Again ? Yes, again… Not a month goes by without the gazettes announcing that an “asteroid is about to graze the Earth”. The last time ? It was October 26, with asteroid 2011 UC190 passing less than 700 kilometers away. But this time, this evening of November 000, it's serious. Because the asteroid 8 YU2011, compared to the small pebbles which generally pass over our heads, is enormous... This object, discovered in 55 and which is currently heading towards the Earth, measures 2005 meters in diameter and weighs more than fifty million tons! Do not panic, however: the trajectory of 400 YU2005 is known precisely, the asteroid will pass 55 kilometers from the Earth's surface, at half past midnight.
No risk of collision, therefore, of 2005 YU55 with the Earth. But for planetary scientists, it's a godsend, the opportunity to observe a star in detail almost as well as if a space probe had been dispatched to it. With their telescopes, astronomers will mainly study its physico-chemical characteristics, but there is no question of obtaining a good photograph of the asteroid: an object of 400 meters, seen at 324 kilometers, it is tiny, even for Hubble. In round numbers, 000 YU2005 will measure, at its closest to the Earth, 55 seconds of arc, which represents only a few pixels of a Hubble camera or the European Very Large Telescope. So, to find out more, astronomers will use major means: the Arecibo radio telescope, in Puerto Rico. This antenna, 0,2 meters in diameter, will be used as a radar. It will emit powerful radio radiation around 300 YU2005 and the echo of this radiation will then be received by the 55-meter antenna of the Deep Space Network, in Goldstone, California, before being analyzed by specialists. The objective aimed by the astronomers is ambitious, since it is a question of mapping the asteroid with an accuracy of about ten meters!
The fact remains that 2005 YU55 belongs to the list, which grows day by day, of "PHAs" (Potentially Hazardous Asteroid) that is to say asteroids potentially dangerous for the Earth: they approach less than eight million kilometers from the Earth's orbit and measure more than a hundred meters. Celestial mechanics specialists already know that, in the coming century, asteroid 2005 YU55 will not collide with Earth. Then we don't know. The gravitational disturbances caused by the stars it will encounter (it will pass 280 kilometers from Venus in 000, for example) make its future unpredictable.
Asteroid monitoring has made incredible progress in recent years. Today, more than 570 asteroids are known in the solar system, and among them, more than 000 near-Earth objects, that is to say objects, which, as their name suggests, cut the Earth's orbit... Finally, among these latter objects, more than 8000 asteroids are PHAs, including Apophis, which will pass within 1200 kilometers of the Earth's surface in April 40...
However, to reassure us, NASA has just published a study based on observations from its infrared satellite, Wise (Wide field Infrared Survey Explorer), which has tracked nearly one hundred thousand asteroids: according to American researchers, the census of near-Earth asteroids is on the right track… Nearly 90% of these objects, for those whose size exceeds 500 meters, would be already listed. For smaller objects, more difficult to detect, of course, the statistics are less good: we would know, still according to NASA, nearly 30% of objects 300 to 500 meters in diameter. Remain the bodies of less than 300 meters. For these small asteroids, the total number of which probably exceeds one million, almost nothing is known: generally, they are discovered when they literally pass under our noses! So, 2011 MD went 12 kilometers only last June, but that rock was only 000 meters tall. 20 TS2008 has passed 26 km, 7100 FU2004 has 162 km, and 6500 CQ2011 has only 1 km. All these last stars were large pebbles, measuring only a few meters. In recent history, only one star has - literally - brushed past the Earth: it is the "Montana meteor", a block of 5500 meters, which, in 15, passed 1972 kilometers above the United States, igniting the atmosphere in its path! Sometimes, of course, these rocks are on a collision course with Earth. When they do not measure more than thirty meters, they heat up, burn and explode in the atmosphere, and leave behind them only a harmless shower of shooting stars. In the end, in contemporary history, there has been only one spectacular collision with the Earth; it was in June 60. Nobody knows what fell then in the Siberian forest, shaving hundreds of kilometers of forest: probably a comet, or an asteroid of about fifty meters...
In the next quarter century, astronomers hope to have identified virtually every object potentially dangerous to us. We have time to see it coming: an asteroid of about fifty meters must fall on Earth every two hundred and fifty years. As for objects the size of 2005 YU55, one falls every twenty-five thousand years, on average. Enough to give time, hopefully, to the space agencies to prepare a parade.
Serge Brunier
source: Science & Life
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