[meteorite-list] WISE Finds Fewer Asteroids Near Earth

[Ron Baalke]

Sept. 29, 2011

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

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


RELEASE: 11-333

NASA SPACE TELESCOPE FINDS FEWER ASTEROIDS NEAR EARTH

WASHINGTON -- 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 (JPL) 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 (NEO) 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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