Comet compositions show striking differences

Two of the most common materials found in Tempel 1 are an iron-silicon mineral called ferrosilite and a glassy form of a magnesium-iron mineral called olivine, which make up 33% and 17% of the comet, respectively, according to observations by the Spitzer Space Telescope. However, these minerals are entirely absent from the Wild 2 samples analysed so far... It is not clear how to explain this difference, says Stardust mission leader Donald Brownlee of the University of Washington in Seattle, US. But he says one possibility is that the material on Tempel 1 was chemically modified by ancient collisions -- the Deep Impact spacecraft struck Tempel 1 near two impact craters. Another possibility is that the two comets were born with different compositions, he says. Both are thought to have formed in the Kuiper Belt, a ring of icy objects beyond Neptune, and both are a few kilometres wide... By analysing atomic isotopes from Wild 2, the Stardust team has also verified that the comet is made up of material from our own solar system as well as that expelled from other stars. But the team has proven that some minerals once suspected of coming from other stars were actually locally grown. Pyroxene, which forms at temperatures greater than 1000 K, is one such mineral: "It almost certainly came from the innermost region of the solar system," Brownlee told New Scientist. That bolsters a theory that in the first few million years after the Sun formed, magnetic currents near the infant Sun lifted material out of the disc from which the planets formed and channelled it to the outer solar system. There, it would have fallen back into the disc to form comets such as Wild 2.

Pyro7480's link shows a comet on a hyperbolic trajectory, which is a comet just passing through. They are unusual, but do occur. Comets on parabolic trajectories are said to be confined to the Solar System. Presumably a comet could (while passing through) have one or more encounters (say, with Jupiter), thus altering their trajectories, leading to a parabolic trajectory. But I dunno.

http://www.thunderbolts.info/tpod/2006/arch06/060217deepimpact3.htm

"Today our view of the comet is greatly enhanced by the technological achievements of the twentieth century; but critical thinking—the ability to question theoretical assumptions—has collapsed to the point that astronomers barely notice the incongruities in coma behavior."

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From An Address Before the Graduate College Forum of Princeton University on October, 1953.

by Immanuel Velikovsky

"I like to tell this story. Once, in the twilight hour, a visitor came into my study, a distinguished-looking gentlemen. He brought me a manuscript dealing with celestial mechanics. After a glance as some of the pages, I had the feeling that this was the work of a mathematical genius. I entered into conversation with my visitor and mentioned the name of James Clerk Maxwell. My guest asked: 'Who is he?"

Embarrassed, I answered: "You know, the scientist who gave a theoretical explanation of the experiments of Faraday."

'And who is Faraday?" inquired the stranger.

In growing embarrassment I said: "Of course, the man who did the pioneer work in electromagnetism."

"And what is electromagnetism?" asked the gentleman.

"What is your name?" I inquired.

He answered: "Isaac Newton."

I awoke. On my knees was an open volume: Newton's Principa.

This story is told to illustrate what I have said before. Would you listen to anybody discuss the mechanics of the spheres who does not know the elementary physical forces existing in nature?

But this is the position adopted by astromomers who acclaim as infallible a celestial mechanics conceived in the 1660's in which electricity and magnetism play not the slightest role."

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