CCNet DIGEST, 15 May 1998

Gerrit L. Verschuur

Syuzo Isobe <>

Z. Sekanina, P.W. Chodas & D.K. Yeomans, CALTECH, JPL


M.A. Barucci et al., PARIS OBSERVATORY


From SKY & TELESCOPE, June 1998, pp. 27-34

By Gerrit L. Verschuur

We’ve been lucky to avoid a recent catastrophic collision with a comet
or asteroid. Blockbuster-movie hype aside, it’s high time we woke up and
dealt with the danger

In recent years the evidence that mass extinctions of life on Earth can
be attributed to the consequences of comet or asteroid impacts has
become overwhelming. Most famous among such catastrophes is the
Cretaceous-Tertiary (K-T) impact that marked the demise of the
dinosaurs some 65 million years ago. While pockets of sceptics remain
who would deny that life on Earth has been affected in this manner, the
attention of many planetary scientists has turned to more immediate
concerns. What is the likelihood that our civilization may be
threatened by the unwanted attention of a rogue comet or asteroid in
the near future, and how would we deal with the threat if it presented
itself? It is perhaps untimely that we should ask this question at the
brink of a new millennium, because critics are only too ready to accuse
us of being victims of that apocalyptic fever that tends to mark the
turn of a century, a fever that shows every sign of heightening further
as the third millennium dawns. In defense of our concerns about cosmic
impacts we should recognize that our civilization has just passed
through an extraordinary era of scientific discovery, which has brought
with it the awareness that planet Earth is profoundly vulnerable to
devastating cosmic collisions. Bear in mind that we have reached our
state of technological sophistication without having been interrupted
by a collision with a comet or asteroid during the last few thousand
years, apparently a remarkable stroke of luck. While major impact
events are capable of destroying species on a worldwide basis, smaller
objects that visit more frequently pose a continuing threat to

Our shared cosmic tranquillity can be abruptly terminated by an
unexpected visit of a near-Earth object (NEO) - an asteroid or comet
whose orbit brings it to Earth’s vicinity. Inevitably a time will come
when an asteroid hunter finds one whose orbit shows a high probability
of passing through our planet, a sure sign of an impending impact.
Rather than have this information communicated to the public via the
media in some overly technical fashion, Richard Binzel (MIT) proposes a
Hazard Index that may be used to precisely describe the likelihood of
an impact in the next 100 years by any newly discovered NEO. The topic
has become "so prone to sensationalism," he says, "that great care must
be taken to assess and publicly communicate the realistic hazard posed
by close approaches."

His index would be applied to objects whose orbits are established to
some degree of certainty. While the categories run from good to bad,
from 0 to 5, the index also needs to describe the consequences of an
impact. This requires data on the size of the object. The table at the
top of the opposite page [see page 29 of Sky & Telescope, June 1998]
offers a broad overview of size categories, including impact energies
in equivalent megatons of TNT and the typical interval between events
in each category.

It is widely agreed that civilization's future will be imperilled by an
impact with an object in the medium-size class. Asteroid hunters
estimate that 9,000 objects 0.5 kilometer or larger are in near-Earth
orbits, of which only 350 have been found to date. Collision with any
one of them is unlikely to kill every human being, but in the aftermath
of having 25 percent of the Earth's population perish, civilization is
likely to topple as a result of the collapse of infrastructure and food
supplies. An event of this magnitude is expected once every 40,000

To date, none of the catalogued NEOs deserve a Hazard Index greater
than 0, but only because no immediate threat has been identified. Does
that mean we should ignore it? Planetary scientists urge us to take
every step necessary to assure that Earth is not paid an unwanted visit
in the near future. Critics say that the likelihood of a catastrophic
impact is so low that we needn’t worry at all.

Duncan Steel is a member of Spaceguard, an international organization
working toward improving public awareness of the NEO threat. He
explains, "There is a one in 1,000 chance that within the next century
humanity's progress will be rudely interrupted by such a cataclysm." As
to whether we will take steps to avoid fate, money is an issue. As
Steel notes, we are dealing with "the economics Armageddon."

In response to critics who minimize the danger, Jonathan Tate (Armagh
Observatory) [Jay, that is Major Tate, will be honoured by Gerrit's
promotion], director of Spaceguard UK, makes this analogy: "If you do a
traffic survey and count 6 cars every 60 seconds, the statistical time
between cars is 10 seconds. But you are unlikely to wait for a car to
pass and then step out into the road thinking you will be safe for the
next 10 seconds. This would be an act of folly, yet that is how we tend
to think about the threat of any natural disaster."

"Since we have no experience of the Earth having been struck by w 0.5
kin asteroid in recorded history," Binzel explains, "the best we can do
to predict consequences is to turn to computer simulations." Dozens of
scientists are engaged in running such models using very powerful
computers. They've determined that the impact by a medium-size asteroid
(0.2 to 1 km) would be terrible beyond words.

The horrors deeply traumatized survivors might witness include the
broiling of creatures within eyesight of the atmospheric fireball, the
widespread triggering of conflagrations as fiery debris originally
blasted into space plummets back to Earth, the darkening of the skies
by soot and dust plunging the world into "winter" lasting months, the
removal of the ozone layer, the formation of poisonous nitrogen oxides
that would create corrosive acid rain, and subsequent global warming
years later courtesy carbon dioxide released by vaporized rock.
Furthermore, the seismic shock of impact would devastate a large area
and earthquake-prone regions could release additional pent-up energy in
the Earth’s crust.

Alain Maury, a dedicated asteroid hunter in France, has pondered
further effects. "Even without an impact winter," he says, "after the
initial fatalities, our banking system goes berserk, all multinational
companies - many of them dealing with food distribution - do the same,
and we do not find our cereal in our supermarket anymore."

Michael Baillie (Queen’s University, Belfast) believes that modern
civilization would collapse after the impact of a 500-meter-wide
object. "The trouble is," he says, "that a significantly smaller impact
could still do the trick, especially if over an ocean. Civilization is
a thin veneer. Take away all air travel, restrict global food supplies,
demonstrate that the military and governments are ineffective,
demonstrate that coastal zones should be avoided, and where would we
all be? That is not to mention the problems when all the dead sea life
washes up."

Alas such scary words, and even evidence from dramatic computer
prognostications, tend to be ignored. The most well-known recent impact
event occurred in the middle of Siberia in 1908. But even Tunguska's
2,000-square-km area of flattened trees only reinforces the sense of
unreality we feel toward the threat. We tend to think that the next
one will also hit somewhere else, not where we are.

Perhaps the best thing that could happen to alert us to the potential
danger would be for a minor impact to do enough damage to frighten us
into action. More specifically, as Jonathan Shanklin (British Antarctic
Survey) notes, "Even a small impact on a politically significant site
might be sufficient to institute a major change of direction for our
present civilization."

The physical consequences of even a small impact will depend crucially
on where the object strikes. A hit on a major city such as Washington,
D.C., or New York would cause additional political and financial
ripples. Should one strike near a nuclear power station, the radiation
hazard created by the release of radioactive material from an
incinerated reactor might be felt worldwide. But no matter the scale of
the catastrophe, the damage to our view of ourselves in the cosmic
context will suffer profound trauma.

Worrying About the Big Splash

As Baillie hinted, the scenario of an ocean impact is drawing great
scientific interest lately. Jack G. Hills and Charles Mader (Los
Alamos National Laboratory) estimate that a 5-km-wide asteroid striking
the middle of the Atlantic Ocean would produce a tsunami that would
swamp the entire upper East Coast of the United States back to the
Appalachian Mountains. Delaware, Maryland, and Virginia would be
inundated, as well as Long island and all other coastal cities. It
would also drown much of France and Portugal.

According to Vitaly Adushkin and Ivan Nemchinov, two Russian scientists
involved in modelling explosive events, a small-to-medium 200-meter
object smashing into a 5-km-deep ocean at 50 Ion per second would raise
a splash 35 km high in 40 seconds. It staggers the imagination to
conceive of such a large curtain of water and debris.

Owen Toon (NASA/Ames Research Center) and his colleagues calculate that
a 10,000-megaton impact (by a 1-km object) in a 4-km-deep ocean would
create devastating tsunamis over an area the size of the Pacific Ocean.
Even a mere 5-megaton impact by a very small object in the ocean would
generate tsunamis comparable to those produced by the largest

Those scientists who have thought most seriously about the consequences
of impacts appear to be the most vocal in urging that we take the
threat seriously. Even if the likelihood of an impact in any given year
is very small, the consequences are so terrible that we should not
play with fate. That is why several dedicated groups are searching the
skies in order to identify and plot the orbits of NEOs, a task that is
continually frustrated by lack of funds and most recently by the
complete shutdown of the Southern Hemisphere studies carried out in

Steel notes that the Australian NEO program had particular
significance: "As the only Southern Hemisphere program, there was a
special responsibility to track objects discovered by the U.S. search
programs but not accessible to them later." The Australian program
supplied about 30 percent of all NEO positional measurements and made
about 60 percent of recoveries (spotting NEOs on predicted visits)
during its final three years. With this gaping hole in the coverage of
southern skies, many if not most of the NEOs that are detected will not
benefit from significant follow-up observations. After finding the
needles in the haystack we throw most of them back in again, says Steel.

"Current programs amount to a bootstrapping exercise" says Mark E.
Bailey, director of Armagh Observatory, "and are being done mostly on a
shoestring." Nevertheless, he emphasizes, these on-the-cheap programs
are extremely efficient.

Part of the problem is that those in charge of funding astronomical
research prefer to pour money into the glamour topics such as cosmology
and black-hole studies rather than into the apparently boring business
of finding asteroids and comets. To counter this, says Bailey, "We must
continue to develop high-quality solar-system research programs that
can compete for funds against other areas of astrophysics; and ensure
that the current skill base is not eroded by the present extreme
shortage of full-time research positions and catastrophic career
structure." He explains that the Spaceguard Survey - proposed by a 1992
NASA study - is a long-term project and thus needs measures to preserve
key personnel and the highly skilled junior research staff.

Andrea Carusi, chairman of the International Astronomical Union’s NEO
Working Group, adds that the situation is critical. "A few projects are
doing fine, though not at an optimal level, but we are far from any
reasonable worldwide program. It is my opinion that we would need a
quantum leap in the level of funding, as well as in the personal
involvement of individuals and teams."

James V. Scotti, ace asteroid hunter in Arizona, agrees. "There should
be more done, but if you look at all of the NEO survey programs,
improvements are being made despite the difficulties in funding. He
notes that not only is the University of Arizona's own Spacewatch
Project gaining a new 1.8-meter telescope, but several other programs
are coming online in the U.S. "Combined with European efforts and a few
others, we are getting better at finding NEOs with time, and I foresee
continued improvements even in the current budget climate."

Others are less cordial. Steven J. Ostro (Jet Propulsion Laboratory),
one of the few people engaged in making radar observations of asteroids
in order to refine their orbits, thinks that "the failure of the
world's space agencies to conduct a Spaceguard Survey is
incomprehensible and reprehensible." Similarly, Baillie decries, "Given
the small amount of money necessary to monitor asteroids greater than
500 meters in diameter, it is incredible that this is not an
international priority." The cost would be about $100 million for a
10-year program.


Imagine for a moment that one of the intrepid asteroid hunters finds an
object that appears to deserve a Hazard Index of 4 or 5. After the
discovery was filed with Brian Marsden at the Minor Planet Center in
Cambridge, Massachusetts, Marsden would issue an announcement,
encouraging others to take follow-up observations to refine the
object's orbit. What then?

Scotti, who has keenly followed more than one close encounter between
an NEO and Earth, has pondered this situation. "I would inform my
network of follow-up observers of the need for more observations," he
says, "but that is the same thing I would do for any NEO that we find.
It is not likely that I could predict a possible collision with enough
certainty to get really worried. More likely, I would note the possible
close approach - or really close approach - and be sure the likes of
Steve Ostro know about it so that radar can be done on the object."

Ostro himself would "publish the astrometric data, the orbital
predictions, and the uncertainties, along with analysis of how further
astrometry might reduce the uncertainties, as soon as possible, in a
peer-reviewed journal." Assuming there is enough time, of course.

What if the observations showed that an object was about to hit? "There
is no chance of such a thing being kept quiet," Steel concedes. Given
that word would quickly leak out, Binzel's Hazard Index might be
helpful in minimizing sensationalist reporting that might lead to panic.

But scientists are human beings too. How might an asteroid hunter
respond to the awareness of impending doom? Bailey thought he’d first
warn his wife and family and then tell others. Maury, who hunts
asteroids from the Cote d'Azur in France, says it would depend on the
probability of impact and the time remaining before the impact. If he
found a small object likely to strike his region of France in a few
days he would take his family to a southern Pacific island and then
tell everybody. On the other hand, if it were a large object likely to
strike Earth in 50 years, one that had a low probability of making him
look like an idiot 10 days later, he would grab his bow tie and tell

Maury makes the point that was a theme in the movie version of Carl
Sagan’s CONTACT. "When a serious announcement is made to the public,
those who have done the work are very rarely in contact with the
press.... I feel this is a very likely scenario. When a large comet is
around, you will see a lot of people who usually never care about
comets on TV."

In the public eye, speculating about the probability of an impact does
no more than arouse mild interest. Even seeing a movie about the danger
titillates for a few hours and then we forget about it. It doesn't
matter whether we warn about a 100-year event such as Tunguska, one
that could wipe out a city if better aimed, or a 1,000-year
super-Tunguska capable of creating a regional catastrophe. The scale of
a truly devastating impact is simply beyond our comprehension. That is
why we tend to deal with such fears by fictionalizing them. We let the
hero, like a Bruce Willis, take care of us in imagination, which also
implies we do not have to take the threat seriously after emerging from
the theater. I am personally horrified by the idea that civilization
could be wiped out by an impact event at almost any time. Yet, wouldn't
that be an interesting time to be alive, despite the destruction? Just
as our species was beginning to solve the most profound mysteries of
the universe something brings its endeavors to an end. How frustrating
that would be. If we were lucky, we'd have to start all over. If not,
there would be no rebirth.

It would be poor recompense for those who could, to say, "I told you
so," after the collision. Nevertheless, to observe the consequences of
a major impact firsthand - ignoring the tragedy, terror, and pain -
would he fascinating. The most spectacular view I could imagine having,
and unfortunately also die last, would be flying over the Atlantic at
35,000 feet (6.6 km) and seeing the splash created by an impact of a
1-km object. The water would reach up to the plane's altitude.
Unfortunately that would be the last thing the passengers would see as
the blast wave blew the rapidly melting plane to shreds moments before
the wave dragged the debris from the sky.

Living Dangerously

I believe that the most significant longterm consequence of the
discovery of the cosmic impact threat, assuming we avoid destruction,
will probably involve the increased awareness on the part of Earth's
inhabitants of our shared vulnerability in the cosmos. It is difficult
enough to accept the inevitability of personal mortality, but it is
virtually impossible to imagine that an impact event could wipe the
planet clean of all human beings in a matter of weeks. But once we
begin to take this possibility seriously, will not our view of
ourselves in the cosmic scheme of things inevitably change?

Benny Peiser (Liverpool John Moores University) recently organized a
conference on the disappearance of the Bronze Age civilizations, which
occurred in three distinct episodes between 2500 and 800 B.C. Evidence
is accumulating that impact events may have been the cause. He observes
that some people involved in NEO research tend to be very conservative
in uttering warnings about the future. He explains, "Scientists who
wish to be truthful to interested laypeople and the general public,
should readily admit that mankind will continue to live in a world of
cosmic uncertainties as long as we fail to spend more time, more
research, and much more money on gathering the vital information which
is not only necessary for any tenable calculation of impact
probabilities but moreover, for the establishment of a global system of
planetary defense."

I suspect that we will be prepared to survive the next major impact
only if we overcome human nature. In order for civilization to continue
to evolve we must take responsibility for our destiny. We must define
the NEO hazard precisely by charting where every NEO is at all times
and, when threatened, take steps to avoid an impact by whatever means
are then at our disposal.

Consider that communities all over the world live happily on the sides
of volcanoes, atop active geologic faults, in the central United States
"tornado alley," and along the hurricane-prone Atlantic coast. These
are all manifestations of the syndrome that human nature does not allow
us to take seriously a low-probability, high-consequence risk - a trait
that may someday spell the extinction of civilization, and probably of
our species.

As Peiser says, by learning to turn away NEOs, "and the threat they
pose to civilization, humans have acquired the capability to change the
course of nature and halt the vicious cycle of cosmic cataclysms.
Scientists have the responsibility to take this challenge head-on and
to ensure that humankind takes its fate into its own hands."

Until a medium-to-large NEO is found that deserves a Hazard Index of 4
to 5, asteroid hunters will do well to consider how the results of
their searches are reported to the public. While predictions of future
visits should be framed to avoid undue alarm or panic, the role of the
asteroid hazard in the long-term future of a technological civilization
on this vulnerable planet should not be underplayed either.

A radio astronomer by training, GERRIT VERSCHUUR is an adjunct
professor of physics at the University of Memphis. He wote the 1996
book "Impact! The Threat of Comets and Asteroids" (Oxford University

Copyright 1998, SKY & TELESCOPE


From Syuzo Isobe <>
National Astronomical Observatory of Japan.

Dear Dr. Peiser

An international conference on "The Future of Human Beings and the
Cosmos" was held in Sapporo, Hokkaido, Japan, on April 25, 1998. 13
main speakers addressed the meeting, 6 of whom were members of the
Japanese House of Representatives. In addition, there were
contributions by Glyn Ford (member of the European Parliament's
research committee), Xuan Ping (General secretary of the Committee
of Science and Technology, China), James H. Hall (Minister Counselor
for Science, American Embassey), Gilbert Kirkham (NASA's representative
in Japan), two other speakers from China and one from Rossia. There
were 100 people in the audience, including a number of reporters from
the mass media. Most of the papers were highly interesting. I have
attached the concluding message of the Sapporo meeting and a
presentation by Mr. Hajime Funada, the former Minister of Economy
Evaluation. You will notice that there are certainly some members of
Japanese House of Representatives who are greatly concerned with regard
to our common problem.

Syuzo Isobe

A Message from Sapporo
April 25, 1998

At the dawn of the new millennium, it is clear that mankind faces a
series of problems that can only be tackled globally, namely peace,
environmental protection and provision of natural resources, in
particular food and water. It is clear that space activities will have
a key and central role in tackling each of these.

The participation of Japan, the United States, China, the European
Union, and Russia in the 'International Conference on the Future of
Human Beings and the Cosmos' has provided an opportunity to discuss
peaceful international cooperation in space in addressing global issues.

We hereby announce the result of today's discussion as 'A Message
from Sapporo.'

This century will go on record as one where mankind both tested the
sustainability of the globe and became capable of transcending those

After 2000, through space activities based on worldwide understanding,
humanity should contribute to solving global problems such as
environment, food and energy and to establishing peace of the world,
and also should expand human activities into space on a large scale.

Continuous development of civilization in the next millennium should be
based on firm trust in all peoples of the world, and it is
indispensable to establish trusting relationships among nations.

We find it greatly significant that each nation concerned with space
development is actively involved in international cooperation while
promoting space activities through the originality and creativity of
each nation.

In order to promote space activities from global viewpoints, it is
necessary to encourage information sharing and establish trustworthy
relations among leaders including parliamentary members of governments
across the world. Hence, we agreed to advocate 'The International
Conference on Human Beings and Cosmos', and to organize a steering
committee for its preparation.

Sapporo, April 25, 1998

The Prevention of Global Catastrophe
Hajime Funada, member of the Japanese House of Representatives

Politicians and public officials with interest in space and space
development from Japan, the United States, Europe, and China have
gathered here in Sapporo to discuss the infinite possibilities that the
future of space holds. I think this is an extremely significant
conference, and is being closely watched by those with high

This conference encourages various deaccessions on space development
that has already begun and the future of the use of space. Many of
these are probably rosy topics that promise the progress of the
mankind: What I would like to talk to you about today is the opposite;
a catastrophe that space may deliver upon Earth.

As you know, between the orbits of Mars and Jupiter in our solar
system, there are countless numbers of asteroids. Some of these
asteroids may enter a near-Earth orbit due to Jupiter's gravity or
collision with other asteroids and, however small the probability,
collide with Earth. The energy released by such a collision would be
enormous: an asteroid 100 m in diameter would level the Kanto Region
and one that is 500 m in diameter would endanger the entire human
species. Naturally, the damage would not only come from the collision,
but also large earthquakes, large tsunami, and the massive amount of
dust raised into the upper layers of the atmosphere that would shield
sunlight over a long period of time and cause a global food shortage.
One of the most widely accepted theories for the extinction of
dinosaurs is an asteroid. According to this theory, the 10-km wide
asteroid fell on Yucatan Peninsula in Mexico some 65 million years ago
and forced dinosaurs that ruled Earth into extinction eventually
leading to the evolution of mammals and mankind.

Fortunately, we won't see such a global catastrophe happen very often.
The frequency with which an asteroid of around 100 m in diameter may
strike Earth is once in around several hundred years and that of a 500
m-asteroid is once in about a million years. It is certainly
infrequent, but once it happens, it would cause tremendous damage.
While the chance of someone dying from an asteroid is much lower than
that of a traffic accident, it is about the same as that of an airplane
accident. There is no need for fear, but we should not ignore it. I
feel that we should spend as much capital and pay as much attention to
asteroids as we do to preventing airplane accidents.

Then what can we do to prevent asteroids from striking Earth, which
rarely happens, but may endanger the existence of the mankind when it
does? If there is time until collision, we can gradually change the
asteroid's orbit by shooting it with lasers or striking it with
projectiles. If there is no time, we would have no choice but to strike
the asteroid with a number of nuclear missiles to pulverize it, at the
risk of causing radiation contamination. In other words, to protect the
mankind from asteroids, it is essential that we calculate the orbit of
every asteroid as early as possible and forecast any collisions.

Under the SpaceGuard Plan proposed by NASA's committee in January 1992,
five 2.4 m telescopes installed on the ground will be used to observe
only asteroids for 20 years and detect more than 99% of the asteroids
larger than 1 km in diameter that are near Earth. Fortunately,
University of Arizona and the U.S. Air Force have put their telescopes
to this enormous project. Two more telescopes in the United States and
one in Europe will be used for this purpose. There is also a plan to
use a telescope in Japan for this project.

As important as this project is, we also need to install several space
watch telescopes on manmade satellites and on the moon, where
observation conditions are favorable, to detect asteroids of 100 m or
larger in diameter and other objects approaching Earth from the Sun.
The Moon naturally has a much better environment to operate several
telescopes consistently over a long period of time. In any case, I
would very much like to see the construction of telescopes on the moon
in early stages of space development projects around the world.

The asteroidal collision 65 million years ago caused the extinction of
dinosaurs and later led to the birth of mankind. The next asteroid to
hit Earth, however, may cause the extinction of mankind. With the
intelligence, that should be superior to that of dinosaurs, and the
effort of mankind, it should not be impossible to avoid this
catastrophe. I would like to ask you, leaders with an interest in space
development, to take leadership in such a space guard project.

While it is not realistic to have to have five independent space
programmes, neither should we expect or want a single global programme
anywhere remotely close to the market. In the commercial,
quasi-commercial, arena competition is vital. We need to have two
or three competing programmes in each sector.

Where we must have a global overview, but not necessarily a single
approach, is with the 'Spaceguard' project to investigate how the Earth
might best be defended against possible asteroid collision.

Yet to achieve this we need a new approach. We need a dialogue between
the technologists and the politicians, industry and the public. Space
is always going to require an element of public funding. In the past it
was sufficient for lobbyists to convince the politicians, now when
money is tight and public funding of all activities is shrinking, when
people are skeptical and our vision is dim, we must build a wider
consensus. This will only be achieved when the public feel part of the
process. That is our most important joint task.


Z. Sekanina, P.W. Chodas & D.K. Yeomans: Secondary fragmentation of
comet Shoemaker-Levy 9 and the ramifications for the progenitor's
breakup in July 1992. PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.1,


Comprehensive analysis of discrete events of secondary fragmentation
leads to a conceptually new understanding of the process of
disintegration of comet Shoemaker-Levy 9. We submit that the jovian
tidal forces inflicted extensive cracks throughout the interior of the
original nucleus but did not split it apart. The initial disruption was
apparently accomplished by stresses exerted on the cracked object by
its fast rotation during the early post-perijove period of time. We
argue that this disruption was in fact a rapid sequence of episodes
during July 1992 that gave birth to the 12 on-train, or primary,
fragments: A, C, D, E, G, H, K, L, Q (later Q(1)), R, S, and W. The
discrete events of secondary fragmentation, which gave birth to the
off-train fragments, are understood in this scenario as stochastic
manifestations of the continuing process of progressive disintegration.
Of the 13 off-train fragments considered, nine were secondary-B, F,
G(2), M, N, P (later P-2 or P-2a), Q(2), U, and V-and four tertiary (J,
P-1, P-2b, and T). The separation parameters of 11 off-train fragments
were determined. The vectorial distribution of separation velocities of
these fragments shows a strong concentration toward a great circle,
unquestionably an effect of the approximately conserved angular
momentum of the progenitor comet since the time of its initial
disruption. Also apparent is their clumping (except for P-1) to a
segment along the great circle, implying that the fragments were
consistently released from one side of their parents, thus explaining
for the first time why the off-train fragments preferentially appeared
on one side of the nuclear train. In order to obtain a consistent
solution, our model requires that the points of separation be on the
antisolar side of the parent fragments, where thermal stresses are
likely to enhance the effect of rotation. The episodes of secondary
fragmentation are found to have occurred in a period of time from a few
weeks to at least nine months after the close encounter with Jupiter in
early July 1992, and the separation velocities ranged between 0.36 and
1.7 m s(-1). The spin-axis position is determined to have been nearly
in the jovicentric orbit plane, which rules out the Asphaug-Benz-Solem
strengthless aggregate model as a plausible breakup hypothesis; Since
the separation velocities are rotational in nature, they cannot
substantially exceed the critical limit for centrifugal breakup and
offer an estimate for the original nuclear dimensions. The comet's
nucleus is found to have been approximately 10 km in diameter and
spinning rapidly. With the exception of P-1, and apparently also P-2
and F, no nongravitational deceleration was detected in the motions of
the off-train fragments. Serious doubts are cast on continuing
appreciable activity of any of these fragments. Indeed, when it was
necessary to introduce a deceleration into the equations of motion, the
effect appears to have been due to the action of solar radiation
pressure on the centroid of centimeter-sized particulates in the
disintegrating condensations. (C) 1998 Elsevier Science Ltd.


D.F. Lupishko & M. Di Martino*): Physical properties of near-Earth
asteroids. PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.1, p.47


More than 400 near-Earth asteroids (NEAs) have been discovered and
their discovery rate is continuously increasing. The study of the
physical properties of these objects is necessary in order to
understand their history and relations with comets and meteors, for the
analysis and solution of new applied topics, such as the asteroid
hazard problem, and the possibility of using NEAs as future sources of
raw materials in near-Earth space. The present review article collects
and summarizes the most important new data on the physical properties
of Earth-approaching asteroids, obtained by photometric, polarimetric,
spectroscopic, radar and other techniques. The analysis of all the
available data gives clear indications that asteroid main-belt is the
main source of NEAs. (C) 1998 Elsevier Science Ltd.


M.A. Barucci*), A. Doressoundiram, M. Fulchignoni, M. Florczak, M.
Lazzarin, C. Angeli: Compositional type characterization of Rosetta
asteroid candidates. PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.1,


The final selection of the two Rosetta target asteroids will be made
in a successive phase of the Rosetta project development, when the
engineering parameters will be frozen. In this paper we present
spectroscopic observations of the possible Rosetta candidates and we
discuss the results obtained, particularly the definition of their
compositional type. We examine the possibility to select some more
'primitive' candidates. On the basis of its size and spectral type, we
suggest including the asteroid 140 Siwa as one of the asteroid targets
of the Rosetta mission. (C) 1998 Elsevier Science Ltd.

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