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(meteorobs) Excerpts from "CCNet 125/2001 - 26 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 125/2001 - 26 November 2001
Date: Mon, 26 Nov 2001 11:50:33 -0000
CCNet 125/2001 - 26 November 2001
=================================
[...]
(2) WHEN SPACE ROCKS COLLIDE: VIOLENT CREATION OF ASTEROID FAMILIES
Space.com, 23 November 2001
(3) SUNLIGHT MAY NUDGE ASTEROIDS TOWARD EARTH
Andrew Yee <ayee@nova.astro.utorontodot ca>
[...]
(6) INDIANA LEONID METEORITES?
Calvin Shipbaugh <res04m7h@gtedot net>
(7) ERATUUM
Matthew Genge <M.Genge@nhm.acdot uk>
[...]
==============
(2) WHEN SPACE ROCKS COLLIDE: VIOLENT CREATION OF ASTEROID FAMILIES
>From Space.com, 23 November 2001
http://www.space.com/scienceastronomy/solarsystem/asteroid_birth_011123.html
By Robert Roy Britt
Senior Science Writer
In this week's journal Science, three new studies explain the violent family
history of asteroids, give a new estimate for how many space rocks exist in
potentially dangerous orbits near Earth, and paint a more precise picture of
where these asteroids roam.
Asteroids known to orbit the Sun in family groups are likely the result of
tremendous collisions between two rocks, both larger than Rhode Island,
according to a new computer simulation. A huge shock wave reverberates
through the asteroids and splinters them into myriad fragments, but gravity
gathers some of the pieces together again to create somewhat loosely bound
"rubble piles."
These individual asteroids then continue orbiting the Sun, but now instead
of two asteroids there are many.
New Near-Earth Asteroid estimate
In a separate study, MIT researcher Joseph S. Stuart developed yet another
estimate for the number of asteroids 1-kilometer (0.62 miles) or larger
orbiting the Sun roughly at the same distance as Earth. These Near-Earth
Asteroids, or NEAs as astronomers call them, are a top priority for
discovery because they stand the greatest chance of colliding with Earth
sometime in the future.
None of the roughly 500 known NEAs is on a course that will hit Earth
anytime in the next century. But scientists are unsure exactly how many more
NEAs are out there. Most estimates for the total have ranged from 900 to
1,200 and have been revised many times in recent years.
The new study, based on data from the highly successful Lincoln Near-Earth
Asteroid Research (LINEAR) project at MIT, puts the count at between 1,137
and 1,397. It is based on a larger sample of known NEAs compared with
previous studies.
Donald Yeomans, an asteroid expert at NASA's Jet Propulsion Laboratory, said
the MIT estimate is the result of new data and methods and is roughly in
line with other recent estimates, though slightly higher than some. The most
widely accepted estimate in recent months has been about 1,000, plus or
minus 200 or 300, he said. Other studies have put the count as high as
1,400.
Where they roam
Stuart also looked into the locations of NEAs. Earth, the other planets and
most asteroids orbit the Sun roughly in the same imaginary plane in space,
called the ecliptic. But Stuart found that more NEAs are farther above or
below this plane than previously thought. This greater orbital
"inclination," as it is called, may be good news for Earth.
"NEAs with higher inclinations are less likely to impact the Earth," Stuart
said.
Yeomans said the result reaffirms the need to continue looking for asteroids
in the entire sky, as the LINEAR search program does.
"If all you want to do is discover the most NEAs, you look in the ecliptic,"
Yeomans said. "But if everyone does that, you miss some that are in higher
inclinations."
Colliding space rocks
Farther out in space, well beyond NEAs, some 20 families of asteroids are
known to orbit the Sun in the main asteroid belt, between Mars and Jupiter.
Millions of asteroids populate the main belt, leftovers of the solar
system's formation more than 4 billion years ago.
The study of how asteroids might collide and create family groups was led by
Patrick Michel and a colleague at Tanga at Observatoire de la Ctte d'Azur in
Nice, France.
Michel explained his group's computer simulation:
Travelling at 11,180 mph (5 km/s), an asteroid 30 miles (48 km) in diameter
slams into another that is 177 miles (284 km) wide. A shock wave sends
cracks propagating through the larger asteroid. Within minutes, it shatters
into 100,000 pieces, none larger than 2 miles (3 km).
The bits are strewn through space, some heading in slightly new directions
at slightly different speeds. But the mutual gravity of the hoard of giant
boulders begins pulling some back together, a process that lasts roughly two
weeks.
The simulation could explain the developmental histories of many asteroids,
which researchers believe are loosely bound "rubble piles" rather than solid
rocks.
"Since a big majority of real asteroids with sizes above a few kilometers
should already have suffered a collision during their lifetime, our result
suggests that many should be rubble piles," Michel told SPACE.com.
And because many of the rocks got back together after being blown apart,
Michel said the study should help scientists better understand the collision
energy required to divert asteroids onto a potentially threatening
trajectory to the Earth. Researchers suspect that many NEAs may have begun
their lives in the asteroid belt and been bumped inward by collisions.
Onward to Earth
In fact, a third study provides further clues as to how some main-belt
asteroids might be bumped into near-Earth orbits.
William F. Bottke, Jr., of the Southwest Research Institute, led a team that
found that the smaller members of an asteroid family spread out and undergo
a change in orbital momentum caused by their re-radiation of solar energy.
This "Yarkovsky effect," as it is called, has been shown to send small rocks
to Earth but was previously thought to be ineffective at changing the orbits
of larger asteroid.
But Bottke's team found, again in computer simulations, that this Yarkovsky
effect leads some family members to the edges of gaps in the main asteroid
belt -- regions of the belt that have been swept clean by the gravitational
effects of Jupiter.
And what does Jupiter do with asteroids that enter these gaps? Sends them to
near-Earth orbits.
Copyright 2001, Space.com
============
(3) SUNLIGHT MAY NUDGE ASTEROIDS TOWARD EARTH
>From Andrew Yee <ayee@nova.astro.utorontodot ca>
Southwest Research Institute (SwRI)
Boulder, Colorado
For more information, contact:
Maria Martinez
Communications Department
Southwest Research Institute
P.O. Drawer 28510
San Antonio, Texas 78228-0510
Phone (210) 522-4630, Fax (210) 522-3547
or
Dr. Bill Bottke
Phone (303) 546-9687
November 22, 2001
Sunlight may nudge asteroids toward Earth
The Earth has long resided among swarms of asteroids. Many of these objects
are miles across, large enough that an impact with the Earth could present a
significant hazard to life. Researchers believe that the starting location
for these bodies is the main asteroid belt, a stable reservoir of huge,
hurtling boulders located between the orbits of Mars and Jupiter. An
on-going puzzle, however, is how these giant rocks escape the asteroid belt
to reach orbits bound for Earth.
A new study led by Dr. William Bottke of Southwest Research InstituteTM
(SwRI) in Boulder, Colo., suggests the ultimate solution may be much more
slow and subtle than anyone suspected. Bottke is the lead researcher on a
U.S.-Czech-French team that has shown that large asteroids are gently nudged
over hundreds of millions or even billions of years by the absorption and
re-emission of sunlight, enough so the asteroids may eventually fall into
orbital zones where the combined gravitational kicks of the planets can
force them onto Earth-crossing orbits.
The team's report, "Dynamical Spreading of Asteroid Families via the
Yarkovsky Effect," appears in the Nov. 23 edition of the journal Science.
The researchers have carefully studied asteroid families, formations of
large and small rocks believed to be the fragments of tremendous collisions
between the largest asteroids in the main asteroid belt. The rocks produced
by these collisions tend to have similar orbits, making it possible to piece
together how the family members have evolved since their formation long ago.
Computer models showing how the asteroid break-up events work are the
subject of a paper written by a team led by Patrick Michel of the
Observatoire de la Cote d'Azur in the same issue of Science. Michel's team
found that collision fragments are frequently thrown far from the impact
site, but not so far that they can reproduce the orbital distribution of
observed asteroid families. The biggest mismatches occur among the smaller
family members, which are less than 10 miles across. Many small family
members also appear to be corralled by narrow chaotic
zones known as resonances, where tiny gravitational kicks produced by nearby
planets such as Mars, Jupiter, or Saturn can push asteroids out sof the
asteroid belt.
The solution arrived at by Bottke's team explains the unusual orbits of the
smaller family members, related to a radiation effect named for Russian
engineer I.O. Yarkovsky, who first described it a century ago. Like a sunlit
sidewalk on Earth, a body spinning in space would be
expected to heat up slowly and reradiate the energy back into space. Because
radiation carries some momentum, Yarkovsky theorized that the reradiated
energy slowly propels the body like a comet spewing off gas. Bottke's team
speculates that this gentle push, if applied to small asteroid family
members for hundreds of millions or even billions of years, could move them
great distances.
The team uses computer simulations to show that the Yarkovsky Effect can
indeed move small family asteroids far enough to place them on their
observed orbits. Moreover, asteroids migrating long and far enough are found
to fall into resonances capable of pushing them onto Earth-threatening
orbits. One such asteroid, which probably evolved in this fashion, is (433)
Eros, the subject of an intensive investigation by the Near-Earth Asteroid
Rendezvous (NEAR) spacecraft over the last several years.
Thus, for the first time, the observed orbital distribution of asteroid
families and the presence of very old asteroids near Earth can be understood
using a combination of Michel's model, which describes how families are
born, and Bottke's model, which describes how families evolve and spread out
over long timescales.
Other authors of this study were David Vokrouhlicky and Miroslav Broz of
Charles University, Czech Republic; David Nesvorny of the Southwest Research
Institute, Boulder; and Alessandro Morbidelli of the Observatoire de la Cote
d'Azur, France. NASA and the European Space
Agency funded the study.
SwRI is an independent, nonprofit, applied research and development
organization based in San Antonio, Texas, with more than 2,700 employees and
an annual research volume of more than $315 million.
Editors: Animations showing an asteroid break up and the spread of an
asteroid family are available at http://swrinews.worldpost.com/asteroids/
============================
* LETTERS TO THE MODERATOR *
============================
(6) INDIANA LEONID METEORITES?
>From Calvin Shipbaugh <res04m7h@gtedot net>
Benny,
I saw your report on the Cambridge-Conference net mentioning the news claim
of possible meteorites during the recent meteor shower. Although only a good
analysis can clear up the issue, as a meteorite science hobbyist I am
skeptical for one outstanding reason. There was no
mention of fusion crust in the news articles and the shiny, flakey
description of the material sounded suspect. Meteorites typically experience
high temperatures sufficient to melt/ablate a thin outer layer which is
quite evident in new falls. Only in the most exceptional cases does there
not appear a striking crust, as with the clean green surfaces of the
Tatahouine diogenite shower of 70 years ago. That, however, was the case of
a stone breaking apart at altitudes such that time and speed did not permit
secondary fusion crust formation (in any great way, at least). The very high
speed of the Leonid shower fragments should, if any survived to ground, show
evidence for crust. Even a soft material like Pasamonte developed a good
crust.
- Calvin Shipbaugh
===========
(7) ERATUUM
>From Matthew Genge <M.Genge@nhm.acdot uk>
Benny,
Apparantley I don't need sub-zero temperatures to stop my brain working
properly. The ANSMET expedition can be followed at
http://www.webexpeditionsdot net not the address I provided on Friday.
Regards,
Matt Genge
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