PLEASE NOTE:


*

CCNet DIGEST, 23 June 1998
--------------------------


    "To me, the comet-shower theory is exciting because it is not just
    a theory about the Sun and the stars. It is also a theory about the
    history of life on Earth. If the theory is right, it changes the
    way we think about life and its evolution. [...] Creatures which
    were too successful in adapting themselves to a stable environment
    were doomed to perish when the environment suddenly changed.
    Creatures which were unspecialized and opportunistic in their
    habits had a better chance when Doomsday struck. We humans are
    perhaps the most unspecialized and the most opportunistic of all
    existing species. We thrieve on ice ages and environmental
    catastrophes (sic). Comet showers must have been one of the major
    forces that drove our evolution and made us what we are."
    (Friedman J Dyson, 1988)


(1) COMMENT ON GRONDINE'S REPORT
    David Morrison <dmorrison@arc.nasa.gov> wrote:

(2) WHAT SPEAKERS FAILED TO MENTION AT THE LUNCHEON: THE TAURID METEOR
    STREAM & DISINTEGRATED GIANT COMETS CAN ALSO AFFECT YOUR HEALTH
    James Marlin <rmarlin@network-one.com>

(3) SPACEGUARD TM
    Duncan Steel <dis@a011.aone.net.au>

(4) FAQ ON FIREBALLS AND METEORITE FALLS
    Jim Bedient (wh6ef@pixi.com)

=============================
(1) COMMENT ON GRONDINE'S REPORT

From David Morrison <dmorrison@arc.nasa.gov> wrote:

Comment on Grondine's report

Ed Grondine sees fit to criticize Carl Pilcher and NASA generally for
the estimates given of impact rates, for both Tunguska-class impacts
and larger civilization-threatening impacts. In fact, Pilcher's
estimates at the Congressional forum on impact rates and their
uncertainties are exactly those of the scientific consensus, as
reflected in peer-reviewed scientific papers such as Chapman and
Morrison (Nature 1994), Morrison, Chapman and Slovic (Hazards book
1994), Rabinowitz, Bowell, Shoemaker and Muinonen (Hazards book 1994),
Shoemaker, Weissman and Shoemaker (Hazards book 1994) and Toon et al.
(Rev of Geophysics 1997). These are also the values quoted in the two
official NASA reports (Morrison et al. 1992; Shoemaker et al. 1995). 
If Grondine or others think these estimates are too low, they should
write up their arguments and submit them to peer-reviewed scientific
journals.  Until that happens, they (and the rest of your readers)
should recognize that the figures quoted by Pilcher do correctly
represent the scientific consensus as it is today (including Gene
Shoemaker's extensive contributions to these studies). That doesn't
mean they are necessarily correct, but they are the best we have to go
on.

On another subject, I would add that the favorable characterization
Grondine gives of the Russian planetary defense proposals as presented
at the Space Protection of the Earth conferences in Snezhinsk in 1994
and 1996 (not 1992 as stated by Grondine) do not agree with what I
heard at these meetings. I attended both the SPE-94 and SPE-96
meetings, which were interesting and constructive, but I am afraid that
Grondine, who did not attend them, may have been misinformed on some of
these areas.

David Morrison

+++++++++++++++++++++++++++++++++++++++++++

David Morrison, NASA Ames Research Center
Tel 650 604 5094; Fax 650 604 1165
david.morrison@arc.nasa.gov or dmorrison@mail.arc.nasa.gov
website: http://space.arc.nasa.gov
website: http://astrobiology.arc.nasa.gov
website: http://impact.arc.nasa.gov

=====================
(2) WHAT SPEAKERS FAILED TO MENTION AT THE LUNCHEON: THE TAURID METEOR
    STREAM & DISINTEGRATED GIANT COMETS CAN ALSO AFFECT YOUR HEALTH

From James Marlin <rmarlin@network-one.com>

Just a reminder:

We are entering the Taurid daytime shower.

It is interesting to note that during this shower, the Farmington, KS,
meteorite fell, the Tunguska Event occured, and five monks saw an impact
on the Moon.

According to IRAS, there may be small asteroids in the core of this
shower, though we will not pass through this core anytime soon. What
does the List think of a possible link between Tunguska and this shower?

==================
(3) SPACEGUARD TM

From Duncan Steel <dis@a011.aone.net.au>

Dear Benny,

Regarding: "By the way, is Spaceguard any kind of a NASA trademark?"

During the deliberations of the NASA NEO Detection Workshop (actually,
whilst the report was being drafted), I mentioned the project
'Spaceguard' as visualized in Sir Arthur C. Clarke's book 'Rendezvous
with Rama' (1973), and that was adopted as the real (one hopes) project
name. I have had the business/company name 'Spaceguard' (now
'Spaceguard Australia') legally registered in Australia since 1988. I
assume that in other countries the people involved have protected their
fledgling organisations by similarly registering the names (Spaceguard
UK, Spaceguard Germany etc.).

Duncan

================
(4) FAQ ON FIREBALLS AND METEORITE FALLS

From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>

Forwarded from Jim Bedient (wh6ef@pixi.com)
Subject: FAQ - Fireballs and Meteorite Dropping Fireballs
 
-----------------------------------------------------------------------------
The American Meteor Society, Ltd.
 
Frequently Asked Questions (FAQ)
About Fireballs and Meteorite Dropping Fireballs
 
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).
 
From that point onward, the stones will rapidly decelerate further
until they are falling at their terminal velocity, which will generally
be somewhere between 0.1 and 0.2 km/sec (200 mph to 400 mph). Moving at
these rapid speeds, the meteorite(s) will be essentially invisible
during this final "dark flight" portion of their fall.
 
9. 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.
 
For the obvious reason, however, exact data on meteorite impact
temperatures is rather scarce and prone to hearsay. Therefore, we are
only able to give you an educated guess based upon our current
knowledge of these events.
 
10. How frequently do meteorite falls occur?
 
Our best estimates of the total incoming meteoroid flux indicate that
about 10 to 50 meteorite dropping events occur over the earth each day.
It should be remembered, however, that 2/3 of these events will occur
over ocean, while another 1/4 or so will occur over very uninhabited
land areas, leaving only about 2 to 12 events each day with the
potential for discovery by people. Half of these again occur on the
night side of the earth, with even less chance of being noticed. Due to
the combination of all of these factors, only a handful of witnessed
meteorite falls occur Each year.
 
As an order of magnitude estimation, each square kilometer of the
earth's surface should collect 1 meteorite fall about once every 50,000
years, on the average. If this area is increased to 1 square mile, this
time period becomes about 20,000 years between falls.
 
11. How big are most meteorites, and do they fall as single objects or
clusters of objects?
 
Meteorite finds range in size from particles weighing only a few grams,
up to the largest known specimen: the Hoba meteorite, found in South
Africa in 1920, and weighing about 60 tons (54,000 kg). As with the
magnitude distribution of meteors, the number of meteorites decreases
exponentially with increasing size. Thus, the majority of falls will
produce only a few scattered kilograms of material, with large
meteorites being quite rare.
 
Meteorites are known to fall as single, discreet objects; as showers of
fragments from a meteor which breaks up during the atmospheric portion
of its flight; and (rarely) as multiple individual falls. The initial
mass and composition of the meteoroid primarily determine its eventual
fate, along with its speed and angle of entry into the atmosphere.
 
12. How fast are meteorites traveling when they reach the ground?
 
Meteoroids enter the earth's atmosphere at very high speeds, ranging from
11 km/sec to 72 km/sec (25,000 mph to 160,000 mph). However, similar to
firing a bullet into water, the meteoroid will rapidly decelerate as it
penetrates into increasingly denser portions of the atmosphere. This is
especially true in the lower layers, since 90 % of the earth's atmospheric
mass lies below 12 km (7 miles / 39,000 ft) of height.
 
At the same time, the meteoroid will also rapidly lose mass due to
ablation. In this process, the outer layer of the meteoroid is
continuously vaporized and stripped away due to high speed collision
with air molecules. Particles from dust size to a few kilograms mass
are usually completely consumed in the atmosphere.
 
Due to atmospheric drag, most meteorites, ranging from a few kilograms
up to about 8 tons (7,000 kg), will lose all of their cosmic velocity
while still several miles up. At that point, called the retardation
point, the meteorite begins to accelerate again, under the influence of
the Earth's gravity, at the familiar 9.8 meters per second squared. The
meteorite then quickly reaches its terminal velocity of 200 to 400
miles per hour (90 to 180 meters per second). The terminal velocity
occurs at the point where the acceleration due to gravity is exactly
offset by the deceleration due to atmospheric drag.
 
Meteoroids of more than about 10 tons (9,000 kg) will retain a portion
of their original speed, or cosmic velocity, all the way to the
surface. A 10-tonner entering the Earth's atmosphere perpendicular to
the surface will retain about 6% of its cosmic velocity on arrival at
the surface. For example, if the meteoroid started at 25 miles per
second (40 km/s) it would (if it survived its atmospheric passage
intact) arrive at the surface still moving at 1.5 miles per second (2.4
km/s), packing (after considerable mass loss due to ablation) some 13
gigajoules of kinetic energy.
 
On the very large end of the scale, a meteoroid of 1000 tons (9 x 10^5
kg) would retain about 70% of its cosmic velocity, and bodies of over
100,000 tons or so will cut through the atmosphere as if it were not
even there. Luckily, such events are extraordinarily rare.
 
All this speed in atmospheric flight puts great pressure on the body of
a meteoroid. Larger meteoroids, particularly the stone variety, tend to
break up between 7 and 17 miles (11 to 27 km) above the surface due to
the forces induced by atmospheric drag, and perhaps also due to thermal
stress. A meteoroid which disintegrates tends to immediately lose the
balance of its cosmic velocity because of the lessened momentum of the
remaining fragments. The fragments then fall on ballistic paths, arcing
steeply toward the earth. The fragments will strike the earth in a
roughly elliptical pattern (called a distribution, or dispersion
ellipse) a few miles long, with the major axis of the ellipse being
oriented in the same direction as the original track of the meteoroid.
The larger fragments, because of their greater momentum, tend to impact
further down the ellipse than the smaller ones. These types of falls
account for the "showers of stones" that have been occasionally
recorded in history. Additionally, if one meteorite is found in a
particular area, the chances are favorable for there being others as
well. 13. How can I recognize a meteorite, and where should I hunt for
them?
 
The classic concept of a meteorite is a heavy, black rock. This
stereotype is true in some cases, but many, many more meteorites
resemble nothing more than mundane terrestrial rocks. These will
attract attention only by being different from all others around them.
 
To understand what a meteorite might look like on the ground, we must
first examine the numerical distribution of the three major types of
meteorites. Of the known meteorite classes (combining falls and finds):
 
   * stones (Aerolites) comprise about 69 per cent;
   * irons (siderites) comprise about 28 per cent;
   * and stony-irons (siderolites) comprise the remaining 3 per cent.
 
First of all, if a meteorite is found fairly quickly after it falls,
most will exhibit an overall blackened surface, called a fusion crust.
This fusion crust is a souvenier of ablation heat from the meteorite's
rapid atmosphere transit. Depending on the composition of the
meteorite, the fusion crust may appear glassy, or dull. Irons develop a
fusion crust consisting of magnetite, and having the appearance of a
fresh weld on steel.
 
Once a meteorite is on the surface, all the normal weathering effects
that erode earthly rocks affect meteorites, too. A fusion crust will
weather, and on a stone, lighten in color to a brownish hue. Chemical
weathering, or oxidation, will attack meteorites. Irons will quickly
rust. Stones will lose their fusion crusts entirely. Water will seep
into the interior, and chemically alter the minerals. Mechanical
weathering, by frost, sun, and wind will reduce the meteorite further.
This is why most ancient meteorites found are irons, most able to
resist these processes.
 
Most suspected meteorites, by the percentages above, are stony, and the
finder's attention was drawn to them by their contrasting appearance
with their surroundings. The indisputable identification of a stony
meteorite requires chemical tests which are beyond the scope of this
article.
 
Iron meteorites may frequently be recognized by their shape. The
melting of the exterior of the body will sometimes cause iron
meteoroids to arrive at the surface carved into fantastic shapes.
Complete rings and segments of arcs have been found. An iron will be
pitted, as portions of the alloy with a lower melting temperature will
be scooped out by the heat and pressure. There will sometimes be sharp
points surrounding these pits, an ablation effect. Positive
identification of an iron requires a grinding and acid etching process
that is again, beyond the scope of this article.
 
Anyone with a serious interest in searching for meteorites should
arrange a visit to a large museum with a meteorite collection, in order
to view not the spectacular specimens on display, but the more
"ordinary" specimens kept in the institutions' collection.. By examining
many specimens, the seeker will gain a good understanding of the varied
appearance that meteorites may present.
 
The most successful areas for hunting for meteorites are open, flat,
arid regions, usually having a light background color. Such regions
have the lowest rates of mechanical and chemical weathering, preserving
the meteorite for much longer periods of time. Some irons and
stony-irons have been found in desert regions more than 10,000 years
after the fall which produced them. Arid regions also offer great
advantages in visual searches due to the relative lack of vegetation or
bodies of water, as well as a light contrasting background color.
 
The best areas for meteorite searching (although rather impractical for
most persons) are the regions of the earth covered by continental
glaciers, such as Greenland and Antarctica. These ice packs offer the
highest degree of preservation of a meteorite after its fall, high
background contrast, and few competing terrestrial rocks. Many of the
meteorites used in research today were recovered during Antarctic
expeditions.
 
For those without access to arid deserts or continental glaciers,
perhaps the best place to do meteorite hunting is in freshly plowed
farmer's fields, especially following a recent rain. Native-American
arrowhead hunters frequently employ this technique as well. Farmers
have plowed up many of the more famous meteorite finds in history. Iron
meteorites are the easiest to recognize and are most frequently found.
Stony meteorites are more difficult to recognize and to differentiate
from terrestrial rocks, such as (ice age) glacial erratics.
 
The majority of meteorites, including the stone varieties, contain
sufficient amounts of iron (Fe) and nickel (Ni) to cause them to be
paramagnetic. Meteorite hunters often employ metal detectors, or very
strong magnets attached to a walking stick, to aid them in their
searches. Meteorites have been known to literally "jump" out of loose
soil in the presence of a strong magnet.
 
14. Where can I get a potential meteorite authenticated?
 
Below is a brief list of academic institutions and museums which might
be contacted about authenticating a potential meteorite find.
 
Readers are highly advised to first contact the institution and obtain
information about their individual policies regarding such testing and
potential fees prior to shipping any actual material. Since the
American Meteor Society does not normally deal in meteorites, we cannot
make recommendations or give advice on the selection of a testing
facility. Readers must use their own discretion in this matter.
 
     Academic Institutions:
 
     Center for Meteorite Studies
     Arizona State University
     Temple, AZ 85281
 
     Institute of Geophysics and Planetary Sciences
     University of California
     Los Angeles, CA 90024
 
     Institute of Meteoritics
     Department of Geology
     University of New Mexico
     Albuquerque, NM 87131
 
     Lunar and Planetary Laboratory
     Space Sciences Building
     University of Arizona
     Tucson, AZ 85721
 
     Museums:

     The American Museum of Natural History
     Central Park West at 79th St
     . New York, NY, 10024
 
     The Field Museum of Natural History
     S. Lake Shore Dr.
     Chicago, IL 60605
 
     National Museum of Natural History
     Dept. of Mineral Sciences
     Smithsonian Institution
     Washington, DC 20560
 
15. What do fireballs and meteorites tell us about their origins?
 
Most of our current knowledge about the origin of meteoroids comes from
photographic fireball studies (meteors > magnitude -4) done over the
last 50 years or so. This may sound like a long time, but good data has
been collected on only about 800 fireballs so far. Of these, only 4
have been recovered on the ground as meteorites. A meteorite-causing
fireball is very rare and must be at least magnitude -8 to have
sufficient mass to survive the trip. Even with an accurate photographic
or video trajectory, it is still a matter of finding a needle in a
haystack once the meteorite is on the ground. In recorded scientific
history, un-photographed (eyewitnessed) falls have resulted in only
about 900 meteorite finds.
 
Studies of meteoroid parent bodies, comets and asteroids, have been
more successful, using space probes and infrared telescope studies to
greatly increase our knowledge of these objects. What we have found is
that, rather than distinct differences between these two smaller solar
system members, there exists an entire spectrum of parent bodies,
ranging from low-density comets to large differentiated asteroids. The
similarities between asteroids and comets is made more apparent by the
recent discovery of a coma (a distinctly cometary phenomena) around the
asteroid Chiron, at its perihelion.
 
At the present time, meteoroid parent bodies can be roughly divided
into the following classes:
 
COMETS:
 
By far the most prevalent parent body of meteoroids, cometary
meteoroids form about 95% of the total meteor population, and include
nearly ALL of the shower meteor population. These parent bodies are
composed of frozen methane (CH4), ammonia (NH3), water (H2O), and
common gases (such as carbon dioxide, CO2), carbon dust and other trace
materials. As a comet passes near the sun in its orbit, the outer
surface exposed to sunlight is vaporized and ejected in spectacular
jets and streams, freeing large amounts of loosely aggregated clumps of
dust and other non-volatile materials.
 
These freshly generated cometary meteoroids, often called "dustballs"
will roughly continue to follow the orbit of the parent comet, and will
form a meteoroid stream.
 
Based upon photographic fireball studies, cometary meteoroids have
extremely low densities, about 0.8 grams/cc for class IIIA fireballs,
and 0.3 grams/cc for class IIIB fireballs. This composition is very
fragile and vaporizes so readily when entering the atmosphere, that it
is called "friable" material. These meteoroids have virtually no chance
of making it to the ground unless an extremely large piece of the comet
enters the atmosphere, in which case it would very likely explode at
some point in its flight, due to mechanical and thermal stresses. If a
piece did survive the journey, it would be extremely black, float in
water, and would rapidly melt and sublime away (with the exception of
some dust particles) in a short amount of time.
 
NON-DIFFERENTIATED ASTEROIDS:
 
These parent bodies are the smaller asteroids, constructed of denser
and less volatile materials than the comets. Small meteoroids of this
type are produced through collisions. This class of parent bodies
generate about 5% of the total meteor population, generally as part of
the non-shower, or "sporadic" meteors. These meteoroids can make it
through the atmosphere, and as meteorites, they make up about 84% of
all falls.
 
Stony meteorites from this source are called Chondrites, due to the
rounded nodules of material found within their structure, which are
called chondrules. Chondrite meteorites have two major groupings:
 
The first group, the Class II fireballs, are the carbon-rich
Chondrites, or Carbonaceous Chondrites, which help bridge the gap
between comets and asteroids. They make up about 4% of all observed
falls, and have densities of around 2.0 grams/cc. They are
characterized by the presence of 2% or more carbon, partly present as
complex hydrocarbons, and of considerable hydrogen (hydroxyl groups,
OH-1, and water, H2O).
 
The second group, the Class I fireballs, are what is called the
Ordinary Chondrites, making up about 80% of all observed falls. They
have an average density of 3.7 grams/cc, and generally fall into two
general types: Olivine-Bronzite Chondrites (about equal amounts of
bronzite and olivine) and Olivine-Hypersthene Chondrites (less pyroxene
than olivine).
 
DIFFERENTIATED ASTEROIDS:
 
These asteroids are physically the largest parent body for meteoroids,
but generate only a small fraction of the overall meteor population:
less than 1%, and have no fireball classification. Due to their hardier
composition, however, they make up about 16% of the observed falls. A
differentiated asteroid is one with sufficient size to cause internal
temperatures high enough to melt and stratify the asteroid. The higher
density materials (mainly iron) gather in the core, the lighter
basalt/silicate materials gather in the outer layers, with thinner
layers of various concentrations of other materials stratified in
between. Small meteoroids of these types have been produced by what
must have been spectacular collisions, breaking up even the iron core
of the asteroid.
 
The three major groups for these meteors are:
 
  1. Achondrites (Basalt/Silicate non-chondritic stones); with a 3-4
     grams/cc density, and comprising about 8% of observed falls. These
     formed in the outer and crustal layers of the asteroid.
  2. Siderolites (Stony-Irons); with a 5-7 grams/cc density, and comprising
     about 2% of observed falls. These formed a thin layer between the core
     and outer layers of the parent bodies. They generally consist of
     round, translucent green crystals of olivine imbedded in a matrix of
     iron.
   3. Siderites (Irons); with a 7.9 grams/cc density, and comprising
     about
     6% of observed falls. These are the remains of the core of a
     differentiated asteroid, and show signs of extremely slow cooling
     (1-10 deg C per million years), and extremely high shock stresses,
     presumably from collisions. These meteorites weather so well once on
     the ground, they make up 54% of all meteorite finds despite their
     small percentage of the fall population.
 
DIFFERENTIATED PLANETOIDS:
 
The very rarest of meteorites are those which are thought to have
originated from large differentiated bodies, such as moons and planets.
These Achondritic stones (basalt/silicate) are believed to have been
ejected from a moon or planet's surface, due to the impact of another
very large meteoroid. One sub-class of Achondrites show a very similar
composition to that of the earth's moon, and are believed to be Lunar
meteorites. Another class, the SNC (shergottite-nakhlite-chassignite)
meteorites, are believed to have been ejected from the crust of the
planet Mars.
 
16. Author's note on fireball / meteorite statistics.
 
Readers of this FAQ will notice that those particles which make up the
majority of the meteoroid population are those which are the least
likely to make it to the ground as a meteorite. Conversely, those
particles which make up a minority of the meteoroid population are the
most likely to reach the ground as a meteorite. This disparity becomes
even more skewed when weathering conditions on the ground are
considered. Thus, the meteors which are most often seen are not found
on the surface, and the ones which are most often found are uncommon in
the sky.
 
It took scientists many years to realize this disparity, and published
texts frequently seem to conflict with one another with regard to the
percentile breakdown of meteorite types. This is especially true if the
author has combined old meteorite finds with fresh, observed falls. In
an attempt to help alleviate this confusion, we present a current
breakdown of the different meteoroid/meteorite types, in their various
stages:
 
OVERALL METEOR POPULATION:
 
As a general rule, the smaller (fainter) is the meteoroid population
under
consideration, the more likely is a cometary origin. As a very rough
estimation, the visible meteor population is composed of about 19 cometary
meteors for every 1 asteroidal meteor. This yields the following breakdown:
 
   * Cometary meteoroids: ~95%
   * Chondritic meteoroids: ~5%
   * Non-chondritic meteoroids: <1%
 
FIREBALL POPULATION:
 
When only the population of meteors of > -4 magnitude are considered, the
more sturdy asteroidal meteoroids begin to make up an increasingly
higher percentage when compared to fainter magnitudes. There are four
basic fireball classes which are divided as follows:
 
   * Cometary meteoroids: 38%
        o Type IIIb fireballs, low density comets: 9%
        o Type IIIa fireballs, high density comets: 29%
   * Chondritic meteoroids: 62%
        o Type II fireballs, Carbonaceous Chondrites: 33%
        o Type I fireballs, Ordinary chondrites: 29%
   * Non-chondritic meteoroids: <1%
        o No fireball class
 
OBSERVED METEORITE FALLS / FRESH FINDS:
 
When only very fresh meteorite falls are considered, it becomes
instantly apparent how important the density and sturdiness of the
meteoroid material is to its likelihood of reaching the ground. The
cometary meteoroid population disappears, and the carbonaceous
chondrite population is greatly reduced. Thus, the ordinary chondrites
and non-chondritic meteorites become the primary constituents of this
population:
 
   * Cometary meteoroids: 0%
   * Chondritic meteoroids: 84%
        o Carbonaceous chondrites: 4%
        o Ordinary chondrites: 80%
   * Non-chondritic meteoroids: 16%
        o Achondrites: 8%
        o Siderolites: 2%
        o Siderites: 6%
 
METEORITE FINDS:
 
Once they are on the ground, meteorites instantly begin to undergo
mechanical and chemical weathering. Again, those meteorites which are
more sturdy and dense tend to withstand these processes much better. In
this case, the iron meteorites (siderites) fare the best, despite their
very small proportion of the overall meteoroid population:
 
   * Cometary meteoroids: 0%
   * Chondritic meteoroids: 37%
        o Carbonaceous chondrites: <1%
        o Ordinary chondrites: 37%
   * Non-chondritic meteoroids: 63%
        o Achondrites: 3%
        o Siderolites: 6%
        o Siderites: 54%
 
This is an active field of study, and readers are reminded that all of 
the above numbers are estimates, and subject to revision as our 
knowledge level increases. We have attempted to select the most
representative values for each.
 
FAQ compiled by:
 
James Richardson
AMS Operations Manager / Radiometeor Project Coordinator
 
James Bedient
AMS Electronic Information Coordinator
 
FAQ References:

     Ceplecha, Z., (1985). "Photographic Fireball Networks".
     Astronomical Institute of the Czechoslovak Academy of Sciences,
     251 65 Ondrejov Observatory, Czechoslovakia.
 
     Ceplecha, Z., (1985). "The Valec Fireball and Predicted Meteorite
     Fall". Astronomical Institute of the Czechoslovak Academy of
     Sciences, 251 65 Ondrejov Observatory, Czechoslovakia.
 
     Ceplecha, Z. (1991). "Meteors depend on Meteoroids", Proceedings
     of the IMC 1990, Violau.IMO, Veitsbronn, Germany, (p.13-21).
 
     Borovicka, J., (1993). "A fireball spectrum analysis", Astron.
     Astrophys. 279, 627-645.
 
     Hey, M. H., & Rea, D. G., (1986), "Solar System / Meteors,"
     Encyclopedia Britannica (Vol 27, pg. 587).
 
     McKinley, D. W. R., (1961). "Meteor Science and Engineering". New
     York: McGraw-Hill Book Co.
 
     Meisel, D. D., (1990). "Meteor", McGraw-Hill Encyclopedia / EST
     7th Ed.
 
     Meisel, D. D., Getman, V., Mathews, J., Jacobs, S. C., and Roper
     R., (1995). "Bolide Aida: Death of an Aubrite Meteoroid," Icarus
     (116, 227-255).
 
     Nininger, H. H., (1972). "Find a Falling Star". New York: P. S.
     Erikson.
 
     Norton, O. R., (1994). "Rocks from Space". Missoula: Mountain
     Press Publishing Co. (449 p.)
 
     Pugh, R. N., (1995). "The Diamond Lake Fireball of March 28,
     1994", Meteor News, No. 110 (Fall 1995).
 
1997 American Meteor Society, Ltd.


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