Abstract. The Deep Impact spacecraft is on
its way to an encounter with Comet Tempel 1. Before arrival, it will release a
370 kg probe that will impact on the comet at a very high speed. The impact
should reveal the true nature of the comet nucleus, which is poorly understood
in the current mainstream model. Predictions (in order of increasing strength of
nucleus material) range from the probe passing all the way through the comet, to
fracturing and splintering the nucleus into many fragments, to getting stuck in
a soft rubble pile comet, to making a huge, deep crater, to making a small,
shallow crater. Mainstream experts are divided about which outcome to expect.
But one model, the satellite model for comets, a corollary of the exploded
planet hypothesis, is unambiguous in its prediction: The comet nucleus is a
single, solid asteroid. The impact will leave a small, shallow crater perhaps
10-20 meters in diameter, will produce no new jet, and will have no lasting
consequences on the comet. It will simply produce an impact flash as the probe
vaporizes, then will cause the comet’s coma to temporarily brighten as new
carbonaceous dust is ejected from the asteroid regolith and the impact crater.
Satellites of the comet will probably not be seen because the nucleus is so
elongated that, like Eros, the local gravity field of the nucleus is unstable
for large satellites.
Mainstream astronomers tell us that comets are
“dirty snowballs”, made mostly of frozen water ice with lots of dirt mixed in.
That theory was advanced in 1950 by astronomer Fred Whipple to explain the basic
properties of comets – a central nucleus surrounded by a vast, spherical coma
streaming off into a long tail pointing away from the Sun. Since the late 18th
century, comets were hypothesized by Laplace to be leftovers from the primeval
solar nebula, and would therefore be the oldest and most pristine objects in the
solar system. But recent physical, chemical, and photometric evidence suggests
that perhaps comets are more like snowy dirtballs than dirty snowballs.
Not long after the first and second asteroids
(Ceres and Pallas) were discovered at the start of the 19th century,
Heinrich Olbers hypothesized that asteroids were the products of a former major
planet orbiting between Mars’s and Jupiter’s orbits that exploded into tens of
thousands of fragments. In 1814, Lagrange noticed that the exploded planet
hypothesis (EPH) also provided a natural explanation for the extreme elongation
of comet orbits, many of which are so thin as to be nearly linear. As the EPH
model has evolved in modern times, it implies that comets started out identical
to certain types of asteroids at the time of the most recent explosion, 3.2
million years ago. [[i],[ii],[iii],[iv]]
But by spending most of that time far from the Sun, comets have retained most of
the volatile elements that have long since been baked out of asteroids by their
proximity to the Sun. So in the EPH, comets are young, solid, rocky bodies
surrounded by gravitationally bound dust and debris fragments of all sizes from
the original explosion. The water ice present is mostly interstitial. This
vision of the origin and nature of comets is called the “satellite model”. [[v],[vi]]
The physics of exploding planets is now arguably better understood than that of
stellar explosions. [[vii]]
These two possibilities are strongly
contrasting views of the nature of comets. But if all goes well, on 2005 July 4,
we will have answers. A NASA/JPL spacecraft named Deep Impact is now on
its way to Comet Tempel 1. The spacecraft carries a probe that will be released
one day before encounter and will then impact the comet nucleus while the main
spacecraft speeds past and photographs what happens during a critical 13-minute
period. What the camera will see depends on the nature and composition of the
nucleus. The anticipated possibilities are: [[viii]]
- Probe flies through comet and comes out the other side
- Probe fractures comet into thousands of pieces that
- Probe enters jello-like, compression-controlled rubble
pile and makes a small, deep crater
- Probe impacts on weak, gravity-controlled surface and
makes a huge, medium-depth crater
- Probe vaporizes on rocky, strength-controlled surface
and makes a small, shallow crater
These are listed in order of increasing strength of
nucleus. The dirty snowball model is not sure what will happen, but has an
explanation ready for each possibility. In that way, the model will not be
placed at risk of falsification. The gravity-controlled scenario might be
considered the model’s “best guess”. The satellite model, by contrast, is clear
that the nucleus is solid rock and will follow the last of the five scenarios.
Size, shape, rotation: Comet Tempel 1’s mean nucleus
diameter is about 6 km. Its elongation is apparently more than 3:1, as judged by
the comet’s lightcurve. That would make the nucleus’s own gravity field fairly
unstable over the long term. Large satellites able to raise tidal forces would
be removed from orbit, many of them falling gently onto the nucleus as boulders,
and only coma dust and small debris would remain in orbit. The rotation period
appears to be 41.5 hours, but the rotation axis remains unknown.
Strength: The “breakup” of Comet Shoemaker-Levy 9 in
Jupiter’s gravity field implied that comets are virtually strengthless because
the tidal forces involved were so weak. The strength of that comet inferred by
the dirty snowball model was less than 1000 dynes/cm2, corresponding
to tidal forces comparable to gently blowing on a bit of cigar ash. In the
satellite model, the fragments were orbiting and never part of the nucleus, so
nuclear strength is irrelevant to what happened. The tidal forces from Jupiter
were easily sufficient to strip away satellites of the nucleus because they were
never pieces of the nucleus in the first place, and were only loosely bound to
it by the nucleus’s weak gravity. The spacecraft probe crash will reveal
primarily the strength of the nucleus of Comet Tempel 1.
Density: The dirty snowball model sometimes argues
for comet nucleus densities as low as 0.1 g/cc, and sometimes as high as 1.0
g/cc – the density of water or less. The satellite model indicates that comets
have typical asteroid densities of 1-3 g/cc – the density of light rock. Neither
the spacecraft nor the probe is expected to be able to determine this comet’s
Composition: The dirty snowball nucleus is primarily
water ice with carbonaceous material mixed in. The satellite model nucleus is a
rocky asteroid with a light dust regolith perhaps a meter or so deep, and some
salt water or ice, mostly interstitial. In the latter model, the nucleus surface
would be loaded with boulders and roll marks, as was the surface of asteroid
Eros, because the comet nucleus is too elongated for the fragment satellites of
significant size to remain in orbit. They would decay onto the irregular-shaped
nucleus through the action of tidal forces. As for the water or ice, many kinds
of salt can occur in planetary oceans. But sodium chloride is perhaps one of the
most common. Sodium has already been detected in comets along with “an unknown
parent molecule”. The satellite model expects that the parent molecule is most
probably chlorine, which should then be seen in the impact ejecta as a fairly
unique indicator of a planetary origin for comets. However, there are 10 major
salts that have sodium as a constituent, and any one of them is a possibility,
depending on the unknown composition of the comet parent body. Nevertheless most
of the possibilities, like chlorine, are indicators of processing in a planet,
rather than of the pristine, unprocessed, solar-composition material expected by
the dirty snowball model. Salts would be a surprise to that model.
Activity: Comets can be active or dormant. Some
asteroids are believed to be extinct comet nuclei. There is presently no known
way to tell the difference between “real” asteroids and ex-comets. In the dirty
snowball model, real activity ceases as the supply of volatiles diminishes over
time. In the satellite model, apparent activity ceases when most of the orbiting
debris cloud has escaped or decayed onto the nucleus and most of the remaining
coma has been removed by solar radiation pressure and solar wind.
Jets: The dirty snowball model hypothesizes that the
coma surrounding a comet nucleus is created and fed by jets and geysers on the
nucleus. These are unpredictable because little is known about their cause or
location except that the location should rotate with the comet. In the satellite
model, the coma is pre-existing, orbiting debris from the parent body explosion.
“Jets” are seen in spacecraft close-up images of a few comets. The satellite
model interprets these as “flashlight beams” from bright spots on the nucleus
(probably frozen volatiles) that preferentially reflect sunlight back toward the
Sun. It is a known property of asteroids that they reflect far more light back
toward the light source than in any other direction, potentially producing beams
of light pointing sunward that become visible when shining through a dusty coma.
As each bright spot rotates away from the mid-day portion of the nucleus, the
beam it generates will dim. But as soon as other bright spots rotate into the
mid-day region, they will create their own beams.
The probe to be released from the
spacecraft weighs 370 kg. The release will occur about a day before impact at a
distance of 864,000 km. When they encounter one another, the relative speed of
probe and comet will be 10.2 km/s. The impact should then release 19 gigajoules
of energy, which corresponds to the explosion of about 4.8 tons of TNT.
Impact is scheduled for about 0600
UTC on 2005 July 4. In some of the dirty snowball scenarios, the impact itself
will not be seen, but the debris thrown up might raise the comet to theoretical
naked-eye visibility. (Hawaii is in the most favored visibility zone.) But in
the satellite model, the probe will be completely vaporized on impact, producing
a bright flash. And the crater-forming event should throw up debris and regolith
dust, but hundreds of times less of it than in a gravity-dominated scenario.
The impact crater is predicted to be 100-200 m
wide and 25-50 m deep with a wide ejecta cone in the gravity-dominated scenario.
But the crater will be more like 10-20 m wide and a few meters deep with a
narrow ejecta cone in the satellite model scenario. The dirty snowball scenario
might make the comet reach naked-eye brightness. But that is much less likely in
the satellite model scenario.
Interpretation of results
the following event interpretation from the official JPL/U. of Maryland web
“The cratering process will help
reveal what type of material makes up the nucleus (or at least the outer layer),
and therefore how the comet formed and evolved. If the crater turns out to be
gravity-dominated, this lends evidence to the theory that the comet's nucleus
consists of porous, pristine, unprocessed material, and that the comet formed by
accretion. If, however, the crater turns out to be strength-dominated, then this
suggests that the material of the nucleus is processed somehow, resulting in a
comet that can hold together better under impact. This would mean that it is not
the pristine, untouched material of accretion. A combination is also possible:
The initial crater formation might be strength-dominated (suggesting a processed
outer shell to the nucleus) while the bulk of the crater is gravity-dominated
(suggesting that the impactor has punched through this outer shell into the
pristine material below).”
In brief, there is a
general expectation that the impact will be gravity-dominated, while respecting
the possibility that it might be strength-dominated. Either result will be
argued as supporting the dirty snowball model for comets, even though the latter
implies that the comet has been heavily processed and is not primordial. Just as
Kuhn predicts for the nature of paradigm shifts, one model will not overthrow
the other on merit, but the incumbent model will be patched until it so closely
resembles the competitor that no important distinction remains. That helps to
maintain the illusion that the progress of science is always forward. [[x]]
The exploded planet hypothesis has already made
a number of successful predictions in defiance of conventional wisdom at the
time, and has made no failed predictions. This ability to predict new phenomena
is the main characteristic expected of any scientific hypothesis: usefulness for
prediction and understanding. The satellite model for comets is another
prediction of the EPH, which is the reason why the satellite model can make
predictions about the Deep Impact mission with confidence, whereas the
dirty snowball model must hedge its bets.
Here are some recent examples of successful
predictions made by the EPH related to comets and asteroids. The initial
citations in each paragraph are to the findings, and the citation following the
verbatim quotes is to actual prediction.
- that satellites of asteroids are “numerous and
p. 445: “… such satellites are both numerous and commonplace.”; p. 459: “As
debris flies away from the disintegrating planet in all directions with a
great range of velocities, the radius of the gravitational sphere of
influence of each body expands with its increasing distance from the parent
planet, limited only by the proximity of another body, or ultimately by the
sun. As these radii expand, many other smaller bodies and much debris are
trapped within the sphere of influence; for a certain range of relative
velocities, escape will then no longer be possible. Hence numerous minor
satellites around each minor planet would seem to be an inevitable result of
such a planetary breakup process.” [[xiv]]
- that satellites of comets exist too [[xv],[xvi],[xvii],[xviii]];
“This paper explores several puzzling features of comets which do not fit
easily into conventional cometary models, but which can be satisfactorily
explained if it is assumed that comets have a full range of gravitationally
bound masses, from dust size to the size of the nucleus, in orbit around the
principal nucleus.” 
- salt water in comets and associated meteorites [[xix],[xx]];
“Has anyone analyzed the spectrum of Comet Hale-Bopp for the presence of
- numerous boulders and roll marks on Eros [[xxii],[xxiii],[xxiv]];
“The most common fate of objects in unstable orbits is to impact gently on
the surface, usually at a grazing angle, followed by rolling until the
orbital angular momentum (from orbital speeds of typically a few meters per
second) is dissipated, then coming to rest on the surface. … The chances of
intact objects coming to rest on the surface are nil except for satellites
because the typical relative speeds between field asteroids are of order 5
km/s. Such speeds would result in highly destructive, crater-forming
impacts. So finding surface "satellites", especially with tell-tale roll
marks, when the NEAR spacecraft goes into orbit around Eros next year is
still a good way to distinguish between the standard model and the exploded
planet hypothesis.” 
- times, locations, and meteor rates for meteor storms
and outbursts [[xxv],[xxvi],[xxvii]];
“The plot shows both the outburst at the predicted time with the predicted
duration, and enhanced activity for the annual shower over the two days
plotted as well. (ZHRs are 50-60 in a typical year.) This of course lends
additional credence to the larger model underlying the predictions, the
“exploded planet hypothesis”, which has these meteors escaping from orbits
in a “debris cloud” around a comet nucleus instead of ejected into space via
jets on the comet nucleus.” 
These additional lines of evidence
are direct corollaries of the EPH, and argue strongly for the EPH over the
several standard models it would replace. Further explanations and supporting
data appear in reference [[xxviii]]:
- explosion signatures for asteroids (found first for
exploded artificial satellites)
- strongly spiked energy parameter for new comets
- distribution of black material on slowly rotating
airless bodies (the “black axiom”)
- splitting velocities of comets (indicates fragments
were in orbit before separation)
- Mars is a former moon of an exploded planet
(hemispheric dichotomy, pole shift, etc.)
Finally, it has just recently been
discovered, to the amazement of all cosmologists, that the cosmic microwave
radiation has a local component, as the EPH predicted. [[xxix]]
This 1993 prediction reads as follows:
“The planetary breakup hypothesis implies
that a fireball originated in the solar system a few million years ago,
expanding in all directions. Although the flux from it would remain uniform as
seen from the inside, it should eventually cool to the limiting temperature set
by interstellar radiation: about 3 degrees Kelvin. Radiation from this fireball,
just as for the hypothetical Big Bang fireball, should be of the blackbody type.
In other words, the planetary breakup hypothesis seems to predict a uniform
microwave blackbody radiation just like the one which is observed, but of
relatively local origin.” [[xxx]]
With the documented track record of successful predictions
the EPH has now established, it is small wonder that professional astronomers
are no longer willing to make wagers with EPH proponents about the outcome of
EPH predictions where the results are not yet known. But sadly, research funding
is still being poured almost exclusively into mainstream theories, with none
designated to validate (or invalidate) the EPH or satellite model.
The current spacecraft location
and mission status, and images following the impact, are available at the
NASA/JPL Deep Impact mission site. [[xxxi]]
Artist's concept of Deep Impact mission. ©2005
by Boris Starosta
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