Witnesses across southwest have reported seeing a bright fireball meteor, slightly after 8.00 pm on Wednesday 4 October 2017. The object was detected by NASA's Fireball Detection Network, which estimates that the object was travelling at about 14.6 kilometres per second and exploded 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) with an energy equivalent to 540 tons of TNT, roughly 164 km to the northwest of the city of Lijiang in Yunnan Province. This would imply an object 17-18 m across exploding at an altitude of about 25 km.
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. Such a fireball typically disappears when the meteor causing it explodes in an airburst; this typically happens several km in the air, with fragments of the rock falling to the ground as meteorites, though they can drift some distance from where the fireball was last seen before reaching the ground, though there is little hope of finding any such meteorites in the mountainous terrain of northwest Yunnan Province.
Fireball meteor over Yunnan Province, China, on 4 October 2017. South China Morning Post.
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 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).
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