(meteorobs) re "Forecast" of fireballs from NEO visit - huh?

[Wayne Hally]

This is off topic, but since the Orionids haven't started up yet, I will
entertain the conversation.

Elton, I will address each of your 3 most recent posts in order.

I will precede my comments with MW>>>

"
Part 1 The significance of Green Fireballs 
Long time list members on the meteorobs list are aware of the concentration
of reported fireballs this past spring and of "green" fireballs-- 
there has been much discussion.  This two part post is to address the
possible significance and relationship of green fireballs and Near Earth 
Objects(NEOs) especially Earth-crossing bodies. Be it remembered that all
meteoroids entering our atmosphere are/were in Earth-crossing orbits 
and represent the closest of the close of NEOs!  The meteorite chasers
community may take interest as the mix of successive colors of a meteor my
provide clues as to the meteorite dropping potential of a particular event.
There is a documented higher flux of both fireballs and meteorite 
falls in early spring( citation later)"

MW>>>

OK, so what. Cometary meteoroids are not the same as asteroidal meteoroids.


"The purpose of meteor observation traditionally has been to document meteor
flux from cometary sources and identify new showers to identify the
existence of long period comets.  Thinking wholly within a particular box,
the long standing focus has been to take snapshots of meteor flux with not
mucg (sic)statistical evaluation og (sic)the bulk data.  I surmise someone
somewhere someday might eventually correlate this data to comets  past and
present, to whatever ends that research might tell us of recent comet
populations and orbits in the inner solar system."

MW>>> OK so far... except you surmise with no basis.

" Virtually no effort is going on in the meteor observer community to
identify asteroid-ally derived debris streams as the low density/frequency 
of observations places any potential pattern there may be into the category
"sporadic".  Couple this with lock box thinking that "such a thing 
could not exist" and no one may ever undertake such an attempt to make
annual correlations. More on that in part two."

MW>> Well part 2 hasn't occurred because of some imaginary refutation, which
had not occurred until John's post, because I haven't had time to respond.
Very busy. Actually, some of us (like me) look for asteroid streams as
suggested in Peter Jenniskens book.

"A minuscule portion of observational effort has been capturing meteor
spectral emissions.  Without a spectrometer, fireball colors are not easy 
to reliably categorize beyond basic color sequences as only the spectrometer
and camera can record them with specificity. We should all realize 
how difficult it is to even capture a fireball on camera and how even more
rare it is to capture the spectral emissions of a meteor through an 
even smaller aperture.  "

MW>>>Actually, we do realize that. Fireball spectra are very rare, certainly
far too small a sample to draw any conclusions...probably less than a dozen
in recorded history.

"In fact, most  "color discussions", have favored the "subjective perception
of the observer" paradigm and not the specific atomic sources of the 
light emissions.  When color is noted, it is largely taken as a "whole"
rather than collection of specific emission lines of light viewed through 
a spectrometer and that is just the way the eye perceives color--by blending
multiple wavelengths into a single hue.  When we mix enough 
wavelengths together we will always favor the "white fireball" when
describing a meteor--unless there is a specific overriding hue. Well 
frequently there is such a natural bias and frequently it is green."

MW>> Indeed. That is part of the major problem. Color judgments are totally
subjective.

"What vignette examples we do have of meteor spectra do not seem to have
been methodically worked into a sound scientific theory regarding the
significance of meteoritical light. Better late than never."

MW>> Huh?

"For clarity a quick review of the spectral sources might be in order:
I find at least three "sources" of light in a meteor's flare and two of
those overlap.
1) The spectral emission lines of the constituent elements/molecules of the
meteoroid proper when they change phase into gas/plasma.
2) The excited state of the atmospheric gases/dust which the meteoroid acts
upon-- mainly 5-6 species: N2, O2, O3,NO, N and O but at high 
velocity expands to at least 19 species of atoms/molecules including CO2,
argon, H2O and so forth.( NOTE: this has implications for a different 
"typical" color of a meteor when viewed from the surface of Mars)

MW>> Unfortunately, you have provided no scientific evidence for this
assertion

"3) The change in chemical composition as existing molecules are
disassociated and  new molecules form in a flash( pun intended) owing to 
recombination which may form species such as (CN)2 ,CO, Fe2O, FeC, Mg2O etc.
not normally seen in the auroral spectra.
The atoms/molecules from the meteoroid emit light because they are heated in
an induced plasma stream when entering the atmosphere; they 
incandesce as well as chemically oxidize, emitting a more complicated
assembly of spectral lines loosely conceptualized  such as the way that 
different compounds in fireworks provides for different colors. "

MW>> Very loosely, with no scientific support

"The atoms/molecules of the atmosphere are ionized in the super-hot bow wave
ahead 
of the meteoroid, causing them to emit photons of certain specific
wavelengths, depending on what elements are present--and what compounds
reform in the furnace of entry as a mist of melted meteoroid enters the slip
stream. "

MW>> OK here

"Color saturation/intensity is also a function of density for the various
atoms/molecules.  Nitrogen will tend to dominate over oxygen which will 
dominate over CO2 owing to bulk percentages in the makeup of the atmosphere.
We will tend to see colors associated with Fe, Mg, N2 and O2 with a generic
meteor counter-mixed with atmospheric plasma.  To the eye, it will seem that
there no more than two colors at once but usually a single overall hue
within a bright overall flash. ( NOTE: human eye physiology-- cone and rod
density, night vision,dazzle--  all come to play in 
perceived color)"

MW>> Well, not really. Rods don't perceive color, that is the job of the
cones. Rods perceive brightness, not color.

"Moving on to specific spectral emissions, the common emissions for metallic
atoms in meteors and for atmospheric atoms can be seen at. 
<http://leonid.arc.nasa.gov/meteor.html>"

MW>>Yes, but that is for the cometary Leonids, which has NOTHING to do with
the alleged asteroid derived green fireballs. The Leonids hit the atmosphere
at ~ 71 km/sec and are comet dust. Any asteroid derived fireballs impact at
30 km/sec at best. KE= 1/2*m*(V^2). Apples and Potatos.

"Combinations of the two sources of emissions' produce the colors one sees
in the fireball. "Colors of meteors: The color of many meteors is 
caused (sic)by light emitted from metal atoms from the meteoroid (blue,
green, and yellow) and light emitted by atoms and molecules of the air 
(red). The metal atoms emit light much like in our sodium discharge lamps:
sodium (Na) atoms give an orange-yellow light, iron (Fe) atoms a 
yellow light, magnesium (Mg) a blue-green light, ionized calcium (Ca+) atoms
may add a violet hue, while molecules of atmospheric nitrogen (N2) 
and oxygen atoms (O) give a red light.( Note: See discussion of spectral
changes in  atomic vs molecular oxygen and nitrogen with decreasing 
altitude below)  The meteor color depends on whether the metal atom
emissions or the air plasma emissions dominate"...NASA
This simplistic model so far described is good for starters but the
atmosphere is more dense with lower altitude and a "Real (non-equilibrium) 
gas model" is required to explain emission behavior.
<http://en.wikipedia.org/wiki/Atmospheric_entry>"

MW>> You seem to be unable to differentiate between actual colors and
perceived colors. 

"We might be able to stop here were it not for the fact that some atoms
actually "color shift" their emissions with altitude.  A curious paradox 
exists for atmospheric oxygen and nitrogen emissions which vary with
altitude owing perhaps to atomic and molecular densities and the effects 
partial pressures might have on average atomic radii.( e.g. O2 vs O and N2
vs N) Illustration at:
<http://www.flickr.com/photos/11304375@N07/2844511020/>. "

MW>> Now wait, you are seriously suggesting that the diameter of atoms is
different at different heights in the atmosphere??????????

"As I annotate: "Nitrogen Oxygen Emissions with changes in altitude: Density
of nitrogen and oxygen varies by altitude. This affects the colors of 
an aurora" (which is a substitute for meteor spectral behavior--not because
it is complete but because it is far more deeply studied).
"Oxygen atoms above 200 km produces a red hue, while below 200km a green hue
is produced. Below 100 km not enough atomic oxygen exists to have an
effect".( Curiously, O2  has about 19 spectral lines which are distributed
seesaw fashion towards the ends of the visible spectrum with 3 green lines
at the fulcrum.  Single atoms of oxygen have about three peaks under lab
conditions)."Nitrogen produces blue and violet when it decays( e.g.
molecular bonding broken by going plasma) at the middle altitudes and
magenta at the lowest altitudes."

MW>>>Which has what to do with asteroid remnants? You do realize that the
excitation mechanism is completely different, right?

"The few meteor spectrographs captured so far suggest a combination of
peaks.  An generic illustration can be seen at 
<http://leonid.arc.nasa.gov/meteor.html>.  What is not differentiated in
this illustration is the fact that the total spectral output of a given 
meteor ( meteoroid plus atmosphere) are going to change as velocity and
altitude decreases."

MW>>> Not relevant. As discussed above, the Leonids are cometary debris, and
have nothing to do with asteroids.

"When we combine what we understand from auroral and meteor spectra, we may
infer that the deeper a meteoroid makes it into the atmosphere, the color
will trend from reddish white, then longer at green perhaps capped by
orange/red/magenta at the end of incandesence. Meteor spectra will differ
from auroral spectra being doped by the composition of the meteoroid itself
(i.e. Fe blue and Mg green, Na yellow). Be it also remembered that blue and
yellow are seen as green.  In fact there are several factors which tend to
make a fireball appear green much of the time below 200 km --more likely
than not!"

MW>>> A statement with no scientific support whatsoever.

"All that said, I believe we may be able to use "color" as a coarse
indicator of the depth of penetration into the atmosphere. To survive a
deeper 
plunge requires a larger mass.  Ergo a green/bluish green fireball  "tends
to suggest" an asteroidal origin vs the sand grain-sized meteor of 
cometary origin-- all other things being equal.  Couple color with sporadics
and these are the fireballs which need to be evaluated against 
similar sightings around the same time year after year to seek out orbits of
asteroidal debris streams.  As I'll debate in part two, these 
presently "random" fireballs may ultimately be associated with heliocentric
earth-crossing NEOs. It should not be a surprise that asteroid debris 
will be strung out along an orbit stream much as a cometary stream only less
densely and  spread more widely."

MW>>> Again, you say so, but provide no evidence for your assertoin.