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Flood and mudslides in Algeria       Leonid magic     Survey predicts somewhat safer skies

Flood and mudslides in Algeria
Between Friday morning on Nov. 9 and noon the following Saturday, 220 millimeters of rain inundated the capital of Algeria, creating mudslides that left more than 750 people dead. The torrential, 36-hour downpour flooded 11 small rivers, called oueds in Arabic, which cross the city of Algiers and lead to the neighborhood of Bab El Oued, or in English, “The Door of the River.”


 “I believe it is not the size of the flood or rainfall that produced the disaster, but the weakness of the construction and the bad state of the sewage system,” says civil engineer Djillali Benouar, who teaches at the University of Algiers and is a member of the Alliance for Disaster Reduction. “The city center is located at the foot of a 306-meter high hill,” he says. “The water washed away everything it met on its way down to the sea, passing through the streets of the capital and taking with it cars, buses, people, mud, rocks and houses.”
 

[Residents walk through a devastated street in Algiers after torrential rains lashed the Algerian capital in November, sending water through its streets and mud down its hilly terrain. A 24-hour deluge pounded northern Algeria, killing at least 287 people and injuring hundreds more. AP Photo/Nabil]


The houses built on the flanks of the hill had shallow foundations, if any, and were without water evacuation routes, Benouar says. “These were weak structures vulnerable to heavy rain. Across the country, these kind of housing constructions have suffered much damage from rain because of degradation, inadequate repair, aging and neglect.”
 
The disaster in Algiers forced thousands of families to flee their homes. In some parts of the city mud rose 13 feet. Morocco and France responded immediately with rescue workers. France provided supplies such as tents, beds, blankets and water purification equipment. Tunisia also responded with medicine, blankets and food.
 
But local newspapers report outrage from residents over what they consider a lax response from Algeria’s government, even though the government said it would offer housing and financial assistance to those hardest hit.
 
“The floods laid bare the absence of political courage,” wrote the newspaper Le Matin, adding that “the men leading us don’t even know how to react to a weather bulletin.” From the capital, the Associated Press reported that armed soldiers had dispersed a spontaneous demonstration of several hundred people complaining about Algeria’s inadequate response to the crises and their own poor living conditions.
 
The heavy rainfall broke a drought that had forced residents to ration water since mid-October. Rainfall was expected to continue throughout late November, feeding the small oueds, which are typically dry in the summer and flooded in winter.

Christina Reed

Leonid magic
Every 33 years, the comet Tempel-Tuttle races around the sun, shedding a dusty cloud of debris in its wake. And every year, as Earth spins its way through the remnant material, stargazers hope to see blazing fireballs streak across the night sky roaring, it seems, from the constellation of Leo the lion. On Nov. 18, with a Cheshire-cat smile of a moon and clear skies in much of North America, the Leonid meteor shower put on a full-fledged storm with more than 1,000 shooting stars an hour.

[At right, the Leonid that created this smokey debris was very large and bright, illuminating the ground like lightning. The debris train was about 1 minute old when the exposure was started. Courtesy of Jim Bryan of Gaston, Ind.]

In one night, Earth crossed four different dust trails from the comet’s previous trips. In North America, the first witnesses of the storm caught a spectacular sight of falling stars from a trail the comet left during a trip around the sun in 1766. Those in Hawaii watched meteoroids leftover from the year 1799 turn to meteors in the sky. A midnight gathering of 150 people in China on the roof of a 550-year-old Ming Dynasty stone observatory saw early morning meteors on Nov. 19, made from an 1866 cloud, that outshone the city lights of Beijing.
 
Those in Sydney, Australia, reported disappointment as rain clouds blocked the view of shooting stars from dust left in space in 1699. But in other parts of Australia and Indonesia, the storm jumped to nearly 3,000 meteors per hour, according to Spaceweather.com.


Christina Reed


Survey predicts somewhat safer skies
A team of Princeton University astronomers has lowered Earth’s estimated risk of suffering a catastrophic impact from an asteroid. They have reduced the odds to about one in 5,000 over the next century.
 
Using data from the Sloan Digital Sky Survey (SDSS), the Princeton team, headed by Zeljko Ivezic, estimated that the solar system contains about 700,000 asteroids large enough to destroy a civilization. Previous estimates put the number at about 2 million, suggesting a 1 in 1,500 chance of impact. These large asteroids have a minimum diameter of 1 kilometer.

[Above, asteroids Mathilde (left) and Eros are shown at the same scale, as they were imaged by NEAR Shoemaker from about 1,800 kilometers on June 27, 1997, and Feb. 12, 2000, respectively. Mathilde is 56 kilometers across, and Eros is 33 kilometers long and 13 kilometers wide. Scientists have now lowered the estimate of impact risk from large asteroids. Courtsey of NASA Jet Propulsion Laboratory.]
 
The Princeton team’s estimate draws on observations of 10,000 asteroids, particularly faint ones — many more than past studies utilized. More importantly, Ivezic says, the SDSS five-color band imaging allowed them to gauge the size of the asteroids five times better than surveys using single-band data. Astronomers can identify asteroid type and size based on the color and amount of light they reflect.
 
The new impact risk, like those from most prior studies, relies on assumptions about a single impact event 65 million years ago when a 10-kilometer asteroid collided with Earth and created the Chicxulub crater. The researchers assume that such impacts occur on roughly 100 million-year intervals, using that statistic to calculate the impact odds for the more common, smaller asteroids.
 
Even with an accuracy margin of error of roughly a “factor of two,” the new estimates are lower than previous ones, Ivezic says, with the odds ranging from 1 in 2,500 to 1 in 10,000. “The main problem with the impact hazard estimate is its statistical nature: there is no guarantee that an impact won’t happen on a much shorter, or much longer, time scale,” Ivezic says. “In order to protect ourselves from a possible impact, we need to know precisely the trajectory of that particular object that is on the collision course.”
 
Ivezic’s team published their work in the November Astronomical Journal. SDSS is a multi-institutional collaboration that is mapping one-quarter of the sky. Ivezic says their asteroid database will soon be available to the public on the SDSS Web site: www.sdss.org.

Lisa M. Pinsker



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