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September
15, 2009: Ninjas knew how to be stealthy: Be dark.
Emit very little light. Move in the shadows between bright
places.
In
modern warfare, though, ninjas would be sitting ducks. Their
black clothes may be hard to see at night with the naked eye,
but their warm bodies would be clearly visible to a soldier
wearing infrared goggles.
To
hunt for the "ninjas" of the cosmos — dim objects
that lurk in the vast dark spaces between planets and stars
— scientists are building by far the most sensitive set of
wide-angle infrared goggles ever, a space telescope called
the Widefield Infrared Survey Explorer (WISE).
 Right:
An artist's concept of the Widefield Infrared Survey Explorer
(WISE). [more]
WISE
will scan the entire sky at infrared wavelengths, creating
the most comprehensive catalog yet of dark and dim objects
in the cosmos: vast dust clouds, brown dwarf stars, asteroids
— even large, nearby asteroids that might pose a threat to
Earth.
Surveys
of nearby asteroids based on visible-light telescopes could
be skewed toward asteroids with more-reflective surfaces.
"If there's a significant population of asteroids nearby
that are very dark, they will have been missed by these previous
surveys," says Edward Wright, principal investigator
for WISE and a physicist at the University of California in
Los Angeles.
The
full-sky infrared map produced by WISE will reveal even these
darker asteroids, mapping the locations and sizes of roughly
200,000 asteroids and giving scientists a clearer idea of
how many large and potentially dangerous asteroids are nearby.
WISE
will also help answer questions about the formation of stars
and the evolution and structure of galaxies, including our
own Milky Way.
And
the discoveries won't likely stop there.
"When
you look at the sky with new sensitivity and a new wavelength
band, like WISE is going to do, you're going to find new things
that you didn't know were out there," Wright says.
Stars
emit visible light in part because they're so hot. But cooler
objects like asteroids emit light too, just at longer, infrared
wavelengths that are invisible to the unaided eye. In fact,
any object warmer than absolute zero will emit at least some
infrared light.
Unfortunately,
this fact makes building an infrared telescope rather difficult.
Without a coolant, the telescope itself would glow in infrared
light just like all other warm objects do. It would be like
building a normal, visible-light telescope out of Times Square
billboard lights: The telescope would be blinded by its own
glow.
 To
solve this problem, WISE will cool its components to about
15°C above absolute zero (or -258°C) using a block of solid
hydrogen. Mission scientists chose solid hydrogen over liquid
helium, which is often used in research for cooling materials
to near absolute zero, because a smaller volume of solid hydrogen
can do the job. "The cooling power is much higher for
hydrogen than for helium," Wright explains. When launching
a telescope into space, being smaller and lighter saves money.
Right:
The WISE 2-stage solid hydrogen dewar resembles R2D2 from
Star Wars. [more]
Previous
space telescopes such as the Infrared Astronomical Satellite
(IRAS) have mapped the sky at infrared wavelengths before,
but WISE will be hundreds of times more sensitive. While other
missions could only see diffuse sources of infrared light
such as large dust clouds, WISE will be able to see asteroids
and other point sources.
After
it launches into orbit as early as this December, WISE will
spend 6 months mapping the sky, during which it will download
its data to ground stations 4 times each day. Analyzing that
data should give scientists some new insights into the cosmos.
For
example, one theory posits that most of the stars in the Universe
were formed in the press of colliding galaxies. When galaxies
collide, interstellar clouds of gas and dust smash together,
compressing the clouds and starting a self-perpetuating cycle
of gravitational collapse. The result is a flurry of starbirth.
Newborn stars are usually concealed by the dusty clouds they
are born in. Ordinary light cannot escape, but infrared light
can.
Above:
Colliding spiral galaxies NGC 2207 and IC 2163. Image credit:
Hubble Space Telescope. [more]
WISE
will be able to detect infrared emissions from the most active
star-forming regions. This will help scientists know how rapidly
stars are formed during galactic collisions, which could indicate
how many of the universe's stars were formed this way.
WISE
will also target dim "failed stars" called brown
dwarfs that outnumber ordinary stars by a wide margin. Mapping
brown dwarfs in the Milky Way may reveal much about the structure
and evolution of our own galaxy.
And
this could be just the beginning of the discoveries scientists
make once WISE puts the spotlight on stealthy denizens of
the dark.
Author: Patrick Barry | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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