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Dec 06, 2004
"D-Day" for Comet Theory

"Deep Impact" is not just the name of a Stephen Spielberg disaster flick. It is now the name of a spacecraft scheduled to depart later this month for an unprecedented encounter with a comet nucleus. And not just a quiet rendezvous, but an explosive meeting in space.

The object of the mission is Comet Tempel 1, which the Deep Impact craft should reach when the comet is some 133 million km (83 million miles) from Earth. If all goes as planned, the craft will launch an 370 kg (820-pound) copper "impactor" toward the nucleus, striking the surface at about 23,000 miles per hour. According to NASA scientists, the result should be a release of energy equivalent to that of exploding 4.8 tons of TNT. Fittingly, the scheduled date for the celestial fireworks is July 4, 2005.

If proponents of the electric universe are correct, however, the fireworks will reverberate through science for years to come. These theorists expect one of the great scientific surprises of the 21st century.

This will be the first experiment to see how a comet reacts to a man-made event. It will also be the first time we have witnessed a hyper-velocity impact on a celestial body. Astronomers expect the impact to form a crater. It is uncertain, however, what size and type of crater will form because it is uncertain exactly what a comet is made of. There are three likely scenarios that the crater formation can take:

Scenario 1. If the strength of the comet nucleus is weak the ejecta cone spreads outwards at an angle of around 45º-50º from the surface of the comet. The cone's base remains attached to the comet nucleus. The majority (roughly 75%) of the material will fall back down onto the surface of the comet, forming a large-diameter ejecta blanket. In this model, the crater may be as large as a football stadium (around 200 meters in diameter), and 30-50 meters deep.

Scenario 2. If the comet nucleus is rocky and strong the ejecta cone will be at a higher angle (around 60º). The cone's base will detach from the crater, and may detach from the comet entirely. Less material (around 50%) will fall back to the surface of the comet in this scenario, yielding a smaller ejecta blanket. In this model, the crater will be much smaller, on the order of 10 meters or less. The predictions of the volume of ejecta produced then differ by roughly a factor of 1000.

Scenario 3. If the comet material is porous then most of the impactor's energy and momentum are absorbed in the process of compression and heating. There is less energy available for excavation, which results in a much smaller diameter crater than expected. The crater will be deep, but produce a very small ejecta cone.

The comet material projected into space will cause the halo to expand and its brightness to increase dramatically. A gaping hole in the nucleus will then allow astronomers to observe pristine material in the comet's interior, believed to represent the constituent "stuff" from which planets were formed billions of years ago.

NASA scientists assure us that comets are chunks of ice, mixed with an undetermined amount of dust and gravel or rocky debris. They are "dirty snowballs." What else could create the cometary display than sublimating ice? So scenarios 1 and 3 are most likely in that model.

Those who support the electric model answer that most comets are solid bodies derived from planets and not the reverse. Scenario 2 is more likely, although 1 and 3 are not ruled out. Their model of a comet does not require evaporation of ices to produce comet tails. Comet surface are excavated by electric arcing as comets move radially through the weak electric field of the Sun, Dust and vaporized surface material is ionized and accelerated into space like the "volcanoes" of Io. Short-lived negative ions are observed emitting forbidden spectroscopic lines indicative of a strong electric field.

Unfortunately, Tempel 1 is not a very active comet. It's elliptical orbit keeps it between Jupiter and Mars. So its radial motion with respect to the Sun is limited and electrical activity is likely to be subdued. The copper impactor may hit as planned without first suffering an electric discharge from the comet nucleus. The most likely anomalies will then be found in the motion of the impact ejecta, which will move under the influence of electrical forces rather than gravity and not form the expected fallout pattern. It will also give wrong estimates of the nucleus material's density. This kind of thing has happened in the past, where rocky looking asteroids appear to have the density of fluff.

However, in the event there is a discharge before impact we may expect to see an early high-energy flash like those of the fragments of comet Shoemaker-Levy 9 at Jupiter. If the arc vaporizes the copper projectile before impact a new electrical crater should appear on the comet's surface.

On the other hand, any copper metal reaching the surface of the comet will act as a focus for a cathode arc. And copper can sustain a much higher current density than rock or ice. There would then be the likelihood of a single arc with a single jet, until the copper is electrically "machined" from the comet's surface. Other signatures of an electrical discharge could be a burst of electrical noise across a wide spectrum, a "flash" from infra-red to ultraviolet, the enhanced emission of x-rays and anomalous ionization in the vicinity of the projectile. The energy of a mechanical impact is not sufficient to generate these effects.

Electrical discharges through the body of a poor conductor can be disruptive and are probably responsible for the breakup of comets. It is not necessary for them to be poorly consolidated dust and ice and to simply fall apart. So there is a finite chance that astronomers will be surprised to see the comet split apart, if the projectile reaches the surface of the comet and results in an intense arc.

Deep Impact, therefore, could provide an acid test of competing comet models.

Whatever happens, the view should be stunning. The High Resolution Imager aboard Deep Impact will be one of the largest interplanetary telescopes ever flown in space. Through the antennas of the Deep Space Network, the Deep Impact craft will transmit visual and spectroscopic data on the event to Earth. For the cosmic electricians, the real "snowball" in all of this will be the effects on theoretical science. In an instant, an electrically discharging comet will change the big picture of space.



David Talbott, Wallace Thornhill
Amy Acheson
  CONTRIBUTING EDITORS: Mel Acheson, Michael Armstrong, Dwardu Cardona,
Ev Cochrane,   Walter Radtke, C.J. Ransom, Don Scott, Rens van der Sluijs, Ian Tresman
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