Researchers unlock the secret of the meteorite fell in Russia in February

Twice in just over a century , Russia was visited she could well have done with the entry into the atmosphere of two large meteorites . The first , in 1908, is known as the Tunguska event name and had only distant witnesses. The car was sprayed a few kilometers above sea level, above which it taiga trees lying on the ground, like spaghetti dropped a box, over 2000 km2. The second visit was less physical impact but has instead made ​​the rounds of the media world on February 15 by the grace of mobile phones with integrated cameras picture. Just the meteorite she had crossed the sky of the city of Chelyabinsk, on the eastern slope of the Urals, a burst of shots and movies strafing Internet .

This is another burst , less spectacular but more scientific , which triggered this Wednesday, November 6 , with the publication of three studies on " superbolide Chelyabinsk " as researchers call it. Two papers in Nature and in Science , which is rare enough to be noticed , has advanced the publication of this study of a day to be in line with the high concurrent review . So it took less than nine months astronomers to discover the secret of this celestial object that nobody saw it coming and of unknown origin. Researchers have extensively relied on the video posted on the Web in order to reconstruct , using astronomical software , the final trajectory of the object but also its orbit before his arrival on Earth.

Here is the scientific reconstruction of what happened that February 15, 2013 . It is the dawn of the Urals , but it is an unexpected star , shining like 30 suns that suddenly illuminates the morning. The asteroid is between a little less than 20 meters in diameter and weighs between 12,000 and 13,000 tons, more than the Eiffel Tower . He was spotted for the first time at 97 km altitude and drove almost 69,000 km / h. His entry into the layers of increasingly dense atmosphere warms quickly and door to the infernal temperatures , while making it very bright , will be because of it.

A 83 km altitude , abrasion and fragmentation of the meteor begin and it will leave a cloud of debris in its wake. Multiple dislocations occur between 40 and 30 km altitude and 29 km is about twenty pieces of about 10 tons each that can be found in the sky. They themselves will disintegrate as and when they fall , scattering a shower of small meteorites along the final trajectory. Broken into pieces , the fireball slows significantly even if its speed remains significant for us : 48 600 km / h to 20 km altitude, 23 000 km / h to 15 km, 11,500 km / h to 12 km ... Only a large fragment of more than 600 kg remains unresolved and falls into a lake (then frozen ), where he was drafted in October.

This trio of studies we draw several lessons . Firstly the energy carried by the meteorite , estimated at more than 500 kilotons of TNT , or about 35 times the Hiroshima is higher than what was originally estimated atomic bomb. There are fortunately had no shot at the event even though more than a thousand people were injured , usually cut by glass that shockwave shattered . The survey population that traces the Science study , says that several people have experienced " sunburn " . Over 60% of respondents said they felt the heat from the fireball and some had sore eyes so the light was intense.

Another lesson : the origin of the meteorite. During the six weeks preceding his arrival on Earth, she was hiding in the sunlight , so that it was invisible to programs used to detect asteroids that could collide with our planet. This has not prevented teams of researchers to reconstruct its orbit. The object in question comes from the vast asteroid belt between Mars and Jupiter. Especially , its trajectory is almost modeled on that of a much larger body, the asteroid 86039 , which is 2.2 km in diameter. For astronomers , it is probably more than a coincidence : there is only one chance in 10,000 that this is the effect of chance. The researchers therefore hypothesize that the asteroid 86039 was hit by one of his colleagues and, in shock, a large rock is detached, which came to rest on top of the Urals. Monitored for several years, meanwhile 86039 is part of these celestial wanderers potentially dangerous , the path may cross ours one day. His next visit is scheduled for March 2014 at a safe distance comfortable about 50 million kilometers.

Finally, two studies published in Nature looks at the statistics of the last twenty years on entry into the atmosphere similar to that of Chelyabinsk asteroids, that is , those with a diameter between 10 and 50 meters. Although the analysis restricted to a relatively small sample is limited , the results are surprising since it appears that this kind of event is ten times more common than was thought until now! However, unlike the very massive asteroids ( and very dangerous ) that are followed , smaller objects are much less known , although they are likely to cause locally severe damage . Of the approximately 10,000 asteroids identified likely to walk in the vicinity of Earth , less than 1500 entering class of objects less than 30 meters in diameter while it is estimated the total number ... several million. A recognition that we are not equipped to detect correctly.

Pierre Barthélémy ( follow me on Twitter here or on Facebook here )