NASA's Terra Satelite detects huge fireball meteor over Bering Sea.

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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Eruptions on Bogoslof Island.

The Alasaka Volcano Observatory reported an eruption on Bogoslof Island in the Aluetian Chain at about 4.10 pm local time on Wednesday 21 December 2016, the first such eruption since 1992. The eruption produced an ash column which rose about 35 000 m above the uninhabited island and was witnessed from a number of aircraft. This was followed by a second eruption at about 1.20 am on Thursday 22 December, which was not directly observed but detected by the seismic network than monitors Earthquakes in the region; a steam plume about 150 m high was later seen over the island.

Eruption on Bogoslof Island observed from an aircraft. Yahoo News.

Bogoslof Island forms the tip of a submarine volcano that rises from the Bering Sea floor, 1.8 km below the surface. The island only rises 150 m above the surface, but is 1.76 km in length and 500 m wide, forming part of the rim of the caldera (crater) of the volcano; all eruptive activity occurs beneath the surface to the northeast of the island itself.

The nature of the island means that it is frequently reshaped by eruptions, and this week's events have been no exception, with much of the northeastern side of the volcano having been removed by a collapse into the caldera, while new material has been added on the south and southwest sides of the volcano, and a new island has formed to the northeast of the submerged caldera.

An  analysis of material added to and lost from Bogoslof Island during the December 2016 eruptions. Areas marked with a '-' have been lost during the event, while areas marked with a '+' have gained material, rising in altitude where land is indicated and rising from the sea in areas marked as sea in the base image. Alaska Volcano Observatory/USGS.

Bogoslof Island is uninhabited, and the islands around is are home to at most sparse Human populations, however it still presents a serious threat to air-traffic, as the Aleutian Islands lie in the path of a number of commercial aircraft routes, connecting the US to East Asia. Volcanic ash is extremely hazardous to aircraft in a number of ways. At its most obvious it is opaque, both visually and to radar. Then it is abrasive, ash particles physically scour aircraft, damaging components and frosting windows. However the ash is most dangerous when it is sucked into jet engines, here the high temperatures can melt the tiny silica particles, forming volcanic glass which then clogs engine. When this happens the only hope the aircraft has is to dive sharply, in the hope that cold air passing through the engine during the descent will cause the glass to shatter, allowing the engine to be restarted. Obviously this is a procedure that pilots try to avoid having to perform.

The approximate location of Bogoslof Island. Google Maps.

The volcanoes of the Alaskan Peninsula and Aleutian Islands are fed by magma rising from the Pacific Plate, which is being subducted beneath the North American Plate to the south along the Aleutian Trench. As the subducting plate sinks into the Earth it is subjected to enormous heat and pressure, causing more volatile minerals to melt. These then rise through the overlying North American plate as magma, fuelling the Alaskan volcanoes.

 How the subduction of the Pacific Plate beneath the North American Plate fuels the volcanoes of Alaska. Alaska Volcano Observatory.

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