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.
See also...
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