[meteorite-list] WISE Asteroid Survey Will Continue Through January

[Ron Baalke]

http://www.spaceflightnow.com/news/n1012/19wise/

WISE asteroid survey will continue through January
BY STEPHEN CLARK 
SPACEFLIGHT NOW
December 19, 2010

NASA has granted funding for the WISE infrared telescope to finish an
extra full sky scan, giving scientists an additional opportunity to comb
the inner solar system for hard-to-see asteroids that could threaten Earth.

The observatory has been circling Earth since launching in December
2009, but the instrument lost some of its sensitivity when a frozen
block of solid hydrogen was used up this fall.

The mission was supposed to end when with the loss of hydrogen, but NASA
budgeted $1.6 million to keep the telescope working through the end of
January, according to Trent Perrotto, a NASA spokesperson.

Tasked with finding near-Earth objects, or NEOs, the extended mission is
utilizing two shorter wavelength channels inside the Wide-field Infrared
Survey Explorer to scan the cosmos for an extra few months.

WISE's sensors are embedded inside a cryostat, a Thermos-like insulated
canister that contained solid hydrogen to chill the telescope's
instrument down to about -430 degrees Fahrenheit. Such cold temperatures
were necessary to make the observatory sensitive enough to see the
infrared light from cold star-forming dust clouds, distant galaxies and
nearby asteroids and comets invisible to conventional visible telescopes.

The hydrogen gradually sublimated into gas and escaped into space, and
WISE exhausted its cryogenic tanks around the end of September,
according to Amy Mainzer, principal investigator for the NEOWISE mission
extension.

Without the chill of hydrogen, the temperatures inside WISE's instrument
bay started to rise.

"The telescope is still warming up," Mainzer said.

NEOWISE was funded to continue operating the spacecraft through the end
of January, enough time to finish an additional survey of the solar
system. WISE's 16-inch telescope observes the universe in a criss-cross
pattern that takes about six months to see the whole sky.

The hydrogen lasted long enough to finish one-and-a-half sky scans, but
scientists wanted to finish the second survey with two of WISE's four
infrared channels unaffected by the loss of hydrogen.

The mission is designed to see some of the coldest parts of the
universe. WISE's detectors were immersed in cryogenic fluid to ensure
the observatory was not overwhelmed by its own heat.

It costs about $400,000 per month to operate WISE. A one-month extension
through October confirmed WISE's 3.4-micron and 4.6-micron detectors,
designed to observe in shorter wavelengths, still functioned with enough
sensitivity to discover main belt asteroids, objects crossing Earth's
orbit and nearby brown dwarfs, or failed stars.

"For asteroids and solar systems objects, this means that we do have
some reduced sensitivity because these things tend to show up most
brightly in the longer wavelengths," Mainzer said. "However, there is
still plenty we can see in the shorter wavelengths. We're still
detecting asteroids."

According to Mainzer, WISE has observed more than 155,000 asteroids,
comets and other minor planets. WISE is responsible for more than 34,000
new solar system discoveries, mostly asteroids in the main belt between
the orbits of Mars and Jupiter.

The mission has spotted about 500 near-Earth objects and discovered
approximately 120 asteroids and comets that regularly pass close to Earth.

WISE is the only space-based telescope capable of discovering asteroids.
Other observatories, such as Hubble and Spitzer, are better suited for
detailed follow-up studies of known objects.

"What we're doing is sort of a blind search," Mainzer said. "We're
carrying out a predetermined survey pattern, and whatever happens to
cross into our path, we see. That has the great benefit of being an
independent survey, meaning that we're not depending on other people to
supply us targets. It's a survey that is not as biased by other people's
prior knowledge."

Ground-based observatories see the sky in visible wavelengths, but a
large fraction of asteroids are easiest to see through radiated heat,
giving infrared missions like WISE an advantage.

"The data sets are highly complementary, because if you have both
infrared and visible data on a particular asteroid, then you can
determine its albedo, which is its reflectivity," Mainzer said. "That's
an important number to know because it helps you understand a little bit
about its physical properties."

Mainzer said new WISE discoveries are disseminated to the scientific
community within 10 days, giving ground observatories and other space
telescopes a chance for focused imaging.

The mission's ground segment includes a system that automatically
searches imagery for potential asteroids, then compares the data with
known objects before declaring a new discovery.

WISE has downlinked about one million images since running out of
hydrogen this fall, according to Mainzer.

"The sky is a very big place, and we've taken images of all of it,"
Mainzer said. "It's just an enormous quantity of data that people are
going to be sifting through for decades."

Perrotto said NASA has received no requests to extend NEOWISE beyond
January, "though that could change."

According to Mainzer, controllers will put WISE in hibernation after the
extended mission is complete. It will continue orbiting more than 325
miles above Earth.

"It will be safe, so if anyone does want to use it again in the future,
it will be there for quite a while until the orbit decays and it will
burn up," Mainzer said. "At this time, we don't have any plans to
continue operating it after the end of January."