The Multi-angle Imaging SpectroRadiometer (MISR) ad Moderate-Resolution Imaging Spectroradiometer (MODIS) systems on the Terra (EOS AM) captured an image of a huge fireball over the Bering Sea (which separates Alaska from Russia) on 19 December last year, according to data released by NASA this week. The object was seen to explode about 26 kilometres above the sea, releasing roughly the same amount of energy as 173 kilotonnes of TNT (an explosion. about 10 times as powerful as the Hiroshima bomb), suggesting an object about 18 m in diameter, the largest such object known to have entered the atmosphere since the Chelyabinsk Meteor in February 2013.
Animation made up of successive MISR images of the 18 December 2018 meteor. The meteor is the yellowish object in the lower centre of the screen, the longer, darker object above is its shadow on the cloud tops. NASA/Godard Space Flight Center/Langley Research Center/Jet Propulsion Laboratory/California Institute of Technology.
Objects of this size probably enter the Earth's atmosphere several times a century, though unless they do so over populated areas they are unlikely to be noticed. They are officially described as fireballs if they produce a light brighter than the planet Venus. The brightness of a meteor is caused by friction with the Earth's atmosphere, which is typically far greater than that caused by simple falling, due to the initial trajectory of the object. Such objects typically eventually explode in an airburst called by the friction, causing them to vanish as an luminous object. However this is not the end of the story as such explosions result in the production of a number of smaller objects, which fall to the ground under the influence of gravity (which does not cause the luminescence associated with friction-induced heating).
These 'dark objects' do not continue along the path of the original bolide, but neither do they fall directly to the ground, but rather follow a course determined by the atmospheric currents (winds) through which the objects pass. Scientists are able to calculate potential trajectories for hypothetical dark objects derived from meteors using data from weather monitoring services. On this occasion any such objects are likely to have fallen into the Bering Sea, making it highly unlikely they will be recovered.
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