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NASA Survey Counts Potentially Hazardous Asteroids
May 17, 2012
WASHINGTON -- 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
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.