WISE mission finds fewer asteroids near Earth

Sep 29, 2011
NEOWISE observations indicate that there are at least 40 percent fewer near-Earth asteroids in total that are larger than 330 feet, or 100 meters. Image credit: NASA/JPL-Caltech

(PhysOrg.com) -- 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 that orbit within 120 million miles (195 million kilometers) of the sun into Earth's orbital vicinity. WISE observed 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 .

This chart shows how data from NASA's Wide-field Infrared Survey Explorer, or WISE, has led to revisions in the estimated population of near-Earth asteroids. Image credit: NASA/JPL-Caltech

"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.

This chart illustrates how infrared is used to more accurately determine an asteroid's size. Image credit: NASA/JPL-Caltech

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.

This chart illustrates why infrared-sensing telescopes are more suited to finding small, dark asteroids than telescopes that detect visible light. Image credit: NASA/JPL-Caltech

"The risk of a really large 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 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."

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User comments : 6

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Jaeherys
not rated yet Sep 29, 2011
How do they estimate the total amount of asteroids?
eachus
4.3 / 5 (6) Sep 29, 2011
How do they estimate the total amount of asteroids?


You look at the rate at which they are found. In a simplified model--constant effort and technology over time--the number discovered per year will best be estimated by a constant fraction of the asteroids remaining to be discovered. Take two year's numbers and solve for the unknown, or use regression techniques. NEOWISE can come pretty close to this model.

The fun (for me at least) is in the statistics part. You want to do your regression using non-least square methods, or transform the data so that the errors are normally distributed. (If you haven't studied non-parametric statistics, don't worry about it. Least squares and normal theory should come close enough. ;-)
that_guy
5 / 5 (2) Sep 29, 2011
How do they estimate the total amount of asteroids?

In simpler terms: This satellite is capable of detecting most or all of the larger asteroids in our neighborhood, even if it would take a while. Since they know how much power this thing has, they extrapolate how many asteroids there are based on how many they've already found in a given amount of time.

omatumr
1 / 5 (4) Sep 29, 2011
Does the gravitational field of a planet "sweep up" nearby asteroids?

With kind regards,
Oliver K. Manuel
http://myprofile....anuelo09
Jaeherys
5 / 5 (1) Sep 29, 2011
@eachus
Haven't done non-parmetric statistics but least squares I can handle, with the textbook in front of me :P. Thanks for the explanation!

@that_guy
It's never as simple as it seems is it :D? But makes sense, thanks for the further clarification.
that_guy
5 / 5 (2) Sep 29, 2011
@eachus
Haven't done non-parmetric statistics but least squares I can handle, with the textbook in front of me :P. Thanks for the explanation!

@that_guy
It's never as simple as it seems is it :D? But makes sense, thanks for the further clarification.

You're right...my explanation is more analagous to how they do it than specifically accurate to how they do it. But I figure, unless you want to do the actual math, it's easier to get the general concept across.

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