CCNet 124/2001 - 23 November 2001: LEONID METEORITES?

    Andrew Yee <>

    Benny J Peiser <>

    Duncan Steel <>

    David Dunham <>

    (CCNet 8 December 1999)
    David Dunham <>

    Ron Baalke <>
    3 November 1999

    IAU Circular 7320

    Matthew Genge <>


>From Andrew Yee <>

Institute of Geological & Nuclear Sciences
Lower Hutt, New Zealand

Institute of Geological & Nuclear Sciences
Phone: (04) 570 1444
Fax: (04) 570 4600



A New Zealand-led group of scientists has found the first evidence for
global destruction of forests when a large asteroid hit the Earth 65
million years ago, killing off the dinosaurs.

Until now scientists believed that destruction of forests due to an
"impact winter" or impact-ignited wildfires was largely confined to the
American continent, within a radius of several thousand kilometres of
the inferred impact site on the Yucatan Peninsula, Mexico.

The New Zealand finding of a sudden death of a mixed forest and rapid
recolonisation by ferns, on the opposite side of the Earth to the
impact site, is compelling evidence that the asteroid impact caused
sudden destruction of terrestrial plants worldwide.

The study by paleontologists Chris Hollis and Ian Raine of the
Institute of Geological and Nuclear Sciences Limited, and Swedish
researcher Vivi Vadja, is published in the latest issue of the
international magazine Science.

The trio focused on pollen grains preserved in exposed coal seams in a
stream bank adjacent to the Moody Creek coal mine, north of Greymouth.

GNS scientists have a good knowledge of the Greymouth coalfield having
mapped it in detail over many years.

Working on a hunch that the coal might contain the evidence they were
looking for, Dr Raine chipped off pieces of the coal seam and brought
them to Wellington where the microscopic pollen grains were studied.

The scientists found a mixed forest community had been abruptly
replaced by a few species of fern directly after the meteorite impact.
The types of fern identified are known as early colonisers of open

Geochemical analysis of the coal showed extremely high concentrations
of the elements iridium, cobalt, and chromium. The iridium
concentration of 71 parts per billion is the highest known from
non-marine rocks anywhere in the world.

These three elements are known to be much more abundant in meteorites
than in the Earth's crust. They have been found at high concentrations
before in New Zealand, but only where the impact layer is preserved in
marine sedimentary rocks in eastern Marlborough.

"Whether the forest destruction was caused by prolonged darkness and
freezing conditions associated with the impact winter, or by global
outbreaks of wildfires, is a matter for further study by the research
team," Dr Hollis said.

"Either way, however, it is no longer difficult to explain the mass
extinction of large herbivorous dinosaurs and their predatory cousins,
especially in the southern hemisphere."

The research was supported by the New Zealand Marsden Fund, and the
Swedish Wenner-Gren Foundation and Royal Physiographic Society.


Note: Sixty-five million years ago there were at least four types of
dinosaur living in New Zealand. They included a sauropod, theropod,
hypsilophodont, and ankylosaurid. There were also numerous marine
reptiles. Bones of these creatures have been found in a stream north
of Napier. Sixty-five million years ago New Zealand was about 1000km
closer to the South Pole than it is today, and several degrees warmer
than today. The Hawke's Bay stream where the remains of New Zealand's
dinosaurs have been found is one of four sites in the southern
hemisphere where the so-called "polar dinosaurs" have been found. The
other locations are in Australia, the Beardmore Glacier Antarctica,
and the Antarctic Peninsula.


>From Benny J Peiser <>

Dear David, Jay and Duncan

After all the fuss regarding recent claims in the US about meteorite
falls during this year's Leonids, I was wondering whether you would like
to comment on the question whether or not meteor streams may include
'larger' pieces of debris. I plan to remind CCNet subscribers about the
1999 lunar impacts related to the Leonids and whether this has any
implications for the terrestrial influx. I would much appreciate your

Kind regards, Benny


>From Duncan Steel <>

Dear Benny,

You have asked me to comment on the basic idea of the Leonid meteor
shower dropping meteorites on the Earth's surface (as per recent
reports) in comparison with reports in 1999 of flashes observed
on the lunar surface coincident with the shower in that year. As
you note, the latter indicates large-mass meteoroids in the stream.
However, there is a fundamental fallacy here in drawing a link between
the two.

Let me start with the lunar flashes. I accept the excellent
observational evidence for these, presented by several groups, as being
impacts by large Leonid meteoroids. In a message back in 1999 Jay Melosh
wrote: "a half meter diameter projectile would mass about 500 kg", and I
would differ there. Jay has assumed a stony-type density, whereas for a
comet-derived meteoroid we might anticipate a far lower density, and a
fluffy structure. Working backwards from a required mass of 500 kg to
explain the flash intensity (and noting that there are a host of assumed
parameters in that conversion), I would expect the meteoroids to be a few
metres across (car-sized, if you like) but very low density, and very dark.

They would be composed of the non-volatile material remaining from a
chunk broken off the comet's surface: small silicate fragments held
together by a heavy organic "glue" after all the water, other inorganic
volatiles and lighter organics had sublimated away in interplanetary
space due to solar heating. As they hit the lunar surface, with no
atmosphere to speak of, of course they make a flash, impacting at
around 70 km/sec.

The presence of large meteoroids in the Leonid stream is irrelevant,
though, from the perspective of whether *meteorites* (on Earth) may be
deposited by such a shower. This is the fallacy I mentioned earlier.

For a meteoroid to survive atmospheric entry and have a substantial
remnant mass decelerated until it drops vertically at the
free-fall speed (just like it had fallen from an aeroplane) there is
a stringent set of conditions that must be met (with some
interdependence between them):

(a) The arrival speed must be low: the minimum possible at the top of the
atmosphere is 11 km/sec (terrestrial escape speed) and certainly those
having speeds below 15 km/sec are much favoured. This implies prograde,
low-inclination, low-eccentricity heliocentric orbits.
(The Leonids arrive at 71 km/sec because they are in quite the opposite

(b) The entry angle must be low: zenith angles of the radiant of circa 80
degrees. Any shallower an angle (>85 degrees) and the object may "bounce
off" the top of the atmosphere and not enter at all (cf. the object filmed
over the Grand Tetons in 1972). At a steeper angle the object will be
totally ablated away. (I'll note why this is the case below, in terms of
entry physics.)

(c) The object must be strong. Thus iron meteorites occur disproportionally
in collections, compared to their fractional population in space. (Of course
there is also their greater likelihood of being noticed on the ground.) Next
in terms of strength are stony meteoroids. Pretty much last come the fragile
carbonaceous chondrites. For one of those to survive, it must have a very
low speed and just the right arrival angle. My bet would be that carbonaceous
chondrites in fact represent a large fraction of the very small meteoroids in space
(see my penultimate paragraph below).

Now let's look at the entry physics. The meteoroid reaches the top of
the atmosphere with a very substantial kinetic energy: even at the lowest
feasible arrival speed of 11 km/sec, it has about 14 times more KE than
the chemical energy of the same mass of TNT. If any mass is to survive
entry (and so produce a meteorite) then it must dump in essence all of
that KE. How does it do it? The KE is dissipated in three ways:

(i) Heating the meteoroid from its initial temperature (ca. 250 K) to
the temperature at which its vapour pressure is substantial and so
sublimation starts in earnest. The temperature involved may be between (say)
500 K for tarry organics through to 1500 K plus for stony and metallic
constituents. The energy involved in this heating is only a small fraction
of the initial KE.

(ii) Radiating away energy due to the meteoroid being heated (a term
that looks like A epsilon sigma T^4, where A is the surface area, epsilon
its emissivity, sigma the Stefan-Boltzmann constant and T the temperature).
This is a heat loss *rate* (in watts, or joules per second).

(iii) Ablating away the meteoroidal material: this is the sublimation
term, which becomes large once the temperature has grown large enough for
the vapour pressure to take off (cf. item i above). (Note that I have
ignored energy losses through other minor mechanisms - cracking the object
apart, generating sound etc. - as these consume very
small fractions of the KE.)

Now think about what actually happens, if a meteorite is to be dropped
intact (some solid mass survives entry). First, you want the KE to be as
small as possible, and thus the speed as low as possible (and of course
the KE rises as the square of the arrival speed). As it enters the upper
atmosphere, drag slows the meteoroid down and the KE it is losing
initially goes to heating the meteoroid (term i above dominates, and terms
ii and iii are essentially zero). Once the meteoroid reaches a sufficiently
high temperature, term i disappears (no more heating occurs, just as a
boiling kettle never exceeds 100 deg C) and the KE being lost as the
meteoroid is decelerated goes to terms ii and iii.

(This would be the case for a very small meteoroid, which may be
considered to be isothermal. For larger meteoroids (bigger than millimetres)
the thermal inertia is large enough such that it takes a long time, compared
to the atmospheric entry episode, for the heat to be conducted to the
middle. Thus it is only a surface layer which is hot, and ablation of that
occurs with the centre remaining cold. That's why meteorites picked up
within minutes of a fall feel cold, not hot.)

If the entry angle is too steep, the deceleration is very great (some
tens of g's) and the shock breaks the meteoroid apart, whereupon all
the fragments ablate away to practically nothing (via item iii). With
an entry angle which is slightly less steep but still substantial (70
degrees, say), item iii leads to complete ablation: that is, the KE
goes mostly to sublimation, and rather little is radiated away. If the
entry angle is shallow enough, though, the duration of the entry phase
is long enough for item ii to radiate away sufficient energy (note ii is
a heat loss *rate* so that making the time t long results in ii
dissipating a large fraction of the KE: it gets hot enough to radiate away
energy but not hot enough for complete sublimation to happen) such that
comparatively little goes to iii, and so the meteoroid does *not* completely
ablate away.

The above really is a quite simple set of arguments. Of course it is
precisely what is involved in controlled re-entries of spacecraft: the
need to hit the "entry corridor". I remember Christmas 1968, when Apollo
8 was on its way back from the Moon, and TV programmes were discussing how
if it came in too shallow then the capsule would bounce off the top of
the atmosphere, whereas if it came in too steep it would break apart and
burn up. (The Apollo capsules had ablation shields which did indeed ablate,
but as they did so their rate of heat dissipation through radiation was 65
MW/m2, and that's where most of the KE went; and they were coming in at
close to the minimum speed, around 13 km/sec.)

OK, now back to the Leonids. Even those meteoroids coming in at shallow
angles do not survive because of their extreme speeds, coupled with
their weakness. It's easy to do numerical experiments along those lines, as
I have. Even a steel ball-bearing entering at 71 km/sec at a shallow angle
does not survive: it sublimates away to essentially nothing (you may
get a tiny metallic sphere left which will take days to fall out, and
these are found - remnants of iron meteoroids - all over the globe;
first identified in deep-ocean samples collected on the Challenger
expedition in the 1870s).

Is it possible *at all* for the Leonids to produce a meteorite? The only
way I could imagine would be multiple bounces off the upper atmosphere
(through very shallow approaches) which cause it to gradually slow down
(as is being done now with the Odyssey satellite at Mars) and eventually
enter at 11 km/sec. The trouble with that idea is that the Leonid
meteoroid would be moving so fast after its bounce that it would not be
trapped in a geocentric orbit, but would be off again into a (slightly
changed) heliocentric orbit. One could imagine other peculiar mechanisms
involving the terrestrial and lunar gravitational fields conjointly causing
some slow-down, but again these are inadequate to cause any substantial drop
from the high Leonid speed.

Getting back to lunar impacts, to conclude, one might ask: why don't we
see tremendously bright fireballs on Earth due to the same sorts of objects
entering the atmosphere? My answer to that comes from the structure and
composition of Leonid meteoroids I mentioned above. I believe they are
fluffy, fragile, and composed largely of heavy organics. Therefore they
would disintegrate at extreme altitudes (as recent optical Leonid
observations show: altitudes above 150 km), and largely ablate away at low
temperatures such that there is comparatively little optical

A variety of radar observations of meteors in general, including my own
at HF and VHF in Australia but also by others at UHF (Arecibo and EISCAT),
have shown that indeed there is a very large meteoroidal population that
ablates too high to be stony or metallic. They're tar-balls. Not like
meteorites at all, in fact.

Do the Leonids produce meteorites? My answer is no, of course not. There
will be micrometeorites right now falling from extreme altitudes that
are tiny (tens of micron-sized) remnant masses from the Leonids, but
those can take years to reach the surface. If we could identify which
were Leonid-derived that would be wonderful for science. But even then,
as with all meteorites of all sizes, the processes taking place during
atmospheric entry mean that what reaches the ground intact is a highly-
selective minor portion of what started out on the tortuous voyage from
the parent object.


Duncan Steel


>From David Dunham <>


- The quote from below was an early estimate that I passed on soon
after my announcement of the discovery of confirmed impacts.  It is now
generally thought that the brightest lunar Leonid impact flashes
were by objects with a mass of a few kilograms and perhaps 20 cm in
diameter.  At 71 km/sec, such objects would surely burn up in the
Earth's atmosphere, probably with a brightness approaching that of
the full Moon.  From what I've heard about this year's shower in
the U.S.A., the brightest meteors were around mag. -5 or -6, not as
bright as many of the meteors during the 1998 fireball shower.  I
observed the shower myself, but unfortunately through varying amounts
of fog, so that I saw only a few dozen, rather than the many hundreds
that others with clear sky saw.  The brightest that I saw was about
mag. -5.  I haven't seen reports of Leonid meteorite claims this year,
but from the above, I would tend to discount them.  There is still a
rather large uncertainty in knowledge of the sizes of the lunar
Leonid impactors due to the uncertainty in the luminous efficiency
at velocities far above anything achievable in laboratories, or
even in low-Earth orbit.  I'm copying this to others besides Jay
Melosh who might have other thoughts on this.
     This year, I recorded the dark side of the Moon with cameras more
sensitive than I had in 1999, but haven't had time to review them.
I've received a few reports of lunar Leonids from others, but no
confirmations yet.

- my early quote below:

>From Joan and David Dunham <>
>The objects that caused the brightest flashes that we observed were
>probably a few hundred kilograms, according to the messages below,
>since very little of the impact energy is converted into light.  I've
>added a few comments about the observations in the messages, using
>" - " to preface my remarks. 

   (from CCNet 8 December 1999)

>From Joan and David Dunham <>

The objects that caused the brightest flashes that we observed were
probably a few hundred kilograms, according to the messages below,
since very little of the impact energy is converted into light.  I've
added a few comments about the observations in the messages, using
" - " to preface my remarks. 

There was a suggestion that sunglints from artificial satellites might
be involved, but this is unlikely since the observations were made late
at night local time when most of these would be deep in the Earth's
shadow. Also, with six events simultaneously recorded at two or more
separated locations, the chances are much greater that they are lunar
phenomena than something closer. In the cases where lunar location
information is available in the separate video records, there is also
good agreement.

Brian Cudnik reports that he observed from the Houston Astronomical
Society's site near Columbus, TX, at long. 96 deg. 39' 50" W., lat. 29
deg. 37' 07" N., h 98m, about 100 km west of downtown Houston.

There are many previous observations of probable lunar impacts,
although none of them apparently were confirmed. Many of these were
published in a NASA Technical Report on transient lunar phenomena that
is on the Web at
One can also find there a link to a modern (about one year old) effort
to videorecord TLP's simultaneously from different locations, a
"lunascan" project, which has other useful links, but so far they don't
have news of the Nov. 18th lunar impacts. Apparently their project has
concentrated more on the terminator and sunlit side of the Moon.  Also,
published in the Proceedings of the 48th convention of the Association
of Lunar and Planetary Observers (Las Cruces, NM, June 25-29, 1997) is
a good paper by John Westfall on "Worthy of Resurrection: Two Past ALPO
Lunar Projects", including one on "Lunar Meteor Search" that includes a
table of meteor size, frequency, flash magnitude, and crater diameter
that is in rather good agreement with the messages below, as well as a
good history of efforts up to 1997.

Does anyone know of a Web (or other) reference to an account of the
large impact observed by Canterbury monks in 1178 that apparently
caused the near-far-side crater Bruno? That's probably the first
observation of a lunar impact, although not confirmed from 
observations elsewhere.

David Dunham, 1999 Dec. 7

Date: Mon, 6 Dec 1999 11:30:36 -0700
To: Joan and David Dunham <>
From: Jay Melosh <jmelosh@LPL.Arizona.EDU>
Subject: References & calcs. of lunar meteor impacts

Dear Joan and David:

I just heard from Paul Weissman that he estimates that your m = 3
flashes must have been made by an object "about half a meter" in diameter. I
have to agree with this estimate--which implies masses *much* larger
than you have mentioned! (a half meter diameter projectile would mass
about 500 kg). The problem is that the luminous efficiency of an impact
onto a solid surface is *much* lower than the ca. 10% Mike Mazur
estimates for a bolide. This is discussed in detail in the Nemtchinov
paper I mentioned in my last email, but let me work out the
consequences using Mazur's estimates for the energy released by the
various flashes on the moon (i.e. L_obj=10^[(m-26.98)/-2.5] J/s, and an
estimated duration of 33 milliseconds). I use Nemtchinov et al.'s
luminous efficiency estimate of

impactor mass,kg    crater diameter,m    luminous energy,J
      100                9.8                  2.5e7              4.8
      300               13.                   7.6e7              3.6
      500               15.                   1.3e8              3.0

I used my web program for computing crater sizes at for the crater size
computations, assuming a projectile density of 1000 kg/m^3, impact
angle of 45 degrees, impact velocity of 71 km/sec and a target of loose
sand (lunar regolith) with a mean density of 2500 kg/m^3.

A potentially serious problem in these estimates is the duration of the
flash. Nemtchinov and I computed that most of the light is emitted in a
single millisecond for a 1 m radius impactor--much shorter than the 1/30
sec Mazur estimates!  Smaller objects will produce correspondingly
flashes.  However, I presume that your video camera (is it a CCD?)
   - Yes
integrates the light emitted over the duration of one frame (1/30
   - Actually, with interlaced video, the even lines are scanned in
     1/60th of a second, then the odd lines are scanned in the next
     1/60th of a second to form a 1/30th-second frame.  But some VCR's,
     including the ones we used, can work with the half-frames to
     achieve 1/60th second time resolution.
so Mazur's estimates for total energy emitted may be correct--but this
has to be verified before these estimates can be accepted.  If the
actual integration time was much smaller than assumed, that will reduce
the mass of the projectile fragment accordingly.
   - No, the integration time per half-frame is close to 1/60th second;
     as I understand, there is very little "dead time" between scans
     but there is some.  The E flash is curious in that I think it
     peaked between two scans in my tape, where it is almost equally
     bright, but rather faint, around 7th mag., on two successive
     half-frames.  But it must have been brighter, around 5th mag.,
     on a single half-frame for it to show up so well in Sada's tape.

The other uncertainty is that Nemtchinov et al. assumed an impact
velocity of 30 km/sec. It is possible that the luminous efficiency is
higher at 70 km/sec, and this is something that should be looked into,
but it seems unlikely it will be off by as much as a factor of 4.

I hope this is of help to you.
    - Yes, certainly, many thanks.

Sincerely,  Jay Melosh


Jay Melosh                              Tel:   (520) 621-2806
Professor of Planetary Science          Fax:   (520) 621-4933
Lunar and Planetary Lab                 email:
University of Arizona
Tucson AZ 85721-0092

Date: Mon, 06 Dec 1999 20:00:35 -0800
From: R Clark <>
Subject: size of lunar leonid impacts

Hello Dr. Dunham,

I was very excited to hear that several impacts on the lunar surface
had been detected during the Leonids. The possibility of such
observations has been examined several times over the years, and
generally ruled out as a difficult project with little likelihood of a
quick success. However the high efficiency and capabilities of modern
sensors and their widespread use by amateurs has now made the
observation a reality.

In the discussion of the observations you mention questions about the
size of the impactors that produced these flashes. You mention size
estimates ranging from >1000 kg to ~100 grams. I am curious about how
the latter figure reached you.
    - The lower estimates were from well-intentioned astrophysical
      calculations by others who, however, did not realize the
      very low fraction of energy that is transformed into visible
      light during these impacts.

For the size of the objects that produced these flashes, I have to
agree with the earlier figure... even though I am probably the source
of the latter. In my thesis at the University of Arizona I studied the
detectability of a different feature associated with lunar impacts.

High velocity lunar impacts produce several phenomena that may be
observed. The impact produces shockwaves in the target that may be
detected as seismic energy. The Apollo missions left a network of
seismometers which detected numerous impacts between 1969 and 1977 when
they, and the remaining active Apollo surface instruments were
foolishly shut down. (the old story about spending $40 billion to plant
a flag but not being able to afford the $50K/yr to receive and archive
the low but unending volume of science data still being returned)
Impacts of objects down to a few kg were detected with this networ
   - It sure would have been nice to have had ALSEP observations
     of the Nov. 18th impacts!  Someone should have thought to try
     to look for flashes from separate observatories before shutting
     down the network.  Of course, the widespread availability of
     inexpensive sensitive CCD video cameras was key to this effort
     (the cameras we used only cost $80), and these didn't exist
     in 1977.

Another impact phenomenon, probably the most obvious thing to look for,
is the flash produced by the impact fireball. At velocities above ~12
km/sec (virtually all impacts of asteroidal or cometary material at
Earth) the impacting object and some ammount of target material
(increasing with higher velocities) will be vaporized to incandescent
temperatures. The radiation from this fireball will have its peak
intensity at visible or UV wavelengths, quickly dropping into the IR as
the gasses disperse and cool. The fraction of the impact energy
partitioned into the initial fireball is generally at most 10%. Only a
small fraction of this energy is released as 'visible' radiation while
the fireball gas is still hot and dense enough to radiate efficiently.
This has now been observed!

A very large fraction of the total impact energy (~60%) ends up as
thermal energy in the immediate vicinity of the newly formed impact
crater. This is what I was studying. After modeling cooling craters to
determine their radiative characteristics, I considered how to detect
them against the background of the cold lunar nightside with
groundbased, LEO, and lunar orbiting sensors. For space based sensors
the optimum wavelength range is in the 1-6 micron range. In the case of
a lunar orbiting sensor I concluded that an impact <100gm may be
detetable. For groundbased observations most of this wavelength range
is unavailable, although the 2 micron window might allow impacts of a
few kg or less to be detected, depending on scattering of light from
the sunlit portion of the disk and skyglow.

I am very pleased, and more than a little surprised, at how quickly the
(groundbased!) detection of any lunar impact events has come within the
grasp of modern instruments and sensors.

Richard Clark

Joan and David Dunham
7006 Megan Lane
Greenbelt, MD 20770
(301) 474-4722


>From Ron Baalke <>

Leonids on the Moon
Marshall Space Flight Center

Leonid meteorite impacts on the Moon might be visible from Earth and
provide a means for long-distance lunar prospecting.

Nov. 3, 1999: When the Leonid meteor shower strikes on the morning of
November 18, 1999, our planet won't be the only place in the cross
hairs. The Moon will also pass very close to the debris stream of comet
Tempel-Tuttle. Here on Earth, space-borne meteoroids will plummet into
the atmosphere and burn up, creating streaks of light called meteors.
The vast majority of meteoroids will burn and disintegrate well before
they hit the ground. The situation on the Moon, where there is no
appreciable atmosphere, is different. Every bit of comet debris that
rains down on our satellite will hit its surface. Some meteor
enthusiasts hope that will create a different sort of display. Rather
than streaks of light in lunar skies, there could be flashes of light
on the Moon's surface each time a sizable meteoroid hits the ground.

Last year, during the 1998 Leonid meteor shower, the phase of the
moon was new. It was so close to the sun in the sky that observing
faint lunar meteorite flashes was impossible. This year is different.
During the 1999 Leonid shower the phase of the Moon will be just 2
days past first quarter. That means the moon will visible in the
night sky during the early evening on November 17, and approximately
35% of the lunar disk as seen from Earth will not be illuminated by
sunlight. There will be plenty of dark lunar terrain where flashes
might be visible.

Is it possible to observe such flashes?

Maybe, say researchers. It depends a great deal on the mass spectrum
of particles in the Tempel-Tuttle debris stream and how efficiently
kinetic energy is converted into optical light as a result of the
impacts. Both factors are poorly known. Although flashes are unlikely
to be seen with the naked eye, they may be detectable through amateur

"The impact of a one gram particle would generate of the order of
1023 to 1024 photons in the peak sensitivity range of the human eye,"
says Dr. Bo Gustafson of the University of Florida Laboratory for
Astrophysics. "Given the distance to the Moon, we could expect a few
times 106 photons per square meter at the Earth. This should be
barely detectable using a small telescope."

In June 1999, Ciel & Espace reported that a Spanish team of
astronomers led by J.L. Ortiz had reached similar conclusions:

     Watching meteorites fall on the moon ... is within reach of
     (modest) amateur telescopes. Because the Moon doesn't have a
     substantial atmosphere, meteorite impacts there are much more
     violent than here on Earth liberating much more energy: 20 million
     joules for a 1-kg block. As seen from the Earth, this would
     produce a flash of magnitude 9 to 15. From Ciel & Espace, No. 349
     - Juin 1999, p. 17: Si, c'est possible! (Translation courtesy
     Bernd Pauli HD).

"The Leonid debris stream is in a retrograde orbit, and it's inclined
just 22 degrees from the plane of Earth's orbit around the sun," says
Professor George Lebo of the University of Florida Department of
Astronomy. "That's why the Leonids enter the atmosphere with such a
high velocity [72 km/s]. The Earth and the Leonids hit head-on, like
a head-on collision between two speeding automobiles."

"If you put yourself in the reference frame of the Earth it's pretty
easy to figure out where these meteoroids will hit the Moon,
"continued Lebo. "On November 18, at 0h UT the lunar sub-Leonid point
[the spot where Leonid meteoroids rain directly down on the Moon's
surface] will be 9.4 degrees north of the lunar equator and 9.5
degrees sun ward of the day-night terminator. In other words, the
greatest flux of Leonids are going to hit nearly dead center on the
lunar disk as seen from Earth, just over the terminator on the sunlit

It won't be possible to see flashes on the Moon's sunlit surface, so
amateurs will have to look where the terrain is dark. The best
approach will be to train a telescope -- higher powers are best for
discerning faint flashes -- at a spot near the lunar equator on the
night side of the terminator, keeping the sunlit side of the moon
completely out of the field of view. Flashes observed with the naked
eye would certainly be exciting, but might have little scientific
value. Instead, experienced observers suggest using a low-light
astronomical CCD video camera to make a permanent record.

The Leonids radiant, in the constellation Leo, rises above the
horizon at mid-northern latitudes around midnight on November 17/18.
That's about the same time that the Moon sets. It's an ideal
situation for observers who can monitor the Moon for the first half
of the night and then enjoy the Leonid meteor shower from midnight
until dawn.

Leonid Lunar Prospecting

Although optical flashes were not observed on the moon during last
year's meteor shower, a team of scientists from the Boston University
Center for Space Physics discovered indirect evidence for Leonid

The Moon has an extremely tenuous atmosphere that contains, among
other things, sodium atoms. Just above the Moon's surface the density
of sodium is 50 atoms per cubic centimeter. For comparison, the
sodium density in Earth's lower atmosphere is 1019/cc! Although the
Moon's atmosphere is incredibly thin, researchers at Boston
University's space physics lab have built sensitive cameras that can
trace its sodium component out to several lunar radii.

In mid-November 1998 the Boston University group were using their
sodium camera to monitor Earth's atmosphere for changes due to Leonid
meteors. To their surprise they detected a bright sodium spot on
November 17 that grew in brightness, peaked on November 19, and then
faded away. The spot was almost 180 degrees away from the new Moon in
the night sky. Nevertheless, the source of the sodium was apparently
Earth's satellite. When Leonid meteoroids crashed into the Moon's
dusty soil they kicked up an extra helping of sodium atoms,
increasing the density of the Moon's thin atmosphere. A long lunar
sodium tail formed (much like the tail of a comet) which swept by our
planet two days later.

The Boston University experiment showed for the first time that
intense meteor showers might be one way of "lunar prospecting" from a
distance -- by looking at materials blasted off the surface as
meteoroids strike. A team of scientists from the University of Texas
and NASA tried something similar earlier this year when they crashed
NASA's Lunar Prospector spacecraft into the Moon. The probe was sent
hurtling into a south polar crater on July 31 in hopes that the
impact would vaporize shadowed water-ice and send a cloud of water
vapor and OH flying over the lunar limb. Telescopes, including the
Hubble Space Telescope, looked near the impact site after the crash,
but failed to detect evidence for water. That doesn't mean there's no
water on the moon, say scientists. Lunar Prospector may simply have
hit a dry spot, or perhaps the water vapor didn't rise high enough to

Dr. David Goldstein, a professor at the University of Texas who
proposed the Lunar Prospector impact experiment, is wondering if the
Leonids might succeed where the Lunar Prospector crash failed. Data
from Lunar Prospector's neutron spectrometer indicate that water-ice
on the moon is concentrated around the Moon's poles where shadowed
areas would allow pockets of water to remain frozen (see the figure
below). The 1999 Leonids won't reach the Moon's south pole, but many
meteoroids should strike the north pole.

"The Leonids will be coming in from above the ecliptic plane," says
Goldstein. "Given the Earth-moon geometry on November 18th that means
that the lunar north pole will be exposed, but not the south pole.
That's unfortunate because there's thought to be more water around
the south pole where we crashed Lunar Prospector. There's no chance
of a Leonid meteoroid hitting the crater where Prospector crashed.
Near the north pole the meteoroids will be coming in at several
degrees above the horizon -- very similar to the Lunar Prospector

"Compared to Lunar Prospector, Leonid meteoroids are light weight and
tiny, but they move a lot faster," Goldstein continued. "The mass of
Lunar Prospector was 160 kg and it was moving 1.7 km/s when it hit
the moon on July 31. Leonid particles are going about 72 km/s. That
means that a Leonid the mass of a golf ball (about 0.1 kg) would
deliver the same kinetic energy as the Lunar Prospector crash."

"If a Leonid meteoroid did hit a spot near the north pole with frozen
water, it's not clear what we would see. The Lunar Prospector
collision was like a car crash -- it was moving at relatively slow
speed. When it hit, we hoped it would kick up water vapor that would
be dissociated into OH by ultraviolet sunlight. In theory we would
then see the OH by looking above the sunlit lunar limb with
appropriate spectrometers. A Leonid crash would be much more violent.
Instead of water vapor gently wafting above the lunar limb, we might
see ionized, hot plasma. It's possible that we would also get some
warm water vapor that didn't sustain such a damaging shock wave, but
it's really hard to say. We haven't done the high speed simulations

Goldstein says that he and his colleagues may not have time to
organize a search for signs of water kicked up by Leonids this year,
following so closely on the heels of the Lunar Prospector experiment.
However, with some experts predicting significant Leonid activity
into the next millennium, there will be time to arrange an observing
campaign for next year and beyond.


>From the IAU Circular 7320

On Nov. 19 D. W. Dunham, Applied Physics Laboratory, Johns Hopkins
University, reported the visual observation by B. Cudnik (Houston, TX,
0.36-m telescope) on Nov. 18 of a brief flash near the center of the
moon's dark limb, at least as bright as psi1 Aqr nearby.  This event,
1'.7 from the moon's edge, was apparently confirmed by Dunham (Mount
Airy, MD, 0.13-m telescope) on two half-frames of a videotape that
showed fading by about 5 mag during the intervening 1/60 second.  On
Nov. 23 and 24 Dunham reported his confirmation of two lunar flashes
videorecorded by P. V. Sada (Monterrey, Mexico, 0.13-m telescope) half
an hour after Cudnik's observation, as well as of two lunar flashes
videorecorded by D. Palmer (Greenbelt, MD) up to an hour or so earlier;
there was also a probable untimed additional visual confirmation of the
Cudnik event by S. Hendrix (Cameron, MO, 0.11-m telescope).  Dunham has
summarized his own measurements of the five Nov. 18 events as follows:

Disc.            UT            m1  m2  lambda beta    Lunar location
         h  m   s        s             deg    deg
Palmer   3 49 40.5  +/- 0.4    3   7   48 W    1 N   175 km SW of Kepler
Palmer   4 08 04.1  +/- 0.6    5   8   70 W   15 S   175 km S of Grimaldi
Cudnik   4 46 15.2  +/- 0.1    3   8   71 W   14 N    50 km ENE of Cardanus
Sada     5 14 12.93 +/- 0.05   7   8   58 W   15 N   200 km WNW of Marius
Sada     5 15 20.23 +/- 0.05   4   7   59 W   21 N    75 km S of Schiaparelli

The magnitude m1 is that on the first frame showing the event, m2 that
on the following half-frame; the first event listed also seems to be
present on a third half-frame at mag 9.  The selenographic coordinates
(longitude lambda and latitude beta) and lunar location for the first
two events are uncertain by 5 deg or more, but the others should be
accurate to within about 2 deg (50 km). Following Dunham's suggestion
that the flashes resulted from Leonid impacts on the moon, D. J. Asher,
Armagh Observatory, computed that the center of the 1899 dust trail
that evidently produced the 1999 Nov. 18 Leonid activity (cf. IAUC
7311) by nominally passing 0.0007 AU from the geocenter would have
passed 0.0002 AU from the selenocenter around 4h49m UT.


A. C. Gilmore provides further photometry of DD Cir = Nova Cir 1999,
obtained as before (see IAUC 7249): Sept. 3.415 UT, V = 10.42, U-B =
-0.43, B-V = +0.38, V-R = +2.01, V-I = +1.65, airmass = 1.50; 4.389,
10.38, -0.43, +0.35, +2.00, +1.57, 1.42; 13.368, 10.98, -0.46, +0.14,
+1.82, +1.05, 1.43. Standard deviations are 0.01 mag or less.

                      (C) Copyright 1999 CBAT
1999 November 26               (7320)              Brian G. Marsden

Reproduced by permission.


>From Matthew Genge <>

Dear Benny,

I'm off to the Antarctic to collect meteorites on Monday and thought
that CCNet might be interested in the following.

The internet, it seems, gets everywhere. Web cameras capture virtually
every human endeavor from DIY to sky diving. Now the internet has
finally reached even the remotest region of the Earth's surface. Over
the next two months the Antarctic Search for Meteorites expedition to
the frozen continent will be covered by
You'll be able to see images of our daily hunting escapades and read our
journal entries. If you've ever wondered how cold meteoriticists have to get
before they stop talking about meteorites then this is your chance to
find out.

Minus 30 ought to do it.

Matthew Genge
Dr Matthew J. Genge
Researcher (Meteoritics)
Department of Mineralogy, The Natural History Museum
Cromwell Road, London SW7 5BD, UK.

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