PLEASE NOTE:
*
Subject:
Yet more on the El Paso airburst
To:
cambridge-conference@livjm.ac.uk (Cambridge Conference)
Date sent: Thu, 9 Oct
1997 23:02:06 -0500 (CDT)
From:
pib@nwu.edu
A news report at
http://www.msnbc.com/local/ktsm/9198.asp
suggests the El Paso authorities have identified
an impact site about an acre in size some 27 miles
east of El Paso and 20 miles north of a border patrol
checkpoint.
It will be interesting to see if this really turns out
to be an impact site or just an unrelated fire (shades
of the Honduras "impact" of last year).
*
Subject:
More on El Paso airburst
To:
cambridge-conference@livjm.ac.uk (Cambridge Conference)
Date sent: Thu, 9 Oct
1997 21:09:05 -0500 (CDT)
From:
pib@nwu.edu
Here is a link with a report on the El Paso airburst.
Looks like a nice
bolide, but no damage resulted. Evidently the early media
reports
exaggerated the effects.
http://www.startext.net/news/doc/1047/1:STATE56/1:STATE56100997.html
*
Subject:
Meteorite over El Paso, Texas (fwd)
To:
cambridge-conference@livjm.ac.uk (Cambridge Conference)
Date sent: Thu, 9 Oct
1997 19:14:42 -0500 (CDT)
From:
pib@nwu.edu
David Early says the Dallas media reported an airburst over El
Paso, Texas
today, October 9, 1997. I haven't found any online reports
about this event
yet.
Forwarded message:
Date: Thu, 09 Oct 1997 17:42:59 -0500
Reply-To: d.g.early@worldnet.att.net
To: meteorite-list@meteoritecentral.com
Subject: Meteorite over El Paso, Texas
A local news program here in Dallas just reported that a large
meteor
exploded in the air over El Paso, Texas today. Apparently the
explosion
was fairly strong and frightened a number of people.
The news report stated that the staff at McDonald Observatory
confirmed
that the explosion was caused by a meteor, but they have not
found any
meteorites at this point.
Does anyone have any addition information about this? I have
checked the
McDonald website but they do not have a press release about it. I
also
checked the website for the only newspaper in El Paso, but the
paper
ceased operation this week so their website hasn't been updated
in several
days.
Any information would be appreciated.
David
*
Date sent: Thu, 09 Oct
1997 12:21:05 -0400 (EDT)
From:
Benny J Peiser <B.J.PEISER@livjm.ac.uk
Subject:
NEO News (10/8/97)
To:
cambridge-conference@livjm.ac.uk
Priority: NORMAL
from: David Morrison <dmorrison@mail.arc.nasa.gov
Dear Friends and Students of NEOs:
Following are two items of interest.
First is a press release this summer from the European
Southern
Observatory reporting the photometric detection of a satellite
orbiting the NEO Dionysus. IN a recent report, Stefano Mottola
writes
that "we are still observing Dionysus from Chile, and
although the
eclipse/occultation events are not visible any more (since the
geometry has changed) we hope we can determine the pole
orientation
of the main body and thus the orientation of the orbital plane
(which
we believe is close to the object's equatorial plane). We plan to
submit a publication as soon as all the data have been collected,
and
this should happen by the end of the year. The most up-to-date
reference is the ESO press release and the references therein.
You can find an on-line version of the PR (with graphs) at the
following URL:
http://www.hq.eso.org/outreach/press-rel/pr-1997/pr-08-97.html"
Second is a listing with brief abstracts of several recent
publications dealing with NEOs.
David Morrison
=======================================================
DISCOVERY OF A SATELLITE AROUND A NEAR-EARTH ASTEROID
In the course of the major observational programme of
asteroids by
the Institute of Planetary Exploration of the German Aerospace
Research Establishment (DLR) in Berlin, two of the staff
astronomers,
Stefano Mottola and Gerhard Hahn, have discovered a small
satellite
(moon) orbiting the asteroid (3671) Dionysus. The new
measurements
were obtained with the DLR CCD Camera attached at the 60-cm
Bochum
telescope at the ESO La Silla Observatory in Chile.
This is only the second known case of an asteroid with a moon.
Moons and planets
Until recently, natural satellites were only known around the
major
planets. The Moon orbits the Earth, there are two tiny moons
around
Mars, each of the giant planets Jupiter, Saturn, Uranus and
Neptune
has many more, and even the smallest and outermost, Pluto, is
accompanied by one.
However, the new discovery now strengthens the belief of many
astronomers that some, perhaps even a substantial number of the
many
thousands of minor planets (asteroids) in the solar system may
also
possess their own moons. The first discovery of a satellite
orbiting
an asteroid was made by the NASA Galileo spacecraft, whose
imagery,
obtained during a fly-by of asteroid (253) Ida in August 1993,
unveiled a small moon that has since been given the name Dactyl.
(3671) Dionysus: an Earth-crossing asteroid
In the framework of the DLR asteroid monitoring programme,
image
sequences are acquired to measure an asteroid's brightness
variations
caused by the changing amount of sunlight reflected from the
asteroid's illuminated surface as it spins, due to its irregular
shape. The brightness variations may be used to derive the
asteroid's
rotational properties, such as speed of rotation and spin axis
orientation.
Asteroid Dionysus was put on the observing list because it
belongs to
a special class of asteroids, the members of which occasionally
come
very close to the Earth and have a small, but non-negligible
chance
of colliding with our planet. Most of these objects move in
highly
elliptical orbits that lie partly inside, partly outside that of
the
Earth. They are accordingly referred to as `Earth-crossing
asteroids'
or Apollo-type asteroids, after the proto-type of this group,
(1862)
Apollo, that was discovered in 1932 by Karl Reinmuth in
Heidelber.
The orbital characteristics of Dionysus lead to moderately
close
approaches to the Earth every 13 years, with the one in 1997
being
the first since its discovery that is favourable for extensive
observations. On July 6, 1997, it passed within 17 million km of
our
planet. At that time it was visible from the southern hemisphere
with
a moderately-sized telescope as a relatively fast-moving object.
The strange lightcurve of asteroid (3671) Dionysus
The observers hypothesised that these lightcurve features were
due to
an eclipse by an unknown object moving in an orbit around (3671)
Dionysus, thereby covering part of the illuminated surface of the
asteroid at regular time intervals. Fortunately, this hypothesis
can
be checked, because the phenomenon should then repeat itself
periodically. Accordingly, the DLR scientists made a prediction
for
the next occurences of dips in the lightcurve, based on the time
difference between the two observed events.
Confirmation of the satellite
Contacts were made with observers located at other
observatories, in
order to secure lightcurve coverage over a longer period of time
than
was possible from La Silla alone. As a result, a series of
lightcurve
measurements were performed from June 3 to 9 in close cooperation
with Petr Pravec and Lenka Sarounova working at the Ondrejov
Observatory, near Prague in the Czech Republic.
Luckily, the weather conditions were favourable at both sites
and the
dips in the lightcurve were indeed observed at the predicted
times.
Based on the four well observed events, it was then possible to
determine a period of 1.155 days for their occurence. Thus, the
hypothesis of a satellite orbiting around Dionysus was confirmed.
As
a result, the International Astronomical Union's Minor Planet
Center
located in Cambridge (MA, USA) promptly gave a provisional
designation to the new satellite - S/1997 (3671) 1.
How big is Dionysus?
Meanwhile, in Hawaii, the world's largest infrared telescope
was
being trained on Dionysus to obtain information about its size
and
composition. Alan Harris, also a scientist from the DLR in
Berlin,
and John Davies from the Joint Astronomy Centre in Hilo, Hawaii,
observed the thermal infrared radiation emitted by Dionysus with
the
3.8-m United Kingdom Infrared Telescope (UKIRT) situated on Mauna
Kea. Similar observations over a broader spectral range were also
made by the European Space Agency's orbiting Infrared Space
Observatory.
The thermal or "heat" radiation emitted by an
asteroid depends on its
size and the amount of sunlight it absorbs (darker bodies being
warmer). In the case of Dionysus the measured radiation was much
weaker than expected, indicating that the asteroid has an
intrinsically bright (reflective) surface and is only about 1 km
in
diameter. This is much smaller than (253) Ida, the only other
asteroid known to have a moon, which is about 60 km across.
Further observations
Eventually it should be possible to determine the orbital
radius of
the satellite, its size and the inclination of its orbital plane.
In
order to obtain the data necessary for these determinations,
observations will be continued during the present period of good
visibility that lasts until September-October 1997. For this
reason
the discoverers have initiated an international observation
campaign
devoted to the study of this intriguing object and now involving
astronomers from many countries.
How common are such satellites?
Satellites in orbit around small bodies in the solar system -
asteroids and cometary nuclei - have been predicted on
theoretical
grounds for a long time, even though there is no consensus among
planetary scientists about the actual numbers of such systems.
Hints about the existence of asteroid satellites also come
from the
presence of double impact craters on the Moon and other planetary
surfaces. This suggests that the projectiles forming these
craters
were `double' asteroids. Moreover, measurements obtained when an
asteroid passes in front of a relatively bright star (a so-called
`occultation') have on a few occasions shown features which could
be
interpreted as due to the presence of a satellite. However,
because
of the difficult nature of such measurements, it has never been
possible to draw unambiguous conclusions.
The existence of double asteroids was invoked earlier by Petr
Pravec
and Gerhard Hahn to explain the unusual features observed in the
lightcurves of two other Earth-approaching asteroids 1991 VH and
1994
AW1. In the case of Dionysus, however, it is possible to predict
eclipse events and to confirm them by subsequent measurements.
There is therefore mounting evidence that asteroid binary
systems
might be comparatively common. Observational programmes like the
present one by the DLR and Ondrejov groups will help to verify
this
possibility.
END ESO PRESS~RELEASE
==================================
[The abstracts of recent articles on NEOs were circulated
yesterday
on the CC-list; Benny J Peiser].