Thursday 20 December 2018

Fireball over northern California.

The American Meteor Society has received reports of a bright fireball meteor being seen off the coast of northern California at about 5.35 pm Pacific Standard Time on Wednesday 19 December 2018 (about 1.35 am  on Thursday 20 December GMT). The majority of the reports came from California, but sightings were also reported from southern Oregon. 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. The object appeared to move northeast-to-southwest.

Fireball meteor off the coast of northern California on 19 December 2018. R Abiad/American Meteor Society.

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. 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).
 Map showing areas where sightings of the meteor were reported, and the route of the object (blue arrow). American Meteor Society.
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 the meteor is reported to have caused a noctilucent cloud, a meteorological phenomenon caused by dust or ice crystals in the high atmosphere, at altitudes of between 70 and 100 km. Because they are so high in the atmosphere these clouds continue to be lit by the Sun for some time after sunset from the perspective of people on the ground, making them appear to glow in the dark.

Diagram showing how noctilucent clouds can be lit from below after sunset. National Weather Service.

Witness reports can help astronomers to understand these events. If you witness a fireball-type meteor over the US you can report it to the American Meteor Society here 
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