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The little asteroid 2012 KA passes near Earth today before heading back out into space. Credit: JPL

Heads up: NASA says 4,700 potentially hazardous asteroids are out there

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This afternoon around 3 o'clock Central time, asteroid 2012 KA will fly silently past Earth at a distance of about 139,433 miles. That's a little more than halfway to the moon and well out of harm's way. 2012 KA was discovered only yesterday by the Mt. Lemmon Observatory as part of the larger Catalina Sky Survey. The survey's goal is to find and determine orbits of larger, potentially hazardous near-Earth asteroids (PHAs) from 100 meters (330 feet) on up. PHAs are a subset of a larger group of near-Earth asteroids or NEAs.

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This fly speck of an asteroid is only 25 feet across. Even if it were to hit the Earth - which it isn't - most of it would burn up on entry. Any fragments left over would be insignificant in terms of causing damage on the ground.

Tiny asteroids like this one are discovered routinely, sometimes only days before closest approach. There are a couple reasons for that. Being small, they're very faint. To spot one, the asteroid has to get close enough to be detectable. That doesn't allow much lead time between discovery and flyby. They also may approach Earth from the direction of the sun and be lost in the glare of daytime. Only when an asteroid appears at night are astronomers able to find, photograph and determine an orbit. At that point, only days or hours may remain before the rock buzzes the planet.

The results of a new study based on data gathered by NASA's Wide-field Infrared Survey Explorer (WISE) have led to a better assessment of potentially hazardous asteroids (PHAs). These are the ones measuring 330 feet across or larger that pass within 5 million miles of Earth and have the potential to cause significant destruction in the event of a collision.

Asteroids are warmed by the sun and radiate heat. WISE studied the sky in the light of infrared or heat waves and was able to see both light and dark asteroids (ones that might be missed by regular telescopes), resulting in a more representative look at the entire population. Based on studies of 107 PHAs with WISE, astronomers estimate there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger than 330 feet. To date, an estimated 20 to 30 percent of these objects have been found. Obviously, many remain to be discovered and orbits determined. Considering that we've only recently begun faint asteroid surveys with large telescopes, we've come a long way.

"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. 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," according to the study.

Because of their similar size and brightness, it's suggested that a collision between two asteroids with low-inclination orbits in the main asteroid belt liberated fragments that drifted into Earth's vicinity to become PHAs.

The diagrams, which are simulations based on real data, look very crowded. That's only because they squeeze an enormous volume of space into a box six inches wide. It's important to keep in mind there's a great deal of space between those dots!

While we're on the topic of small solar system bodies, Comet Garradd, the comet that's been visible in small telescopes and binoculars for more than a year, will soon disappear in twilight and fade away as it travels into the deep southern sky. It's still 9th magnitude and viewable in 4-inch and larger scopes. Larger instruments will show its faint dust tail pointing east. Garradd will slowly trek through the constellation Cancer the Crab through June. Get a last look while you can. It's still the brightest comet in the sky.

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