archive: SETI CCNet DIGEST 04/02/99
SETI CCNet DIGEST 04/02/99
Larry Klaes ( lklaes@bbn.com )
Fri, 05 Feb 1999 11:57:19 -0500
>From: Benny J Peiser <b.j.peiser@livjm.ac.uk>
>Sender: humbpeis@livjm.ac.uk
>To: cambridge-conference@livjm.ac.uk
>Subject: CCNet DIGEST 04/02/99
>Date: Thu, 4 Feb 1999 10:56:25 -0500 (EST)
>Priority: NORMAL
>X-Mailer: Simeon for Windows Version 4.0.5
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>
>CCNet DIGEST, 4 February 1999
>------------------------------------------------
>
>(1) NASA: SHOEMAKER NEO SEARCH FUNDED,
> BUT YEOMANS GETS NO ADDITIONAL MONEYS
> E.P. Grondine <epgrondine@hotmail.com>
>
>(2) UNIVERSITY OF CHICAGO PREPARING INSTRUMENTS FOR ASTEROID LANDING
> Steve Koppes <s-koppes@uchicago.edu>
>
>(3) STARDUST MISSION SET TO BRING BACK A PIECE OF A COMET
> Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>
>(4) MINOR PLANET WORKSHOP: CALL FOR PAPERS & POSTERS
> Richard A Kowalski <bitnik@bitnik.com> wrote:
>
>(5) CRATERING RATES ON THE GALILEAN SATELLITES
> Z. Zahnle et al., NASA, AMES RES CTR
>
>(6) DUST EMISSION FOR COMETS SHOEMAKER-LEVY AND McNAUGHT-RUSSELL
> W. Waniak*), S. Zola, JAGIELLONIAN UNIVERSITY, KRAKOW,POLAND
>
>(7) LEONID METEOR SHOWER
> D.M. Hunten et al., UNIVERSITY OF ARIZONA
>
>(8) STATISTICAL PROPERTIES OF ENCOUNTERS AMONG ASTEROIDS
> A. DellOro and P. Paolicchi, UNIVERSITY OF PISA
>
>
>==========================
>(1) NASA: SHOEMAKER NEO SEARCH FUNDED,
> BUT YEOMANS GETS NO ADDITIONAL MONEYS
>
>>From E.P. Grondine <epgrondine@hotmail.com>
>
>Benny -
>
>I attended the NASA Fiscal Year 2000 budget briefing on Monday. NASA
Adminstrator
>Dan Goldin's presentation was brief, and he was clearly showing signs of
>exhaustion, which he acknowledged.
>
>While Goldin did not mention the NEO search in his remarks, he did mention
briefly
>the need for determining the composition of both asteroids and comets, in
case it
>became necessary to stop one.
>
>Afterwards I asked him about the NEO search, and he told me that the NASA
>Comptroller had the numbers on that. The NASA Comptroller told me that
the program
>was continuing and that funding had been held at 1999 levels. I asked the
Director
>of Space Sciences about the NEO search, and recieved the same reply.
>
>I examined the NASA Budget request hoping to get a more detatiled
breakdown, but
>found that the NEO search was not listed as a separate subhead. I then
asked Doug
>Isbell, from the NASA Space Sciences Public Affairs Office if he could
provide me
>with details, and received today the following reply, which sets the
number at
>$3.5 million:
>
>Ed --
>
>Tom Morgan reminded me that NASA FY99 funding for NEO searching is
>approx.$3.5m, and he is not aware of any change to that number for '00.
>
>-- Doug
>
>Douglas Isbell
>NASA HQ Public Affairs Officer
>Space Science (Planetary)
>202/358-1753 -3093 fax
>douglas.isbell@hq.nasa.gov
>
>I know I would find a detailed description by Don of the budget
negotiations quite
>interesting, but I have a feeling that it is a lot like hotdotgs: They're
tasty,
>but I don't know if you really want to visit the plant.
>
>Best wishes -
>EP ,
>3 February 1999
>
>=====================
>(2) UNIVERSITY OF CHICAGO PREPARING INSTRUMENTS FOR ASTEROID LANDING
>
>>From Steve Koppes <s-koppes@uchicago.edu>
>
>February 2, 1999
>For immediate release
>
>Contact: Steve Koppes
> (773) 702-8366
> s-koppes@uchicago.edu
>
>University of Chicago preparing instruments for first asteroid landing, two
>missions to Mars
>
> The first object that humans will land on the surface of an
>asteroid will contain a miniature version of the University of Chicago
>instrument that helped make aerospace history during the Mars Pathfinder
>mission in 1997.
> Chicago's alpha proton X-ray spectrometer, or APXS, which was
>carried aboard the Sojourner rover, provided the first-ever chemical
>analysis of native Martian rock during the Pathfinder mission. Now a much
>smaller, Chicago-built alpha X-ray spectrometer, AXS, will provide similar
>data during the joint U.S.-Japanese MUSES-C mission scheduled for launch to
>asteroid Nereus in 2002. The AXS may even play a role in selecting a sample
>of the asteroid for return to Earth during the mission in 2005.
> "We are now in the final stages of design," said Thanasis "Tom"
>Economou, Senior Scientist at Chicago's Enrico Fermi Institute. The
>prototype is being built, and in a few months, it will be integrated with
>the nanorover "to make sure everything works in coordination," he said.
> The AXS instrument is key to accomplishing mission objectives, said
>Donald Yeomans, a senior research scientist at NASA's Jet Propulsion
>Laboratory and U.S. science team leader for the asteroid lander mission.
> "As the rover wanders around the surface of the asteroid, we're
>counting on Tom's instrument to tell us what the various soils and rocks
>are made of," Yeomans said. "It makes a big difference as to how the object
>formed. Is it a chip off a bigger object or is it an accumulation of
>various bits and pieces of asteroids, sort of a rubble pile?"
> The chemical composition of the asteroid can also be compared to
>that of meteorites found on Earth. "Once we make the link between a certain
>type of asteroid and a certain type of meteorite, we can remotely observe
>asteroids and infer what they're made of," Yeomans said.
> This is possible because asteroids, planets and stars have specific
>spectral characteristics -they give off certain types of electromagnetic
>radiation that depend on the object's chemical composition. "Presumably,
>every other asteroid having the same spectral characteristics will be made
>of the same stuff," Yeomans said.
> MUSES-C is a cooperative effort between the National Aeronautics
>and Space Administration and Japan's Institute of Space and Astronautical
>Science. The mission is scheduled for launch from Japan in January 2002.
>The spacecraft will arrive at Nereus in early April 2003 and will land
>later that month. Nereus, discovered in 1982, measures less than a mile in
>diameter.
> "When the spacecraft is 50 feet above the surface, it will gently
>drop the nanorover. The spacecraft will then land, take some samples, then
>take off and hover nearby for a couple months while we're doing the surface
>analysis and investigations," Economou said. "There will be an attempt to
>do two landings, so it is conceivable that we will tell mission controllers
>where to land for the second time to grab a sample of particular interest."
> The solar-powered nanorover, mounted on four wheels, will measure
>only 6 inches square. It will be equipped with a camera and a near-infrared
>spectrometer as well as the AXS. "The nanorover is a nice little toy,"
>Economou said. "It can go over much larger rocks than itself. It can also
>automatically right itself up after falling on its back."
> The APXS that Economou built for Pathfinder's rover weighed 570
>grams (1.2 pounds). But for MUSES-C, Economou had to think on the
>Lilliputian scale of Gulliver's Travels. His weight limit for the
>nanorover's chemical analyzer is no more than 100 grams (3.5 ounces),
>packed into an area measuring less than 3 cubic inches.
> Together the AXS's alpha and X-ray detectors can identify any
>chemical element except hydrogen at concentrations as low as a fraction of
>1 percent. The instrument's design saves weight by dispensing with the
>proton detector, which mostly duplicates data collected by the X-ray
>detector, and by sharing capabilities with the nanorover.
> In November, Economou was one of 11 Mars Pathfinder team members
>and instrument developers to receive special recognition from the National
>Air and Space Museum of the Smithsonian Institution. The Pathfinder team
>received the 1998 National Air and Space Museum Trophy for Current
>Achievement for its demonstration of technologies and concepts for use in
>future Mars missions.
> Economou still is analyzing data collected during the Pathfinder
>mission, which officially ended in 1997. He also is designing and building
>two more APXS instruments in collaboration with the
>Max Planck Institute in Germany for the rovers of the Mars Surveyor 2001
>and 2003 missions. The rovers of these two missions will visit more Martian
>sites to collect samples, some of which will be returned to Earth during a
>mission set for 2005.
> The rover for Mars 2001, called Marie Curie, is similar to Sojourner.
>"For 2003, there will be the Athena rover, a new type of rover,
>capable of traveling farther distances, carrying more instruments and
>carrying a drill bit to collect samples," Economou said.
> The Athena rover's robotic arm will pick up a sample and bring it
>to the APXS and other instruments. After examination, the robot arm will
>either drop the sample on the surface or place it in a special container
>for return to Earth.
>"It will be exciting times," Economou said.
>###
>Editor's Note: An image of Economou with a prototype of his AXS instrument
>is available upon request.
>Radio stations: The University of Chicago has an ISDN line. Please call
>for information.
>For more news from the University of Chicago, visit our Web site at
>http://www-news.uchicago.edu
>
>===========================
>(3) STARDUST MISSION SET TO BRING BACK A PIECE OF A COMET
>
>>From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>
>MEDIA RELATIONS OFFICE
>JET PROPULSION LABORATORY
>CALIFORNIA INSTITUTE OF TECHNOLOGY
>NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
>PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
>http://www.jpl.nasa.gov
>
>Contact: Franklin O'Donnell
>
>FOR IMMEDIATE RELEASE February 3, 1999
>
>STARDUST MISSION SET TO BRING BACK A PIECE OF A COMET
>
> NASA's Stardust mission, scheduled for launch Saturday, February 6,
from Cape
>Canaveral, FL, will send a spacecraft flying through the cloud of dust that
>surrounds the nucleus of a comet - and, for the first time ever, bring
cometary
>material back to Earth.
>
> Launch is scheduled at 4:06 p.m. Eastern time, with live coverage on
NASA
>Television beginning at 2:30 p.m. Eastern. A post-launch briefing is
planned to be
>broadcast on NASA Television at 6 p.m. Eastern.
>
> Comets, which periodically grace our sky like celestial bottle
rockets, are
>thought to hold many of the original ingredients of the recipe that
created the
>planets and brought plentiful water to Earth. They are also rich in organic
>material, which provided our planet with many of the ready-to-mix
molecules that
>could give rise to life. They may be the oldest, most primitive bodies in
the
>solar system, a preserved record of the original nebula that formed the
Sun and
>the planets.
>
> "Scientists have long sought a sample directly from a known comet
because of
>the unique chemical and physical information these bodies contain about the
>earliest history of the solar system," said Dr. Edward Weiler, NASA's
associate
>administrator for space science. "Locked within comet molecules and atoms
could
>be the record of the formation of the planets and the materials from which
they
>were made."
>
> Stardust is the first U.S. mission dedicated solely to a comet and
will be the
>first to return extraterrestrial material from outside the orbit of the
Moon.
>Stardust's main objective is to capture a sample from a well-preserved
comet called
>Wild-2 (pronounced "Vilt-2").
>
> The spacecraft will also collect interstellar dust from a recently
discovered
>flow of particles that passes through our solar system from interstellar
space. As
>in the proverbial "from dust to dust," this interstellar dust represents the
>ultimate in recycled material; it is the stuff from which all solid
objects in the
>universe are made, and the state to which everything eventually returns.
>Scientists want to discover the composition of this "stardust" to
determine the
>history, chemistry, physics and mineralogy of nature's most fundamental
building
>blocks.
>
> Because it would be virtually impossible to equip a spacecraft with
the most
>sophisticated lab instrumentation needed to analyze such material in
space, the
>Stardust spacecraft is more of a robotic lab assistant whose job it is
pick up and
>deliver a sample to scientists back on Earth. The spacecraft will,
however, radio
>some on-the-spot analytical observations of the comet and interstellar dust.
>
> "The samples we will collect are extremely small, less than a micron,
or
>1/25,000th of an inch, in size, and can only be adequately studied in
laboratories
>with sophisticated analytical instruments," said Dr. Donald C. Brownlee of
the
>University of Washington, principal investigator for the Stardust mission.
>
> "Even if a ton of sample were returned, the main information in the
solids
>would still be recorded at the micron level, and the analyses would still
be done a
>single grain at a time."
>
> Stardust will meet up with Comet Wild-2 on January 2, 2004. A
gravity assist
>flyby of Earth will put Stardust on a trajectory that will allow it to
capture
>cometary dust intact at a low relative speed of 6.1 kilometers per second
(about
>13,600 miles per hour). An onboard camera will aid in navigating the
spacecraft as
>close as about 150 kilometers (100 miles) from the comet's nucleus,
permitting the
>capture of the freshest samples from the heart of the comet.
>
> Dressed for survival behind armored shields, Stardust will document
its
>10-hour passage through the hailstorm of comet debris with scientific
instruments
>and the navigation camera. On approach to the dust cloud, or "coma," the
>spacecraft will flip open a tennis-racket-shaped particle catcher filled
with a
>smoke-colored glass foam called aerogel to capture the comet particles.
Aerogel,
>the lowest-density material in the world, has enough "give" in it to slow
and stop
>particles without altering them too much. After the sample has been
collected, the
>aerogel capturing device will fold down into a return capsule, which
closes like a
>clamshell to enclose the sample for its safe delivery to Earth.
>
> In addition, a particle impact mass spectrometer will obtain
in-flight data on
>the composition of both cometary and interstellar dust, especially very fine
>particles. The optical navigation camera should provide excellent images
of the
>dark mass of the comet's nucleus. Other equipment will reveal the
distribution in
>both time and space of coma dust, and could give an estimate of the
comet's mass.
>
> On January 15, 2006, a parachute will set the capsule gently onto the
salt
>flats of the Utah desert for retrieval. The scientifically precious
samples can be
>studied for decades into the future with ever-improving techniques and
analysis
>technologies, limited only by the number of atoms and molecules of the
sample
>material available. Many types of analyses now performed on lunar
samples, for
>example, were not even conceived at the time of the Apollo missions to the
Moon.
>
> Comets are small, irregularly shaped bodies composed of a mixture of
grains of
>rock, organic molecules and frozen gases. Most comets are about 50
percent water
>ice. Typically ranging in size up to about 10 kilometers (6 miles) in
diameter,
>comets have highly elliptical orbits that bring them close to the Sun and
then
>swing them back out into deep space. They spend most of their existences
in a deep
>freeze beyond the orbit of Pluto - far beyond the Sun's dwindling
influence, which
>is why so much of their original material is well-preserved.
>
> When a comet approaches within about 700 million kilometers (half
billion
>miles) of the Sun, the surface of the nucleus begins to warm, and material
on the
>comet's nucleus heats and begins to vaporize. This process, along with
the loss of
>rocky debris or other particles that fly off the surface, creates the
cloud around
>the nucleus called the coma. It is the glowing, fuzzy coma that appears
as the
>head of a comet when one is observed from Earth. A tail of luminous
debris and
>another, less apparent, tail of gases flow millions of miles beyond the
head in
>the direction away from the Sun.
>
> Comet Wild-2 is considered an ideal target for study because, until
recently,
>it was a long-period comet that rarely ventured close to the Sun. A
fateful pass
>near Jupiter and its enormous gravity field in 1974 pulled Comet Wild-2
off-course,
>diverting it onto a tighter orbit that brings it past the Sun more
frequently and
>also closer to Earth's neighborhood. Because Wild-2 has only recently
changed its
>orbit, it has lost little of its original material when compared with other
>short-period comets, so it offers some of the best-preserved comet samples
>that can be obtained.
>
> Stardust was competitively selected in the fall of 1995 under NASA's
Discovery
>Program of low-cost, highly focused science missions. As a Discovery
mission,
>Stardust has met a fast development schedule, uses a small Delta launch
vehicle, is
>cost-capped at less than $200 million, and is the product of a partnership
>involving NASA, academia and industry.
>
> Principal investigator Brownlee is well-known for his discovery of
cosmic
>particles in Earth's stratosphere known as Brownlee particles. Dr. Peter
Tsou of
>NASA's Jet Propulsion Laboratory, Pasadena, CA, an innovator in aerogel
technology
>and maker of aerogel, serves as deputy investigator. JPL, a division of the
>California Institute of Technology, manages the Stardust mission for
NASA's Office
>of Space Science, Washington, DC. Dr. Kenneth L. Atkins of JPL is project
manager.
>The spacecraft is designed, built and operated by Lockheed Martin
Astronautics,
>Denver, CO. JPL provided the spacecraft's optical navigation camera, and
the Max
>Planck Institute of Germany provided the real-time dust composition analyzer.
>
> NASA Television is broadcast on the satellite GE-2, transponder 9C, C
band, 85
>degrees west longitude, frequency 3880.0 MHz, vertical polarization, audio
monaural
>at 6.8 MHz.
>
>==========================
>(4) MINOR PLANET WORKSHOP: CALL FOR PAPERS & POSTERS
>
>>From Richard A Kowalski <bitnik@bitnik.com> wrote:
>
>Ladies and gentlemen.
>
>As you have read on the (MPML) mailing list, we are planning a workshop
for the
>minor planet researcher. This workshop will be held on April 23rd and
24th, 1999
>at Lowell Observatory in Flagstaff, Arizona, USA. There will be a
reception held
>on the evening of April 22.
>
>The intent of this meeting is to provide an outlet for discussion and
>collaboration between the amateur and professional communities. It is
hoped that
>this workshop will strengthen the ties between these related groups and
result
>in better understanding of this field of research.
>
>The general plan is as follows. The first day will be devoted entirely to
>questions of astrometry, including: the scope of the follow-up problem,
>follow-up strategies, astrometry techniques, and how best to organize the
>amateur efforts, plus a review of on-line resources available to amateurs,
and
>how amateurs can obtain grants. The second day will cover mainly questions of
>photometry and photometric techniques.
>
>A number of well known individuals in this field will be in attendance and
will
>be presenting papers. To allow for a greater range of participation and
ideas,
>the Organizing Committee (OC) is now requesting the submission of
abstracts for
>contributed talks for possible inclusion in the workshop.
>
>Contributed talks will be limited to 15 minutes. All abstracts for
contributed
>speakers MUST be submitted no later than February 28th, 1999 to be considered
>for inclusion in the workshop.
>
>The OC is also requesting submission of poster presentations. The idea of
poster
>presentations is so that participants who do not wish to speak or can not
fill
>15 minutes worth of time can make presentations. These posters may be
elaborate
>or can simply be a description of your observing program, techniques and
>instrumentation.
>
>We have set a deadline for submission of poster requests of April 1st, 1999.
>Please advise Paul Comba that you will be a poster presenter and the
approximate
>amount of space your posted information will require. This will then allow
the
>OC to insure that there will be sufficient space for all presentations.
>
>Commercial presentations of any sort will not be permitted. However,
attendees
>with items and products of a commercial nature will be permitted to bring
along
>handouts for placement on tables held in reserve for handouts, flyers and the
>like.
>
>All abstracts and requests for poster presentations should be sent to:
>
> Paul G. Comba
> 1411 Galaxy Lane
> Prescott, AZ 86303
> e-mail: comba@northlink.com
>
>Richard Kowalski bitnik@bitnik.com
>http://www.bitnik.com/QHO Quail Hollow Observatory 761 Zephyrhills
>
>=====================
>(5) CRATERING RATES ON THE GALILEAN SATELLITES
>
>Z. Zahnle*), L. Dones, H.F. Levison: Cratering rates on the Galilean
>satellites. ICARUS, 1998, Vol.136, No.2, pp.202-222
>
>*) NASA, AMES RES CTR, MS 245-3, MOFFETT FIELD,CA,94035
>
>We exploit recent theoretical advances toward the origin and orbital
>evolution of comets and asteroids to obtain revised estimates for
>cratering rates in the jovian system. We find that most, probably more
>than 90%, of the craters on the Galilean satellites are caused by the
>impact of Jupiter-family comets (JFCs). These are comets with short
>periods, in generally low-inclination orbits, whose dynamics are
>dominated by Jupiter. Nearly isotropic comets (long period and Halley-
>type) contribute at the 1-10% level. Trojan asteroids might also be
>important at the 1-10% level; if they are important, they would be
>especially important for smaller craters. Main belt asteroids are
>currently unimportant, as each 20-km crater made on Ganymede implies
>the disruption of a 200-km diameter parental asteroid, a destruction
>rate far beyond the resources of today's asteroid belt,
>Twenty-kilometer diameter craters are made by kilometer-size
>impacters; such events occur on a Galilean satellite about once
>in a million years. The paucity of 20-km craters on Europa indicates
>that its surface is of order 10 Ma. Lightly cratered surfaces on
>Ganymede are nominally of order 0.5-1.0 Ga. The uncertainty in these
>estimates is about a factor of five. Callisto is old, probably more
>than 4 Ga. It is too heavily cratered to be accounted for by the
>current flux of JFCs. The lack of pronounced apex-antapex asymmetries
>on Ganymede may be compatible with crater equilibrium, but it is more
>easily understood as evidence for nonsynchronous rotation of an icy
>carapace. (C) 1998 Academic Press.
>
>===============
>(6) DUST EMISSION FOR COMETS SHOEMAKER-LEVY AND McNAUGHT-RUSSELL
>
>W. Waniak*), S. Zola: Dust emission for Comets Shoemaker-Levy 1991a1
>and McNaught-Russell 1993v. ICARUS, 1998, Vol.136, No.2, pp.280-297
>
>*) JAGIELLONIAN UNIVERSITY,ASTRON OBSERV,PL-30244 KRAKOW,POLAND
>
>We present CCD photometric results for the dust comae of the
>dynamically new Comet Shoemaker-Levy 1991a1, carried out at
>heliocentric distances from 1.2 to 0.8 AU pre-perihelion, and
>the high-eccentricity, long-period Comet McNaught-Russell 1993v
>obtained at a heliocentric distance close to 1.0 AU post-perihelion.
>Maps of the directional distribution of the dust emission rate from
>these cometary nuclei were obtained using the directional deconvolution
>method (Waniak 1994, Icarus 111, 237-245). For Comet Shoemaker-Levy the
>prominent region of enhanced dust production was situated between the
>solar terminator and the nucleocentric meridian opposite the subsolar
>point. Activity in this region on the night side of the nucleus may be
>explained both by the heating of the nucleus' surface by scattered
>visible and reemitted infrared radiation, which is produced by the dust
>coma, or by non-solar radiation sources of energy, such as chemical
>reactions or phase transitions. During the period of observations the
>dust emission rate for this region decreased in comparison with that of
>another region of enhanced dust production situated on the subsolar
>hemisphere. For Comet McNaught-Russell tno active regions were also
>visible, although the subsolar region was much more active than that on
>the night side of the nucleus. For both comets, dust was emitted from
>the entire surface of the nucleus at a level no lower than 30% of the
>maximum value for the active regions. The total (integrated over a 4 pi
>solid angle) dust emission rate for Comet Shoemaker-Levy changed as
>r(h)(-2.3) for the observed range of heliocentric distance r(h). For
>both comets, the ejection velocity of submicron dust particles was of
>the order of 0.1 km sec(-1) and the power-law size distribution of dust
>particles (a(-n)) had a mean value of exponent n equal to 2.9. The
>power-law dependence of the ejection velocity upon the beta parameter
>(nu similar to beta(k)) was specified by a mean value of k close to
>0.18. (C) 1998 Academic Press.
>
>==========================
>(7) LEONID METEOR SHOWER
>
>D.M. Hunten*), G. Cremonese, A.L. Sprague, R.E. Hill, S. Verani, R.W.H.
>Kozlowski: The Leonid meteor shower and the Lunar sodium atmosphere.
>ICARUS, 1998, Vol.136, No.2, pp.298-303
>
>*) UNIVERSITY OF ARIZONA,LUNAR & PLANETARY LAB,TUCSON,AZ,85721
>
>Observations of lunar sodium were made on November 16-18, 1997, at
>Asiago and Mount Lemmon Observatories. A small enhancement was
>observed, and me suggest that it was related to the Leonid shower.
>Visual observations by members of the Association of Lunar and
>Planetary Observers and the North American Meteor Network show a peak,
>a few hours wide, at about 12:50 UT on the 17th. The possible
>enhancement of the lunar Na appears to last at least 3 days, and must
>be associated with a considerably more extended cloud of particles than
>that responsible for the bright visual meteors. The circumstances of
>the 1997, 1998, and 1999 events are discussed. Large showers may occur
>in 1998 and 1999; the 1998 shower occurs too close to new moon for good
>atmospheric observations, but if the extended, faint shower is also
>enhanced the corresponding lunar Na should be observable for a
>considerably longer period. (C) 1998 Academic Press.
>
>======================
>(8) STATISTICAL PROPERTIES OF ENCOUNTERS AMONG ASTEROIDS
>
>A. DellOro and P. Paolicchi: Statistical properties of encounters
>among asteroids: A new, general purpose, formalism. ICARUS, 1998,
>Vol.136, No.2, pp.328-339
>
>UNIVERSITY OF PISA, DIPARTIMENTO FIS,PIAZZA TORRICELLI 2,I-56127
>PISA, ITALY
>
>The statistical properties of asteroid mutual encounters have been
>studied by several authors, with the main purpose of estimating
>collisional rates (and thus mean collisional lifetimes) and the
>distribution of encounter velocities. In this paper we present a new
>approach, conceptually not really different with respect to the
>classical ones, but implemented with a rather different mathematical
>formalism and consequently more flexible. When a comparison is possible
>our results are very similar to those obtained by means of other
>techniques. We exploited the peculiar flexible features of the present
>formalism to study-in a quantitative way-what happens when special
>dynamical conditions occur, such as a clustering of longitudes of
>perihelia (as in the so-called Kresak effect) or of the longitudes of
>the sample around the longitude (variable in time) of Jupiter, as in
>the case of Trojans. These dynamical situations have never been
>explored in the past using statistical approaches, and the development
>of the present one can avoid the use of heavy N-body integrations.
>Concerning the Trojan asteroids, the results of our analysis, although
>discussed here in a simplified version, are satisfactorily compared
>with those emerging from a detailed numerical integration of the orbits
>(Marzari et al., 1996, Icarus 119, 192-201). Finally, we used our
>approach to analyze the statistical properties of impacts among very
>large samples of objects with a moderate amount of computer time,
>thanks to the numerical algorithm, based on a Monte Carlo technique of
>integration. We have tested this numerical procedure by comparing our
>results with previous ones published in the literature; we find an
>amazing agreement with the more standard and refined numerical
>methods. (C) 1998 Academic Press.
>
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