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FAQ On Fireballs and Meteorite Falls

Forwarded from Jim Bedient (wh6ef@pixi.com)
Subject: FAQ - Fireballs and Meteorite Dropping Fireballs
 
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The American Meteor Society, Ltd.
 
Frequently Asked Questions (FAQ)
About Fireballs and Meteorite Dropping Fireballs
 
Version 1.2
 
Question List:
 
  1. What is a fireball? What is the difference between a fireball and a
     bolide?
  2. How frequently do fireballs occur?
  3. Can you see fireballs in daylight, and will a fireball leave a trail?
  4. I saw a very bright meteor. Did anyone else see it, and to whom should
     I report it?
  5. Can fireballs appear in different colors?
  6. Can a fireball create a sound? Will the sound occur right away, as you
     watch the fireball, or is their some delay?
  7. How bright does a meteor have to be before there is a chance of it
     reaching the ground as a meteorite?
  8. Can a meteorite dropping fireball be observed all the way to impact
     with the ground?
  9. Are meteorites "glowing" hot when they reach the ground?
 10. How frequently do meteorite falls occur?
 11. How big are most meteorites? Do they fall as single objects or
     clusters of objects?
 12. How fast are meteorites traveling when they reach the ground?
 13. How can I recognize a meteorite, and where should I hunt for them?
 14. Where can I get a potential meteorite authenticated?
 15. What do fireballs and meteorites tell us about their origins?
 16. Author's note on fireball / meteorite statistics.
 
Below are some relatively concise answers to the above questions. If you
need further clarification or have further questions, please feel free to
contact us via electronic mail.
 
1. What is a fireball? What is the difference between a fireball and a
bolide?
 
A fireball is another term for a very bright meteor, generally brighter
than magnitude -3 or -4, which is about the same magnitude of the planet
Venus in the morning or evening sky. A bolide is a special type of fireball
which explodes in a bright terminal flash at its end, often with visible
fragmentation.
 
If you happen to see one of these memorable events, we would ask that you
report it to the American Meteor Society, remembering as many details as
possible. This will include things such as brightness, length across the
sky, color, and duration (how long did it last), it is most helpful of the
observer will mentally note the beginning and end points of the fireball
with regard to background star constellations, or compass direction and
angular elevation above the horizon.
 
The table below will aid observers in gaging the brightness of fireballs:
 
    Object            magnitude
    ----------------------------
    Polaris             +2.1
    Vega                +0.14
    Sirius              -1.6
    Bright Jupiter      -2.5
    Bright Mars         -2.8
    Bright Venus        -4.4
    1st Quarter Moon   -10.4
    Full Moon          -12.6
    Sun                -26.7
 
2. How frequently do fireballs occur?
 
Several thousand meteors of fireball magnitude occur in the Earth's
atmosphere each day. The vast majority of these, however, occur over the
oceans and uninhabited regions, and a good many are masked by daylight.
Those that occur at night also stand little chance of being detected due to
the relatively low numbers of persons out to notice them.
 
Additionally, the brighter the fireball, the more rare is the event. As a
general thumbrule, there are only about 1/3 as many fireballs present for
each successively brighter magnitude class, following an exponential
decrease. Experienced observers can expect to see only about 1 fireball of
magnitude -6 or better for every 200 hours of meteor observing, while a
fireball of magnitude -4 can be expected about once every 20 hours or so.
 
3. Can you see fireballs in daylight, and will a fireball leave a trail?
 
Yes, but the meteor must be brighter than about magnitude -6 to be noticed
in a portion of the sky away from the sun, and must be even brighter when
it occurs closer to the sun.
 
Fireballs can develop two types of trails behind them: trains and smoke
trails. A train is a glowing trail of ionized and excited air molecules
left behind after the passage of the meteor. Most trains last only a few
seconds, but on rare occasions a train may last up to several minutes. A
train of this duration can often be seen to change shape over time as it is
blown by upper atmospheric winds. Trains generally occur very high in the
meteoric region of the atmosphere, generally greater than 80 km (65 miles)
altitude, and are most often associated with fast meteors. Fireball trains
are often visible at night, and very rarely by day.
 
The second type of trail is called a smoke trail, and is more often seen in
daylight fireballs than at night. Generally occurring below 80 km of
altitude, smoke trails are a non-luminous trail of particulate stripped
away during the ablation process. These appear similar to contrails left
behind by aircraft, and can have either a light or dark appearance.
 
4. I saw a very bright meteor. Did anyone else see it, and to whom should I
report it?
 
The American Meteor Society (AMS) collects fireball reports from throughout
North America, the Caribbean, and the Pacific islands for use by our
organization and other meteor organizations. Persons who have seen a bright
meteor event are encouraged to report their sighting to us. If multiple
sightings of a single event can be grouped together, it is sometimes
possible to determine the actual trajectory of the object in question.
 
The easiest way to report a fireball to us is to utilize our on-line form,
located at our Internet Web site. This site is located at
http//www.serve.com/meteors.
 
Another feature of this Web site is the "Bright Meteor Diary," which
permits on-line browsing of our electronic fireball database.
 
Implemented in March, 1997, this database permits visitors to search for
reports about a particular fireball event, as sorted by date and location.
Even if others are reporting the same fireball event that you saw, you are
still encouraged to add your own sighting, in order to improve our
information.
 
5. Can fireballs appear in different colors?
 
Vivid colors are more often reported by fireball observers because the
brightness is great enough to fall well within the range of human color
vision. These must be treated with some caution, however, because of
well-known effects associated with the persistence of vision. Reported
colors range across the spectrum, from red to bright blue, and (rarely)
violet. The dominant composition of a meteoroid can play an important part
in the observed colors of a fireball, with certain elements displaying
signature colors when vaporized. For example, sodium produces a bright
yellow color, nickel shows as green, and magnesium as blue-white. The
velocity of the meteor also plays an important role, since a higher level
of kinetic energy will intensify certain colors compared to others. Among
fainter objects, it seems to be reported that slow meteors are red or
orange, while fast meteors frequently have a blue color, but for fireballs
the situation seems more complex than that, but perhaps only because of the
curiousities of color vision as mentioned above.
 
The difficulties of specifying meteor color arise because meteor light is
dominated by an emission, rather than a continuous, spectrum. The majority
of light from a fireball radiates from a compact cloud of material
immediately surrounding the meteoroid or closely trailing it. 95% of this
cloud consists of atoms from the surrounding atmosphere; the balance
consists of atoms of vaporized elements from the meteoroid itself. These
excited particles will emit light at wavelengths characteristic for each
element. The most common emission lines observed in the visual portion of
the spectrum from ablated material in the fireball head originate from iron
(Fe),magnesium (Mg), and sodium (Na). Silicon (Si) may be under-represented
due to incomplete dissociation of SiO2 molecules. Manganese (Mn), Chromium
(Cr), Copper (Cu) have been observed infireball spectra, along with rarer
elements. The refractory elements Aluminum (Al), Calcium (Ca), and Titanium
(Ti) tend to be incompletely vaporized and thus also under-represented in
fireball spectra.
 
6. Can a fireball create a sound? Will the sound occur right away, as you
watch the fireball, or is their some delay?
 
There are two reported types of sounds generated by very bright fireballs,
both of which are quite rare. These are sonic booms, and electrophonic
sounds.
 
If a very bright fireball, usually greater than magnitude -8, penetrates to
the stratosphere, below an altitude of about 50 km (30 miles), and explodes
as a bolide, there is a chance that sonic booms may be heard on the ground
below. This is more likely if the bolide occurs at an altitude angle of
about 45 degrees or so for the observer, and is less likely if the bolide
occurs overhead (although still possible) or near the horizon. 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.
Electrophonic sounds have never been validated scientifically, and their
origin is unknown. Currently, the most popular theory is the potential
emission of VLF radio waves by the fireball, although this has yet to be
verified.
 
7. How bright does a meteor have to be before there is a chance of it
reaching the ground as a meteorite?
 
Generally speaking, a fireball must be greater than about magnitude -8 to
-10 in order to potentially produce a meteorite fall. Two important
additional requirements are that (1) the parent meteoroid must be of
asteroidal origin, composed of sufficiently sturdy material for the trip
through the atmosphere, and (2) the meteoroid must enter the atmosphere as
a relatively slow meteor. Meteoroids of asteroid origin make up only a
small percentage (about 5%) of the overall meteoroid population, which is
primarily cometary in nature.
 
Photographic fireball studies have indicated that a fireball must usually
still be generating visible light below the 20 km (12 mile) altitude level
in order to have a good probability of producing a meteorite fall. Very
bright meteors of magnitude -15 or better have been studied which produced
no potential meteorites, especially those having a cometary origin.
 
8. Can a meteorite dropping fireball be observed all the way to impact with
the ground?
 
No. At some point, usually between 15 to 20 km (9-12 miles or 48,000-63,000
feet) altitude, the meteoroid remnants will decelerate to the point that
the ablation process stops, and visible light is no longer generated. This
occurs at a speed of about 2-4 km/sec (4500-9000 mph).