Telescopes from the Ground Up

Webb will orbit at a point four times farther away from Earth than the Moon. About 940,000 miles (1.5 million km) from Earth is a place called the Second Sun-Earth Lagrange Point, or L2. Here, Earth’s gravity will pull Webb along, so it can “keep up” with Earth in its path around the Sun. Only at the L2 point are the Sun and Earth always on the same side of Webb’s sunshield, giving the telescope almost full protection from their light and heat.

A unique mirror

Webb’s primary mirror is 60 times larger than Spitzer’s mirror and seven times larger than Hubble’s. The bigger the mirror, the more light it can collect, so Webb will be able to see back to about 200 million years after the Big Bang, when the very first stars and galaxies began to form. Hubble, which sees mostly in visible light and has one of the largest mirrors currently in space, can see back to about 800 million years after the Big Bang.

Webb engineers and scientists are getting such a large mirror into space by constructing it out of 18 hexagonal segments, then folding it up inside the rocket that will launch it into space. Once the telescope is in space, ground controllers will send a message ordering the mirror to unfurl, like a butterfly opening its wings.

Webb’s unique mirror is made out of a material called beryllium. Beryllium is lightweight and changes shape much less and more predictably than glass does in cold temperatures — which is important because the Webb telescope will be facing temperatures of -388 degrees Fahrenheit (-233 degrees Celsius). Each segment is also coated with an extremely thin layer of 24- karat gold, which reflects red and infrared light exceptionally well.

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Space Telescopes


Portrait of James E. Webb.
James Webb
The NASA administrator who ushered in a bold new era of space science in the 1960s
Read about him