PLEASE NOTE:
*
CCNet DIGEST 8 July 1998
------------------------
(1) IF YOU WERE WONDERING WHERE I WAS: CCNet MODERATOR IS OVER
THE MOON
Benny J Peiser <b.j.peiser@livjm.ac.uk>
(2) CLARIFICATIONS REGARDING 1998 DK36
Dave Tholen <tholen@galileo.IfA.Hawaii.Edu>
(3) 1998 DK36 & INTRA-EARTH ASTEROIDS
Duncan Steel <dis@a011.aone.net.au>
(4) WHY RUBBLE PILE ASTEROIDS CAN ALSO PRODUCE SINGLE IMPACT
CRATERS
William Bottke <bottke@astrosun.tn.cornell.edu>
(5) TEXAS, LUBBOCK & ARMAGEDDON
David Levy <dhlevy@LPL.Arizona.EDU>
(6) SCIENCE VS HOLLYWOOD
James Perry <AJDPerry@aol.com>
(7) SUMMER METEOR SHOWERS
Lew Gramer <dedalus@latrade.com>
==============
(1) IF YOU WERE WONDERING WHERE I WAS: CCNet MODERATOR IS OVER
THE MOON
From Benny J Peiser <b.j.peiser@livjm.ac.uk>
Gillian and Benny Peiser are overjoyed to announce the birth of
their
first child
R U B Y H A N N A H
born
7 July 1998.
Mother and baby (8lb 8oz) are both well; father is a wee-bit
tired but
over the moon.*
*That's why the CCNet might not be posted every day during the
summer
period.
=====================
(2) CLARIFICATIONS REGARDING 1998 DK36
From Dave Tholen <tholen@galileo.IfA.Hawaii.Edu>
Benny,
I need to clarify some issues regarding 1998 DK36.
The way Gareth Williams' remarks were worded, it makes it sound
like we
measured the trail ends of just two exposures, one of seven
minutes
duration on Feb 23, and the other of four minutes duration on Feb
24.
In reality, we took four exposures. The time interval
between the two
on Feb 23 and Feb 24 is about four minutes. We measured the
middle of
the trails on Feb 24, but on Feb 23, the end of the first trail
fell in
the gap between two CCDs, and the beginning of the second trail
was
still in that same gap, so we had no choice but to use the ends,
which
increased the time base to seven minutes.
Yes, the observational arc is very short, and the resulting
semimajor
axis and eccentricity are very uncertain. But we caught the
object
essentially bang-on aphelion, and the aphelion distance is
remarkably
well constrained, as is the topocentric distance at the time of
the
observations.
To estimate the uncertainty in the aphelion distance, a Monte
Carlo
experiment was performed. Gaussian noise was added to the
observations
and perturbed orbit solutions were performed on fifteen
additional
synthetic sets of data. When the one sigma noise level was
set to
0.5 arcsec, all fifteen orbits (along with the best-fitting orbit
to
the original data) remained inside the Earth's orbit. The
aphelion
distance was remarkably well constrained to 0.980 AU. When the
one
sigma noise level was doubled to 1.0 arcsec, one of the fifteen
orbit
solutions became Earth crossing. I also tried a two-dimensional
noise
model, in which 1.0 arcsec of noise was used along the direction
of
the trailed images and 0.5 arcsec orthogonal to it, and only one
of
the fifteen orbits became Earth-crossing. Brian Marsden had
already
communicated to me the fact that he had found an Earth-crossing
orbit
that satisfied the observations to the arcsecond level, but that
didn't
surprise me, because I had already found a similar Earth-crossing
orbit
during my Monte Carlo experiment.
So, the question really becomes: what is the appropriate
noise model
to use? Well, the RMS error in the astrometric fit to the
reference
stars from the USNO-A1.0 catalog was at most 0.35 arcsec for the
four
observations. I'm not aware of any systematic errors in the
USNO
catalog at this point, though we can't rule them out. Any
clock error
would have manifested itself in the observations of 1998 DV9,
which
we discovered and observed on the same nights, so I don't have
any
reason to expect a systematic error due to the clock. The
human factor
is involved in the measurement of the trail centers on Feb 24,
which
is why I tried a two-dimensional noise model, because the
greatest
uncertainty lies along the trail. I'd be thrilled if others
would like
to also measure the Feb 24 trails, just to see what sort of
standard
deviation we get in the resulting positions, and I'd be willing
to
make the frames available for that purpose, if enough people
express
an interest in doing so to make it a good statistical test.
The reason I used the word "apparently" in the press
release is because
of that one orbit out of fifteen that went Earth-crossing, and
that's
for what I consider a pessimistic noise model. Remember,
none of the
orbits became Earth-crossing for the 0.5 arcsec noise
model. Nevertheless,
the point remains: this object doesn't spend much time
beyond the Earth's
orbit, if any at all, and so the chances it would have been
discovered by
opposition search efforts is small to zero. We really can't
afford to
assume that there are no objects with aphelion distances equal to
the
Earth's heliocentric distance at any longitude.
--Dave Tholen
==================
(3) 1998 DK36 & INTRA-EARTH ASTEROIDS
From Duncan Steel <dis@a011.aone.net.au>
Dear Benny,
Regarding the prototypical (is that a word?) interior asteroid
1998
DK36, I expect that many people will be proposing names for the
general
class, which has of course been long suspected. This note
is just to
draw people's attention to a couple of things on this front.
The idea that the 'anomalous' precession of the orbit of Mercury
(actually due to GTR) might be due to a planet closer to the Sun
than
Mercury was championed by Le Verrier and others in the latter
half of
the 19th century. Many people spotted it (the power of
suggestion), and
it was given a name (Vulcan) even though it does not exist.
See the
handbooks of William R. Corliss for lists of published
announcements of
its discovery.
This name was taken up, for example, by Leake et al. ('The
chronology
of Mercury's geological and geophysical evolution: The vulcanoid
hypothesis', Icarus, 71, 350-375, 1987) as an appelation for a
hypothetical group of asteroids which might have existed on
Mercury-crossing orbits early in the history of the solar system,
producing much of its cratering record.
Richard G. Hodgson ('Searching for inferior minor planets', Minor
Planet Bulletin, 8, 4-5, 1981) discussed how to look for
asteroids with
orbits entirely within that of the Earth. He defined the
following:
Vulcanian planets: aphelia inside the orbit of Mercury.
Mercurian planets: aphelia between the orbits of Mercury and
Venus.
Cytherian planets: aphelia between the orbits of Venus and
Mercury.
1998 DK36 would come into the latter class. Hodgson wrote:
"The
Cytherian planets are of particular interest to us here on Earth
(or
should be of interest) not only because of their suitability for
future
space exploration, and their possible useful mineral deposits,
but also
because their orbits probably lack long-term stability, and could
shift
to present a hazard to the Earth."
Myself, I don't like 'Cytherian' as a class name: it must start
with an
'A'. Here's a possibility: Adrastea. I have not
checked yet to see
whether there is a minor planet with this name already. It
is an
alternative to Nemesis in classical mythology, and has a twang
suggestive of an object closer than us to our local star. What
is the dative/ablative of star (singular)? 'Ad astra' is 'to the
starS'
I think. Why didn't I pay more attention in Latin O-level?
Duncan
================
(4) WHY RUBBLE PILE ASTEROIDS CAN ALSO PRODUCE SINGLE IMPACT
CRATERS
From William Bottke <bottke@astrosun.tn.cornell.edu>
Hello,
Benny J Peiser wrote:
> There is another interesting aspect of the work by Asphaug
et al. which
> might have significant implications on how impact rate
statistics are
> currently calculated. One of the most important variables
used in these
> calculations are the known hyper-velocity impact craters on
earth. Due
> to the existence of (mostly) individual impact structures,
researchers
> see these single craters as evidence for monolithic
impactors.
> Obviously, the collision of a rubble pile asteroid with
earth would
> result in multiple impacts both in the atmosphere and on the
surface of
> the earth depending on the features and composition of such
an object.
> Instead of impacting in a particularl locality (and thereby
limiting
> the overall environmental knock-on effects), such an object
would cause
> multiple impacts of a Super-Tunguska kind, releasing
explosive energy
> over a wide area of the globe.
Your statement is inaccurate. The existence of "individual
impact
structures" on the terrestrial planets does not place
meaningful
constraints on the nature of the impactor.
There are only two mechanisms capable of disrupting a rubble pile
body
before impact: (1) tidal forces and (2) aerodynamic forces.
We have extensively tested (1) using sophisticated N-body codes,
and
our results show that mass shedding *only* occurs after a rubble
pile
has made close approach with the planet (periapse). Since,
on an
impact encounter, periapse is *inside* the planet, tidal forces
would
not prohibit the formation of a normal impact crater.
Atmospheric break-up (2), on the other hand, does occur, but it
has the
greatest effect on small bodies (i.e., in the case of Earth,
"Tunguska"-sized objects having a diameter of 50 or
so). It is not
difficult to show that the maximum separation between components
entering a terrestrial planet's atmosphere (if it has one) is
small.
We estimate this distance to be 2 km for Venus, 150 m for Earth,
and 18
m for Mars, independent of the size or velocity of the incoming
projectile(s). Separations several times larger are
possible for low
angle entries, and atmospheric drag on the trajectories may
multiply
separations by a factor a few, but our values give a reasonable
approximation of the degree to which aerodynamic forces can
separate the
components of an impacting asteroid or comet. Thus, since
asteroids
tend to make craters 10 times their own diameter, the small
separation
produced by (2) can be considered negligible.
Finally, numerical hydrocode codes capable of simulating asteroid
impacts shows that, at the moment of contact between a planetary
surface and a projectile, a shock wave is generated exceeding the
yield
strength of both the target and projectile (i.e., hundreds of
GPa).
Thus, since even a monolithic impactor is immediately fragmented
after
planetary contact, it would produce the same kind of impact
structure
as an impacting rubble pile.
If you need references, I would be happy to provide them.
Regards,
- Bill
------------------------------------------------------
Bill Bottke
Center for Radiophysics & Space Research
306 Space Sciences
Cornell University
Ithaca, NY 14853-6801
Tel: (607) 255-3934
Fax: (607) 255-9002
E-mail: bottke@astrosun.tn.cornell.edu
http://astrosun.tn.cornell.edu/staff/bottke/index.html
================
(5) TEXAS, LUBBOCK & ARMAGEDDON
From David Levy <dhlevy@LPL.Arizona.EDU>
Benny,
When people ask us about Armageddon's asteroid the size of Texas,
we
say that we think it was a mistake in editing. An asteroid the
size of
Lubbock, Texas, was what they might have meant. Then, in final
editing,
someone might have figured that no one would know where Lubbock
is, so
they just deleted it!
Also, a reminder to watch ABC's programme Armageddon: Target
Earth
on Wednesday, July 9,1998 at 9:30 pm. EDT. It is an
attempt to place the concept of the film in a scientific context.
Thanks for posting this!
David Levy and Peter Jedicke
==================
(6) SCIENCE VS HOLLYWOOD
From James Perry <AJDPerry@aol.com>
Dear Dr. Peiser,
Criticizing "Armageddon" for Bad Science is like
criticizing "Lethal
Weapon" for unrealistic police procedures,
"Braveheart" for historical
inaccuracy, or "Friday the 13th" for its
less-than-credible portrayal
of a serial killer. For a fact, I don't think I've ever seen
Bruce
Willis in anything that wasn't wildly implausible, and to expect
otherwise only sets you up for a major disappointment. But does
he
care? No, he's laughing all the way to the bank!
Sincerely,
James Perry
==================
(7) SUMMER METEOR SHOWERS
From Lew Gramer <dedalus@latrade.com>
[as posted on the meteororb-list]
Following are two short articles on the Meteor Showers of Summer.
The
first covers observing the Perseids, while the second describes
some of
the other worthwhile meteor events this summer.
The Perseids are NOT destined to be at their best this particular
year
because of the moon's light interference, so other showers
are very
worth watching. In particular, you are from a more southerly
location
in the US. So if you take the time to watch AFTER MIDNIGHT, you
should
see nice activity from a shower memorably named "South Delta
Aquarids",
around July 28-30!
If you have any other questions, or would like to learn more
about
meteor observing in general, don't hesitate to write us again!
You can
also always visit our Web site at:
http://www.tiac.net/users/lewkaren/meteorobs
Clear skies in LA, and take care!
Lew Gramer
Medford, MA
------------------------------------------------------------------
"Perseids", from Gary Kronk's Annual Meteor Calendar
at:
http://medicine.wustl.edu/~kronkg/calendar.html
Activity from this meteor shower is visible from about July 23
until
August 22. At the time of Maximum on August 12/13, the radiant
[where
all the meteors appear to trace back to] is located [in the
constellation Perseus, in the northern sky]. The hourly rate
typically
reaches 80, although some years have been as low as 4 and as high
as
200. The meteors tend to be very fast, quite bright, and about
45%
leave persistent trains...
History
This is the most famous of all meteor showers. It never fails to
provide an impressive display and, due to its summertime
appearance, it
tends to provide the majority of meteors seen by non-astronomy
enthusiasts. The earliest record of its activity appears in the
Chinese
annals, where it is said that in 36 AD "more than 100
meteors flew
thither in the morning." Numerous references appear in
Chinese,
Japanese and Korean records throughout the 8th, 9th, 10th and
11th
centuries...
------------------------------------------------------------------
Hot Summer Falling Stars (by Lew Gramer)
------------------------
As most Northerners know, the night skies of Late Summer are
frequently
hazy, often offering poor views through telescopes. On the other
hand,
with the warm weather and visits to dark-skied vacation spots on
our
calendars, these hot months can be an excellent time for meteor
watching - casual or otherwise!
Many people may be unnecessarily discouraged from meteors this
Summer,
since the Perseids - that ancient shower of mid-August which
never
fails to produce high rates - will be sorely affected by bright
moonlight. But what many fail to realize, even as they stand at
their
telescopes on dark Summer nights, is that the Perseids are not
the only
Summer shower which puts on a good show!
Among the several "minor" meteor showers active during
Summer months,
the "Aquarid/Capricornid complex" is far the most
prominent. Throughout
July and August, careful watchers under dark skies after midnight
will
see many meteors falling all over the sky, the majority appearing
to
trace back to (or "radiate from") the constellations of
the Water
Bearer and the Seagoat in the South.
In particular, moonless mornings leading up to July's end - when
the
Southern Delta Aquarids reach their peak - will show avid
watchers as
many as 40 or more meteors per hour. (This number includes
meteors from
other showers too, as well as the non-shower meteors or
"sporadics"
visible all year round.)
For dark AM hours in August and September this year, the
"Aurigid
complex" and late Perseids will provide many swift meteors.
The
Perseids may still be seen throughout August, while the Aurigids
peak
in early September.
FOUR THINGS TO KEEP IN MIND when planning a Meteor Watch: 1)
Comfort -
it gets mighty COLD lying under the night sky, even in Summer! 2)
Meteors will be seen in all directions and areas of the sky - NOT
just
near radiants. 3) Meteor rates go up rapidly after midnight -
usually
the later the better! 4) If you have any interest at all, try
NOTING
WHAT YOU SEE: not only will you be doing a potential service to
meteor
science, but you may actually be the ONLY METEOR OBSERVER IN THE
WORLD
gathering data on that particular night...
For information on scientific meteor watching, drop me a line or
check out:
html://www.tiac.net/users/lewkaren/meteorobs
Lew Gramer
77 Magoun Ave.
Medford, MA 02155
USA
dedalus@latrade.com
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