Huge Sun Shield Built for Space Telescope

The largest sun shield ever made has been created for the Hubble Space Telescope's successor. The enormous sun blocker will unfold in space into the size of a tennis court, despite being hundreds of thousands of miles away from the help of a human hand.

The 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. ... - space