Paris - A year ago on Saturday, inhabitants of the Russian city of Chelyabinsk looked skyward, some frozen in fear that a nuclear war had begun.Overhead, an asteroid exploded in a ball of fire, sending debris plummeting to Earth in brilliant streaks.The shockwave blew out windows, hurting about 1 600 people, and the burst of ultraviolet light was so strong that more than two dozen people suffered skin burns.Today, enshrined in Russia's folk memory as a big scare, the Chelyabinsk Meteorite, for space scientists, is a boon.They say it has yielded unprecedented insights into the makeup and orbit of asteroids and the risks that a rogue rock may pose to Earth.Treasure trove"It was a remarkable event", Mark Bailey, director of the Armagh Observatory in Northern Ireland, told AFP.Only a few asteroids ever cross Earth's path. Fewer still survive the fiery contest of friction with the atmosphere. Those that do are likely to fall in the sea, which covers more than two-thirds of the planet, or in a remote area, such as desert, tundra or Antarctica.So for a meteorite to explode over a city, where mobile phones and dashboard cameras recorded the event and in a country with a rich scientific tradition, was an extraordinary plus for researchers.The event "has provided colossal information", said Viktor Grokhovsky of the Russian Academy of Sciences."Thanks in particular to the video recordings, Chelyabinsk is one of only 18 meteorites where it has been possible to make a backwards calculation of its trajectory, to find where it came from in the asteroid belt", he told the newspaper Rossiiskaya Gazeta.Its age was calculated at 4.5 billion years, the same as that of the Solar System meaning it consisted of primordial matter.Digging into this treasure trove of evidence, scientists estimate the asteroid must have measured about 20m across and weighed some 13 000 tons.As it entered the Earth's atmosphere, that mass would have translated into the energy equivalent of half a million tons of TNT, or 30 times the explosive yield of the Hiroshima atomic bomb."The asteroid broke into small pieces between the altitudes of around 45 and 30km preventing more serious damage on the ground", scientists in the Czech Republic and Canada reported last November.The trajectory, they added, suggested it was once part of a massive 2km rock called 86039, a nasty "geocruiser" first spotted in 1999 that regularly comes close to Earth's orbit.Meteorite hunters have gone into overdrive to pick up fragments, including a mighty half-ton chunk recovered from a lake near Chelyabinsk.Chemical analysis revealed the meteorite was a so-called LL chondrite, which constitute the minority of asteroids, but with a high content of cobalt.Grokhovsky said the fragments had intriguing differences. Some are blackened on top, others all the way through and others are half-and-half."Maybe it is radiation from space, the impact, the melting", he said.Investigators will meet in Houston, Texas in March at the annual Lunar and Planetary Science Conference, which is devoting a special session to the Chelyabinsk event.