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CATCH A FALLING STAR

PITY THE poor meteorite researcher. To catch a falling star is a complex task indeed. The challenges and rewards of studying such extraterrestrial objects are described in “All eyes on shooting stars,” by Eric Hand, in Science magazine, published by the American Association for the Advancement of Science on September 19, 2014 (http://goo.gl/CXLKYP).

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Most of us know a meteorite from its fireball phase, where friction encountering the Earth’s atmosphere heats its edges into a glowing plasma. This happens at an altitude of 12 to 20 miles, roughly 65,000 to 100,000 ft.

A falling star is, at best, a fleeting sight. Friction slows the object, it cools and enters a “dark flight” phase. A dark-flight meteorite can be tracked at altitudes from about 6 miles down to 1 mile, 33,000 to 5000 ft. Radar is used, the same technology that weather stations scan for rain, snow and hail.

Starting with a fireball and continuing with its radar trace, researchers can identify a meteorite’s size and orbit. Data can narrow the probable ground strike to as small as a couple hundred acres.

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Weather radar shows a shower of fragments (arrow) from an Illinois meteorite fall in 2007. Image from Science, September 19, 2014.

Nevertheless, the odds are 50 to 1 against finding any pieces of a falling star. Worse, time is of the essence, especially if it rains.

The Sutter’s Mill meteorite is an example. This one caused a fireball and sonic boom above central California in April 22, 2012. It broke into fragments on entry; dozens of pieces were later retrieved.

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Fragments of the Sutter’s Mill meteorite, obtained from Henningsen Lotus Park, Lotus, California, about 130 miles northeast of San Francisco.

The first of the Sutter’s Mill fragments examined contained oldhamite, a fragile reactive mineral left over from the early days of the solar system (and, thus, a great find). Other fragments, discovered after rain had fallen, exhibited no oldhamite.

The reason? As soon as it’s exposed to water vapor, oldhamite readily transforms into ordinary calcium sulfate.

Eighty percent of the world’s weather stations are located outside the U.S., and international cooperation in meteorite data sharing is crucial. For example, there’s a European Fireball Network.

Researchers in Australia have set up state-of-the-art monitoring in the Outback, in an area the size of Alaska. Their Desert Fireball Network contains remote cameras taking high-resolution images of the sky at 30-second intervals. These time-lapse images allow calculation of trajectories and brightness of any meteorite in the area. Duration of the fireball phase can be used to estimate its speed.

This last bit of data is useful to separate extraterrestrial objects from our own space junk falling out of orbit. Stuff lofted by humans enters the atmosphere slower than 6.8 miles/second, about 24,600 mph.

The stations of the Australian Desert Fireball Network are solar-powered and collect data, unattended for months on end. Researchers expect to record five to ten falling stars a year. Meteorite recovery should be more straightforward than with encounters in other parts of the world. The Outback lacks precipitation. It is largely barren, buff-colored desert. And researchers will use a camera-equipped drone to aid the search for dark-colored fragments.

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The Catalonia meteorite, dawn, September 7, 2014, northeastern Spain. Image from Science, September 19, 2014.

There’s an element of intrigue in meteorite research. On September 7, 2014, a massive fireball brighter than the full moon disintegrated over Sant Antoni de Calogne, Catalonia, in northeastern Spain. It entered the international data base as SPMN070914. See http://goo.gl/nnXqkw for its fleeting image; http://goo.gl/J1dyLH for another.

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Another view of SPMN070914. Image from Jessica Lleonart via Vimeo.

Researchers at the Institute of Space Sciences in Barcelona had data from five fireball network cameras and a pair of pictures from eyewitnesses. They used these data to identify a likely impact area for fragments, but kept it secret until their own people could get there for a search.

Somewhere in Catalonia…. ds

© Dennis Simanaitis, SimanaitisSays.com, 2014

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