CCNet, 26 August 1999

    Larry Klaes <>


    A. Morbidelli) et al., OBSERVATORY COTE AZUR

    L.A.M. Benner et al., CALTECH,JET PROP LAB,4800 OAK GROVE


    P. Michel, et al., OBSERVATORY COTE AZUR

    S.J. Ostro et al., CALTECH

    R.H. Brown et al., UNIVERSITY OF ARIZONA


     Doug Keenan <>

     Steve Drury <>


From Larry Klaes <>

August Hot Idea:
Asteroids have lower densities than expected, probably because they have
been disrupted and then reassembled into porous rubble piles.

Honeycombed Asteroids
Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

Asteroids seem to have lower densities than the rocks scientists
believe compose them. This implies that there is quite a bit of empty
space inside the typical asteroid or small moon. Lionel Wilson
(Lancaster University, UK), Klaus Keil (University of Hawaii), and
Stanley Love (astronaut candidate, Johnson Space Center) investigated
two possibilities for producing the high percentage of pore space. In
one case they calculated that if an asteroid were broken apart and then
reassembled, the resulting rubble pile would have a porosity of 20 to
40%, hence a density 20 to 40% lower than it had to begin with. They
also calculated how fractures would form on bodies that contained water
ice that was heated to steam, concluding that the fractures would be
pervasive and, hence, decrease the density of the object.


Wilson, L., K. Keil, and S. J. Love, 1999, The Internal Structures and
Densities of Asteroids, Meteoritics and Planetary Science, vol. 34, p.

Asteroid and Meteorite Densities

We have fairly accurate measurements of the densities of four
asteroid-sized bodies that have been visited by spacecraft: Phobos and
Deimos, the small moons of Mars, and asteroids 243 Ida and 253
Mathilde. Each has been visited close enough by a spacecraft to allow
calculation of its mass from the way it deflected the spacecraft from
its course. Also, each of these bodies has been photographed well
enough to be able to determine its volume. Because density is mass
divided by volume, these data allow us to determine the densities of
the objects. See the table below (Adapted from Wilson et al., 1999.)

Density is a useful parameter. An asteroid made of pure iron-nickel
metal, for example, would have a density of about 7.9 grams per cubic
centimeter, so if we found an asteroid with such a high density we
could conclude it was made of metallic iron. Of course, things are
never that simple. An asteroid might have empty spaces in it (called
pore spaces) which would lower the density in proportion to the
percentage of pore spaces. Most planetary surface materials have pore
spaces. Sand, for example, consists typically of about 40% pore spaces.
(To get an idea of how much pore space a material can have, dump some
sand into a volumetric beaker, up to say the 500 milliliters level.
Then pour water from another calibrated container into the sand until
you finally begin to get a puddle on top. You may be able to pour 200
milliliters of water into the 500 milliliters of sand.)

Even individual meteorites have some pore spaces. Guy Consolmagno SJ
(Vatican Observatory) and Daniel Britt (University of Tennessee) have
measured the density and porosity (the percentage of pore spaces is
called porosity) of hundreds of meteorites and assembled a database of
measurements made by other investigators [see PSRD supplement on:
measuring density and porosity].

They find that the porosity of meteorites ranges from 5 to 35%. The
porosity is in the form of cracks and spaces between mineral grains.
Carbonaceous chondrites, which are dark rocks that contain carbon
compounds (including organic chemicals) and water-bearing minerals,
have porosities between 15% to 35%. In contrast, ordinary chondrites,
which do not contain carbon compounds or water, have porosities of
about 10%. Stony-iron meteorites have porosities of only about 5% and
iron meteorites have essentially no porosity.

To compare the meteorite data with a given asteroid, we need to know
what type of meteorite could have come from it. Wilson and coworkers
did that by abopting a classification scheme used by astronomers. the
classification is based on the amount and color of the light reflected
off an asteroid, and on comparisons with the way powdered meteorite
samples reflect light. Astronomers measuring the reflected light have
classified 253 Mathilde as a C type asteroid, thought to be similar to
carbonaceous chondrites. So, Wilson and colleagues assume 253 Mathilde
is a carbonaceous chondrite, with a porosity of about 15 to 35% (see
table above). Phobos and Deimos are classified as D or P types, which
are darker than even the darkest carbonaceous chondrite, and do not
correspond to any known type of meteorite. Nevertheless, Wilson assumes
that the Martian moons are more likely to be similar to CI or CM
carbonaceous chondrites, with porosity of 15-35%. 243 Ida is classified
as an S type asteroid. What type of meteorite these correspond to
depends on who one talks to: the issue is hotly contested. They could
be either like ordinary chondrites or stony-iron meteorites. Wilson and
colleagues assume 243 Ida is an ordinary chondrite.

Putting all this together, the two asteroids and two moons of Mars have
somewhat lower densities, hence higher porosities, than do the
meteorites to which they appear to be most closely related (see table
above). This indicates that the asteroids and small moons have more
empty spaces inside them than do small rocks removed from them. In
other words, they appear to be gravitationally bound piles of rocks
with empty spaces between the rocks.

Breaking Up Asteroids

The idea that asteroids are rubble piles is not new. Theoretical
studies of the evolution of asteroids by impacts indicate that for a 
large range in impact energies, the bodies break up, but the fragments
do not move apart fast enough to escape each other. Their mutual
gravity causes the fragments to promptly reassemble into a mixed-up,
fragmented body; see graphic below.

Chondrite meteorites provide proof that this process actually happens.
Some chondrites are called "regolith breccias." These are rocks
reworked by impacts on the surfaces of asteroids. We know they formed
in the uppermost part of the asteroid because regolith breccias contain
gases implanted from the solar wind, which penetrate only micrometers
into rock surfaces. These breccias also contain grains of metallic
iron-nickel. It is possible to determine how fast these metallic
particles cooled by measuring their compositions and sizes.

Numerous analyses indicate that in a given regolith breccia, the
metallic particles cooled at rates ranging from 1 to 1000 degrees
Celsius per million years. Using the laws of heat conductivity, we can
calculate how deep a rock must be buried to cool at a given rate. The
range of cooling rates of the particles in regolith breccias indicates
original burial depths of a few kilometers (those cooling at 1000
degrees per million years) to 100 kilometers (those cooling at 1 degree
per million years). Clearly, there has been a lot of scrambling of the
asteroids on which these breccias formed!

At first glance, one would think that craters of different sizes could
dig up rocks from a large range of depths and deposit them onto the
surface where they could be incorporated into regolith breccias. It is
not so easy, however. In 1987 my colleagues and I calculated that
craters large enough to excavate rock from a depth of 60 kilometers
would demolish asteroids smaller than 500 kilometers in diameter (see
graph below). The easier way to deposit rocks from great depths and to
mix them with rocks from shallow depths is to bust up the asteroid and
reassemble it into a rubble pile.

Increasing Porosity by Fragmentation and Reassembly

Lionel Wilson and his colleagues wondered if the high porosities
(hence, low densities) of 243 Ida, 253 Mathilde, and the Martian moons,
could be due to break up and reassembly of the bodies. To test this
idea, Wilson wrote a computer program that followed the paths of
numerous asteroid fragments after an impact large enough to demolish
the asteroid. A critical piece of information he needed was the sizes
and shapes of the fragments created by the disruption of an asteroid.
For that information, Wilson used data from a series of experiments by
Akiko Nakamura and Akira Fujiwara (Kyoto University, Japan). They broke
up rock spheres by whacking them with projectiles moving at 3-4
kilometers per hour (appropriate for velocities in the asteroid belt),
and examined the fragments produced.

Using Nakamura and Fujiwara's data as a guide, Wilson assumed that the
asteroid broke up into elongated pieces with one large fragment, 4
smaller fragments, 32 still smaller, and 128 even smaller. About half
the object breaks into even smaller pieces but most of them do not fall
back onto the reassembled object. (A real asteroid, of course, would
break up into millions of fragments, but computers can only keep track
of a small number of particles. Too many fragments and the program
will take forever to run.) The results are shown in the illustration
The blocks in this diagram, which vaguely resembles a cubist painting,
represent the relative abundances and sizes of the fragments produced
by a catastrophic impact. (Physicists frequently depict objects as
recognize that natural objects are not all cubes and spheres!) Orange
is a single large fragment, blue the 4 smaller ones, green the 32 next
smallest, purple the 128 next smallest, and red the smallest. The
captions in the left corners show the time elapsed since the impact,
which takes place at the asterisk by a projectile approaching from the
direction of the arrow. Very few of the smallest fragments closest to
the point of impact fall back to the shattered asteroid. The largest
fragments, because they are accelerated the least, fall back first,
followed by progressively smaller fragments, and most of the object is
reassembled in about half an hour. Of course, it is missing a lot of
mass - the red region that was fragmented into small pieces that were
accelerated too fast by the impact to return to the scene of the crime.

Wilson and colleagues needed to determine the porosity of a
reconstructed object. No elaborate computer program or cubist 
illustrations were required for this. Wilson simply cut up a chunk of
polystyrene (StyrofoamŪ) into a range of sizes, with the number of
fragments increasing as the size decreased (as in his computer
calculations and in Nakamura and Fujiwara's experiments). He dumped the
pieces into a beaker, putting the largest particles in first, then the
smaller ones, and finally the smallest ones, to simulate the way his
calculations indicated fragments fell onto the reassembling asteroid in
the calculations. After adding the fragments, he gave the beaker a
shake, then looked at the calibrations on the side of the beaker to
determine the volume of the reassembled polystyrene "asteroid." Because
he knew how much polystyrene he had used in making the fragments, the
volume of empty space (the volume of pores) was simply the difference
between the volume of the beaker taken up by the pile of fragments
minus the volume of polystyrene particles. He repeated the measurement
numerous times, and found that the porosity varied between 20% and 40%.
This is about what Wilson and his co-authors estimated for the porosity
of 243 Ida, 253 Mathilde, and the Martian moons.

Double Bubble, Toil and Trouble

Wilson and his colleagues also examined another way to increase the
porosity of a small asteroid that originally contained lots of water
ice mixed with rock. When such a mixture is heated (by decay of
radioactive elements, for example), the water reacts with the rocky
grains and forms hydrated minerals, releasing hydrogen gas, and 
possibly other gases as well. Wilson's calculations indicate that these
gases would build up faster than they could escape because the outer
parts of the asteroid may have had solid ice layers and the interior
was compacted.. As a result, the pressure built up, eventually exceeding
the strength of the surrounding rock, causing cracks to form. This
process may have caused pervasive fracturing of the interiors of
asteroids, greatly increasing their porosities. Wilson estimates that
this cracking mechanism could have operated in asteroids smaller than
about 50 kilometers. Higher pressure inside larger asteroids prevented

Rubble Piles

These latest results, which build on a great deal of work during the
past 15-20 years, suggest that most asteroids are gravitationally bound
rubble piles. This may make future mining of asteroids easier than if
they were solid rock. It might also make it more difficult to predict
the trajectories of fragments if earthlings try to blow up an asteroid
headed for our planet.

We will soon be able to do another test of the rubble-pile hypothesis.
The NEAR spacecraft will visit the asteroid 433 Eros early in 2000.
Sensors onboard NEAR will try to determine the mineralogy and chemical
composition of Eros, from which we will be able to estimate the
asteroid's density if it were solid rock. The rubble-pile idea predicts
that Eros will have a density much lower than the rock it is composed

Asteroids: from The Nine Planets by Bill Arnett.

Asteroids: from the National Space Science Data Center.

Consolmagno, G. J. and D. T. Britt, 1998, The Density and Porosity of
Meteorites from the Vatican Collection, Meteoritics and Planetary
Science, vol. 33, p. 1231-1241.

Galileo Mission homepage.

Nakamura, A. and A. Fujiwara, 1991, Velocity Distribution of Fragments
Formed in a Simulated Collisional Disruption, Icarus, vol. 92, p.

NASA Solar System Exploration.

Near Earth Object Program homepage.

NEAR Mission homepage.

Taylor, G. J., P. Maggiore, E. R. D. Scott, A. E. Rubin, and K. Keil,
1987, Original Structures, and Fragmentation and Reassembly Histories
of Asteroids: Evidence from Meteorites, Icarus, vol. 69, p. 1-13.

Wilson, L., K. Keil, and S. J. Love, 1999, The Internal Structures and
Densities of Asteroids, Meteoritics and Planetary Science, vol. 34, p.


N. McBride*), J.K. Davies, S.F. Green, M.J. Foster: Optical and
infrared observations of the Centaur 1997 CU26. MONTHLY NOTICES OF THE
ROYAL ASTRONOMICAL SOCIETY, 1999, Vol.306, No.4, pp.799-805


Minor planet 1997 CU26 is a Centaur, and is probably undergoing
dynamical evolution inwards from the Kuiper Belt. We present optical
and infrared (VRIJHK) photometry which gives mean colours of V - R =
0.46 +/- 0.02, V - I = 1.02 +/- 0.02, V - J = 1.74 +/- 0.02, V - H =
2.15 +/- 0.02 and V - K = 2.25 +/- 0.02. The resulting relative
reflectance spectrum lies between those of Chiron and Pholus (although
closer to that of Chiron). A 1.6-2.6 mu m spectrum confirms the broad
absorption feature at 2.05 mu m associated with water ice reported by
Brown et al.1997 CU26 displays no significant light curve variation and
(unlike Chiron) has no observable coma. We place an upper limit to the
dust production rate of 1.5 kg s(-1). J-band data taken at phase angles
of 1.degrees 7 to 4.degrees 0 give a phase parameter of G(J) = 0.36 +/-
0.1, and are consistent with a phase parameter of G = 0.15 in the V
band (a value often assigned to low-albedo objects when no other
information is available) if we assume a phase reddening of 0.017 mag
deg(-1) in the J band. We find V(1; alpha = 4.degrees 1) = 7.022 +/-
0.02, from which we deduce, by assuming G = 0.15 +/- 0.1, an absolute
visual magnitude of H-V = 6.64 +/- 0.04. Copyright 1999, Institute for
Scientific Information Inc.


A. Morbidelli*), D. Nesvorny: Numerous weak resonances drive asteroids
toward terrestrial planets orbits. ICARUS, 1999, Vol.139, No.2,


A systematic exploration of the chaotic structure of the asteroid belt
is presented, first taking into account only the perturbations provided
by the four giant planets and then including also the effects of the
inner planets. We find that both the inner belt (a < 2.5 AU) and the
outer part of the main belt (a > 2.8 AU) are mostly chaotic. In the
outer part of the belt, chaos is due to the presence of numerous
mean-motion resonances with Jupiter and three-body resonances, Jupiter-
Saturn-asteroid. In the inner belt; chaos is generated by mean motion
resonances with Mars and three-body resonances, Mars-Jupiter-asteroid.
Due to the chaoticity of the belt, asteroids tend to slowly migrate in
eccentricity. This phenomenon of 'chaotic diffusion' allows many bodies
in the inner belt to become Mars-crossers. The number of asteroids
leaking out from the inner belt is large enough to keep the population
of Mars-crossing asteroids in steady state, despite of the short
dynamical lifetime of the latter (similar to 25 Myr). We speculate that
chaotic diffusion could have substantially eroded the high-eccentricity
part of the asteroid belt, thus providing the impactors responsible for
the Late Heavy Bombardment phase of the early Solar System. (C) 1999
Academic Press.


L.A.M. Benner*), R.S. Hudson, S.J. Ostro, K.D. Rosema, J.D. Giorgini,
D.K. Yeomans, R.F. Jurgens, D.L. Mitchell, R. Winkler, R. Rose, M.A.
Slade, M.L. Thomas, P. Pravec: Radar observations of asteroid 2063
Bacchus. ICARUS, 1999, Vol.139, No.2, pp.309-327


We report Doppler-only (cw) and delay-Doppler radar observations of
Bacchus obtained at Goldstone at a transmitter frequency of 8510 MHz
(3.5 cm) on 1996 March 22, 24, and 29. Weighted, optimally filtered
sums of cw and delay-Doppler echoes achieve signal-to-noise ratios of
similar to 80 and similar to 25, respectively, and cower about 180
degrees of rotation phase (period = 14.90 h; Pravec et al, 1998), Our
cw observations place up to four 2-Hz-resolution cells on Bacchus at
echo powers greater than two standard deviations of the noise.
Delay-Doppler observations typically place about ten 0.5-mu s (75-m)x
1-Hz cells on Bacchus above the same threshold. A weighted sum of all
cw spectra gives an OC radar cross section of 0.12(-0.02)(+0.06) km(2)
and a circular polarization ratio of 0.21 +/- 0.01. The dispersion of
the echoes in time delay indicates a lower bound on Bacchus' maximum
pole-on breadth of 0.6 km that is consistent with the echo bandwidth
(6+/-2 Hz) and rotation period. Echo spectra on March 22 and
delay-Doppler images on all three days show a central deficit of echo
power that provides strong evidence for a bifurcation in the shape.
Inversion of delay-Doppler images, cw spectra, and optical lightcurves
obtained at Ondrejov Observatory yields single-lobe and two-lobe models
that define lower and upper bounds on the degree of bifurcation. Both
shape models have a prominent central concavity, modestly asymmetric
shapes, and similar physical dimensions, spin vectors, and radar and
optical geometric albedos, We adopt the more conservative single-lobe
shape model as our working model and explore its implications. It has a
radar-derived sidereal rotation period P-sid = 15.0 +/- 0.2 h and a
north pole within a few tens of degrees of ecliptic longitude lambda =
24 degrees and ecliptic latitude beta = -26 degrees; retrograde
rotation is likely, It has dimensions of 1.11 x 0.53 x 0.50 km, an
effective diameter (the diameter of a sphere with the same volume as
the model) D-eff = 0.63(-0.06)(+0.13) km, and radar and optical
geometric albedos <(sigma)over cap> = 0.33(-0.11)(+0.25) and p(v) =
0.56(-0.18)(+0.12), respectively, that are larger than most estimated
for other asteroids. Bacchus' low circular polarization ratio and high
radar albedo are consistent with nearly regolith-free ordinary
chondrite and basaltic achondrite compositions, but its high optical
geometric albedo seems inconsistent with an ordinary chondrite
composition and may favor a V-class composition. Bacchus has less
structural complexity at centimeter-to-decimeter spatial scales and its
near-surface is more dense (either more metal, lower porosity, or both)
than the average radar-detected near-Earth asteroid. (C) 1999 Academic


E. Asphaug*), D..J. Scheeres: Deconstructing Castalia: Evaluating a
postimpact state. ICARUS, 1999, Vol.139, No.2, pp.383-386


We describe the rotational and translational states of the fragments of
a disrupted asteroid. All the fragments are found to be in complex
rotation and have widely varying rotation periods. Although this is
only one example of a possible collision, the numerical results for
small fragment rotation rates are consistent with recent observations,
including the small asteroid 1998KY26, and they lend insight into the
latter stages of impact disruption. In particular, fragment 'memory' of
parental rotation appears to be negligible. This is the most
comprehensive description to date of the state of an asteroid or comet
immediately following a disruptive collision. (C) 1999 Academic Press.


P. Michel*), R. Gonczi, P. Farinella, C. Froeschle: Dynamical evolution
of 1036 Ganymed, the largest near-Earth asteroid. ASTRONOMY AND
ASTROPHYSICS, 1999, Vol.347, No.2, pp.711-719


We have studied numerically the dynamical evolution of 1036 Ganymed,
the largest near-Earth asteroid, by integrating the orbits of tens of
'clone' particles with similar initial conditions. Typically, the orbit
initially undergoes large, coupled oscillations of the eccentricity and
inclination; then, Mars encounters random-walk the semimajor axis until
it reaches a strong Jovian resonance; and eventually, resonant effects
pump up the eccentricity until the orbit becomes Sun-grazing or
hyperbolic (after encountering Jupiter). The median dynamical lifetime
is of about 10 Myr. Most orbits become Earth-crossing within 10 Myr of
evolution. The origin of Ganymed and a few other sizable Mars-crossing
asteroids with similar orbital elements is an open problem, since the
main-belt asteroid population in the neighbouring lower-eccentricity
portion of the phase space is quite sparse. Although Ganymed's
reflectance spectrum has some similarity to those of the ordinary
chondrites, the Earth delivery efficiency from bodies with this type of
orbits is low, because they are short-lived after they  become
Earth-crossing. Copyright 1999, Institute for Scientific Information


S.J. Ostro*) et al.: Radar and optical observations of asteroid 1998
KY26. SCIENCE, 1999, Vol.285, No.5427, pp.557-559


Observations of near-Earth asteroid 1998 KY26 shortly after its
discovery reveal a slightly elongated spheroid with a diameter of about
30 meters, a composition analogous to carbonaceous chondritic
meteorites, and a rotation period of 10.7 minutes. which is an order of
magnitude shorter than that measured for any other solar system object.
The rotation is too rapid for 1998 KY26 to consist of multiple
components bound together just by their mutual gravitational
attraction. This monolithic object probably is a fragment derived from
cratering or collisional destruction of a much larger asteroid.
Copyright 1999, Institute for Scientific Information Inc.


R.H. Brown*), D.P. Cruikshank, Y. Pendleton: Water ice on Kuiper Belt
object 1996 TO66. ASTROPHYSICAL JOURNAL, 1999, Vol.519, No.1 Pt2,


The 1.40-2.45 mu m spectrum of Kuiper Belt object 1996 TO66 was
measured at the Keck Observatory in 1998 September. Its spectrum shows
the strong absorptions near 1.5 and 2.0 mu m that are characteristic of
water ice-the first such detection on a Kuiper Belt object. The depth
of the absorption bands and the continuum reflectance of 1996 TO66
suggest the presence of a black- to slightly blue-colored, spectrally
featureless particulate material as a minority component mixed with the
water ice. In addition, there is evidence that the intensity of the
water bands in the spectrum of 1996 TO66 varies with rotational phase,
suggesting a ''patchy'' surface. Copyright 1999, Institute for
Scientific Information Inc.


A. Milani*), S.R. Chesley, G.B. Valsecchi: Close approaches of asteroid
1999 AN(10): Resonant and non-resonant returns. ASTRONOMY AND
ASTROPHYSICS, 1999, Vol.346, No.3, pp.L65-L68


The Earth passes very close to the orbit of the asteroid 1999 AN(10)
twice per year, but whether or not this asteroid can have a close
approach depends upon the timing of its passage across the ecliptic
plane. Among the possible orbits there are some with a close approach
in 2027. The period of the asteroid may be perturbed in such a way that
it returns to an approach to the Earth at either of the possible
encounter points. We have developed a theory which successfully
predicts the 25 possible such returns up to 2040. We have also
identified 6 more close approaches resulting from the cascade of
successive returns. Because of this extremely chaotic behaviour there
is no way to predict all possible approaches for more than a few
decades after any close encounter, but the orbit will remain
dangerously close to the orbit of the Earth for about 600 years.
Copyright 1999, Institute for Scientific Information Inc.


From Doug Keenan <>

Hi Benny,
Regarding your posting yesterday on the cause of the 4000 BP collapse
of civilisations: the cause was drought. The point is that people need
water to live--without it, they must migrate or die.
Perhaps Egypt illustrates this best. Palaeoecological evidence for
greatly reduced Nile flows at the time is conclusive. And Egyptian
records tell of the Nile being so low that people could walk across it;
even if exaggerated, it is clear that Nile flows were far from adequate
for many decades, possibly over a century.  Without the Nile, the
Egyptians could not live (Egyptian records even tell of people eating
their own children). So the people either left or died.
The main topic over which there can be debate is the cause of the
droughts. But, the palaeodata from Europe, North Africa, and the
Atlantic fits the pattern of a specific climatic state--a high phase of
the North Atlantic Oscillation (with accompanying changes in NADW
production). Also, there is a likely mechanism by which an eruption
could have pushed the NAO into that high phase, and ample evidence for
a large eruption. By contrast, no one has suggested how a cosmic impact
could trigger a high NAO, and there is no direct evidence of such an
impact. The paper is on my web site.
Doug Keenan


From Steve Drury < >

Dear Benny

The conjecture of coincident collapse of Early Bronze Age cultures in
association with volcanic- or impact-induced climate change could be
tested (if it has not already been) from Greenland and Antarctic
ice-core records and pollen analysis of Holocene sediments.  The former
would, through layer counting, give precise ages for any detectable
event, either in ice-cap air temperatures or the dust records. If such
studies have been conducted I would be interested in hearing of any
published articles.

Steve Drury

The CCNet is a scholarly electronic network. To subscribe/unsubscribe,
please contact the moderator Benny J Peiser < >.
Information circulated on this network is for scholarly and
educational use only. The attached information may not be copied or
reproduced for any other purposes without prior permission of the
copyright holders. The fully indexed archive of the CCNet, from
February 1997 on, can be found at

CCCMENU CCC for 1999