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(meteorobs) Excerpts from "CCNet DIGEST, 15 June 1999"
------- Forwarded Message
From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST, 15 June 1999
Date: Tue, 15 Jun 1999 10:29:42 -0400 (EDT)
CCNet DIGEST, 15 June 1999
==========================
[...]
(4) OBSERVATION OF 10-100 m BODIES
Luigi Foschini <L.Foschini@isao.bo.cnrdot it>
[...]
(10) WATCHING METEORITES FALL ON THE MOON
Bernd Pauli HD <bernd.pauli@lehrer1.rz.uni-karlsruhedot de>=20
===================================================
(4) OBSERVATION OF 10-100 m BODIES
>From Luigi Foschini <L.Foschini@isao.bo.cnrdot it>
Dear Benny,
I have greatly appreciated your note of 14 June 1999 about "THE
SPACEGUARD PARADOX: OR WHY WE NEED TO SEARCH FOR THOSE ASTEROIDS MOST
LIKELY TO HIT US HARD". I agree with you when you said that although
small asteroids can produce local damage, they are much more dangerous
because of their high impact frequency and their smallness (therefore,
they are very difficult to observe and to follow).
Already Paolo Farinella, during his talk in the plenary session at the
Torino workshop, underlined this fact.. Specifically, he said that
bodies in the range 10-100 m are poorly observed, in spite of their
dangerousness. For example, the Tunguska cosmic body was about 60 m,
but it destroyed an area of 2200 square kilometers.
In my short presentation during the Subgroup 2 session, I have added
also other things and I asked for more studies and observations of
these bodies. Indeed, we have also theoretical problems: from studies on
meter-sized bodies we know that they break up at dynamical pressures
lower than their mechanical strength. Ceplecha et al. (Astron.
Astrophys. 279, 1993, 615) found five classes of mean fragmentation
pressure, from 0.08 to 1.1 MPa. Comparing these data with known
mechanical strength of cosmic bodies (from 1 to 200 MPa), we can see
that small asteroids break up at dynamical pressures up to 200 times
lower than their mechanical strength.
It is worth noting that actual models for the asteroid fragmentation in
the Earth's atmosphere consider that the body break up when the
dynamical pressure is *equal* to the body mechanical strength (e.g.
Hills and Goda, Astron. J. 105, 1993, 1114). But observations suggest
that this is not true. Even though almost all known cases are in the
range 1-10 m, there are some episodes, such as Marshall Islands (Febr.
1, 1994; r~13 m) that suggest that this feature may be valid also for
larger bodies. Therefore, we cannot set a quantitative threshold and we
need also for theoretical studies, in order to assess the hazard.
Recently, I have proposed a new approach to the problem based on the
stagnation temperature, rather than the pressure (Foschini, Astron.
Astrophys. 342, 1999, L1). But we have only some preliminary
(encouraging) data and there is still a lot of work to do. In any way,
we need for observations of airburst (explosion with energy greater than
10 kton, at least) in order to better understand what occur when a small
asteroid enters the Earth's atmosphere.
Moreover, in collaboration with Paolo Farinella, Christiane Froeschle',
Robert Gonczi, Tadeusz J. Jopek and Patrick Michel, we are studying the
interplanetary dynamics of these bright bolides. Paolo spoke about first
results in his talk and you can download a preprint at my home page:
http://www.fisbat.bo.cnrdot it/homepp/dinamica/foschini.html
or at Paolo's home page:
http://tycho.dm.unipidot it/~paolof/homefarinella.html).
Cheers,
Luigi
===================================================
(10) WATCHING METEORITES FALL ON THE MOON
>From Bernd Pauli HD <bernd.pauli@lehrer1.rz.uni-karlsruhedot de>
>From Ciel & Espace, No. 349 - Juin 1999, p. 17: Si, c'est possible !
[B. Pauli's translation] Watching meteorites fall on the Moon -
just a joke to impress first graders? No, says the study group of J.L.
Ortiz - it 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 (the faintest stars visible to the
unaided eye in a non-polluted environment are about mag. 6). Tests that
are being undertaken with a telescope of 250 mm aperture (a 10-inch
scope) have yielded the first results. The Spanish team says they have
already observed several impacts - this will still have to be
confirmed. According to J.L. Ortiz, the Leonids in November will offer
a suitable opportunity for research for both amateurs and
professionals. As the Moon will be in its first-quarter, observing the
regions that are still in the lunar dark will do the job.
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