NASA Space Telescope Finds Fewer Asteroids Near Earth

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
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Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
Whitney.clavin@jpl.nasa.gov

Dwayne Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown@nasa.gov

News release: 2011-304 Sept. 29, 2011

NASA Space Telescope Finds Fewer Asteroids Near Earth

PASADENA, Calif. -- New observations by NASA's Wide-field Infrared Survey Explorer, or WISE,
show there are significantly fewer near-Earth asteroids in the mid-size range than previously thought.
The findings also indicate NASA has found more than 90 percent of the largest near-Earth asteroids,
meeting a goal agreed to with Congress in 1998.

Astronomers now estimate there are roughly 19,500 -- not 35,000 -- mid-size near-Earth asteroids.
Scientists say this improved understanding of the population may indicate the hazard to Earth could
be somewhat less than previously thought. However, the majority of these mid-size asteroids remain
to be discovered. More research also is needed to determine if fewer mid-size objects (between 330
and 3,300-feet wide) also mean fewer potentially hazardous asteroids, those that come closest to
Earth.

The results come from the most accurate census to date of near-Earth asteroids, the space rocks that
orbit within 120 million miles (195 million kilometers) of the sun into Earth's orbital vicinity. WISE
observed infrared light from those in the middle to large-size category. The survey project, called
NEOWISE, is the asteroid-hunting portion of the WISE mission. Study results appear in the
Astrophysical Journal.

"NEOWISE allowed us to take a look at a more representative slice of the near-Earth asteroid
numbers and make better estimates about the whole population," said Amy Mainzer, lead author of
the new study and principal investigator for the NEOWISE project at NASA's Jet Propulsion
Laboratory in Pasadena, Calif. "It's like a population census, where you poll a small group of people
to draw conclusions about the entire country."

WISE scanned the entire celestial sky twice in infrared light between January 2010 and February
2011, continuously snapping pictures of everything from distant galaxies to near-Earth asteroids and
comets. NEOWISE observed more than 100 thousand asteroids in the main belt between Mars and
Jupiter, in addition to at least 585 near Earth.

WISE captured a more accurate sample of the asteroid population than previous visible-light surveys
because its infrared detectors could see both dark and light objects. It is difficult for visible-light
telescopes to see the dim amounts of visible-light reflected by dark asteroids. Infrared-sensing
telescopes detect an object's heat, which is dependent on size and not reflective properties.

Though the WISE data reveal only a small decline in the estimated numbers for the largest near-Earth
asteroids, which are 3,300 feet (1 kilometer) and larger, they show 93 percent of the estimated
population have been found. This fulfills the initial "Spaceguard" goal agreed to with Congress.
These large asteroids are about the size of a small mountain and would have global consequences if
they were to strike Earth. The new data revise their total numbers from about 1,000 down to 981, of
which 911 already have been found. None of them represents a threat to Earth in the next few
centuries. It is believed that all near-Earth asteroids approximately 6 miles (10 kilometers) across, as
big as the one thought to have wiped out the dinosaurs, have been found.

"The risk of a really large asteroid impacting the Earth before we could find and warn of it has been
substantially reduced," said Tim Spahr, the director of the Minor Planet Center at the Harvard
Smithsonian Center for Astrophysics in Cambridge, Mass.

The situation is different for the mid-size asteroids, which could destroy a metropolitan area if they
were to impact in the wrong place. The NEOWISE results find a larger decline in the estimated
population for these bodies than what was observed for the largest asteroids. So far, the Spaceguard
effort has found and is tracking more than 5,200 near-Earth asteroids 330 feet or larger, leaving more
than an estimated 15,000 still to discover. In addition, scientists estimate there are more than a
million unknown smaller near-Earth asteroids that could cause damage if they were to impact Earth.

"NEOWISE was just the latest asset NASA has used to find Earth's nearest neighbors," said Lindley
Johnson, program executive for the Near Earth Object Observation Program at NASA Headquarters
in Washington. "The results complement ground-based observer efforts over the past 12 years. These
observers continue to track these objects and find even more."

WISE is managed and operated by JPL for NASA's Science Mission Directorate in Washington. The
principal investigator, Edward Wright, is at the University of California, Los Angeles. The WISE
science instrument was built by the Space Dynamics Laboratory in Logan, Utah, and the spacecraft
was built by Ball Aerospace and Technologies Corp. in Boulder, Colo. Science operations and data
processing occur at the Infrared Processing and Analysis Center at the California Institute of
Technology.

For more information about the mission, visit: http://www.nasa.gov/wise .

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