New Count of Potentially Hazardous Asteroids

A New Count of Potentially Hazardous Asteroids

May 16, 2012:  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. Also known as "PHAs," these asteroids have orbits that come within five million miles (about eight million kilometers) of Earth, and they are big enough to survive passing through Earth's atmosphere and cause damage on a regional, or greater, scale.

The asteroid-hunting portion of the WISE mission, called NEOWISE, sampled 107 PHAs to make predictions about the 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.

In this simulated view of the near-Earth asteroid population, potentially hazardous asteroids (PHAs) are denoted in orange. Less dangerous near-Earth asteroids are blue. Earth's orbit is green. [more]

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. Because the WISE space 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 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. "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."

This orbit diagram illustrates the difference between a PHA and less hazardous near-Earth asteroids (NEA). [more]

The new analysis suggests that about twice as many PHAs as previously thought reside in low-inclination orbits, which are roughly aligned with the plane of Earth's orbit.

"Our team was surprised to find the overabundance of low-inclination PHAs," said Amy Mainzer, NEOWISE principal investigator, at NASA's Jet Propulsion Laboratory. "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 exploration."

The NEOWISE analysis suggests a possible origin for the low-inclinaton PHAs: Many of them could 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.

The lower-inclination PHAs appear to be somewhat brighter and smaller than other near-Earth asteroids. The discovery that 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 NEOWISE project, which wasn't originally planned as part of WISE, has turned out to be a huge bonus," said Mainzer.  "Everything we can learn about these objects helps us understand their origins and fate."

The NEOWISE results have been accepted for publication in the Astrophysical Journal.

SOURCE: http://science.nasa.gov/science-news/science-at-nasa/2012/16may_pha/

On May 17, 2012 there were 1287 potentially hazardous asteroids.

Recent & Upcoming Earth-asteroid encounters:

Asteroid	Date(UT)	Miss Distance	Mag.	Size
2012 JU	May 13	0.5 LD	--	10 m
2012 KA	May 17	0.6 LD	--	8 m
2010 KK37	May 19	2.3 LD	--	31 m
4183 Cuno	May 20	47.4 LD	--	5.7 km
2012 JV11	May 22	6.7 LD	--	69 m
2002 VX94	May 26	72.8 LD	--	1.1 km
2002 AC	Jun 16	62.2 LD	--	1.2 km
1999 BJ8	Jun 16	68.8 LD	--	1.1 km
2005 GO21	Jun 21	17.1 LD	--	2.2 km
2003 KU2	Jul 15	40.3 LD	--	1.3 km
2004 EW9	Jul 16	46.8 LD	--	2.1 km
2002 AM31	Jul 22	13.7 LD	--	1.0 km
37655 Illapa	Aug 12	37 LD	--	1.2 km

Notes: LD means "Lunar Distance." 1 LD = 384,401 km, the distance between Earth and the Moon. 1 LD also equals 0.00256 AU. MAG is the visual magnitude of the asteroid on the date of closest approach.
SOURCE:http://spaceweather.com/

Increased Meteor and Meteorite Activity 2012. Fireball February.

The Fireballs of February

Feb. 22, 2012:  In the middle of the night on February 13th, something disturbed the animal population of rural Portal, Georgia. Cows started mooing anxiously and local dogs howled at the sky. The cause of the commotion was a rock from space.

"At 1:43 AM Eastern, I witnessed an amazing fireball," reports Portal resident Henry Strickland. "It was very large and lit up half the sky as it fragmented. The event set dogs barking and upset cattle, which began to make excited sounds. I regret I didn't have a camera; it lasted nearly 6 seconds."

Strickland witnessed one of the unusual "Fireballs of February."

A fireball over north Georgia recorded on Feb. 13th by a NASA all-sky camera in Walker Co., GA. [video]

"This month, some big space rocks have been hitting Earth's atmosphere," says Bill Cooke of NASA's Meteoroid Environment Office. "There have been five or six notable fireballs that might have dropped meteorites around the United States."

It’s not the number of fireballs that has researchers puzzled. So far, fireball counts in February 2012 are about normal. Instead, it's the appearance and trajectory of the fireballs that sets them apart.

"These fireballs are particularly slow and penetrating," explains meteor expert Peter Brown, a physics professor at the University of Western Ontario. "They hit the top of the atmosphere moving slower than 15 km/s, decelerate rapidly, and make it to within 50 km of Earth’s surface."

The action began on the evening of February 1st when a fireball over central Texas wowed thousands of onlookers in the Dallas-Fort Worth area.

"It was brighter and long-lasting than anything I've seen before," reports eye-witness Daryn Morran. "The fireball took about 8 seconds to cross the sky. I could see the fireball start to slow down; then it exploded like a firecracker artillery shell into several pieces, flickered a few more times and then slowly burned out."

Another observer in Coppell, Texas, reported a loud double boom as "the object broke into two major chunks with many smaller pieces."

The fireball was bright enough to be seen on NASA cameras located in New Mexico more than 500 miles away. "It was about as bright as the full Moon," says Cooke. Based on the NASA imagery and other observations, Cooke estimates that the object was 1 to 2 meters in diameter.

So far in February, NASA's All-Sky Fireball Network has photographed about a half a dozen bright meteors that belong to this oddball category. They range in size from basketballs to buses, and all share the same slow entry speed and deep atmospheric penetration. Cooke has analyzed their orbits and come to a surprising conclusion:

This camera is part of NASA's All-Sky Fireball Network. [more] 

"They all hail from the asteroid belt—but not from a single location in the asteroid belt," he says. "There is no common source for these fireballs, which is puzzling."

This isn't the first time sky watchers have noticed odd fireballs in February. In fact, the "Fireballs of February" are a bit of a legend in meteor circles.

Brown explains: "Back in the 1960s and 70s, amateur astronomers noticed an increase in the number of bright, sound-producing deep-penetrating fireballs during the month of February. The numbers seemed significant, especially when you consider that there are few people outside at night in winter. Follow-up studies in the late 1980s suggested no big increase in the rate of February fireballs. Nevertheless, we've always wondered if something was going on."

Indeed, a 1990 study by astronomer Ian Holliday suggests that the 'February Fireballs' are real. He analyzed photographic records of about a thousand fireballs from the 1970s and 80s and found evidence for a fireball stream intersecting Earth's orbit in February. He also found signs of fireball streams in late summer and fall. The results are controversial, however. Even Halliday recognized some big statistical uncertainties in his results.

NASA's growing All-Sky Fireball Network could end up solving the mystery. Cooke and colleagues are adding cameras all the time, spreading the network's coverage across North America for a dense, uninterrupted sampling of the night sky.

"The beauty of our smart multi-camera system," notes Cooke, "is that it measures orbits almost instantly. We know right away when a fireball flurry is underway—and we can tell where the meteoroids came from." This kind of instant data is almost unprecedented in meteor science, and promises new insights into the origin of February’s fireballs.

Meanwhile, the month isn't over yet. "If the cows and dogs start raising a ruckus tonight," advises Cooke, "go out and take a look."

source: NASA