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A global network of microphones could help scientists discover exactly how many small NEOs enter our atmosphere reports Nature Science Update. The microphones are designed to help police the Comprehensive Test Ban Treaty (CTBT) that forbids the testing of nuclear weapons but will also be able to detect the detonation of small asteroids and comets in the atmosphere. Small weak NEOs between roughly 1 and 100 metres in size do not survive their fall through the Earth's atmosphere and are broken apart explosively. At smaller sizes fragments of the meteoroids can reach the ground as 'bag of sugar' sized meteorites but still often produce a bright fireball and sonic booms during their fall. Objects 10 m in diameter detonate with energies equivalent to 10,000 megatonnes of TNT and yet occur at high altitude and do not cause damage on the Earth surface. Five or six of these events occur each year. Larger objects can penetrate lower in the atmosphere and cause local damage such as the 60 m diameter object that devastated 2000 kilometres square of forest in Siberia in 1908. Such events are thought to occur every 100 to 300 years.
The ground-based network will detect the faint rumbles of NEO explosions high in the atmosphere and allow them to be distinguished from nuclear blasts. At present there are 12 networks of microphones, however, sixty will be constructed in the next 5 years. The data from the network will be freely available to everyone.
Another network that will listen for meteorites entering the atmosphere is currently being tested by Dr Phil Bland, a meteorite researcher at the Open University. Bland's sensors will, however, detect sound and electromagnetic disturbances caused by meteorite fireballs over Australia. Because meteorites heat the surrounding atmosphere intensely they produce an electromagnetic pulse which can sometimes be detected by the human ear as a crackling noise known as an electrophonic effect. Bland's sensors, together with high speed cameras, will be used to locate where meteorites fall and allow the nature of their asteroids to be studied in the laboratory.
More info: Nature Science Update
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