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kite-shaped layers of the sun shield for NASA's James Webb Space Telescope (JWST) will block out the sun's ultraviolet radiation and protect the machinery from high-speed impacts of tiny space debris.No other space observatory has had such an enormous or membrane-based sun shield, said Mark Clampin, NASA's Webb Telescope Observatory project scientist at Goddard Space Flight Center in Greenbelt, Md. The Hubble Space Telescope is equipped with a light shield, but it is nothing like the new sun shield in the works for the Webb. In addition, other solar shields, such as the one aboard the infrared Spitzer Space Telescope, are fixed and so don't need to be deployed once in orbit, as the Webb will be.
"If you could imagine, this would be like going outdoors applying sunscreen of 'SPF' of 1.2 million on your skin," said Martin Mohan, program manager for the telescope at Northrop Grumman in California. "Previous to this crucial technology, materials with thermal properties that reflect the sun without being heated did not exist."
The James Webb Space Telescope, set for launch in 2013, will make observations mainly in the infrared range of the electromagnetic spectrum, with some capability in visible light. In space, JWST will reside in an orbit that's 1 million miles (1.5 million km) from Earth at what is called the second Lagrange point.
Giant sun blocker
Once it is erected, the sun shield will sit below and perpendicular to the telescope's upright, primary mirror, which will span 21.3 feet (6.5 meters) in diameter. (For comparison, Hubble's primary mirror measures about eight feet, or 2.4 meters, in diameter.) The sun shield consists of five layers of Kapton (a mylar-like material made by DuPont) with aluminum and special silicon coatings to reflect the sun's heat back into space.
While the sun-facing side of the sun shield will block out sunlight, the other side facing away from the sun will minimize light scattering as well as heat.
"The other side of the telescope you think of as being dark," Clampin said. "But when you're trying to find the first galaxies in the universe, even a small amount of light that's scattering from different places on the telescope can really hurt you."
Clampin added, "And we're talking about infrared light here, so heat sources on that side of the telescope can actually scatter light into the telescope and be seen as a signal, so you have to be very careful."
Overall, the sun shield will keep the telescope at a cryogenic minus 387 degrees Fahrenheit (minus 233 degrees Celsius). Any warmer than that and the heat given off from the telescope would corrupt the infrared observations.
"Infrared is heat radiation. In order to see the faint glow of infrared heat from distant stars and galaxies, the telescope has to be very cold," said Jonathan Gardner, NASA's deputy senior project scientist for the Webb Telescope at Goddard. "If the telescope were heated by sunlight or the warm glow of the Earth, the infrared light emitted by the telescope would outshine its targets, and it wouldn't be able to see anything."
New technologies
The sun shield technology is new in many more ways, its developers said.
For instance, complete passive cooling, in which refrigerants aren't used to keep temperatures down, is new to the field of telescopes and should keep JWST alive for its mission lifetime of no less than five and a half years, with an optimistic goal of 10 years, Clampin said. The other infrared observatories relied on cryogens, such as liquid nitrogen or liquid helium as their primary means of cooling, Clampin said.
"The advantage of that is it gives you a pretty long lifetime for the telescope," Clampin told SPACE.com, referring to the no-refrigerant approach for JWST. "The other approach is to put a telescope in a big fridge and fill it with coolant but that coolant slowly boils off and you have a limited life."
(For comparison, Spitzer relies partially on liquid helium to keep its instruments chilled.)
Due to its size, the sun shield will have to be compactly folded, much like a parachute, around the telescope in order to fit into its launch rocket.
"There have been a lot of missions that have unfolded large antenna," Clampin said. "To our knowledge it's one of the first missions where we are unfolding a really big membrane."
And so the engineers had to figure out how to fold the coated membranes, which make up the layers of the sun shield, to make sure they didn't get tangled upon opening and so that the unfolding didn't rub off any of the coatings, Clampin said.
Once the shield and the rest of the telescope are on their way to final orbit, engineers at Northrop Grumman will issue commands to the Webb Telescope to unfold the sun shield.
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