[meteorite-list] NASA Survey Counts Potentially Hazardous Asteroids (WISE)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 16 May 2012 14:59:00 -0700 (PDT)
Message-ID: <201205162159.q4GLx0pW011798_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2012-138

NASA Survey Counts Potentially Hazardous Asteroids
Jet Propulsion Laboratory
May 16, 2012

PASADENA, Calif. -- Observations from NASA's Wide-field Infrared Survey
Explorer (WISE) have led to the best assessment yet of our solar
system's population of potentially hazardous asteroids. The results
reveal new information about their total numbers, origins and the
possible dangers they may pose.

Potentially hazardous asteroids, or PHAs, are a subset of the larger
group of near-Earth asteroids. The PHAs have the closest orbits to
Earth's, coming within five million miles (about eight million
kilometers), and they are big enough to survive passing through Earth's
atmosphere and cause damage on a regional, or greater, scale.

The new results come from the asteroid-hunting portion of the WISE
mission, called NEOWISE. The project sampled 107 PHAs to make
predictions about the entire population as a whole. Findings indicate
there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger
than 330 feet (about 100 meters). So far, an estimated 20 to 30 percent
of these objects have been found.

While previous estimates of PHAs predicted similar numbers, they were
rough approximations. NEOWISE has generated a more credible estimate of
the objects' total numbers and sizes.

"The NEOWISE analysis shows us we've made a good start at finding those
objects that truly represent an impact hazard to Earth," said Lindley
Johnson, program executive for the Near-Earth Object Observation Program
at NASA Headquarters in Washington. "But we've many more to find, and it
will take a concerted effort during the next couple of decades to find
all of them that could do serious damage or be a mission destination in
the future."

The new analysis also suggests that about twice as many PHAs as
previously thought are likely to reside in "lower-inclination" orbits,
which are more aligned with the plane of Earth's orbit. In addition,
these lower-inclination objects appear to be somewhat brighter and
smaller than the other near-Earth asteroids that spend more time far
away from Earth. A possible explanation is that many of the PHAs may
have originated from a collision between two asteroids in the main belt
lying between Mars and Jupiter. A larger body with a low-inclination
orbit may have broken up in the main belt, causing some of the fragments
to drift into orbits closer to Earth and eventually become PHAs.

Asteroids with lower-inclination orbits would be more likely to
encounter Earth and would be easier to reach. The results therefore
suggest more near-Earth objects might be available for future robotic or
human missions.

"NASA's NEOWISE project, which wasn't originally planned as part of
WISE, has turned out to be a huge bonus," said Amy Mainzer, NEOWISE
principal investigator, at NASA's Jet Propulsion Laboratory in Pasadena,
Calif. "Everything we can learn about these objects helps us understand
their origins and fate. Our team was surprised to find the overabundance
of low-inclination PHAs. Because they will tend to make more close
approaches to Earth, these targets can provide the best opportunities
for the next generation of human and robotic exploration."

The discovery that many PHAs tend to be bright says something about
their composition; they are more likely to be either stony, like
granite, or metallic. This type of information is important in assessing
the space rocks' potential hazards to Earth. The composition of the
bodies would affect how quickly they might burn up in our atmosphere if
an encounter were to take place.

The WISE spacecraft scanned the sky twice in infrared light before
entering hibernation mode in early 2011. It catalogued hundreds of
millions of objects, including super-luminous galaxies, stellar
nurseries and closer-to-home asteroids. The NEOWISE project snapped
images of about 600 near-Earth asteroids, about 135 of which were new
discoveries. Because the telescope detected the infrared light, or heat,
of asteroids, it was able to pick up both light and dark objects,
resulting in a more representative look at the entire population. The
infrared data allowed astronomers to make good measurements of the
asteroids' diameters and, when combined with visible light observations,
how much sunlight they reflect.

JPL manages, and operates the Wide-field Infrared Survey Explorer for
NASA's Science Mission Directorate, Washington. The principal
investigator, Edward Wright, is at UCLA. The mission was competitively
selected under NASA's Explorers Program managed by the Goddard Space
Flight Center, Greenbelt, Md. The science instrument was built by the
Space Dynamics Laboratory, Logan, Utah, and the spacecraft was built by
Ball Aerospace & Technologies Corp., Boulder, Colo. Science operations
and data processing and archiving take place at the Infrared Processing
and Analysis Center at the California Institute of Technology in
Pasadena. Caltech manages JPL for NASA.

More information is online at http://www.nasa.gov/wise and
http://jpl.nasa.gov/wise .

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin at jpl.nasa.gov

J.D. Harrington 202-358-5241
Headquarters, Washington
j.d.harrington at nasa.gov

2012-138
Received on Wed 16 May 2012 05:59:00 PM PDT