Friday 15 September 2017

Fireball over Saint Petersburg, Russia.

Many witnesses in Russia and Finland have reported a bright fireball meteor on the evening of Tuesday 12 September 2017. The object was seen at about 8.15 pm local time, from southeastern Finland, and the Saint Petersburg, Smolensk, Novgorod and Moscow Oblasts (states) of Russia, though it was apparently over St Petersburg. Based upon data from a number of observatories in Russia and Finland, Viktor Grokhovsky of the Ural Federal University has estimated that the object was at most a meter across and a ton in mass. An object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) in the atmosphere more than 50 km above the ground, with only fragmentary material reaching the Earth's surface, though there is a possibility that some material from this meteorite has fallen in the Lake Seliger area Tver Oblast.

Fireball meteor over Saint Petersburg on 12 September 2017. Real Piter/Instagram/RT.

A fireball is defined as a meteor (shooting star) brighter than the planet Venus. These are typically caused by pieces of rock burning up in the atmosphere, but can be the result of man-made space-junk burning up on re-entry.

Objects of this size probably enter the Earth's atmosphere several times a year, 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. It is possible, though unlikely, that this object will have produced meteorites that reached the surface (an object visible in the sky is a meteor, a rock that falls from the sky and can be physically held and examined is a meteorite), though most meteorites come from larger objects that penetrate further into the atmosphere before exploding, and therefore have a better chance of producing fragments that reach the surface. 

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