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(meteorobs) LEONID excerpts from "CCNet 127/2001 - 30 November 2001"


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From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 127/2001 - 30 November 2001
Date: Fri, 30 Nov 2001 13:47:06 -0000

CCNet 127/2001 - 30 November 2001


(1) LEONID IMPACTS ON MOON OBSERVED AGAIN
    NASA Science News for November 30, 2001

(2) LEONID FLUXES FROM 1994-1998 ACTIVITY PATTERNS
    Josep Maria Trigo i Rodr=EDguez <trigo@exp.uji.es>

[...]

(7) ASTEROID SATELLITES & ASTEROID FAMILIES
    S. Fred Singer <singer@sepp.org>

[...]

======================================================================

(1) LEONID IMPACTS ON MOON OBSERVED AGAIN

NASA Science News for November 30, 2001
http://science.nasadot gov/headlines/y2001/ast30nov_1.htm?list20392

Explosions on the Moon

During the 2001 Leonid meteor storm, astronomers observed a curious flash on
the Moon -- a telltale sign of meteoroids hitting the lunar surface and
exploding.

November 30, 2001: Vivid, colorful streaks of light. A ghostly flash.
Strange crackling noises and twisting smoky trails. Add to those a cup of
hot cocoa, and you have all the ingredients for a delightful meteor
shower... on Earth.

The recent Leonids were a good example. On Nov. 18th our planet plunged into
a debris cloud shed by comet Tempel-Tuttle. Sky watchers saw thousands of
meteors -- each streak of light a tiny bit of comet dust disintegrating in
the atmosphere.

A quarter of a million miles away, another Leonid shower was happening. But
the recipe was different: Blinding flashes of light. Flying debris and
molten rock. Sizzling craters. And certainly no hot cocoa! That's what the
Leonids were like ... on the Moon.

"Like Earth, the Moon also plowed through comet Tempel-Tuttle's debris =
field
on Nov. 18th," says Bill Cooke of the NASA Marshall Space Flight =
Center.
But, unlike Earth, the Moon doesn't have an atmosphere where meteoroids
harmlessly disintegrate." Instead, lunar Leonids hit the ground and =
explode.

David Palmer, an astrophysicist at the Los Alamos National Laboratory, saw
just such an explosion from his backyard in White Rock, New Mexico. The 2001
Leonids were well underway when Palmer trained his 5-inch Celestron
telescope and a low light video camera on the crescent Moon. "It was
twilight," says Palmer. "Even so, the flash was bright enough to detect." He
had spotted a Leonid crashing down near Sinus Media -- a lava plain on the
lunar equator.

Far from New Mexico, observers on the east coast of the United States saw
it, too. Using 8 inch telescopes equipped with video cameras, David Dunham
in Maryland and Tony Cook in Virginia independently recorded the flash --
a double confirmation. "We estimate it was at least as bright as a 4th
magnitude star," says Dunham, director of the International Occultation
Timing Association.

This marks the second year Dunham and Palmer have seen lunar Leonids. They
and others video-recorded six meteoroid impacts on the Moon during the 1999
Leonid meteor storm. The brightness of those flashes ranged from 7th to 3rd
magnitude.

"Actually, we've known for many years that Leonids hit the Moon," notes
Cooke. "Between 1970 and 1977, Apollo seismic stations detected impacts
during the Leonids and several other annual meteor showers. What's new
since 1999 is that we're actually seeing the explosions from Earth."

The first reports of bright lunar Leonids two years ago puzzled many
scientists. Their calculations suggested that a Leonid hitting the Moon
would need to mass hundreds of kilograms to produce an explosion visible
through backyard telescopes. Yet there was little evidence for such massive
fragments in the Leonid debris stream. Hundred-kilogram meteoroids hitting
Earth's atmosphere would produce sensational fireballs, brighter than any
sky watchers actually saw. Furthermore, lunar seismic stations operating
for years had detected nothing larger than 50 kg.

To solve the mystery, Jay Melosh, a planetary scientist at the University
of Arizona's Lunar and Planetary Lab and an expert on planetary impact
cratering, teamed up with Ivan Nemtchinov, a Russian physicist skilled
in computer simulations of nuclear explosions.

Experience with bombs came in handy solving this problem, says Melosh:
"Leonid impacts aren't as potent as a nuclear warhead, but they are
powerful. They hit the Moon traveling 72 km/s or 160,000 mph -- that's
240 times faster than a rifle bullet. In fact, the energy per unit mass
in a Leonid strike is 10,000 times greater than a blast of TNT."

Using computer programs designed to study bomb blasts, Melosh and
Nemtchinov discovered that Leonids didn't have to be so massive to
produce flashes as bright as those detected by Dunham and Palmer.
Impactors massing only 1 to 10 kg could do the job.

"That's more like it," says Cooke. "We occasionally see kg-sized fragments
burning up in Earth's atmosphere. They appear as very bright fireballs that
disintegrate completely before hitting the ground." On the Moon, of course,
there's nothing to stop them from reaching the surface.

According to Melosh, here's what happens when the Moon and a 10 kg Leonid
collide:

Much of the ground within a few meters of the impact point would be
vaporized, and a cloud of molten rock would billow out of a growing crater.
"At first the cloud would be opaque and very hot, between 50,000 K and
100,000 K," explains Melosh. "But the temperature would drop rapidly.
Milliseconds after the initial blast, the cloud would expand to a few meters
in diameter and cool to 13,000 K. That's the critical moment," he says,
"when the vapor becomes optically thin (transparent); then, all the photons
rush out and we can see a flash of light from Earth."

An astronaut watching the event on the Moon, perhaps a hundred meters or so
from the impact, would be momentarily blinded by the Sun-bright explosion.
There wouldn't be a deafening report, however, and onlookers wouldn't be
knocked down. "There's no air on the Moon to carry shock waves," explains
Melosh. "Even so, you might have to pry some nasty bits of molten rock out
of your space suit."

Fortunately for future Moon colonists, there's little chance of being hit.
Cooke explains: "During an intense Leonid meteor storm like the one Earth
experienced in 1966, the lunar flux of meteoroids more massive than 10-5 gm
would be 1 per square-km per hour. If we assume really chubby or bulky
astronauts with a cross-sectional area of 0.5 square-meters, then the
probability of being hit by a 10-5 gm Leonid is only 0.00025." Such tiny
particles carry enough energy to puncture a spacesuit, but the astronaut
inside would remain mostly intact, says Cooke. "The probability of being
hit by something that might totally vaporize you -- like a 10 kg fragment --
is a billion times less."

So ... lunar Leonid meteoroid showers might not be as scary as they sound.
Future denizens of the Moon might even take up a new astronomical hobby:
ground watching. "I saw a hundred puffs of moondust every hour," they might
say after a good spate of Leonids. "And, ooh that fireball... what a blast!"

======================================================================

(2) LEONID FLUXES FROM 1994-1998 ACTIVITY PATTERNS

>From Josep Maria Trigo i Rodr=EDguez <trigo@exp.uji.es>

Dear Benny,

We attach below information about a new Leonid contribution from =
researchers
of the Spanish Fireball and Meteor Network. Our paper has been =
published in
the December 2001 issue of "Meteoritics & Planetary Science".

Please receive our congratulations for your interesting CNNet.

LEONID FLUXES FROM 1994-1998 ACTIVITY PATTERNS
Josep M=AA Trigo-Rodriguez 1,2,
Juan Fabregat2
and Jordi Llorca3,4

1 Depto. Ciencies Experimentals, Universitat Jaume I.
2 Departament Astronomia i Astrofisica, Universitat de Valencia.
3 Institut d'Estudis Espacials de Catalunya.
4 Departament Qu=EDmica Inorg=E0nica, Universitat de Barcelona.

The interest of our detailed 1994-1998 analysis of the Leonids activity
focuses in the reconstruction of the stream spatial structure during =
this
cometary return. We have combined 40 independent ZHR determinations =
obtained
in the last two centuries with their corresponding orbital geometry.
Initially we placed all ZHR points and the software apply a precise
numerical contorning technique to define the averaged density of the =
Leonid
stream. We use historical observations obtained after the 1800 return =
and
the ZHR determinations revised by several authors (see for more details
historical ZHRs revisions Jenniskens 1996; Brown, 1999). The result is =
one
figure that shows an averaged contour plot with the corresponding Log =
(ZHR)
isolines.

We inclose below the abstract. For more details please consult =
Meteoritics
homepage: http://www.uarkdot edu/studorg/meteor/public_html/

ABSTRACT: The Leonid shower was observed in November 1998 worldwide in =
an
intensive campaign without precedent. During this international effort =
near
35,500 meteors were reported by members and collaborators of the
International Meteor Organization (IMO) using a standard methodology.
Despite the absence of a meteor storm in 1998, the rich observational =
data
allows to obtain a detailed unprecedented knowledge of the stream =
structure
between 1994-1998.

********************************************
Josep M. Trigo-Rodr=EDguez
Prof. Dept. Experimental Sciences
Campus del Riu Sec (E.S.T.C.E)
University Jaume I
12071 Castell=F3 (SPAIN)
SPANISH FIREBALL NETWORK
Homepage: www.spmn.uji.es
E-mail: trigo@exp.uji.es
*********************************************

======================================================================
* LETTERS TO THE MODERATOR *
======================================================================

(7) ASTEROID SATELLITES & ASTEROID FAMILIES=20

>From S. Fred Singer <singer@sepp.org>

Dear Benny

Apropos the asteroid studies reported in SciAm (Nov. 27) and in the Nov 23 
issue of Science, I have three comments:

1. The asteroids created in the breakup should preserve (through their
rotation) the angular momentum of the parent body (plus impactor -- if
created that way-- or as an exploding planet - -- a la van Flandern). It
would be interesting to see if the data allow sufficient discrimination.

2. We next come to the formation of asteroidal satellites (or binary
asteroids). This would be quite common if there is complete disintegration
of the parent body (van Flandern, Michel) and relatively rare if only a
small fraction of the parent creates asteroids in an impact. Observations
may help in deciding (Chapman, Merline).

3.  Finally, in interpreting the data, we must consider the lifetime T of
an asteroidal satellite. I have analyzed this stability problem and
concluded that T will be quite short if the satellite's orbital period is
less than the rotation period of the asteroid -- and vice versa.  This
result holds whether the orbit is prograde or retrograde and does not
depend on tidal interaction (which, while present, should be of lesser
importance).

I will try to publish these results soon.

Best              Fred


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