RISC DISC 2/RISC_DISC_2.iso/resources/nasa/shoemaker/info/comet_fact_sheets

Text File | 1995-05-03 | 92.2 KB | 1,493 lines

Periodic Comet Shoemaker-Levy 9 Collides with Jupiter  JULY 1994
Background Material for Science Teachers


Comet Shoemaker-Levy 9 is expected to collide with Jupiter in July 1994.  From
this historic event, scientists hope to learn more about comets, Jupiter, and
the physics of high velocity planetary impacts.

For a period of about six days centered on July 19, 1994, fragments of Comet
Shoemaker-Levy 9 are expected to collide with Jupiter, the solar system's
largest planet.  No such event has ever before been available for study.  The
energy released by the larger fragments during impact will be more than 10,000
times the energy released by a 100-megaton hydrogen bomb!  Unfortunately for
observers, the collisions will occur on the night side of Jupiter, which also
will be the back side as seen from Earth. The collisions can still be studied
in many ways, nevertheless, by spacecraft more advantageously located, by light
of the collisions reflected from Jupiter's satellites, and by the effects of
the impacts upon the Jovian atmosphere. (The impact sites will rotate into view
from Earth about 20 minutes after each collision.)

Stupendous as these collisions will be, they will occur on the far side of a
body half a billion miles from Earth. There will be no display visible to the
general public, not even a display as obvious as a faint terrestrial meteor.
Amateur astronomers may note a few seconds of brightening of the inner
satellites of Jupiter during the impacts, and they might observe minor changes
in the Jovian cloud structure during the days following the impacts.  The real
value of this most unusual event will come from scientific studies of the
comet's composition, of the impact phenomena themselves, and of the response of
a planetary atmosphere and magnetosphere to such a series of "insults."

This booklet offers some background material on Jupiter, comets, what has and
possibly will happen, and how scientists propose to take advantage of the
impact events.

Contents
1.    What Is a Comet?
2.    The Motion of Comets
3.    The Fragmentation of Comets
4.    The Discovery and Early Study of Shoemaker-Levy 9
5.    The Planet Jupiter
6.    The Final Orbit of Shoemaker-Levy 9
7.    The Collisions
8.    How Can These Impacts and Their Consequences Be Studied?
9.    What Do Scientists Expect to Learn from All of This?

Appendices
A.    Comparative Tables
B.    The K-T Event
C.    The Probability of Collisions with Earth

NOTE: Diagrams and illustrations refered to in the text will be produced as GIF
files in the near future.  When they become available, these text files will be
reposted with them.

1.  What is a Comet?

    Comets are small, fragile, irregularly shaped bodies composed of a
mixture of non-volatile grains and frozen gases.  They usually follow highly
elongated paths around the Sun. Most become visible, even in telescopes, only
when they get near enough to the Sun for the Sun's radiation to start subliming
the volatile gases, which in turn blow away small bits of the solid material.
These materials expand into an enormous escaping atmosphere called the coma,
which becomes far bigger than a planet, and they are forced back into long
tails of dust and gas by radiation and charged particles flowing from the Sun.
Comets are cold bodies, and we see them only because the gases in their comae
and tails fluoresce in sunlight (somewhat akin to a fluorescent light) and
because of sunlight reflected from the solids.  Comets are regular members of
the solar system family, gravitationally bound to the Sun. They are generally
believed to be made of material, originally in the outer part of the solar
system, that didn't get incorporated into the planets -- leftover debris, if
you will.  It is the very fact that they are thought to be composed of such
unchanged "primitive" material that makes them extremely interesting to
scientists who wish to learn about conditions during the earliest period of the
solar system.  Comets are very small in size relative to planets.  Their
average diameters usually range from 750 m or less to about 20 km.

Recently, evidence has been found for much larger distant comets, perhaps
having diameters of 300 km or more, but these sizes are still small compared to
planets.  Planets are usually more or less spherical in shape, usually bulging
slightly at the equator.  Comets are irregular in shape, with their longest
dimension often twice the shortest. (See Appendix A, Table 3.) The best
evidence suggests that comets are very fragile.  Their tensile strength (the
stress they can take without being pulled apart) appears to be only about 1,000
dynes/cm^2 (about 2 lb./ft.^2).  You could take a big piece of cometary
material and simply pull it in two with your bare hands, something like a
poorly compacted snowball.

Comets, of course, must obey the same universal laws of motion as do all other
bodies.  Where the orbits of planets around the Sun are nearly circular,
however, the orbits of comets are quite elongated.  Nearly 100 known comets
have periods (the time it takes them to make one complete trip around the Sun)
five to seven Earth years in length.  Their farthest point from the Sun (their
aphelion) is near Jupiter's orbit, with the closest point (perihelion) being
much nearer to Earth. A few comets like Halley have their aphelions beyond
Neptune (which is six times as far from the Sun as Jupiter). Other comets come
from much farther out yet, and it may take them thousands or even hundreds of
thousands of years to make one complete orbit around the Sun. In all cases, if
a comet approaches near to Jupiter, it is strongly attracted by the
gravitational pull of that giant among planets, and its orbit is perturbed
(changed), sometimes radically.  This is part of what happened to Shoemaker-
Levy 9. (See Sections 2 and 4 for more details.)

The nucleus of a comet, which is its solid, persisting part, has been called an
icy conglomerate, a dirty snowball, and other colorful but even less accurate
descriptions.  Certainly a comet nucleus contains silicates akin to some
ordinary Earth rocks in composition, probably mostly in very small grains and
pieces.  Perhaps the grains are "glued" together into larger pieces by the
frozen gases.  A nucleus appears to include complex carbon compounds and
perhaps some free carbon, which make it very black in color.  Most notably, at
least when young, it contains many frozen gases, the most common being ordinary
water.  In the low pressure conditions of space, water sublimes, that is, it
goes directly from solid to gas -- just like dry ice does on Earth. Water
probably makes up 75-80% of the volatile material in most comets.  Other common
ices are carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ammonia
(NH3), and formaldehyde (H2CO).  Volatiles and solids appear to be fairly well
mixed throughout the nucleus of a new comet approaching the Sun for the first
time.  As a comet ages from many trips close to the Sun, there is evidence that
it loses most of its ices, or at least those ices anywhere near the nucleus
surface, and becomes just a very fragile old "rock" in appearance,
indistinguishable at a distance from an asteroid.

A comet nucleus is small, so its gravitational pull is very weak.  You could
run and jump completely off of it (if you could get traction).  The escape
velocity is only about 1 m/s (compared to 11 km/s on Earth). As a result, the
escaping gases and the small solid particles (dust) that they drag with them
never fall back to the nucleus surface.  Radiation pressure, the pressure of
sunlight, forces the dust particles back into a dust tail in the direction
opposite to the Sun. A comet's tail can be tens of millions of kilometers in
length when seen in the reflected sunlight.  The gas molecules are torn apart
by solar ultraviolet light, often losing electrons and becoming electrically
charged fragments or ions.  The ions interact with the wind of charged
particles flowing out from the Sun and are forced back into an ion tail, which
again can extend for millions of kilometers in the direction opposite to the
Sun. These ions can be seen as they fluoresce in sunlight.

Every comet then really has two tails, a dust tail and an ion tail.  If the
comet is faint, only one or neither tail may be detectable, and the comet may
appear just as a fuzzy blob of light, even in a big telescope.  The density of
material in the coma and tails is very low, lower than the best vacuum that can
be produced in most laboratories.  In 1986 the Giotto spacecraft flew right
through Comet Halley only a few hundred kilometers from the nucleus.  Though
the coma and tails of a comet may extend for tens of millions of kilometers and
become easily visible to the naked eye in Earth's night sky, as Comet West's
were in 1976, the entire phenomenon is the product of a tiny nucleus only a few
kilometers across.  Because comet nuclei are so small, they are quite difficult
to study from Earth. They always appear at most as a point of light in even the
largest telescope, if not lost completely in the glare of the coma.  A great
deal was learned when the European Space Agency, the Soviet Union, and the
Japanese sent spacecraft to fly by Comet Halley in 1986.  For the first time,
actual images of an active nucleus were obtained (see Figure 1) and the
composition of the dust and gases flowing from it was directly measured.  Early
in the next century the Europeans plan to send a spacecraft called Rosetta to
rendezvous with a comet and watch it closely for a long period of time.  Even
this sophisticated mission is not likely to tell scientists a great deal about
the interior structure of comets, however.  Therefore, the opportunity to
reconstruct the events that occurred when Shoemaker-Levy 9 split and to study
those that will occur when the fragments are destroyed in Jupiter's atmosphere
is uniquely important (see Sections 4, 7, and 8).

2.  The Motion of Comets

    Comets necessarily obey the same physical laws as every other object.
They move according to the basic laws of motion and of universal gravitation
discovered by Newton in the 17th century (ignoring very small relativistic
corrections).  If one considers only two bodies -- either the Sun and a planet,
or the Sun and a comet -- the smaller body appears to follow an elliptical path
or orbit about the Sun, which is at one focus of the ellipse.  The geometrical
constants which fully define the shape of the ellipse are the semimajor axis a
and the eccentricity e (see Figure 2).  The semiminor axis b is related to
those two quantities by the equation b = a(1-e2).  The focus is located a
distance ae from the center of the ellipse.  Three further constants are
required if one wishes to describe the orientation of the ellipse in space
relative to some coordinate system, and a fourth quantity is required if one
wishes to define the location of a body in that elliptical orbit.

    In Figure 2B several ellipses are drawn, all having the same semimajor
axis but different eccentricities.  Eccentricity is a mathematical measure of
departure from circularity.  A circle has zero eccentricity, and most of the
planets have orbits which are nearly circles.  Only Pluto and Mercury have
eccentricities exceeding 0.1.  Comets, however, have very large eccentricities,
often approaching one, the value for a parabola.  Such highly eccentric orbits
are just as possible as circular orbits, as far as the laws of motion are
concerned.

The solar system consists of the Sun, nine planets, numerous satellites and
asteroids, comets, and various small debris.  At any given time the motion of
any solar system body is affected by the gravitational pulls of all of the
others.  The Sun's pull is the largest by far, unless one body approaches very
closely to another, so orbit calculations usually are carried out as two-body
calculations (the body in question and the Sun) with small perturbations (small
added effects due to the pull of other bodies).  In 1705 Halley noted in his
original paper predicting the return of "his" comet that Jupiter undoubtedly
had serious effects on the comet's motion, and he presumed Jupiter to be the
cause of changes in the period (the time required for one complete revolution
about the Sun) of the comet. (Comet Halley's period is usually stated to be 76
years, but in fact it has varied between 74.4 and 79.2 years during the past
2,000 years.) In that same paper Halley also became the first to note the very
real possibility of the collision of comets with planets, but stated that he
would leave the consequences of such a "contact" or "shock" to be discussed "by
the Studious of Physical Matters."

In the case of Shoemaker-Levy 9 we have the perfect example both of large
perturbations and their possible "consequences." The comet was fragmented and
perturbed into an orbit where the pieces will hit Jupiter one period later.  In
general one must note that Jupiter's gravity (or that of other planets) is
perfectly capable of changing the energy of a comet's orbit sufficiently to
throw it clear out of the solar system (to give it escape velocity from the
solar system) and has done so on numerous occasions.  See Figure 3.  This is
exactly the same physical effect that permits using planets to change the
orbital energy of a spacecraft in so-called "gravity-assist maneuvers" such as
were used by the Voyager spacecraft to visit all the outer planets except
Pluto.

One of Newton's laws of motion states that for every action there is an equal
and opposite reaction.  Comets expel dust and gas, usually from localized
regions, on the sunward side of the nucleus.  This action causes a reaction by
the cometary nucleus, slightly speeding it up or slowing it down.  Such effects
are called "non-gravitational forces" and are simply rocket effects, as if
someone had set up one or more rocket motors on the nucleus.  In general both
the size and shape of a comet's orbit are changed by the non-gravitational
forces -- not by much but by enough to totally confound all of the celestial
mechanics experts of the 19th and early 20th centuries.  Comet Halley arrived
at its point closest to the Sun (perihelion) in 1910 more than three days late,
according to the best predictions.  Only after F. L. Whipple published his icy
conglomerate model of a degassing nucleus in 1950 did it all begin to make
sense.  The predictions for the time of perihelion passage of Comet Halley in
1986, which took into account a crude model for the reaction forces, were off
by less than five hours.

Much of modern physics is expressed in terms of conservation laws, laws about
quantities which do not change for a given system.  Conservation of energy is
one of these laws, and it says that energy may change form, but it cannot be
created or destroyed.  Thus the energy of motion (kinetic energy) of
Shoemaker-Levy 9 will be changed largely to thermal energy when the comet is
halted by Jupiter's atmosphere and destroyed in the process.  When one body
moves about another in the vacuum of space, the total energy (kinetic energy
plus potential energy) is conserved.

Another quantity that is conserved is called angular momentum.  In the first
paragraph of this section, it was stated that the geometric constants of an
ellipse are its semimajor axis and eccentricity.  The dynamical constants of a
body moving about another are energy and angular momentum.  The total (binding)
energy is inversely proportional to the semimajor axis.  If the energy goes to
zero, the semimajor axis becomes infinite and the body escapes.  The angular
momentum is proportional both to the eccentricity and the energy in a more
complicated way, but, for a given energy, the larger the angular momentum the
more elongated the orbit.

The laws of motion do not require that bodies move in circles (or even ellipses
for that matter), but if they have some binding energy, they must move in
ellipses (not counting perturbations by other bodies), and it is then the
angular momentum which determines how elongated is the ellipse.  Comets simply
are bodies which in general have more angular momentum per unit mass than do
planets and therefore move in more elongated orbits.  Sometimes the orbits are
so elongated that, because we can observe only a small part of them, they
cannot be distinguished from a parabola, which is an orbit with an eccentricity
of exactly one.  In very general terms, one can say that the energy determines
the size of the orbit and the angular momentum the shape.


3.  The Fragmentation of Comets

    Every body is held together by two forces, its self- gravitation and
its internal strength due to molecular bonding.  With no external forces on it
(and no initial rotation) a liquid body would form a perfect sphere just from
self-gravitation (and from very weak molecular forces -- surface tension).
Approaching another body, the sphere would begin to elongate toward that body.
Finally, when the difference in gravitational force on the near side and far
side of the former sphere exceeded the selfgravitation, the body would be torn
apart.  The distance from the larger body at which this disruption occurs is
the so-called Roche limit, named for the man who first studied the problem.
The differential gravitational effects of the Moon and the Sun are what raise
the tides in Earth's oceans, and such forces are often referred to as tidal
forces.

Solid bodies have intrinsic strength due to their molecular bonds.  Aluminum
wire may have a tensile strength of 2.4 x 10^9 dynes/cm2 (5 million lb./ft.2)
and good steel wire a tensile strength 10 times larger still, which far exceeds
the tidal force of anything short of a black hole.  As stated in Section 1,
comets have very low tensile strength, near 1 x 10^3 dynes/cm2 (2 lb./ft.2).
They can be pulled apart very easily by tidal force (or any other substantial
force, for that matter).  Some 25 comets have been observed to split over the
past two centuries.  In other cases two or more comets have been discovered in
nearly the same orbit, and calculations have indicated that they were once a
single comet.  A few of these cases have been obviously attributable to the
tidal forces of Jupiter (Comet Brooks 2 and Comet Shoemaker-Levy 9) or the Sun
(the Kreutz comet family), while other splittings have to be attributed to less
obvious causes.  For example, the loss of material from an active comet, which
tends to occur from a few localized areas, is bound to weaken it.  It may be
that a rapidly rotating comet can be weakened to the point where the
centrifugal force is sufficient to cause large pieces to break off.

The Kreutz family is the name given to many comets which closely approach the
Sun from one direction in space.  They always approach the Sun to within 3
million km or less, and some have actually hit the Sun. The family was named
for Heinrich Kreutz who published extensive monographs on three of these comets
and supported the idea that they had a common origin, perhaps in a giant comet
observed in 372 B.C. Today the Kreutz family has eight definite, well-studied
members; 16 probable members (that are listed as probable only because they
didn't survive passage within 800,000 km of the Sun to permit further study);
and three more possible members.  Extensive work by Brian Marsden suggests that
all of these may have resulted from the splitting of two comets around 1100
A.D., which in turn may have been the parts of the great comet of 372 B.C.
Those Kreutz fragments which survive their encounters with the Sun are often
found to have split yet again!

The classic Roche limit for a (fluid) body of density 1 g/cm3 approaching
Jupiter is about 119,000 km above the cloud tops of the planet.  It is about
169,000 km for a body having a density of 0.5 g/cm3.  More complete modern
theories making different assumptions result in a somewhat smaller limit.  In
1886 Comet Brooks 2 came within 72,000 km of Jupiter's clouds and split into
two pieces.  In July 1992 Comet Shoemaker-Levy 9 came within about 25,000 km of
Jupiter's clouds and fragmented into 21 or more large pieces and an enormous
amount of smaller debris down to micron or submicron size.  Details of this
last event follow.

4.  The Discovery and Early Study of Shoemaker-Levy 9

    Comet Shoemaker-Levy 9 was discovered photographically by the husband
and wife scientific team of Carolyn S. and Eugene M. Shoemaker and David H.
Levy on March 24, 1993, using the 0.46-m (18-in.) Schmidt telescope at Palomar
Observatory in California. Its discovery was a serendipitous product of their
continuing search for "near-Earth objects," and the "9" indicates that it was
the ninth short-period comet (period less than 200 years) discovered by this
team.  Near-Earth objects are bodies whose orbits come nearer to the Sun than
that of Earth and hence have some potential for collisions with Earth. The
appearance of the comet was reported as "most unusual"; the object appeared as
"a dense, linear bar about 1 arc minute long" and had a "fainter, wispy 'tail.'
" (A circle is divided into 360 degrees, each degree into 60 minutes, and each
minute into 60 seconds.  The word "arc" is added to denote an angular measure
rather than time.  The diameter of the Moon is near 30 arc minutes, for
example, while the apparent diameter of Jupiter when closest to Earth is 50 arc
seconds.) The comet's brightness was reported as about magnitude 14, more than
a thousand times too faint to be seen with the naked eye.

The existence of this object was soon confirmed by James V. Scotti of the
Spacewatch program at the University of Arizona, and the International
Astronomical Union's Central Bureau for Astronomical Telegrams immediately
issued "Circular No. 5725" reporting the discovery as a new comet, giving it
the provisional designation of 1993e (the fifth comet discovered or recovered
in 1993).  Scotti reported at least five condensations in a "long, narrow train
about 47 arc seconds in length and about 11 arc seconds in width," with dust
trails extending 4.20 arc minutes to the east and 6.89 arc minutes to the west
and tails extending about 1 arc minute from elements of the nuclear train.
Bureau director Brian G. Marsden noted that the comet was some 4 degrees from
Jupiter and that its motion suggested that it could be near Jupiter's distance
from the Sun.

By March 27 Marsden had enough positions to attempt to derive possible orbits.
One elliptical solution gave a close approach to Jupiter in July 1992.  Also on
March 27, Jane Luu and David Jewitt took an image with the 2.2-m telescope on
Mauna Kea in Hawaii that showed as many as 17 separate sub-nuclei "strung out
like pearls on a string" 50 arc seconds long, and this was reported in Circular
No. 5730 two days later.  Figure 4 shows an early image taken by Scotti on
March 30, 1993.  This long exposure (440 seconds on a CCD detector) brings out
the faint detail of the debris field, though it overexposes the individual
nucleus fragments.  Figure 5 is an image from the Hubble Space Telescope (HST),
taken by Harold A. Weaver and collaborators on July 1, 1993 (before the HST
repair mission), that clearly shows at least 15 individual fragments in one
image frame of the train.

In IAU Circular No. 5744, dated April 3, 1993, Marsden used positions covering
a period of 17 days (including two prediscovery positions from March 15) and
was able to report that no orbit of very long period (near parabolic) was
possible.  The orbit had to be an ellipse of rather small eccentricity relative
to the Sun and relatively short period.  Since it was not at all obvious where
the center of mass of this new comet lay, most observers were just reporting
the position of what appeared to be the center of the train.  This made an
accurate orbit (or orbits) difficult to determine.  Marsden suggested that a
very close approach to Jupiter in 1992 continued to be a distinct possibility,
and the orbit he chose to publish was one with the comet "at least temporarily"
in orbit around Jupiter.

By May 22 Marsden had almost 200 positions of the center of the train.  In
Circular No. 5800 he reported on an orbit computed May 18 by Syuichi Nakano
that showed the comet approaching within 120,000 km of Jupiter on July 8, 1992,
and approaching again, this time within 45,000 km of the center of Jupiter, on
July 25, 1994.  Marsden noted that this distance was less than the radius of
Jupiter. In other words, the comet, or at least parts of it, could very well
hit Jupiter.

By October 18, 1993, Paul W. Chodas and Donald K. Yeomans were able to report
at the annual American Astronomical Society's Division of Planetary Sciences
meeting that the probability of impact for the major fragments of
Shoemaker-Levy 9 was greater than 99%.  The fragments apparently would hit over
a period of several days, centered on July 21.2, on the night side of Jupiter
at latitude 44 degrees S and longitude 35 degrees past the midnight meridian,
according to available observations.  This unfortunately is also the back side
of Jupiter as viewed from Earth. The 1992 approach to Jupiter that disrupted
the comet was calculated to have been at a distance of 113,000 km from the
planet's center and only 42,000 km above its cloud tops.  Furthermore, they
found that the comet had been in a rapidly changing orbit around Jupiter for
some time before this, probably for at least several decades.  It did not
fragment during earlier approaches to Jupiter, however, because these were at
much greater distances than that of 1992.

After recovery of the comet on December 9, following the period during which it
was too near to the Sun in the sky to observe, Chodas and Yeomans found that
the probability was greater than 99.99% that all the large fragments will hit
Jupiter. The encounter period is now centered on July 19.5, and orbits for
individual fragments are uncertain by about 0.03 days (1 s).  The impact site
has moved closer to the limb of Jupiter, now near 75 from the midnight meridian
and only a few degrees beyond the dark limb as seen from Earth, but all pieces
still impact on the back side.  The 1992 approach that split the comet is now
calculated to have occurred on July 7.84 and only 25,000 km (15,500 mi.) above
the clouds.  These data now cover a much longer time base and are based upon
calculations for individual fragments.  They are unlikely to change
significantly in the future.  The comet probably approached Jupiter no nearer
than about 9 million km in the orbit prior to that of 1992.

In a comprehensive paper prepared for The Astronomical Journal, Zdenek
Sekanina, Chodas, and Yeomans report on the details of the breakup of
Shoemaker-Levy 9 as calculated from the positions, motions, and brightness of
the fragments and debris.  They used data from Jewitt, Luu, and Chen taken in
Hawaii, Scotti in Arizona, and Weaver's Hubble Space Telescope (HST) observing
team.  For example, the 11 brightest fragments as measured with the HST, visual
(V) magnitude 23.7-24.8 or about 15 million times too faint to be seen by the
naked eye, had the brightness one would expect from spheres 4.3 down to 2.5 km
in diameter, assuming a normal cometary reflectivity for the fragments (about
4%).  Of course the fragments are not spheres, since tidal disruption tends to
occur in planes perpendicular to the direction of the object causing the
disruption (Jupiter) and since comets generally are not spherical to begin
with.  Nevertheless, adding up the sizes of these 11 fragments, the other
fragments not precisely measured, and all of the debris making up the trails
and tails, suggests that the original comet must have been at least 9 km in
average diameter, and it could have been somewhat larger.  This was a
good-sized comet, about the same size as Comet Halley.

When comets split, the pieces do not go flying apart at a high velocity, each
to immediately go into its own independent orbit.  The escape velocity from a
non-rotating spherical comet 5 km in radius with a density of 0.5 g/cm3 (half
that of water) is 2.65 m/s (6.5 mph).  If suddenly freed of gravity and
molecular bonds, a particle at the equator of that 10-km body, assuming a
rotation period of 12 hours, would depart with a velocity of only 0.72 m/s (1.6
mph) relative to the center of the comet.  Some comets appear to rotate more
rapidly than once per half day, while many, such as Halley, rotate more slowly.
In any case the centrifugal force on unattached pieces of material lying on the
surface of a rotating comet is not normally sufficient to overcome the gravity
holding them there.  Pieces do not fly off of the nucleus "spontaneously." Even
when the tidal forces overcome selfgravity the pieces separate slowly, and they
continue to interact gravitationally.  More important, the pieces bang into one
another, changing their velocities and perhaps fragmenting further.

In the case of Shoemaker-Levy 9, Sekanina, Chodas, and Yeomans estimate that
although fragmentation probably began before closest approach to Jupiter,
dynamic independence of the pieces didn't occur until almost two hours after
closest approach.  For a period of at least two-three hours, collisions
dominated the dynamics of all but the largest pieces, with each small grain
suffering some 10 collisions per second and the bigger pieces being subjected
to many times this number of low velocity impacts by the small particles.

All of this converted the original rotational velocities of the bits and pieces
of 0-2 mph into a random "equilibrium" velocity distribution, with some smaller
pieces having velocities several times their original velocity.  Once the
pieces stopped hitting one another, each continued to move in its own
independent orbit determined mainly by the gravity of Jupiter and the Sun. The
pressure of light from the Sun also had a significant effect upon the smallest
particles, creating a broad dust tail just as happens in a normal comet.  There
has been no evidence of the presence of gases from Shoemaker-Levy 9, either
direct spectroscopic evidence or motion of the dust particles that cannot
otherwise be explained.  This is not to say that there are no gases, only that
there is no evidence for them.  The only direct evidence we have that
Shoemaker-Levy 9 is really a comet and not an asteroid is the fact that it
broke up so easily!  Asteroids are not thought to be so fragile.  It is
unlikely that the exact circumstances of the breakup of Shoemaker-Levy 9 will
ever be known with certainty.  However, the physical model needed to reproduce
the train (of individual large fragments), the trails (of debris on either side
of the train), and the tails (of very small particles in the anti-Sun
direction) in many images like those shown in Figures 4 and 5 does set limits
on the separation time, sizes, and velocities of the pieces and particles
making up each element.  The model of Sekanina, Chodas, and Yeomans, the most
complete at this writing, suggests that the original comet cannot have been
much smaller than 9 km in mean diameter, that it probably was rotating quite
rapidly (perhaps once in eight hours), and that the breakup, as defined by
dynamical independence from collisions and limited mutual gravitational
effects, was not completed until about two hours after the closest approach to
Jupiter. The comet nucleus was probably not very spherical or the debris trails
on either side of the train of nucleus fragments would be nearly equal in
length, which they are not.  After the collisions ceased, the motion of the
largest fragments was dominated by Jupiter, with those fragments closest to
Jupiter at breakup remaining closest and therefore moving with a shorter period
in accordance with basic mechanics.  The fragment that started nearest to
Jupiter will be the first to return to Jupiter and hit the planet.

All of the large fragments were soon strung out in nearly a straight line that
pointed at Jupiter, and they will remain so until colliding with the planet
(see Figure 6).  H. J. Melosh and P. Schenk have offered the intriguing
suggestion that linear chains of craters observed on Jupiter's satellites
Ganymede and Callisto are the product of impacts by earlier comets fragmented
by Jupiter.

5.  The Planet Jupiter

    Jupiter is the largest of the nine planets, more than 10 times the
diameter of Earth and more than 300 times its mass.  In fact the mass of
Jupiter is almost 2.5 times that of all the other planets combined.  Being
composed largely of the light elements hydrogen and helium, its mean density is
only 1.314 times that of water.  The mean density of Earth is 5.245 times that
of water.  The pull of gravity on Jupiter at the top of the clouds at the
equator is 2.4 times as great as gravity's pull at the surface of Earth at the
equator.  The bulk of Jupiter rotates once in 9h 55.5m, although the period
determined by watching cloud features differs by up to five minutes due to
intrinsic cloud motions.

The visible "surface" of Jupiter is a deck of clouds of ammonia crystals, the
tops of which occur at a level where the pressure is about half that at Earth's
surface.  The bulk of the atmosphere is made up of 89% molecular hydrogen (H2)
and 11% helium (He).  There are small amounts of gaseous ammonia (NH3), methane
(CH4), water (H2O), ethane (C2H6), acetylene (C2H2), carbon monoxide (CO),
hydrogen cyanide (HCN), and even more exotic compounds such as phosphine (PH3)
and germane (GeH4).  At levels below the deck of ammonia clouds there are
believed to be ammonium hydro-sulfide (NH4SH) clouds and water crystal (H2O)
clouds, followed by clouds of liquid water.  The visible clouds of Jupiter are
very colorful.  The cause of these colors is not yet known. "Contamination" by
various polymers of sulfur (S3, S4, S5, and S8), which are yellow, red, and
brown, has been suggested as a possible cause of the riot of color, but in fact
sulfur has not yet been detected spectroscopically, and there are many other
candidates as the source of the coloring.  The meteorology of Jupiter is very
complex and not well understood.  Even in small telescopes, a series of
parallel light bands called zones and darker bands called belts is quite
obvious.  The polar regions of the planet are dark. (See Figure 7.) Also
present are light and dark ovals, the most famous of these being "the Great Red
Spot." The Great Red Spot is larger than Earth, and although its color has
brightened and faded, the spot has persisted for at least 162.5 years, the
earliest definite drawing of it being Schwabe's of Sept. 5, 1831. (There is
less positive evidence that Hooke observed it as early as 1664.) It is thought
that the brighter zones are cloud- covered regions of upward moving atmosphere,
while the belts are the regions of descending gases, the circulation driven by
interior heat.  The spots are thought to be large-scale vortices, much larger
and far more permanent than any terrestrial weather system.

The interior of Jupiter is totally unlike that of Earth. Earth has a solid
crust "floating" on a denser mantle that is fluid on top and solid beneath,
underlain by a fluid outer core that extends out to about half of Earth's
radius and a solid inner core of about 1,220-km radius.  The core is probably
75% iron, with the remainder nickel, perhaps silicon, and many different metals
in small amounts.  Jupiter on the other hand may well be fluid throughout,
although it could have a "small" solid core (say up to 15 times the mass of
Earth!) of heavier elements such as iron and silicon extending out to perhaps
15% of its radius.  The bulk of Jupiter is fluid hydrogen in two forms or
phases, liquid molecular hydrogen on top and liquid metallic hydrogen below;
the latter phase exists where the pressure is high enough, say 3-4 million
atmospheres.  There could be a small layer of liquid helium below the hydrogen,
separated out gravitationally, and there is clearly some helium mixed in with
the hydrogen.  The hydrogen is convecting heat (transporting heat by mass
motion) from the interior, and that heat is easily detected by infrared
measurements, since Jupiter radiates twice as much heat as it receives from the
Sun. The heat is generated largely by gravitational contraction and perhaps by
gravitational separation of helium and other heavier elements from hydrogen, in
other words, by the conversion of gravitational potential energy to thermal
energy.  The moving metallic hydrogen in the interior is believed to be the
source of Jupiter's strong magnetic field.

Jupiter's magnetic field is much stronger than that of Earth. It is tipped
about 11 to Jupiter's axis of rotation, similar to Earth's, but it is also
offset from the center of Jupiter by about 10,000 km (6,200 mi.).  The
magnetosphere of charged particles which it affects extends from 3.5 million to
7 million km in the direction toward the Sun, depending upon solar wind
conditions, and at least 10 times that far in the anti-Sun direction.  The
plasma trapped in this rotating, wobbling magnetosphere emits radio frequency
radiation measurable from Earth at wavelengths from 1 m or less to as much as
30 km.  The shorter waves are more or less continuously emitted, while at
longer wavelengths the radiation is quite sporadic.  Scientists will carefully
monitor the Jovian magnetosphere to note the effect of the intrusion of large
amounts of cometary dust into the Jovian magnetosphere.

The two Voyager spacecraft discovered that Jupiter has faint dust rings
extending out to about 53,000 km above the atmosphere.  The brightest ring is
the outermost, having only about 800-km width.  Next inside comes a fainter
ring about 5,000 km wide, while very tenuous dust extends down to the
atmosphere.  Again, the effects of the intrusion of the dust from
Shoemaker-Levy 9 will be interesting to see, though not easy to study from the
ground.

The innermost of the four large satellites of Jupiter, Io, has numerous large
volcanos that emit sulfur and sulfur dioxide.  Most of the material emitted
falls back onto the surface, but a small part of it escapes the satellite.  In
space this material is rapidly dissociated (broken into its atomic
constituents) and ionized (stripped of one or more electrons).  Once it becomes
charged, the material is trapped by Jupiter's magnetic field and forms a torus
(donut-shape) completely around Jupiter in Io's orbit.  Accompanying the
volcanic sulfur and oxygen are many sodium ions (and perhaps some of the sulfur
and oxygen as well) that have been sputtered (knocked off the surface) from Io
by high energy electrons in Jupiter's magnetosphere.  The torus also contains
protons (ionized hydrogen) and electrons.  It will be fascinating to see what
the effects are when large amounts of fine particulates collide with the torus.

Altogether, Jupiter has 16 known satellites.  The two innermost, Metis and
Adrastea, are tiny bodies, having radii near 20 and 10 km respectively, that
interact strongly with the rings and in fact may be the source of the rings.
That is, the rings may be debris from impacts on the satellites.  Amalthea and
Thebe are still small, having mean radii of 86.2 and about 50 km, respectively,
but they are close to Jupiter and may serve as useful reflectors of light from
some of the impacts.  The Galilean satellites (the four moons discovered by
Galileo in 1610), Io, Europa, Ganymede, and Callisto, range in radius from
1,565 km (Europa) to 2,634 km (Ganymede). (Earth's Moon has a radius of 1,738
km.) They lie at distances of 421,700 km (Io) to 1,883,000 km (Callisto) from
Jupiter. These objects will serve as the primary reflectors of light from the
impacts for those attempting to indirectly observe the actual impacts.  The
outer eight satellites are all tiny (less than 100-km radius) and at large
distances (greater than 11 million km) from Jupiter. They are expected to play
no role in impact studies.

6.  The Final Orbit of Shoemaker-Levy 9

    The motion of Comet Shoemaker-Levy 9 can technically be described as
chaotic, which means that calculations based upon the input of comet positions
having very tiny differences (small input errors) causes large differences in
the results of calculations of the subsequent motion and the apparent prior
motion.  Large perturbations caused by successive close approaches to Jupiter
have resulted in each orbit being different in size, shape, and orientation.
The orbits have not been the simple result of a small body in orbit around a
large one but rather the product of a "tug of war" between Jupiter and the Sun,
a classic "three-body problem." Near to Jupiter the planet's gravity has
dominated the motion, but far from Jupiter the Sun is more important.  On July
16, 1993, at apojove (the point farthest from Jupiter) in the current orbit,
Shoemaker-Levy 9 was almost 1,200 times as far from Jupiter as at the time of
breakup, a distance of 50 million km, equal to a third of the distance of Earth
from the Sun. The comet has been in a closed orbit around Jupiter for many
decades, but that orbit is far from stable.  Figure 8 by Chodas shows what this
orbit will look like as viewed from the Sun at the time of impact.  It is
tipped (inclined) 53 degrees to Jupiter's equator as measured at apojove and is
bent about 20 degrees more near to Jupiter. (A three- body orbit rarely lies in
one plane like the simple two-body orbit.) During this final orbit, after
breakup in July 1992, Shoemaker- Levy 9 was followed carefully from discovery
in March 1993 until the time the comet's angular distance from the Sun became
too small to permit observations.  The last useful astrometric (positional)
observations reported before the fragments were lost in the glare of the Sun
were made on July 11.  The comet was recovered (found again after almost five
months without observations) by Scotti and Tom Gehrels on December 9, with the
comet rising above the horizon a bit more than three hours before the Sun in
the morning sky. (The comet is so faint that it cannot be observed in twilight
or too low on the horizon.) The quality of the predictions for the time of
impact of the individual fragments on Jupiter will depend upon the number of
high-quality astrometric observations of each comet fragment made between
December 1993 and the time of impact.  A week before the impacts the times
should be known at least to plus or minus 10 minutes (with 50% confidence),
improving to perhaps plus or minus 5 minutes a half day before impact.

At a fall planetary astronomy meeting (DPS) Jewitt, Luu, and Chen exhibited an
image showing 21 distinct fragments in the Shoemaker- Levy 9 nucleus train.  At
discovery in March, this train was about 50 arc seconds or 162,000 km in length
as projected on the sky.  This angular distance had increased by about 40% (and
the true linear distance by about 50%, since Jupiter was then farther from
Earth) by the time the comet was lost in the glare of the Sun in July. The
spreading is caused mainly by the fact that the piece closest to Jupiter at
breakup was some 9 km closer than the farthest piece (the diameter of the
comet) and therefore entered a faster orbit.  The orbits are all so elongated
that from Earth they appear to be nearly a straight line with the fragments
strung out along it.  The fragment nearest to Jupiter at breakup remains
nearest to it and will be the first to impact.  At this writing, Chodas and
Yeomans predict that the train will reach an apparent length of some 1,286 arc
seconds at the time the first of the fragments enters Jupiter's atmosphere.
The true length of the train will be 4,900,000 km at that time, and it will
require 5.5 days for all of the major fragments to impact.

The new data taken following solar conjunction (the closest apparent approach
to the Sun as projected against the sky) more than doubled the length of time
since discovery for which cometary positions were available.  With this new
data, it appears that the impacts will be centered on about July 19.5, a day
and a half earlier than the first predictions.  The approach to Jupiter that
shattered the comet appears to have been even closer than first thought, about
96,000 km from the planet's center and only 25,000 km above the clouds.  The
revised orbit has also moved the impact points closer to the visible
hemisphere, but unfortunately still on the back side as seen from Earth. The
brightest fragment, of which there is some indication that it itself is
fragmented, will impact on about July 20.78 and contains about 10% of the total
mass of the comet.  The other 20 observed fragments contain more than 80% of
the mass.  The remaining mass is contained in all of the dust and small pieces
in the train, the trails, and the tails.  Most of this mass also will hit
Jupiter over a period of several months beginning about July 10, but it
probably will cause few or no detectable effects.

Meanwhile, the dust trails of small debris will continue to spread, as will the
major fragments.  The east-northeast trail is expected to reach a maximum
apparent length of some 70 arc minutes in late June of 1994 and then decrease
again in apparent length with the tip turning around into a "V" shape.  The
west-southwest trail may reach a length of almost 100 arc minutes before
impacts begin.  Only the larger trail material will actually impact Jupiter.
The earliest dust will begin to hit about July 10, and impacts will continue
into October. The smaller dust will be moved into the tail by solar radiation
pressure and will miss the planet completely.

If upon further study it is found that the pieces of ShoemakerLevy 9 have
continued to fragment, then predicting impact times will be much more difficult
and the predictions less reliable.  Such continued fragmentation of pieces
already badly fractured is very possible, but fragmentation in the last day or
two, when it is most likely to occur, will have no significant effect on the
predicted times of impact.  The pieces typically separate with a velocity of
less than a meter per second.  There are 86,400 seconds in a day, so even
pieces separating at a full meter per second would be only 86.4 km apart after
one day.  Moving jointly at a velocity which reaches 60 km/s at impact, the
pieces would hit within a few seconds of each other.  The effect of further
splitting upon the impact phenomena would be far greater and is discussed in
the next section.  Figure 9 by David Seal shows the final segment of the comet
fragment's trajectories.  They will impact near a latitude of 44 degrees S and
a longitude 70 degrees past the midnight meridian, still 10 degrees beyond the
limb of Jupiter as seen from Earth, impacting the atmosphere at an angle of
about 42 degrees from vertical.

Observing conditions from Earth will not be ideal at the time of the impacts,
since there will be only about two hours of good observing time for large
telescopes at any given site after the sky gets good and dark and before
Jupiter comes too close to the horizon to observe.  At least it will be summer
in the northern hemisphere, and there will be a better chance for good weather
where many observatories are located.  With 21 pieces hitting over a 5.5-day
period, there will be an impact on average about every 6 hours, so any given
site should have about one chance in three of observing at the actual time of
an impact each night.  Since the impacts will be on the back side of Jupiter,
light from the impacts can only be observed by reflection from Jupiter's moons
or perhaps from the rings or the dust comae of the comet fragments.  Those
attempting observations of the effects of the impacts on Jupiter can begin
about 20 minutes after the impacts, when the impact area rotates into view from
Earth.

7.  The Collisions

    Exactly what will happen as the fragments of Shoemaker- Levy 9 enter
the atmosphere of Jupiter is very uncertain, though there are many predictions.
If the process were better understood, it would be less interesting.  Certainly
scientists have never observed anything like this event.  There seems to be
complete agreement only that the major fragments will hit Jupiter and that
these collisions will occur on the back side of Jupiter as seen from Earth.

Any body moving through an atmosphere is slowed by atmospheric drag, by having
to push the molecules of that atmosphere out of the way.  The kinetic energy
lost by the body is given to the air molecules.  They move a bit faster (become
hotter) and in turn heat the moving body by conduction.  This frictional
process turns energy of mass motion (kinetic energy) into thermal energy
(molecular motion).  The drag increases roughly as the square of the velocity.
In any medium a velocity is finally reached at which the atmospheric molecules
can no longer move out of the way fast enough and they begin to pile up in
front of the moving body.  This is the speed of sound (Mach 1 -- 331.7 m/s or
741 mph in air on Earth at sea level).  A discontinuity in velocity and
pressure is created which is called a shock wave.  Comet Shoemaker-Levy 9 will
enter Jupiter's atmosphere at about 60 km/s, which would be about 180 times the
speed of sound on Earth (Mach 180) and is about 50 times the speed of sound
even in Jupiter's very light, largely hydrogen atmosphere.

At high supersonic velocities (much greater than Mach 1) enough energy is
transferred to an intruding body that it becomes incandescent and molecular
bonds begin to break.  The surface of the solid body becomes a liquid and then
a gas.  The gas atoms begin to lose electrons and become ions.  This mixture of
ions and electrons is called a plasma.  The plasma absorbs radio waves and is
responsible for the communication blackouts that occur when a spacecraft such
as the Space Shuttle reenters Earth's atmosphere.

The atmospheric molecules are also dissociated and ionized and contribute to
the plasma.  At higher temperatures, energy transfer by radiation becomes more
important than conduction.  Ultimately the temperatures of the plasma and the
surface of the intruding body are determined largely by the radiation balance.
The temperature may rise to 50,000 K (90,000 degrees F) or more for very large
bodies such as the fragments of Shoemaker-Levy 9 entering Jupiter's atmosphere
at 60 km/s.  The loss of material as gas from the impacting body is called
thermal ablation.  The early manned spacecraft (Mercury, Gemini, and Apollo)
had "ablative heat shields" made of a material having low heat conductivity
(through to the spacecraft) and a high vaporization temperature (strong
molecular bonds).  As this material was lost, as designed, it carried away much
of the orbital energy of the spacecraft reentering Earth's atmosphere.

There are other forms of ablation besides thermal ablation, the most important
being loss of solid material in pieces.  In a comet, fragile to begin with and
further weakened and/or fractured by thermal shock and by melting, such
spallation of chips or chunks of material has to be expected.  Turbulence in
the flow of material streaming from the front of the shock wave can be expected
to strip anything that is loose away from the comet and send it streaming back
into the wake.  The effect of increasing temperature, pressure, and vibration
on an intrinsically weak body is to crush it and cause it to flatten and
spread.  Meanwhile the atmosphere is also increasing in density as the comet
penetrates to lower altitudes.  All of these processes occur at an ever
increasing rate (mostly exponentially).

On Earth a sizable iron meteoroid or even some relatively low velocity stony
meteoroids can survive all of this and impact the surface, where we collect
them for study and exhibition. (Small bodies traveling in space are called
meteoroids.  The visible phenomena which occur as a meteoroid enters the
atmosphere is called a meteor.  Surviving solid fragments are called
meteorites.  There is no sharp size distinction between meteoroids and
asteroids.  Normally, if the body has been detected telescopically before
entering the atmosphere, it has been called an asteroid.) Many meteoroids
suffer what is called a "terminal explosion" when crushed while still many
kilometers above the ground.

This is what happened in Tunguska, Siberia, in 1908.  There a body with a mass
of some 109 kg (2.2 billion lb.) and probably 90 to 190 m in diameter entered
Earth's atmosphere at a low angle with a velocity of less than 15 km/s.  It
exploded at an altitude of perhaps 5- 10 km.  This explosion, equivalent to
10-20 megatons of TNT, combined with the shock wave generated by the body's
passage through the atmosphere immediately before disruption, leveled some
2,200 km^2 of Siberian forest.  The Tunguska body had a tensile strength of
some 2 x 10^8 dynes/cm^2, more than 100,000 times the strength of
Shoemaker-Levy 9, but no surviving solid fragments of it (meteorites) have ever
been found.  The fragile Shoemaker-Levy 9 fragments entering an atmosphere of
virtually infinite depth at a much higher velocity will suffer almost immediate
destruction.  The only real question is whether each fragment may break into
several pieces immediately after entry, and therefore exhibit multiple smaller
explosions, or whether it will survive long enough to be crushed, flattened,
and obliterated in one grand explosion and terminal fireball.

Scientists have differed in their computations of the depths to which fragments
of given mass will penetrate Jupiter's atmosphere before being completely
destroyed.  If a "terminal explosion" occurs above the clouds, which are
thought to lie at a pressure level of about 0.5 bar or roughly 0.5 Earth
atmosphere (see Section 5), then the explosion will be very bright and easily
observable by means of light reflected from Jupiter's satellites.  Using
ablation coefficients derived from observation of many terrestrial fireballs,
Sekanina predicts that the explosions indeed will occur above the clouds.
Mordecai-Mark Mac Low and Kevin Zahnle have made calculations using an
astrophysical hydrodynamic code (ZEUS) on a supercomputer.  They assume a fluid
body as a reasonable approximation to a comet, since comets have so little
strength, and they predict that the terminal explosions will occur near the
10-bar level, well below the clouds.  Others have suggested still deeper
penetration, but most calculations indicate that survival to extreme depths is
most unlikely.  The central questions then appear to be whether terrestrial
experience with lesser events can be extrapolated to events of such magnitude
and whether all the essential physics has been included in the supercomputer
calculations.  We can only wait and observe what really happens, letting nature
teach us which predictions were correct.

O.K. So an explosion occurs at some depth.  What does that do?  What happens
next?  Sekanina calculates that about 93% of the mass of a 1013-kg fragment
remains one second before the terminal explosion and the velocity is still
almost 60 km/s.  During that last second the energy of perhaps 10,000
100-megaton bombs is released.  Much of the cometary material will be heated to
many tens of thousands of degrees, vaporized, and ionized along with a
substantial amount of Jupiter's surrounding atmosphere.  The resulting fireball
should balloon upward, even fountaining clear out of the atmosphere, before
falling back and spreading out into Jupiter's atmosphere, imitating in a
non-nuclear fashion some of the atmospheric hydrogen bomb tests of the 1950s.
Once again, the total energy release here will be many thousands of times that
of any hydrogen bomb ever tested, but the energy will be deposited initially
into a much greater volume of Jupiter's atmosphere, so the energy density will
not be so high as in a bomb, and, of course, there will be no gamma rays or
neutrons (nuclear radiation or particles) flying about.  The energy of these
impacts will be beyond any prior experience.  The details of what actually
occurs will be determined by the observations in July 1994, if the observations
are successful.

If differential gravitation (tidal forces) should further fragment a piece of
the comet, say an hour or two before impact, the pieces can be expected to hit
within a second of each other.  In one second a point at 44 degrees latitude on
Jupiter will rotate 9 km (5.6 mi.), however, so the pieces would enter the
atmosphere some distance apart.  Smaller pieces will explode at higher
altitudes but not so spectacularly.  If smaller pieces do explode above the
clouds, they may be more "visible" than larger pieces exploding below the
clouds.  It is also possible that implanting somewhat less energy density over
a wider volume of atmosphere might create a more visible change in Jupiter's
atmosphere.  Sekanina notes that pieces smaller than about 1.3-km mean radius
should not be further fragmented by tidal forces unless they were already
weakened by earlier events.

One of the more difficult questions to answer is just how bright these events
will be.  Terrestrial fireballs have typically exhibited perhaps 1% luminous
efficiency.  In other words about 1% of the total kinetic energy has been
converted to visible light.  The greater magnitude of the Jupiter impacts may
result in more energy appearing as light, but let's assume the 1% efficiency.
Then Sekanina calculates that a 1013-kg fragment, a reasonable value for the
largest piece, will reach an apparent visual magnitude of -10 during the
terminal explosion.  This is 1,000 times Jupiter's normal brilliance and only
10 times fainter than the full moon!  Sekanina, of course, calculates that the
explosions will occur above the clouds.  And, remember that, unfortunately,
these impacts will occur on Jupiter's back side as seen from Earth. There will
be no immediate visible effect on the appearance of Jupiter. The light of the
explosion may be seen reflected from the Galilean satellites of Jupiter, if
they are properly placed at the times of impacts.

Ganymede, for example, might brighten as much as six times, while Io could
brighten to 35 times its normal brilliance for a second before fading slowly,
if the explosions occur above the clouds.  This would certainly be visible in
an amateur telescope and could conceivably be visible to the naked eye at a
dark mountain site as a tiny flash next to Jupiter at the location of the
normally invisible satellite.  Emphasis on "tiny"!  The brightness of
explosions occurring below the clouds will be attenuated by a factor of at
least 10,000, making them most difficult to observe.  In the best of cases,
these events will be spectacles for the mind to imagine and big telescopes to
observe, not a free fireworks display.

The most recent predictions are that at least some of the impacts will occur
very close to the planetary limb, the edge of the planet's disk as seen from
Earth. That edge still has 11 degrees to rotate before it comes into sunlight.
This means that the tops of some of the plumes associated with the rising
fireballs may be just visible, although with a maximum predicted height of
3,000 km (0.8 arc second as projected on the sky) they will be just "peeking"
over the limb.  The newly repaired Hubble Space Telescope (HST), with its high
resolution and low scattered light, may offer the best chance to see such
plumes.  By the time they reach their maximum altitude the plumes will be
transparent (optically thin) and not nearly so bright as they were near the
clouds.  Some means of blocking out the bright light from Jupiter itself may be
required in order to observe anything.

A number of observers plan to look for evidence of plumes and to attempt to
measure their size and brightness.  It also is difficult to predict the effects
of the impacts on Jupiter's atmosphere.  Robert West points out that a
substantial amount of material will be deposited even in the stratosphere of
Jupiter, the part of the atmosphere above the visible clouds where solar
heating stabilizes the atmosphere against convection (vertical motion).  Part
of this material will come directly from small cometary grains, which vaporize
during entry and recondense just as do meteoritic grains in the terrestrial
atmosphere.  Part will come from volatiles (ammonia, water, hydrogen sulfide,
etc.) welling up from the deeper atmosphere as a part of the hot buoyant
fireballs created at the time of the large impact events.  Many
millimeter-sized or larger pieces from the original breakup will also impact at
various times for months and over the entire globe of Jupiter. There is
relatively little mass in these smaller pieces, but it might be sufficient to
create a haze in the stratosphere.

James Friedson notes that the fireball created by the terminal explosion will
expand and balloon upward and perhaps spew vaporized comet material and
Jupiter's entrained atmospheric gas to very high altitudes.  The fireball may
carry with it atmospheric gases that are normally to be found only far below
Jupiter's visible clouds.  Hence the impacts may give astronomers an
opportunity to detect gases which have been hitherto hidden from view.  As the
gaseous fireball rises and expands it will cool, with some of the gases it
contains condensing into liquid droplets or small solid particles.  If a
sufficiently large number of particles form, then the clouds they produce may
be visible from Earth-based telescopes after the impact regions rotate onto the
visible side of the planet.  These clouds may provide the clearest indication
of the impact locations after each event.

After the particles condense, they will grow in size by colliding and sticking
together to form larger particles, eventually becoming sufficiently large to
"rain" out of the visible part of the atmosphere.  The length of time spent by
the cloud particles at altitudes where they can be seen will depend principally
on their average size; relatively large particles would be visible only for a
few hours after an impact, while small particles could remain visible for
several months.  Unfortunately, it is very difficult to predict what the number
and average size of the particles will be.  A cloud of particles suspended in
the atmosphere for many days may significantly affect the temperature in its
vicinity by changing the amount of sunlight that is absorbed in the area.  Such
a temperature change could be observed from Earth by searching for changes in
the level of Jupiter's emitted infrared light.

Glenn Orton notes that large regular fluctuations of atmospheric temperature
and pressure will be created by the shock front of each entering fragment,
somewhat analogous to the ripples created when a pebble is tossed into a pond,
and will travel outward from the impact sites.  These may be observable near
layers of condensed clouds in the same way that regular cloud patterns are seen
on the leeward side of mountains.  Jupiter's atmosphere will be sequentially
raised and lowered, creating a pattern of alternating cloudy areas where
ammonia gas freezes into particles (the same way that water condenses into
cloud droplets in our own atmosphere) and clear areas where the ice particles
warm up and evaporate back into the gas phase.  If such waves are detected,
measurement of their wavelength and speed will allow scientists to determine
certain important physical properties of Jupiter's deep atmospheric structure
that are very difficult to measure in any other way.

Whether or not "wave" clouds appear, the ripples spreading from the impact
sites will produce a wave structure in the temperature at a given level that
may be observable in infrared images.  In addition there should be compression
waves, alternate compression and rarefaction in the atmospheric pressure, which
could reflect from and refract within the deeper atmosphere, much as seismic
waves reflect and refract due to density changes inside Earth. Orton suggests
that these waves might be detected "breaking up" in the shallow atmosphere on
the opposite side of the planet from the impacts.  Others suggest the
possibility of measuring the small temperature fluctuations wherever the waves
surface, but this requires the ability to map fluctuations in Jupiter's visible
atmosphere of a few millikelvin (a few thousandths of a degree).  Detection of
any of these waves will require a very fine infrared array detector (a thermal
infrared camera).

Between the water and other condensable gases (volatiles) brought with the
comet fragments and those exhumed by the rising fireballs, it is fairly certain
that a cloud of condensed material will form at the location of the impacts
themselves, at high altitudes where such gases seldom, if ever, exist in the
usual course of things.  It may be difficult to differentiate between the color
or brightness of these condensates and any bright material below them in
spectra at most visible wavelengths.  However, at wavelengths where gaseous
methane and hydrogen absorb sunlight, a distinction can easily be made between
particles higher and lower in the atmosphere, because the higher particles will
reflect sunlight better.  Much of the light is absorbed before reaching the
lower particles.  Observing these clouds in gaseous absorption bands will then
tell us how high they lie in the atmosphere, and observations over a period of
time will indicate how fast highaltitude winds are pushing them.  The speed
with which these clouds disappear will be a measure of particle sizes in the
clouds, since large particles settle out much faster than small ones, hours as
compared to days or months.

Orton also notes that in the presence of a natural wind shear (a region with
winds having different speeds and/or directions) such as exists commonly across
the face of Jupiter, a long-lived cyclonic feature can be created which is
actually quite stable.  It may gain stability by being fed energy from the wind
shear, in much the same way that the Great Red Spot and other Jovian vortices
are thought to be stabilized.  Such creation of new, large, fixed "storm"
systems is somewhat controversial, but this is a most intriguing possibility!



8.  How Can These Impacts and Their Consequences be Studied?  Space-Based

    There are at least four spacecraft -- Galileo, Ulysses, Voyager 2, and
Clementine -- with some potential to observe the Jovian impacts from different
vantage points than that of Earth. There is also the Hubble Space Telescope
(HST), in orbit around Earth, which will view the event with essentially the
same geometry as any Earth-based telescope.  HST, however, has the advantages
of perfect "seeing" (no atmospheric turbulence), very low scattered light,
ultraviolet sensitivity, and the ability to observe much more than two hours
each day.  HST is scheduled to devote considerable time to the observation of
Shoemaker-Levy 9 before as well as during the impacts.

The Galileo spacecraft has the best vantage point from which to observe the
impacts.  It is on its way to Jupiter and will be only 246 million km away from
the planet, less than a third the distance of Earth from Jupiter at that time.
All of the impacts will occur directly in the field of view of its high
resolution camera and 20 -25 degrees of Jovian longitude from the limb.  Images
of Jupiter will be 60 picture elements (pixels) across, although the impact
site will still be smaller than the resolution of the camera.  Several
instruments besides the camera have potential use, including an ultraviolet
spectrometer, a near infrared mapping spectrometer, and a photopolarimeter
radiometer.  This last suite of instruments could acquire light curves (plots
of intensity versus time) of the entry and fireball at many wavelengths from
ultraviolet to thermal infrared (from wavelengths much shorter than visible
light to much longer).

Using Galileo to make these observations will be challenging.  The amount of
data the spacecraft can transmit back to Earth is limited by the capability of
its low-gain antenna and the time available on the receiving antennas of the
National Aeronautics and Space Administration's (NASA's) Deep Space Network
here on Earth. The "commands" that tell the spacecraft what to do must be sent
up several weeks before the fact and before the impact times are known to
better than about 20 minutes with 95% certainty.  A later command that simply
triggers the entire command sequence may be possible.  A lot of data frames can
be stored in the Galileo tape recorder, but only about 5% of them can be
transmitted back to Earth, so the trick will be to decide which 5% of the data
are likely to include the impacts and to have the greatest scientific value,
without being able to look at any of them first!  After the fact, the impact
times should be known quite accurately.  This knowledge can help to make the
decisions about which data to return to Earth.

The Ulysses spacecraft was designed for solar study and used a gravity assist
from flying close to Jupiter to change its inclination (the tilt of its path
relative to the plane of the planets) so it can fly over the poles of the Sun.
In July 1994 it will be about 378 million km south of the plane of the planets
(the ecliptic) and able to "look" over the south pole of Jupiter directly at
the impact sites.  Unfortunately, Ulysses has no camera as a part of its
instrument complement.  It does have an extremely sensitive receiver of radio
frequency signals from 1 to 1000 kHz (kilohertz, or kilocycles in older
terminology) called URAP (Unified Radio and Plasma wave experiment).  URAP may
be able to detect thermal radiation from the impact fireballs once they rise
sufficiently high above interference from the Jovian ionosphere (upper
atmosphere) and to measure a precise time history of their rapid cooling.

The Voyager 2 spacecraft is now far beyond Neptune (its last object of study
back in 1989 after visiting Jupiter in 1979, Saturn in 1981, and Uranus in
1986) and is about 6.4 billion km from the Sun. It can look directly back at
the dark side of Jupiter, but the whole of Jupiter is now only two picture
elements in diameter as seen by its high-resolution camera, if that instrument
were to be used.  In fact the camera has been shut down for several years, and
the engineers who knew how to control it have new jobs or are retired.  It
would be very expensive to take the camera "out of mothballs" and probably of
limited scientific value.  Voyager does have an ultraviolet spectrometer which
is still taking data, and it will probably be used to acquire ultraviolet light
curves (brightness versus time) of the impact phenomena.  The possibility of
using one or two other instruments is being considered, though useful results
from them seem less likely.

A new small spacecraft called Clementine was launched on January 25 of this
year, intended to orbit the Moon and then proceed on to study the asteroid
Geographos. Clementine has good imaging capabilities, but its viewpoint will
not be much different from Earth's. The impact sites will still be just over
the limb, and Clementine's resolution will be only a few picture elements on
Jupiter. Since the spacecraft will be in cruise mode at the time, on its way to
Geographos and not terribly busy, it seems probable that attempts will be made
to observe "blips" of light on the limb of Jupiter, from the entering fragments
or the fireballs or perhaps light scattered from cometary material (coma) that
has not yet entered the atmosphere.  Useful light curves could result.
Ground-Based

    Many large telescopes will be available on Earth with which to observe
the phenomena associated with the ShoemakerLevy 9 impacts on Jupiter in
visible, infrared, and radio wavelengths.  Small portable telescopes can fill
in gaps in existing observatory locations for some purposes.  Imaging,
photometry, spectroscopy, and radiometry will certainly be carried out using a
multitude of detectors.  Many of these attempts will fail, but some should
succeed.

Apart from the obvious difficulty that the impacts will occur on the back side
of Jupiter as seen from Earth, the biggest problem is that Jupiter in July can
only be observed usefully for about two hours per night from any given site.
Earlier the sky is still too bright and later the planet is too close to the
horizon.  Therefore, to keep Jupiter under continuous surveillance would
require a dozen observatories equally spaced in longitude clear around the
globe.  A dozen observatories is feasible, but equal spacing is not.  There
will be gaps in the coverage, notably in the Pacific Ocean, where Mauna Kea,
Hawaii, is the only astronomical bastion.

Measuring the light curve of the entering fragments and the post- explosion
fireball can be done only by measuring the light reflected from something else,
one of Jupiter's satellites or perhaps the dust coma accompanying the fragment.
That dust coma could still be fairly dense out to distances of 10,000 km or
more around each fragment.  Moving at 60 km/s, it will be almost three minutes
before all of the dust also impacts Jupiter. Proper interpretation of such
observations will be difficult, however, because the area of the "reflector,"
the coma dust particles, will be changing as the observations are made.
Another complication is the brightness of Jupiter itself, which will have to be
masked to the greatest extent possible.  Observations in visible light
reflected from the satellites will be relatively straightforward and can be
done with small telescopes and simple photometers or imaging devices.  This
equipment is small enough that it can be transported to appropriate sites.

Spectroscopy of the entry phenomena via reflected light from one of the
Galilean satellites could be used to determine the composition of the comet and
the physical conditions in the fireball, if the terminal explosions occur above
Jupiter's clouds.  If the explosion occurs below the clouds, there will be too
little light to do useful spectroscopy with even the largest telescopes.

The impact zone on Jupiter will rotate into sight from Earth about 20 minutes
after each impact, though quite foreshortened as initially viewed.  Extensive
studies of the zone and the area around it can be made at that time.  Such
studies surely will include imaging, infrared temperature measurements, and
spectroscopy using many of the largest telescopes on Earth. These studies will
continue for some weeks, if there is any evidence of changes in Jupiter's
atmosphere and cloud structure as a result of the impacts.

For example, astronomers will use spectrometers to look for evidence of
chemical changes in Jupiter's atmosphere.  Some of the species observed might
be those only present in the deep atmosphere and carried up by the fireball (if
the explosion occurs deep enough).  Others will be the result of changes to the
chemistry of the upper atmosphere, taking place because of the energy deposited
there by the impacts or because of the additional particulates.

The faint rings of Jupiter, mentioned in Section 5, can be usefully observed
from the ground at infrared wavelengths.  Shoemaker-Levy 9 debris might bring
in new ring material by hitting the two small satellites embedded in the rings
(Metis and Adrastea). The rings surely will be monitored for some time using
infrared imaging array detectors, which are sensitive to wavelengths more than
eight times as long as red visible light.

In Section 5, note was made of the Jovian magnetosphere, which makes its
presence known at radio wavelengths, and the Io torus of various ions and
atoms, which can be mapped spectroscopically.  Either or both of these could be
affected sufficiently by the intruding dust from Shoemaker-Levy 9 to be
detectably changed.  Radio telescopes will surely monitor the former and
optical telescopes the latter for weeks or months looking for changes.
Jupiter's intense electromagnetic environment is responsible for massive
auroral emission near the planet's poles and less intense phenomena across the
face of the planet.  These may also be disrupted by the collisions and/or the
dust "invasion," making auroral monitoring a useful observing technique.

In summary, the phenomena directly associated with each impact from entry trail
to rising fireball will last perhaps three minutes.  The fallback of ejecta
over a radius of a few thousand kilometers will last for about three hours.
Seismic waves from each impact might be detectable for a day, and atmospheric
waves for several days.  Vortices and atmospheric hazes could conceivably
persist for weeks.  New material injected into the Jovian ring system might be
detectable for years.  Changes in the magnetosphere and/or the Io torus might
also persist for some weeks or months.  There is the potential to keep
planetary observers busy for a long time!


9.  What Do Scientists Expect to Learn from All of This?

    To give a simple and succinct answer to the title of this section,
scientists hope to learn more about comets, more about Jupiter, and more about
the physics of high velocity impacts into a planetary atmosphere.  Something
has already been learned about comets from the behavior of Shoemaker-Levy 9
during its breakup, as discussed in Section 4.  Bits and pieces of what
everyone hopes will be learned have been noted in Sections 5 through 8.  A more
complete summary follows.

If the fragments explode above the clouds, there should be enough light
reflected from various Jovian satellites to take spectra of the explosions.
Since the atmosphere of Jupiter contains very few heavier elements to
contaminate the spectra, they could give a great deal of information about the
composition of cometary solids.  If the fragments explode below the clouds,
then spectroscopy must wait until the impact sites rotate into view from Earth.
By that time everything will have cooled a great deal, and the cometary
component will have been diluted by mixing with the Jovian atmosphere, making
such study difficult.  In that case the Jovian material itself may prove of
interest, with spectroscopic study giving new knowledge of Jupiter's deeper
atmospheric composition.

It seems somewhat more certain that new knowledge of Jupiter's atmosphere will
be obtained, even if predictions differ as to exactly what that new knowledge
will be.  There is nearly unanimous agreement that the impacts will cause
observable changes in Jupiter, at least locally at the impact sites.  These may
include changes in the visible appearance of the clouds, locally or more
widely, measurable temperature fluctuations, again locally or more widely,
composition changes caused by material brought up from below the clouds (if the
fragments penetrate that deeply), and/or chemical reactions brought about by
the thermal pulse and the introduction of cometary material.  Any dynamic
processes such as these will give a new and better understanding of the
structure of Jupiter's atmosphere, perhaps of its motion as well as its static
structure.

If sufficient material impacts Jupiter's rings or especially the ring
satellites, then there should be local brightening caused by the increase in
reflecting area due to the introduction of new material.  These new ring
particles will each take up their own orbits around Jupiter, gradually
spreading out and causing local brightening followed by slow fading into the
general ring background.  Careful mapping of that brightening and fading will
reveal a great deal about the structure and dynamics of the rings.  Many
believe that impacts on those small inner satellites are the source of the
rings, the reason for their existence.  Enhancement of the rings from
Shoemaker-Levy 9 impacts would be strong confirmation of this idea.  Similarly,
the interaction of cometary dust with the magnetosphere and with the Io torus
will be quite informative, if the dust density proves sufficient to cause
observable effects.  Radio telescopes will be active in the magnetospheric
studies, along with optical spectroscopy of the ions and atoms in the torus.

Last, but far from least, the physics of the impact phenomena themselves,
determined from the reflected light curves and from spectra, will be most
instructive.  Note the inability of scientists to agree on the level of
Jupiter's atmosphere at which the terminal explosion will occur. (A few even
believe that there will be no terminal explosion or that it will occur so deep
in that atmosphere as to be completely unobservable.) Entry phenomena on this
scale cannot be reproduced, even by nuclear fusion explosions, and have never
before been observed.  Better knowledge of the phenomena may allow scientists
to predict more accurately just how serious could be the results of future
impacts of various-sized bodies on Earth, as well as to determine their effects
in the past as registered by the fossil record.


Appendix A

Comparative Tables

Table 1. Energy comparisons.*
        Energy, J    Energy, Relative

Two 3,500-lb. cars colliding head-on at 55 mph
        9.6 x 10^5    1

Explosion of 1 U.S. ton of TNT
        4.2 x 10^9    4,271

Explosion of a 20-megaton fusion bomb
        8.4 x 10^16    87,500,000,000

Total U.S. annual electric power production, 1990
        1 x 1019    10,400,000,000,000

Energy released in last second of 10^13 kg fragment of Shoemaker-
Levy 9
    ~ 9 x 10^21    9,375,000,000,000,000

Total energy released by 1013-kg fragment of Shoemaker-Levy 9
1.8 x 10^22    18,750,000,000,000,000

Total sunlight on Jupiter for one day
        6.6 x 10^22    68,750,000,000,000,000

*1 BTU = 252 (small) calories = 1,055 J = 2.93  x 10-4 kWh.



Table 2. Power comparisons.*

Power Producer    Power, MW    Power,

Relative

Hoover Dam    1,345        1

Grand Coulee Dam, final plant
        9,700        7.2

Annual average, sum of all U.S. power plants
320,000    238

Average, impact of 1013-kg fragment of Shoemaker-Levy
9, final sec       ~ 9 x 10^15    6,700,000,000,000

Sun        3.8 x 1020
    280,000,000,000,000,000

*1 horsepower = 745.7 W = 7.457  x 10-4 MW.


Table 3. Size comparisons.*

Object              Radius, km        Volume, km^3

Jupiter            71,350 (Equatorial)        1.4 x 10^15
67,310 (Polar)

Earth            6,378 (Equatorial)        1.1 x 10^12
            6,357 (Polar)

Comet Shoemaker-Levy 9  4.5 (Equivalent sphere)    382

Comet Halley        7.65 x 3.60 x 3.61        365
*1 mi. = 1.609 km


Table 4. Brightness comparisons.

Object  Magnitude Vo(V Mag. at Opposition)

Relative Brightness

Largest fragment of Shoemaker-Levy 9 during last second
    ~ -10
    1,000

Jupiter   -2.5
    1

Ganymede   4.6
    1/692

Io       5.0
    1/1,000


Largest fragment of Shoemaker-Levy 9 as viewed
today
       23.7
    1/30,000,000,000


Appendix B

The K-T Event

    Sixty-five million years ago about 70% of all species then living on
Earth disappeared within a very short period.  The disappearances included the
last of the great dinosaurs.  Paleontologists speculated and theorized for many
years about what could have caused this "mass extinction," known as the K-T
event (Cretaceous-Tertiary Mass Extinction event).  Then in 1980 Alvarez,
Alvarez, Asaro, and Michel reported their discovery that the peculiar
sedimentary clay layer that was laid down at the time of the extinction showed
an enormous amount of the rare element iridium.  First seen in the layer near
Gubbio, Italy, the same enhancement was soon discovered to be worldwide in that
one particular 1-cm (0.4- in.) layer, both on land and at sea.  The Alvarez
team suggested that the enhancement was the product of a huge asteroid impact.

On Earth most of the iridium and a number of other rare elements such as
platinum, osmium, ruthenium, rhodium, and palladium are believed to have been
carried down into Earth's core, along with much of the iron, when Earth was
largely molten.  Primitive "chondritic" meteorites (and presumably their
asteroidal parents) still have the primordial solar system abundances of these
elements.  A chondritic asteroid 10 km (6 mi.) in diameter would contain enough
iridium to account for the worldwide clay layer enhancement.  This enhancement
appears to hold for the other elements mentioned as well.

Since the original discovery many other pieces of evidence have come to light
that strongly support the impact theory.  The high temperatures generated by
the impact would have caused enormous fires, and indeed soot is found in the
boundary clays.  A physically altered form of the mineral quartz that can only
be formed by the very high pressures associated with impacts has been found in
the K-T layer.

Geologists who preferred other explanations for the K-T event said, "show us
the crater." In 1990 a cosmochemist named Alan Hildebrand became aware of
geophysical data taken 10 years earlier by geophysicists looking for oil in the
Yucatn region of Mexico. There a 180-km (112-mi.) diameter ring structure
called "Chicxulub" seemed to fit what would be expected from a
65million-year-old impact, and further studies have largely served to confirm
its impact origin.  The Chicxulub crater has been age dated (by the 40Ar/39Ar
method) at 65 million years!  Such an impact would cause enormous tidal waves,
and evidence of just such waves at about that time has been found all around
the Gulf of Mexico. Similarly, glassy debris of appropriate age called tektites
(and their decomposition products), which are produced by large impacts, have
been found all around the Gulf.

One can never prove that an asteroid impact "killed the dinosaurs." Many
species of dinosaurs (and smaller flora and fauna) had in fact died out over
the millions of years preceding the K-T event.  The impact of a 10-km asteroid
would most certainly have been an enormous insult to life on Earth. Locally
there would have been enormous shock wave heating and fires, a tremendous
earthquake, hurricane winds, and trillions of tons of debris thrown everywhere.
It would have created months of darkness and cooler temperatures globally.
There would have been concentrated nitric acid rains worldwide.  Sulfuric acid
aerosols may have cooled Earth for years.  Life certainly could not have been
easy for those species which did survive.  Fortunately such impacts occur only
about once every hundred million years.  Appendix C

The Probability of Collisions with Earth

    Most bodies in the solar system with a visible solid surface exhibit
craters.  On Earth we see very few because geological processes such as
weathering and erosion soon destroy the obvious evidence.  On bodies with no
atmosphere, such as Mercury or the Moon, craters are everywhere.  Without going
into detail, there is strong evidence of a period of intense cratering in the
solar system that ended about 3.9 billion years ago.  Since that time cratering
appears to have continued at a much slower and fairly uniform rate.  The cause
of the craters is impacts by comets and asteroids.  Most asteroids follow
sensibly circular orbits between the planets Mars and Jupiter, but all of these
asteroids are perturbed, occasionally by each other and more regularly and
dramatically by Jupiter. As a result some find themselves in orbits that cross
that of Mars or even Earth. Comets on the other hand, as noted in Section 2,
follow highly elongated orbits that often come close to Earth or other major
bodies to begin with.  These orbits are greatly affected if they come anywhere
near Jupiter. Over the eons every moon and planet finds itself in the wrong
place in its orbit at the wrong time, many times, and suffers the insult of a
major impact.

Earth's atmosphere protects us from the multitude of small debris, the size of
grains of sand or pebbles, thousands of which pelt our planet every day.  The
meteors in our night sky are visible evidence of bodies of this type burning up
high in the atmosphere.  In fact, up to a diameter of about 10 m (33 ft.) most
stony meteoroids are destroyed in the atmosphere in a terminal explosion.
Obviously some fragments do reach the ground, because we have stony meteorites
in our museums.  Such falls are known to cause property damage from time to
time.  On October 9, 1992, a fireball was seen streaking across the sky all the
way from Kentucky to New York. A 27-lb. stony meteorite (chondrite) from the
fireball fell in Peekskill, New York, punching a hole in the rear end of an
automobile parked in a driveway and coming to rest in a shallow depression
beneath it.  Falls into a Connecticut dining room and an Alabama bedroom are
other well documented incursions in this century.  A 10-m body typically has
the kinetic energy of about five Hiroshima fission bombs, however, and the
shock wave it creates can do considerable damage even if nothing but
comparatively small fragments survive to reach the ground.

Many fragments of a 10-m iron meteoroid will reach the ground.  The only well
studied example of such a fall in recent times took place in the Sikhote-Alin
Mountains of eastern Siberia on February 12, 1947.  About 150 U.S. tons of
fragments reached the ground, the largest intact fragment weighing 3,839 lb.
The fragments covered an area of about 1 x 2 km (0.6 x 1.2 mi.), within which
there were 102 craters greater than 1 m in diameter, the largest of them 26.5 m
(87 ft), and about 100 more smaller craters.  If this small iron meteorite had
landed in a city, it obviously would have created quite a stir.  The effect of
the larger pieces would be comparable to having a supersonic auto suddenly drop
in!  Such an event occurs about once per decade somewhere on Earth, but most of
them are never recorded, occurring at sea or in some remote region such as
Antarctica. It is a fact that there is no record in modern times of any person
being killed by a meteorite.

It is the falls larger than 10 m that start to become really worrisome.  The
1908 Tunguska event described in Section 7 was a stony meteorite in the 100-m
class.  The famous meteor crater in northern Arizona, some 4,000 ft. in
diameter and 600 ft. deep, was created 50,000 years ago by a nickel-iron
meteorite perhaps 60 m in diameter.  It probably survived nearly intact until
impact, at which time it was pulverized and largely vaporized as its 6- 7 x
10^16 joules of kinetic energy were rapidly dissipated.  An explosion
equivalent to some 15 million tons of TNT creates quite a bang!  Falls of this
class occur once or twice every 1,000 years.  There are now over 100 ring-like
structures on Earth recognized as definite impact craters.  Most of them are
not obviously craters, their identity masked by heavy erosion over the
centuries, but the minerals and shocked rocks present make it clear that impact
was their cause.  The Ries Crater in Bavaria is a lush green basin some 25 km
(15 mi.) in diameter with the city of Nrdlingen in the middle.  Fifteen million
years ago a 1,500-m (5,000-ft.) asteroid or comet hit there, excavating more
than a trillion tons of material and scattering it all over central Europe.
This sort of thing happens about once every million years or so.  Another step
upward in size takes us to Chicxulub, described in detail in Appendix B, an
event that occurs once in 50-100 million years.  Chicxulub is the largest
crater known which seems definitely to have an impact origin, but there are a
few ring-like structures that are 2-3 times larger yet about which geologists
are "suspicious."

There are now more than 150 asteroids known that come nearer to the Sun than
the outermost point of Earth's orbit.  These range in diameter from a few
meters to about 8 km.  A working group chaired by D. Morrison estimates that
there are some 2,100 such asteroids larger than 1 km and perhaps 320,000 larger
than 100 m, the size that caused the Tunguska event and the Arizona meteor
crater.  An impact by one of the latter in the wrong place would be a great
catastrophe, but it would not threaten civilization.  An impact by an 8-km
object is in the mass extinction category.  In addition there are many comets
in the 1-10-km class, 15 of them in shortperiod orbits that pass inside Earth's
orbit, and an unknown number of long-period comets.  Virtually any short-period
comet among the 100 or so not currently coming near to Earth could become
dangerous after a close passage by Jupiter.

This all sounds pretty scary.  However, as noted earlier, no human in the past
1,000 years is known to have been killed by a meteorite or by the effects of
one impacting. (There are ancient Chinese records of such deaths.) An
individual's chance of being killed by a meteorite is ridiculously small as
compared to death by lightning, volcanism, earthquake, or hurricane, to say
nothing of the multitude of human- aided events.  That small probability was
unlikely to have been any consolation to the dinosaurs, however.  For this
reason astronomers today are conducting ever-increasing searches for all of the
larger asteroids that could become dangerous.  Once discovered, with a few
years of warning, there is every reason to believe that a space mission could
be mounted "to shove them aside."

Acknowledgments:

    This booklet is the product of many scientists, all of whom have
cooperated enthusiastically to bring their best information about this exciting
event to a wider audience.  They have contributed paragraphs, words, diagrams,
slides, and preprints as well as their critiques to this document, which
attempts to present an event that no one is quite sure how to describe.
Sincere thanks go to Mike A'Hearn, Paul Chodas, Gil Clark, Janet Edberg, Steve
Edberg, Jim Friedson, Mo Geller, Martha Hanner, Cliff Heindl, David Levy,
Mordecai-Mark Mac Low, Al Metzger, Marcia Neugebauer, Glenn Orton, Elizabeth
Roettger, Jim Scotti, David Seal, Zdenek Sekanina, Anita Sohus, Harold Weaver,
Paul Weissman, Bob West, and Don Yeomans -- and to those who might have been
omitted.  The choice of material and the faults and flaws in the document
obviously remain the responsibility of the author alone.

The writing and production of this material was made possible through the
support of Jrgen Rahe and Joe Boyce, Code SL, NASA, and of Dan McCleese, Jet
Propulsion Laboratory (JPL).  For help in the layout and production of this
booklet, on a very tight schedule, additional thanks go to the Design Services
Group of the JPL Documentation Section.

All comments should be addressed to the author:
Ray L. Newburn, Jr.
Jet Propulsion Laboratory, MS 169-237
4800 Oak Grove Dr.
Pasadena, CA 91109-8099