its light-gathering ability and farther we can see across the universe.
From the semi-stable L2 Langrangian point a million miles from Earth opposite the sun, the Webb can both block the sun, Earth and moon from view as well as study deep space 24/7. Credit: NASA
Soon we’ll recall when the Hubble with its 94-inch mirror was the biggest orbiting telescope.
In October 2018, NASA plans to launch the James Webb Space Telescope (JWST) with a behemoth 255-inch (6.5-m) mirror coated in gold.
The Webb will set up stakes one million miles from Earth at the L2 Lagrangian Point, a region of space where the sun’s and Earth’s gravities strike a balance, allowing a spacecraft there to “hover” indefinitely with only an occasional firing of its thrusters to maintain position.
James Webb Credit: NASA
Named after the NASA administrator James Webb, best known for his leadership in the Apollo moon program, it will be the most powerful space telescope ever built. It
will observe the most distant objects in the universe, including the very first galaxies and search for clues left behind by the earliest stars.
Closer to home, it will examine planets in our solar system as well as planets around nearby stars. The telescope will be able to determine the composition of an exoplanet’s atmosphere by studying the light of its host star filtering through the alien air. The Webb telescope is a joint project of NASA, the European Space Agency and the Canadian Space Agency.
NASA engineer Ernie Wright looks on as the first six flight ready James Webb Space Telescope’s gold-coated primary mirror segments are prepped to begin final cryogenic testing at NASA’s Marshall Space Flight Center. Credit: NASA’s MFSC/David Higginbotham
Its unique mirror, made of the lightweight metal beryllium and coated with a golf-ball’s worth of gold spread into an ultra-thin layer across 18-hexagonal mirror segments, is optimized for infrared light. That’s the invisible light just beyond the red of the rainbow that we sense as heat.
Light from celestial objects like galaxies receding rapidly from Earth is stretched and reddened. Light from objects approaching Earth appears bluer.
Why infrared? Our expanding universe got its start in the Big Bang. Because light takes time to travel to our eyes from distant regions of the universe, we also peer back into time when we look into space. Since the universe is expanding, the farther back we look, the faster objects appear to be moving away from us. Like the sound of an ambulance siren dropping in pitch as it races to the hospital, light from a distant star or galaxy “drops in pitch” as it recedes from Earth, becoming redder in color. Astronomers say the star’s light is redshifted.
Since the Webb’s primary mission is to discover the farthest objects to light up the early universe, they’ll also be the ones receding most quickly. Light that left the earliest galaxies started out as visible and ultraviolet but has been redshifted by the expansion of the universe into the near and mid-infrared range, beyond the reach of the human eye and most telescopes.
Earth’s atmosphere happily lets visible light – colors of the rainbow – and radio waves pass to the ground but blocks most of the infrared, ultraviolet, X-rays and gamma rays.
Unfortunately we see little of that light from the ground. Our atmosphere acts as a barricade to much of the infrared beaming from space. The only way to sample this crucial slice of light is to loft a telescope above the atmosphere into space.
Gold is used as a mirror coating instead of the more typical aluminum because gold is an excellent reflector of yellow, red and infrared light. Think about why gold is golden-colored in the first place – it absorbs blue and green light and reflects that delicious buttery yellow back to our eyes.
If the Hubble Space Telescope’s 94-inch (2.4 m) mirror were scaled to be large enough for Webb, it would be too heavy to launch into orbit. The Webb team had to find new ways to build the mirror so that it would be light enough – only one-tenth of the mass of Hubble’s mirror per unit area – yet very strong. The sides of the mirror also fold back like leaves on a table for a compact fit in a rocket. Credit: NASA
Not only does infrared vision help astronomers see back to the universe’s teething years, it also penetrates dust to see otherwise hidden stars and planets cloaked in their dusty birth cocoons. The Webb will spy stars 10 to 100 times fainter than the Hubble Space Telescope. Click HERE for a nice summary of the mission’s primary science goals.
Because all things radiate some amount of heat or infrared light, including the telescope itself, everything must be kept very cold otherwise pictures would look like fogged film. That’s why a large, five-layered sunshield will be deployed toward the sun, blocking both visible and infrared light from the sun, Earth and moon that would otherwise heat up the telescope. Looking like a hi-tech Viennese layer cake, the vacuum of space between each layer serves as fabulous insulation.
Shielded this way, the Webb’s operating temperature will drop to a nippy -370 F (-223 C) or just 50 degrees above absolute zero. From the L2 vantage point described earlier, all three objects are almost in a straight line behind the space telescope and straightforward to block with the sunshield.
Group photo of the Webb Telescope team with a full-scale model of the James Webb Space Telescope at the Goddard Space Flight Center in Maryland. Credit: NASA
Getting a telescope with 255 inches (21.3 feet) of mirror into space means designing the craft and optics to fold up into a compact package that resembles a backpack. Once in orbit, the Webb will be carefully unfolded and tested before observations begin. Electricity generated by solar cells will provide the power needed to run this magnificent machine.
Delays and cost overruns have been part of the project to build the telescope, but work continues and a launch window has been set. The thought of looking back to the time when the universe’s lights first turned on not only gives me the chills but makes it worth the few extra bucks.