Fireballs are normally millimeter-sized meteoroids that have entered the atmosphere. There are various definitions of what constitutes a fireball. The IMO has created a clearinghouse for all fireball reports known as the Fireball Data Center (FIDAC). The definition used by FIDAC for a fireball is any meteor with a brightness of -3.0 or greater, and is the definition used by the NAMN.
The frequency of fireballs throughout the year varies. If you observe every meteor shower during an entire year, you will find that some showers tend to produce more fireballs than others. In addition, fireballs exhibit a seasonal variation just as sporadic meteors. For the northern hemisphere, this amounts to about three times as many around the vernal equinox as there is around the autumnal equinox. A diurnal variation has also been observed among fireballs. More will appear around 18h local time than 06h local time. Careful readers will notice that this is directly opposite the variation of sporadic meteors. The reason is related to velocity.
A meteoroid entering the atmosphere is acted upon by friction. At a lower velocity, the meteor will penetrate to a deeper, denser level of the atmosphere because it is not as susceptible to breaking apart. This results in a brighter meteor being produced. A meteoroid of the same size, entering the atmosphere at a higher velocity, will penetrate to a shallower depth because its velocity will tend to break it apart. In this case, a fainter meteor will be produced. Since bright meteors tend to be produced at low velocities, fireballs are more frequent at 18h local time when the meteoroid must overtake the earth in its orbit. This is because to overtake the earth, a meteoroid must expend a portion of its velocity.
For a fireball to actually reach the surface of the earth, its velocity generally must be lower than 23 km/s. The initial mass must be greater than that which is lost from ablation as it penetrates lower levels of the atmosphere. At speeds greater than 23 km/s, forces normally break apart the original meteoroid into smaller particles that will not reach the surface (this does not hold true for very large particles which produce the meteorite craters of the earth). At some point along the path through the atmosphere, the meteor's velocity becomes zero, and the meteorite falls by gravity alone three or four minutes to the surface.
This brings us to the point of reporting observed fireballs. If sufficient observations are obtained, an orbit for the fireball might be computed. If the fireball was large enough to reach the earth's surface, it may then become possible to recover the meteorite for research. To aid in determining the characteristics of a fireball you observed, certain information is required. (Note: Keep in mind that really bright fireballs may draw the attention of your local media. If so, reports can also be gathered from them. It is far easier for a local meteor observer to do all the leg work here instead of relying on FIDAC to gather this information. Always be alert for media attention, follow it up and forward that data to FIDAC too.)
The following information should be included in your report:
-- Date and Time - Once again, use universal time and a double date.
-- Location of Observation, Geographic Coordinates - Name of the location, including state and country. Be as precise as possible with geographic coordinates and include the elevation. You can get this off of a topographic map.
-- Magnitude - Give the apparent magnitude if possible, or if not known exactly, a magnitude interval (e.g. -5 to -8) or relate to known bright objects such as Jupiter, Venus or the Moon.
-- Apparent Path, Coordinates of First/Last Sighting - Please note right ascension and declination of first and last points using degrees. You can convert right ascension by multiplying each hour by 15 and add 5 degrees for each 20 minutes.
-- Duration - Give the estimated or measured duration in seconds.
-- Color - Give color and any variations, referring only to pure colors, not, for example, bluish-green.
-- Persistent Train - Note appearance and duration, and if observed, color and shape (e.g. gray, white; puffy, wavy, straight).
-- Fragmentation - Give the number of fragments, their brightness, color and position along the trajectory.
-- Apparent Velocity - Note degrees per seconds, or the description: stationary, very slow, slow, medium, fast and very fast.
-- Sounds - Description of sound (compare with: supersonic sound, like an avalanche, swishing, whistling, etc.) and the time between optical and audible observations. Note that it may take a minute or more before any sound reaches you.
-- Observer - Name and address.
-- Source - Should be given if observations of other witnesses are reported. Note the address of other observers to allow additional inquiries.
Keep in mind the following special rules for fireballs greater than -10 in magnitude. There is a very good possibility that fireballs of this brightness will be recorded by U.S. Government satellites. In an effort to retrieve pertinent data from them, all fireballs greater than magnitude -10.00 should be reported to FIDAC immediately. Satellite data is stored for a short time so immediate reports are important. As a simple rule of thumb as to what constitutes greater than -10 magnitudes, consider the following as a guide:
--If the fireball appeared during daylight hours.
--Or, if at night, it appeared to be as bright as a full moon (-12) with distinct shadows of trees, houses, etc. present.
Fireball reports should be either e-mailed or postal mailed as soon as possible to the NAMN Coordinator.
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