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.
"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.
advertisement
.+Credit%3A+NASA%2FESA){kind=link}