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TWO AND A HALF years from now, the James Webb Telescope will surge into space to replace the current best eye in the sky, the Hubble Telescope. The Hubble has brought striking advances in astronomy, and the Webb promises to be 100 times more sensitive. Not without challenges, though: For two decades now, Webb technicalities have involved hundreds of specialists in 17 countries with one of mankind’s most complex scientific endeavors.
The February 19, 2016, issue of Science, weekly magazine of the American Association for the Advancement of Science, describes the amazing scope of this project in “The Next Big Eye,” by Daniel Clery. In particular, a two-page graphic in the article is a perfect example of relaying fascinating details in a compact format.
The project’s timeline could actually stretch back to the mid-1990s, not long after astronauts corrected the Hubble’s optics. Scientists at the time recognized the need for a telescope dedicated to the infrared spectrum—for a very interesting reason.
As shown in one of the Science graphics, our visible spectrum is only a tiny portion of electromagnetic waves. What’s more, as noted in Science, visible light from the earliest stars and galaxies gets stretched so much by expansion of the Big Bang universe that it ends up in the infrared range by the time it reaches us. The Hubble examines only a small portion of this range; the Webb is dedicated to its entirety.
The Hubble orbits at around 7000 km (4300 miles) above Earth. By contrast, for enhanced sensitivity, the Webb will orbit considerably farther out, at L2, the second Lagrange point. This is the point in space on the opposite side of Earth from the Sun where these two bodies’ gravitational forces balance the centrifugal force on the orbiting object.
L2 is about 1.5 million km (932,000 miles) from Earth, roughly four times the distance to the Moon. And, thus, unlike the Hubble, the Webb will be far beyond any fine tuning or repair by astronauts.
The Webb will take 29 days after its October 2018 launch to reach L2. The graphic shows its elaborate orchestration of deployment. Its primary mirror is composed of 18 hexagonal segments, each of gold-plated beryllium and adjustable within a graphite-composite framework.
Like other Webb materials, the graphite backplane and beryllium mirrors were chosen to meet serious design challenges. Graphite is rigid, lightweight and retains its shape in cryogenic temperatures. Beryllium is preferable to glass (the Hubble’s mirror material) for several reasons: It’s strong enough to withstand the immense g-loads of launch. It’s lightweight. And it behaves predictably in warping from Earth clime to near absolute zero of space.
This matter of space clime is paradoxical: Absolute zero, the lowest theoretical temperature, is defined by minimal motion of particles. It’s 0 Kelvin, – 273.15 degrees Celsius, -459.67 degrees Fahrenheit. (As a bit of temperature trivia, saying “degrees Kelvin” is redundant.)
As shown in the Science graphic, Sun load on the Webb would overwhelm faint infrared signals, so the telescope requires its own sunshield. The hot side would be 85 deg C (185 deg F). Its sunshield will reduce this by a factor of a million, to 40 K, (– 233 deg C, -387.4 deg F). The Webb’s sunshield unwraps into a huge multilayered sheet.
Attendees of South by Southwest in Austin, Texas, in 2012 got a first-hand look at a full-scale model of the Webb. Involved in its construction were the Astronomy Department of the University of Texas at Austin and Microsoft Research. Over a three-day run, more than 10,000 people gave the Webb model a close examination.
Such a rare treat will be well nigh impossible come 2018 when the real Webb reaches its L2 home. ds
© Dennis Simanaitis, SimanaitisSays.com, 2016