[Prev][Next][Index][Thread]
(meteorobs) Excerpts from "CCNet 109/2001 - 19 October 2001"
I wonder if anyone on 'meteorobs' happens to know the Dr. Alan Hildebrandt,
mentioned in the first item of this issue? If you do, please ask exactly how
that quote at the beginning became so garbled: "Canadian Fireball Reporting
Centre" certainly sounds like a very official body. (The CSA is mentioned as
some sort of sponsor for it... And is it somehow related to MIAC?) And so do
we now have to conclude that pachyderms have become the official CBAT cutoff
criterion for distinguishing "meteorites" from asteroids? Whoo, lordy... :)
In any case, there is also a wonderfully concise piece below that about the
Yarkovsky effect and the other "secondary forces" which act on meteor*OIDS*
during their long, lonely orbital trips around the sun.
Clear skies,
Lew Gramer
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 109/2001 - 19 October 2001
Date: Fri, 19 Oct 2001 11:53:16 +0100
CCNet 109/2001 - 19 October 2001
================================
"What was first thought to be a meteorite that flashed across
the southern Alberta sky could be an asteroid about twice the
size of an elephant, says a geological expert. "This object was
probably an asteroidal fragment entering the atmosphere, something in
the region of one to 10 tonnes," said Alan Hildebrandt,
co-ordinator of the Canadian Fireball Reporting Centre, which is a
volunteer arm of the Canadian Space Agency." "The loudest sounds
reported so far seem to be from the Banff area, so that presumably means
the terminal burst, the explosions at the end where the object
fragmented, were relatively near there." "Much of the object
disintegrated into dust, but some of the strongest pieces would
survive," he said.
Calgary Sun, 16 October 2001
(1) ASTEROID WITNESSES SOUGHT
Calgary Sun, 17 October 2001
(2) EXPERTS SEARCH FOR ASTEROID
Calgary Sun, 16 October 2001
(3) POLICE CALLED FOLLOWING ASTEROID CRASH
Vancouver Sun, 15 October 2001
(4) METEOR LIGHTS UP NIGHT SKY IN AUSTRALIA
Ron Baalke <baalke@jpl.nasadot gov>
[...]
(6) SPACE DRIFTERS
Andrew Yee <ayee@nova.astro.utorontodot ca>
[...]
================================================================
(1) ASTEROID WITNESSES SOUGHT
>From Calgary Sun, 17 October 2001
http://wwwdot canoedot ca/CNEWSSpace0110/17_witness-sun.html
By PETER SMITH-- Calgary Sun
CALGARY -- Scientists tracking the spectacular asteroid seen by numerous
people in Western Canada want to hear from witnesses who saw it west and
north of Calgary.
"We'd really appreciate hearing reports from anyone in the Rocky
Mountain House, Sylvan Lake, Sundre area who saw it on Sunday," said
Alan Dyer of the Calgary Science Centre.
>From calls scientists have received, they believe they've tracked the
asteroid's path for part of its journey through the atmosphere.
"It passed from B.C. into Alberta, somewhere north of Lake Louise ...
heading in a northeast direction into Alberta," said Dyer. "How far it
got is yet to be determined."
Scientists say it may have been up to 10 tonnes. They hope some pieces
reached Earth and that more sightings will help find them.
Copyright 2001, Calgary Sun
============
(2) EXPERTS SEARCH FOR ASTEROID
>From Calgary Sun, 16 October 2001
http://wwwdot canoedot ca/CNEWSSpace0110/16_asteroid-sun.html
CALGARY -- What was first thought to be a meteorite that flashed across
the southern Alberta sky could be an asteroid about twice the size of an
elephant, says a geological expert.
"This object was probably an asteroidal fragment entering the
atmosphere, something in the region of one to 10 tonnes," said Alan
Hildebrandt, co-ordinator of the Canadian Fireball Reporting Centre,
which is a volunteer arm of the Canadian Space Agency.
Hildebrandt is trying to collect information from as many witnesses as
possible to pinpoint where the asteroid's remnants may have landed after
it flashed across the afternoon sky Sunday.
So far, he's received sightings from Medicine Hat in the eastern part of
Alberta to Mission in the lower mainland of British Columbia.
"It was seen over hundreds of kilometres of area even though the region
was partially cloudy," said Hildebrandt.
"The loudest sounds reported so far seem to be from the Banff area, so
that presumably means the terminal burst, the explosions at the end
where the object fragmented, were relatively near there."
But Hildebrandt says if they fell in the Rocky Mountains, it's unlikely
they'll be found. He is hoping the pieces may have landed in fields
where farmers may find them.
"Much of the object disintegrated into dust, but some of the strongest
pieces would survive," he said.
Randy Ilcisin was about 15 kilometres west of Sundre in south-central
Alberta when he heard the asteroid exploding at 2:30 p.m.
"I heard what sounded like rumbling thunder, but the sky was clear,"
said Ilcisin.
"We had no idea what it was. We thought it might be a distant
landslide."
Copyright 2001, CNews, 2001
============
(3) POLICE CALLED FOLLOWING ASTEROID CRASH
>From Vancouver Sun, 15 October 2001
http://wwwdot canada.com/vancouver/news/story.asp?id={6FDEBC95-E239-462B-BC
45-43C650D17A7E}
Patricia Bailey
Vancouver Sun
Monday, October 15, 2001
A woman who told police she witnessed an airplane crash into a hillside
near Mission probably saw an asteroid, says one of the province's top
astrophysicists.
"I think it was probably a fireball," said Jeremy Tatum, a University of
Victoria astronomer and B.C.'s official representative on the Canadian
Space Agency's meteorites and impacts advisory committee (MIAC).
Mission RCMP say an unidentified woman called their detachment Sunday
afternoon and said she saw an object that "glinted silver in the sun,"
hit the side of a hill north of Deroche. The woman saw no smoke or fire
in the spot where she thought the plane landed, said police.
"It's almost universal for a witness to think it's very close. But
typically it's more than sixty kilometres up in the air," said Tatum.
Shortly after the woman contacted the detachment, police from Mission,
Calgary, Cranbrook, Golden, Banff and Jasper received calls that a
meteor was observed falling to the earth.
"A plane wouldn't have been seen over that wide an area," said the
astrophysicist, who added that if the asteroid didn't break up in the
sky first, it probably crashed to the earth in Alberta.
Asteroids, the source of many meteorites, are rocks, or small planets
that orbit the Sun and range from a few kilometres to hundreds of
kilometres across. They are scattered through the solar system, but most
are between Jupiter and Mars.
Tatum has asked a colleague in Alberta, Allan Hildebrand, who is the
chair of MIAC, to help him investigate the sightings.
"If there are any reports from the West Coast, I would very much like to
know," said Tatum, who has asked that people contact the following
e-mail address with any information:
universe@uvvm.uvicdot ca
) Copyright 2001 Vancouver Sun
==============
(4) METEOR LIGHTS UP NIGHT SKY IN AUSTRALIA
>From Ron Baalke <baalke@jpl.nasadot gov>
http://www.thewest.com.au/20011017/news/latest/tw-news-latest-home-sto28
280.html
Meteor lights up night sky
The West Australian
October 17, 2001
POLICE received a number of reports of a bright flash or explosion in
the skies last night when a meteor vapourised about 100km north-east of
Perth.
Perth Observatory astronomer Peter Birch said the two-second flash was
caused by a rock the size of a cricket ball burning up as it entered the
atmosphere about 10.15pm.
Full story here:
http://www.thewest.com.au/20011017/news/latest/tw-news-latest-home-sto28
280.html
============
(6) SPACE DRIFTERS
>From Andrew Yee <ayee@nova.astro.utorontodot ca>
[http://www.guardian.codot uk/Print/0,3858,4279294,00.html]
Thursday, October 18, 2001
Space drifters
A peculiar pseudo-force has changed the way astronomers think meteorites
reach Earth, explains Duncan Steel.
The Guardian
Meteorites are mostly chips off bigger blocks. Out in the main belt,
between Mars and Jupiter, there are billions of asteroids. Inevitably
there are collisions between them, and some of the debris eventually
reaches us.
A simple picture, but there is a puzzle in the details. Most meteorites
appear to be too young, in terms of the time spent on independent orbits
after escaping their parent asteroids. Subject to the assumption that
the gravitational tugs of the planets are the only forces at play,
astro-mathematicians are able to trace how the paths of interplanetary
objects wander. Such calculations lead to an estimate that meteorites
need about a hundred million years to reach us, much longer than they
actually take.
This transit time is known from a meteorite's space exposure age. This
duration is quite different from the period it may have lain on the
ground before discovery (between seconds and millennia), or its age from
formation as measured using radioactive dating.
Space exposure ages are determined using cosmic rays. Within a much
larger asteroid, an eventual meteorite is shielded by an overlying layer
of rock. After an inter-asteroid collision, the freed meteoroid is
suddenly exposed to the high-energy elementary particles that permeate
space.
When these cosmic rays hit the meteoroid, they penetrate a centimetre or
so. Characteristic tracks are left in the rock, which may be studied
under a microscope. By counting the numbers of tracks it is possible to
determine how long it took for the meteorite to travel from its parent
asteroid to the Earth's surface. Typical values are a few million years.
This implies that meteoroid orbits must evolve much faster than purely
gravity-based computations would indicate. Something else must be going
on. What could it be?
Consider the famous experiment of two cannonballs of different size
being dropped from the Leaning Tower of Pisa. Both reach the ground at
the same time, despite their differing masses: only gravity matters
here.
This is not the case if a feather is substituted, because its large
cross-section compared to its mass means that air resistance is
substantial. In a vacuum the feather falls at the same rate as the iron
balls. Now think again about meteoroids in space. Are there any
influences that are size-dependent, causing them to evolve dynamically
at a rate faster than pure gravity would allow? There is no air, but is
there some other sort of resisting medium affecting their orbits,
helping them migrate inwards on a crash course with Earth?
The solar wind, the stream of charged particles moving outwards from the
sun, imposes a small force. A greater pressure derives from the photons
of sunlight. These two factors are important for tiny interplanetary
dust grains, but a meteoroid the size of a basketball is essentially
unaffected.
The sunlight absorbed by meteoroids can have other effects. They are
heated by this flux, and that energy is then re-emitted as infrared
radiation. The emission is not isotropic, though: it is not the same in
all directions. This leads to two types of pseudo-force affecting
orbiting objects.
The first was discovered in 1903 by a British physicist, John Poynting,
who spent his career at the University of Birmingham. Howard Robertson
of the California Institute of Technology further explored this concept
in 1937: in astronomical jargon it is known as the Poynting-Robertson
effect.
Poynting reasoned that because the meteoroid is moving in its orbit with
a speed of more than 10 miles per second, there are differing Doppler
shifts on the infrared photons emitted in opposing directions.
Forward-emitted radiation is shifted to a shorter wavelength, while
radiation emanating in the reverse direction is pulled out to a longer
wavelength. As a result, more momentum is emitted forwards than
backwards, and there is a retarding force on the meteoroid causing it to
spiral slowly in toward the sun. Although this is important for objects
less than a few centimetres in size, it is not significant for larger
bodies.
The second pseudo-force has only recently been recognised to be of
consequence. The idea is not new: it was simply forgotten over the
several decades since two Russian astronomers, named Yarkovsky and
Radzievskii, explored how a warm object's spin may affect its path.
The easiest way to comprehend the so-called Yarkovsky force is to think
about the Earth rotating on its axis. Split its surface into four time
zones. On the dayside are the morning and afternoon zones, on the
nightside there are pre- and post-midnight segments. Because it takes
some hours for the temperature to rise during the morning, on average
the afternoon zone is hottest, and so a greater share of the radiation
emitted into space emanates from there. Diametrically opposed to that
segment is the post-midnight zone, which is the coolest, and so the
least radiation escapes from that region.
This non-isotropic emission of radiation provides a slight shove in the
direction away from the afternoon segment, accelerating the planet
slightly. In the case of a huge body like the Earth, there is no
measurable effect. But for a small spinning meteoroid, the influence is
substantial.
It appears that the Yarkovsky effect causes a hurry-up of the orbital
evolution, and so can explain the brief space exposure ages of
meteorites.
There is another important implication of this work. In making
predictions of the tracks of asteroids and so possible impacts on the
Earth, we generally assume that only gravity affects their motion. In
such a complicated situation, even a tiny additional perturbation like
the Yarkovsky effect may make the difference between a near-miss and a
bull's-eye, the target being this little sphere in space we call our
home.
[Duncan Steel works at the University of Salford. His most recent book
is Target Earth (Time Life).]
) Guardian Newspapers Limited 2001
-------------------------------------------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK (CCNet)
-------------------------------------------------------------------
The CCNet is a scholarly electronic network. To subscribe/unsubscribe,
please contact the moderator Benny J Peiser <b.j.peiser@livjm.acdot uk>.
Information circulated on this network is for scholarly and educational
use only. The attached information may not be copied or reproduced for
any other purposes without prior permission of the copyright holders.
The fully indexed archive of the CCNet, from February 1997 on, can be
found at:
http://abob.libs.ugadot edu/bobk/cccmenu.html
DISCLAIMER: The opinions, beliefs and viewpoints expressed in the
articles and texts and in other CCNet contributions do not necessarily
reflect the opinions, beliefs and viewpoints of the moderator of this
network.
------- End of Forwarded Message
The archive and Web site for our list is at http://www.meteorobs.org
If you are interested in complete links on the upcoming LEONIDS, see:
http://www.meteorobs.org/storms.html
To stop getting email from the 'meteorobs' list, use the Web form at:
http://www.meteorobs.org/subscribe.html
Follow-Ups: