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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.

Two comet models

            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 18^th 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 19^th 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]]

Nucleus impact predictions

            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 fly off
• 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.

Specific comet properties

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 density.

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 impacting probe

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

We quote the following event interpretation from the official JPL/U. of Maryland web site: [[ix]]

“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]]

EPH prediction history

            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 commonplace” [[xi],[xii],[xiii]]; 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.” [5]
• salt water in comets and associated meteorites [[xix],[xx]]; “Has anyone analyzed the spectrum of Comet Hale-Bopp for the presence of chlorine?” [[xxi]]
• 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.” [23]
• 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.” [27]

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 (Oort’s discovery)
• 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.

Mission status

The current spacecraft location and mission status, and images following the impact, are available at the NASA/JPL Deep Impact mission site. [[xxxi]] 

2005/05/01