CCNet DIGEST, 24 June 1998
"In this great celestial creation, the catastrophe of a world,
such as ours, or even the total dissolution of a system of worlds,
may possibly be no more to the great Author of Nature, than the
most common accident in life with us, and in all probability such
final and general Doomsday may be as frequent there, as even
Birthdays or mortality with us upon earth. This idea has something
so chearful in it, that I own I can never look upon the stars
without wondering why the whole world does not become astronomers;
and that men endowed with sense and reason should neglect a
science they are naturally so much interested in, and so capable
of inlarging their understanding, as next to a demonstration must
convince them of their immortaility, and reconcile them to all
those little difficulties incident to human nature, without the
least anxiety. All this the vast apparant provision in the starry
mansions seem to promise: What ought we then not to do, to
preserve our natural birthright to it and to merit such
inheritance, which alas we think created all to gratify a race of
vain-glorious gigantic beings, while they are confined to this
world, chained like so many atoms to a grain of sand"
(Thomas Wright, 1750)
(1) WHY THE IMPACT RATE IS AT LEAST TWO TIMES HIGHER THAN NASA CLAIMS:
A RESPONSE TO MORRISON'S COMMENTS
E.P. Grondine <email@example.com>
(2) AMS ANSWERS FREQUENTLY WRONG
Duncan Steel <firstname.lastname@example.org> wrote:
(3) TAURIDS & TUNGUSKA
Sir Arthur C Clarke
(4) TUNGUSKA, TAURIDS & COMET ENCKE
Bill Napier <email@example.com>
(5) WHEN WORLDS COLLIDE
DISCOVER MAGAZINE, July 1998
(6) NEO News (6/23/98)
David Morrison <firstname.lastname@example.org>
(7) MONOLITHIC ASTEROID
Duncan Steel <email@example.com>
(1) WHY THE IMPACT RATE IS AT LEAST TWO TIMES HIGHER THAN NASA CLAIMS:
A RESPONSE TO MORRISON'S COMMENTS
From E.P. Grondine <firstname.lastname@example.org>
Knowing Dr. Morrison's penchant for exactitude (which is a requirement
for those who engage in the design, launch, and flight of multi-million
dollar space probes) I was delighted to get off with comments instead
of a detailed critique. I have become even happier since finding that
the problems he had with the dispatch are either not particularly valid
or else trivial.
First, I did not even know that a Space Protection of the Earth
Conference was held in 1996. What I was working from was Russian press
reports of the SPE 1994 Conference (planned in 1992, hence my date
error), which detailed the debate between Dr. Teller and Tom Gehrels.
I'm sure that the members of this Conference would be very interested
in more exact accounts of both SPE conferences from a different
perspective, and I'm certain Morrison could provide these.
Second, while I admire Morrison et al.'s fortitude at getting any
admission that impact events occur much more frequently than supposed
published in the regular academic press, I don't quite understand who
exactly are the "peers" that conducted the reviews that are supposed to
give these estimates so much weight. The only peers who they have who I
can think of are mostly members of this list, and this is one place
where the reviews that matter have been and hopefully will continue to
On estimated impact rates, I think I have seen about 7 different ways to
arrive at one. First, one may take the discovered population and
extrapolate from that. Second, one could make a complete survey of a
limited part of the sky and extrapolate from that. Third, one can
estimate the population of both asteroids and comets, describe
astrophysical models for their orbital determination, and derive an
estimated impact rate. Fourth, one can do surveys of craters on the
Moon, Mars, Mercury, and the moons of Saturn and extrapolate from that.
Fifth, one can look at the Earth's own geologic history for large impact
events and extrapolate from that. Sixth, one can measure the impact
rates of smaller meteorites (including data from reconnaissance
satellites if it is available) and estimate the impact rates for larger
objects from that.
I'm neither an astronomer nor an astrophysicist nor a geologist, and I
certainly don't run reconnaissance satellites, so I can't comment on
these six methods. But I can comment on the seventh method, which is to
take historical records and extrapolate from those. While field work is
currently going on to confirm how many of the records which we have are
actually historical, it is certain that if even 50% of them are
demonstrated unquestionably to have occurred, then we will have an
impact rate of about 1 per 100 years, confirming similar estimates that
have been arrived at by other members of the Conference using other
techniques. In sum, if even 1/2 of the historical reports turn out to
be spurious or mistaken, we will still be left with an observed
historical impact rate 2 to 3 times higher than the estimate which NASA
is using for planning purposes.
I'll plead guilty to making an eye-ball estimate of the consensus of the
members of the Conference. I think it would be most useful if a survey
were done of members as to their estimated impact rates for three
different size impactors: Tunguska class, North Australian Wall of
Water, and Dinosaur Killer.
Knowing what we do know, Morrison rightly calls attention to the need
to review the late Gene Shoemaker's estimated impact rates and to watch
how they evolved over time. Along these same lines, I think it would
also be of interest to learn how Morrison's own views on the Nemesis
hypothesis from U C Berkeley may have changed.
Moving on to a different problem, I don't know how anyone could construe
my estimate of current Russian intentions as being a "favorable
characterization". The reason for including the estimate was simply to
remind people that just as other scientists have different estimates for
the impact rates, other nations have different schemes for planetary
My estimate was based on the press accounts of SPE 1994, the summary of
a TRUD article for 22-23 January, 1998 (which I have not seen in the
original), my general knowledge of the state of the Russian economy and
space program, their general attitude towards impact events (which has
been colored by both the Tunguska and Sikhote Ailin events), and the
working principle of the Soviet and now Russian defense organizations
which is to use massive and overwhelming force for any and all defence.
What I favor is that someday Russia may again be wealthy, wealthy enough
to use a Proton rocket to test launch a small scientific probe, one
equipped with a radar ranging system but without a nuclear charge, on a
fast interception trajectory to either an asteroid of comet at a good
distance from the Earth. If the United States did the same thing with
its new Titan and Delta EELVs, and Japan with its H-2, and Europe with
its Ariane 5 rocket, then given the launch rates for commercial
satellites, there would nearly always be somewhere on the Earth a rocket
on a launch pad capable of launching a nuclear charge capable of keeping
mankind from going the way of the dinosaur.
In closing, I'd like to mention how struck I was by the words of Pliny
on impact events: "quo nihil terribilius mortalium timori est", "than
which no more terrible form of death is to be feared". Context here is
very important, as Pliny wrote for readers in a society which fed people
to animals for entertainment.
Best wishes -
In the last week I saw the section of the HBO television series "From
the Earth to the Moon" which featured the superb job Astronaut Harrison
Schmitt did in securing geological training for the Apollo astronauts.
If the search for head of the new program office were to go outside of
the NASA field centers, and if he were available, Schmitt's skills in
interdisciplinary team work, combined with his tremendous political
experience, may certainly serve to make him an attractive candidate for
(2) AMS ANSWERS FREQUENTLY WRONG
From Duncan Steel <email@example.com> wrote:
I found the FAQ list from The American Meteor Society to contain
several erroneous statements. I will mention just two, the first only
(1) Electrophonic sounds.
>Electrophonic sounds have never been validated scientifically,
>and their origin is unknown. Currently, the most popular theory is the
>potential emission of VLF radio waves by the fireball, although this
>has yet to be verified.
Both sentences are false. I have invited Colin Keay, the world
authority on electrophonic sounds, to comment for the list.
(2) Origin of meteoroids.
>As a general rule, the smaller (fainter) is the meteoroid population
>under consideration, the more likely is a cometary origin.
...and elsewhere it is stated that 95% of meteoroids come from comets.
As a matter of fact, meteoroids for which orbits have been determined
have orbits more similar to Earth-crossing asteroids than comets, and
the similarity is more pronounced for the fainter/smaller
(radar-detected) meteors than for the brighter/larger
(optically-detected) meteors. I discuss this extensively, giving plots
of orbital parameters for most meteoroid orbit surveys conducted
D. Steel, 'Meteoroid orbits', Space Science Reviews, 78, 507-553 (1996).
Let me say a little more. It is true that many of the meteoroid orbits
are 'cometary' rather than 'asteroidal' as defined by Whipple's
K-criterion (see the above review), but mostly the meteoroids are
cis-jovian (aphelion distance Q < 5 AU) whereas we know of only one
cis-jovian comet (2P/Encke). For example both the Geminids and the
apparent 'parent' [asteroid (3200) Phaethon] have 'cometary' orbits (by
the K-criterion), but much smaller orbits than any known active comet.
Time has moved on from when Whipple proposed his criterion as an
One could answer my claim that meteoroids mostly have asteroidal rather
than cometary orbits by saying that those meteoroids with measured
orbits are just those which remain after many millennia of major jovian
perturbations (i.e., the only meteoroids which have survived from their
initial parent comets are those which have managed to migrate to Q <
4-5 AU orbits, so escaping the havoc wreaked by Jupiter), but the same
applies to the asteroids. That is, I interpret the observations as
implying that many of the Earth-crossing asteroids have a cometary
origin [this notion being supported by the Geminid association with
(3200) Phaethon, the apparent association between various other
asteroids and meteoroid streams, the cometary activity shown just 50
years ago by (4105) Wilson-Harrington, some apparent cometary activity
on the part of (2201) Oljato, and so on]. Such asteroids would be
dormant (like 2P/Encke pre-1786) or extinct comets (cometary nuclei
which have run out of steam and other volatiles). But that in effect
provides a unification, speaking for a common origin for comets and (at
least some) Earth-crossing asteroids, so that it is then not sensible
to speak of these as being separate source classes. Again, I have
reviewed the evidence in:
D.I. Steel, 'The association of Earth-crossing asteroids with meteoroid
streams', Earth, Moon & Planets, 68, 13-30 (1995).
Certainly all prominent meteor showers for which we know (with some
security) the parent objects have an active cometary parent, with the
exception of the Geminids. But there are many distinct meteor showers
which have cis-jovian orbits like Earth-crossing asteroids and no
parent is known. Those parents, I expect, are amongst the large number
of Earth-crossing asteroids yet to be discovered, although in many
cases I further expect that orbital dispersal over several millennia
will have meant that a distinct association will not be possible.
Whilst meteor showers are noticeable because of their character and
temporary nature, the overall influx is dominated by the sporadic
meteoroids which are NOT random (as is often mis-stated): sporadic
meteoroids are concentrated amonst near-ecliptic radiants (hence low
orbital inclinations), with semi-major axes mostly 1 to 3 AU and
eccentricities over 0.5. They have orbits like Earth-crossing
(3) TAURIDS & TUNGUSKA
From Sir Arthur C Clarke
DEPT OF MODEST COUGHS
Was I the first to suggest (in my MYSTERIOUS WORLD book and TV series -
1980) the connexion between Tunguska and the Taurids? I can still recall how
excited I was when I came across the date coincidence - 30 June - just a
week from now!
Sir Arthur Clarke 23 Jun 98
(4) TUNGUSKA, TAURIDS & COMET ENCKE
From Bill Napier <firstname.lastname@example.org>
In response to Sir Arthur's query; so far as I know the suggestion of a
possible connection between the Taurids and the Tunguska object was
first made by the late Lubor Kresak in 1978, in a paper entitled `The
Tunguska object: a fragment of Comet Encke?' (Bull Astr. Inst. Czech.
Vol. 29, pp. 129-134).
(5) WHEN WORLDS COLLIDE
From DISCOVER MAGAZINE, July 1998
Science battles Hollywood glitz in two big-budget disaster movies
In the 1960s, Gerry Griffin was the NASA Flight Director in Mission
Control watching over the Apollo missions. In the 1990s, art is
imitating life: Griffin has fallen into a new career as a movie
consultant, making sure that the fantasy versions of space flight have
at least some of the look and feel of the real thing. Consultants like
Griffin help determine the balance between science and fiction in the
$100 million entertainment behemoths that Hollywood churns out each
Two of this summer's most expensive movies--Deep Impact and
Armageddon--illustrate how the process works. Both movies' plots
revolve around the menace of a large celestial object about to strike
the earth; in Deep Impact the body is a comet, in Armageddon an
asteroid. Giant impact certainly occur, and when they do the results
can be devastating. The extinction of the dinosaurs most likely
resulted from the impact of a 10-mile-wide body striking the Yucatan
Peninsula. Researchers are actively studying the risks and effects of
such cosmic collsions (see related story, The Science Behind the Hits.)
But many of the plot details, from the timetable of events to the
technology available to thwart an impact, are severely warped to keep
popcorn munchers on the edges of their seats.
Deep Impact, which opened in theaters on May 8, concerns the effort to
protect the people of the Earth from the fictional Comet
Wolf-Biederman, which is headed on a collision course. The filmmakers,
headed by director Mimi Leder, called on a number of scientific and
technical consultants. Griffin, who had previously assisted with the
movie Apollo 13, teamed up with David Walker, a former Shuttle
astronaut, to verify the plausibility of the equipment and the scenes
depicting weightlessness. Comet hunter Carolyn Shoemaker and her late
husband Gene provided background on what it is like tracking down the
fuzzy interlopers; Chris Luchini of the Jet Propulsion Laboratory
offered advice in modeling the close-ups of the comet; and Joshua
Colwell of the University of Colorado helped out with research.
Griffin and Walker speak about their work on Deep Impact in highly
enthusiastic terms, especially when describing the sets and costumes.
The movie's imaginary spacecraft "could be a follow-on to the space
shuttle," Griffin says; the space suits in the movie, based
on NASA's Mark III prototype, "look incredible." And the look and sound
of the scenes featuring ground controllers are so accurate that "it was
like being back in Mission Control." Walker concurs. "The designers did
an excellent job on the space hardware," he comments.
Having spent years surrounded by NASA's PR machinery, both Griffin and
Walker are familiar with the needs of keeping the public engaged. And
in turn, Griffin says, the filmmakers were responsive to suggestions
"to make the movie as realistic as possible." Griffin's biggest quibble
with Deep Impact is the level of technology depicted in the movie. The
advanced, nuclear-powered Messiah spacecraft in the movie "would take
years to develop--it would be worse than the Shuttle," Griffin notes.
Or as Walker puts it, the threat from Comet Wolf-Biederman would be
"like seeing somebody firing a rifle at you": by the time you see it
coming, it would be far too late to do anything about it.
Michael Tolkin, one of the many writers involved in the Deep Impact
script, has a pragmatic view of how his movie alternately uses and
abuses science. "You make a pact with the truth," he says. He claims he
conducted extensive background research and even made a pilgrimage to
Kitt Peak, though at the same time he admits that the amount of
scientific information that ultimately got into the movie "could have
been boiled down into a 10-page report." Leder likewise Leder professes
the "utmost respect" for scientists and for their contribution to the
movie, but likens them to the teachers whereas "as filmmakers we are
kids--we get to play." Still, asteroid expert David Morrison of NASA's
Ames Research Center gives the film "high marks for understanding the
nature of the impact threat and for the quality of its special effects
Armageddon, which reaches theaters on July 1, takes an even more
relaxed approach to scientific accuracy. (Armageddon was produced by a
division of the Walt Disney Company, which also owns Discover
magazine.) In this case, the body headed toward us is "an asteroid the
size of Texas." Never mind that there is only one asteroid, Ceres, that
is large enough to fit that description, and that it doesn't orbit
anywhere close to the Earth.
Unlike Deep Impact, Armageddon is the brainchild of a single
writer--Jonathan Hensleigh, who also wrote Die Hard with a Vengence and
the upcoming Virus. Hensleigh had for some time wanted to make a movie
about an oil driller but couldn't come up with the right plot. Two
years ago, while he was reading about asteroid impacts, Hensleigh had
an idea: why not send the oil driller into space to bury nuclear
weapons on an incoming asteroid, thereby saving the world from certain
doom? Thus was born Armageddon.
Hensleigh talks in an expletive-laced, energetic style reminiscent of a
character from a David Mamet play. "I read everything," he claims, and
it does not seem a completely idle boast; he reveals at least a passing
familiarity with topics ranging from impact physics to the dynamics of
the Kuiper Belt (a zone of asteroid-like frozen objects lying beyond
the orbit of Neptune). His goal was to keep Armageddon "completely
Armageddon, which is directed by action-monger Michael Bay, relies on a
Rube-Goldberg setup to explain how such a huge asteroid could be on a
collision course with the Earth. (The 1979 impact film Meteor used a
similar plot device.) And again, the movie has to assume the existence
of secret, high-tech space hardware that would be available on short
notice in case of an impending catastrophe. In this case, Hensleigh
dreams up a super-Shuttle capable of deep-space travel, along with a
hidden cache of fuel stored aboard the Russian Mir space station.
Despite the dramatic stretches, "there's no conscious effort to subvert
science," Hensleigh promises. As with Deep Impact, the filmmakers
called on technical experts to keep the action realistic. Joe Allen, a
former Shuttle astronaut who once flew with David Walker, and Ivan
Becky, the director of NASA's Division of Advanced Concepts until last
year, were the movie's main advisors. And as with their counterparts on
Deep Impact, Allen and Becky are bullish about their experience with
the Hollywood crews, whom Allen praises as "very professional people."
The spaceship interiors in Armageddon "are a Cadillac version of the
real shuttle," Allen comments. Becky analyzed the flight path of the
spacecraft in the movie and concluded that "everything there is pretty
realistic." He also worked out the details of how to power a giant
oil-rig drill in space, and confirmed that the best way to deflect a
rogue asteroid is to bury nuclear charges beneath its surface.
The movie's fundamental scenario, however, "is not realistic in the
strict scientific sense," as Becky tactfully puts it. The biggest fudge
is the amount of energy needed to steer the asteroid clear of the
Earth. Armageddon's script gives the astronauts just 18 days of warning
before impact. With such a short lead time to divert such a huge rock,
a couple dozen nuclear weapons would be far too feeble to do the job.
Allen agrees: "It's virtually impossible because of the energy
required." Nevertheless, he is proud of the many other "violations of
Mother Nature" that were excised as the movie progressed. Scenes in
Armageddon now correctly depict the ultra-low gravitational pull on the
surface of an asteroid and the need to put on a space suit before
opening the hatch into a vacuum. Morrison is not impressed; in a curt
review he quips that "the world may be saved in Armageddon but the
credibility of the movie is a casualty."
Of course, the function of movies like Deep Impact and Armageddon is
entertainment, not education. Even so, some of the scientists are
hopeful that such movies will help spread awareness that a comet or
asteroid impact, while unlikely, is not impossible. Becky also suggests
that these impact movies reduce the "giggle factor" that tends to
impede calls for federal funds for detecting and monitoring potentially
Some small fallout has already occurred. On June 15 a group of
scientists will address members of Congress in the Rayburn House Office
Building. The purpose of the briefing is to let the representatives
know whether the disasters seen in Deep Impact and Armageddon could
really happen, and what scientists know about the risks from comets and
Corey S. Powell
Copyright 1998 Discover Magazine
(6) NEO News (6/23/98)
From David Morrison <email@example.com>
A NEW NEO DISCOVERY TEAM IS OPERATIONAL
Ted Bowell of Lowell Observatory reports the first NEO discovered by
the new Lowell Observatory Near Earth Object System (LONEOS), which
uses a dedicated Schmidt telescope with multiple CCD detectors in the
The newly-discovered NEO is designated as 1998 MQ, and is an Amor
(perihelion distance 1.05 AU). Bowell reports that it is also it's
quite big---2 or 3 km diameter---so we'll probably find archived images
of it in our film and plate collections. The asteroid was found
automatically and was confirmed by eye examination of 3 images.
The teams now regularly discovering NEOs include Spacewatch at Kitt
Peak Observatory, The JPL/USAF NEAT group at Heleakala in Hawaii, the
LINEAR system in New Mexico operated by the Lincoln Laboratory, and
LONEOS at Lowell Observatory. All four of these groups use automatic
comparison of CCD frames to locate NEOs.
THE NEO IMPACT HAZARD AS PERCEIVED 30 YEARS AGO
New York Times, Oct. 18, 1967
Monitoring of Asteroids Urged to Warn of Impending Collisions
A lunar expert suggested today that asteroids should be monitored more
closely so that warning could be given if one were about to strike the
Dr. Harold Masursky of the Geological Survey said at a Lunar orbiter
news conference that lunar photographs had convinced most scientists
that the moon's largest features were impact craters caused by
collisions with heavenly bodies.
The large crater Copernicus, 60 miles across, was created by an object
only 1 mile in diameter that struck the moon with tremendous speed,
Since similar objects are known to have struck the earth throughout its
history, man would be prudent to pay more attention to this potential
natural hazard, Dr. Masursky said.
"Man acts as if he were in complete control of his environment, but he
is not, the geologist said.
Asteroids, also called minor planets, are small irregularly shaped
bodies varying in size from 1 to 500 miles across. It is estimated that
there are 80,000 of them large enough to be seen by telescope.
Most asteroids -- about 98 per cent -- circle the sun in orbits between
Mars and Jupiter, but some lie between the earth and Mars, and some
swing in on paths between the earth and the sun.
Icarus, the most eccentric, would hit the earth if its orbit changed by
only one degree, Dr. Masursky said.
The asteroids show only on the best telescopes and are photographed
only infrequently. Dr. Masursky said more thought needed to be given to
monitoring many more of them with much more sophisticated equipment.
"Only 30 small astronomical bodies are being monitored at the present
time," he said. "We could increase this by many orders of magnitude."
"Many of these are coming very, very, very close to earth as
astronomical distances go, he added.
Nuclear weapons could be used to destroy errant asteroids, or small
rocket engines could be used to push them into safer orbits, [?] said.
At worst, people could be warned to evacuate populated areas or
coastlines where an incoming asteroid, striking the ocean, might cause
tidal waves, he added.
[The remaining ~30% of the article is on lunar science.]
DISCOVERY OF A MONOLITHIC ASTEROID
Al Harris passes on the following information on the asteroid with the
shortest known period of rotation (1998 KY26) , with a brief
explanation of its significance: this asteroid spins far too fast to be
held together by its own gravity.
Date: Mon, 15 Jun 1998 19:40:33 +0200 (MET DST)
P. Pravec, Ondvrejov Observatory, reports: "Our photometric
observations of this Apollo object from during UT 1998 June 1.977-2.040
and June 2.875-3.037 revealed a synodic period of (10.702 +/- 0.003)
min; periods other than half-integer multiples of the period given
above are apparently ruled out. The lightcurve has most signal in the
second harmonic (i.e., it shows two maxima/minima pairs per period),
the peak-to-peak amplitude is 0.3 mag. We interpret the period as being
the rotation period of the asteroid. This result is in agreement with
the radar observations by Ostro et al. (IAUC 6935). The quite fast
rotation indicates that the object is a monolithic body; if it would be
composed of pieces held together by self-gravitation only (so called
"rubble pile" structure, see Harris Lunar Planet. Sci. XXVII, 493-494),
then the lower limit on its bulk density would be 60 g/cm^3. Our
composite lightcurve is presented at
(7) MONOLITHIC ASTEROID
From Duncan Steel <firstname.lastname@example.org>
Thanks for the latest NEO News.
* DISCOVERY OF A MONOLITHIC ASTEROID
...perhaps the following paper escaped your attention.
D.I. Steel, R.H. McNaught, G.J. Garradd, D.J. Asher & A.D. Taylor,
"The lightcurve of 1995 HM: a small, fast-rotating, elongated near-Earth
asteroid," Planetary & Space Science, 45, 1091-1098 (1997).
This was, I believe, the first identification of a monolithic asteroid.
Whilst the recent discovery (1998 KY26) has a much shorter spin period (1995
HM we found to have a 97 minute synodic period), the extreme amplitude of
the lightcurve of the latter indicates a highly-elongated shape,
re-inforcing the notion that it must be a monolith apart from the brevity of
its period (which is below the limit for a spherical rubble pile).
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