13 October 2008

Astronomy Picture of the Day - a bolide


On September 30, a spectacular bolide or fireball meteor surprised a group of amateur astronomers enjoying dark night skies over the Oklahoma panhandle's Black Mesa State Park in the Midwestern US. Flashing past familiar constellations Taurus (top) and Orion, the extremely bright meteor was captured by a hillside camera overlooking the 2008 Okie-Tex Star Party. Astronomy enthusiast Howard Edin reports that he was looking in the opposite direction at the time, but saw the whole observing field light up and at first thought someone had turned on their car headlights.
A bolide is a special type of fireball which explodes in a terminal flash, before striking the ground, as explained at this page of FAQs about fireballs and bolides. Excerpts:

Because sound travels quite slowly, at only about 20 km per minute, it will generally be 1.5 to 4 minutes after the visual explosion before any sonic boom can be heard. Observers who witness such spectacular events are encouraged to listen for a full 5 minutes after the fireball for potential sonic booms.

Another form of sound frequently reported with bright fireballs is "electrophonic" sound, which occurs coincidentally with the visible fireball. The reported sounds range from hissing static, to sizzling, to popping sounds. Often, the witness of such sounds is located near some metal object when the fireball occurs. Additionally, those with a large amount of hair seem to have a better chance of hearing these sounds. [?????]

Are meteorites "glowing" hot when they reach the ground? Probably not. The ablation process, which occurs over the majority of the meteorite's path, is a very efficient heat removal method, and was effectively copied for use during the early manned space flights for re-entry into the atmosphere. During the final free-fall portion of their flight, meteorites undergo very little frictional heating, and probably reach the ground at only slightly above ambient temperature.

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