Scientists say that the shock wave from an asteroid that burned up over Russia in February was so powerful that it travelled twice around the globe. They used a system of sensors set up to detect evidence of nuclear tests and said it was the most powerful event ever recorded by the network. More than 1,000 people were injured when a 17m, 10,000-tonne space rock burned up above Chelyabinsk. The researchers studied data from the International Monitoring System (IMS) network operated by the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO).
The detection stations look out for ultra-low frequency acoustic waves, known as infrasound, that could come from nuclear test explosions. But the system can also detect large blasts from other sources, such as the Chelyabinsk fireball. Alexis Le Pichon, from the Atomic Energy Commission in France and colleagues reported that the explosive energy of the impact was equivalent to 460 kilotonnes of TNT. This makes it the most energetic event reported since the 1908 Tunguska meteor in Siberia.
Fallen trees still litter the Taiga forest near the site of the Tunguska airburst. The 1908 fireball, the biggest space impact of modern times, was probably caused by an iron-rich meteorite, a study has confirmed. The Tunguska air blast is estimated to have been equivalent to three to five megatonnes of TNT, hundreds of times more energetic than the Hiroshima explosion, and it flattened trees across 2,000 sq km of forest. Victor Kvasnytsya, from Ukraine's National Academy of Sciences, and colleagues studied microscopic samples of mineral debris from the blast area that have been trapped in peat.
They describe the mineralogy of samples recovered from the peat in the 1970s and 80s. High-resolution imaging and spectroscopy identified carbon minerals such as diamond, lonsdaleite and graphite. Lonsdaleite in particular is found in carbon-rich material subjected to a shock wave, and is typically formed in meteorite impacts. The lonsdaleite fragments contain smaller inclusions of iron sulphides and iron-nickel alloys, troilite and taenite, which are also characteristic meteorite minerals.
The iron to nickel ratio and the precise combinations of minerals assembled in these small fragments all point to a meteorite source, and are nearly identical to similar minerals found in the Canyon Diablo meteor that impacted Barringer Crater in Arizona. The findings ruled out a theory that the Tunguska airburst was caused by a large fragment of Comet Encke. This comet is responsible for a meteor shower called the Beta Taurids, which cascade into Earth's atmosphere in late June and July.