What is a Comet?

Nature:



A comet is a loose collection of ices and grit weakly held together by its own gravity.  Our idea of what it is is best grasped by imagining a loose pile of snowflakes and frost, mixed with dirt.  There may be larger blocks of stone in the mass as well.  By "ices" is meant not only water ice but all manner of things that are not ice here on earth: methane, ammonia, carbon monoxide and dioxide, cyanide, nitrogen, etc. This is important because it is the evaporation of these into gas that produces all of the phenomena we see in comets:  The coma is essentially a temporary "atmosphere" erupted by this evaporating body, filled with dust that was propelled out along with the vapor.  The tail in turn is an extension of the coma blown back by the solar wind.  The "pile of snowflakes and dirt" is called the nucleus of the comet and is so tiny (less than10 km in diameter usually and 1-3 km for Comet Hyakutake, the Great Comet of 1996) that it is rarely observed, being invisible within its coma.  The nucleus of Comet Hale-Bopp may be 40 miles in diameter - enormous for a comet.  Therefore, what we call a comet is actually just the cloud surrounding the important object, which is the nucleus.   The coma varies is greatly size from comet to comet and from time to time.  Hyakutake's was about 300,000 miles in diameter - greater than the distance from the Earth to the moon, and the tail is many millions of miles long.  The coma of Hale-Bopp is now over 2 million miles in diameter, making it one of the largest visible objects in the Solar System.  One of the famous trivia about comets is that all of this volume contains much less actual material than a small lake, but against the blackness of space it shows very brightly.







Origin:




Birth of solar systems: protoplanetary disks against the backdrop of the Orion Nebula (HST WFPC2 Images)


Comet nuclei originate in the outer solar system, usually far beyond the orbit of Pluto, probably from a hypothesized spherical cloud of comets called the Oort Cloud, after the astronomer Jan Hendrick Oort, who recognized its probable existence.  These comets are by nature PLANETESIMALS left over from the origin of the Solar System.  Astronomers have theorized that the Solar System formed from a cloud of gasses, mostly Hydrogen and Helium, about 4.7 billion years ago.  This PROTOPLANETARY DISK contained also all of the heavier elements and many compounds, which condensed into various graind and flakes which in turn condensed into many trillions of small bodies up to a few kilometers across.  Continuing coalescence raised the size of a few to thousands of kilometers, while in the center the largest coalesced to form a body so massive that it collected the majority of even the Hydrogen and Helium to itself, to such a size that the resultant pressure and heat began nuclear FUSION within the object.  At this point it became a star, the Sun, whose new eruption blew out of the inner solar system all material that could not survive its heat and powerful solar wind.  Lighter gasses like Hydrogen were boiled off of the inner planets.  Only at Jupiter and beyond were the distances great enough and planets massive enough to resist this stripping by the sun.  In this period the planets continued to accumulate the remaining solid parts of the planetesimals until most were absorbed.  The remainders in the inner Solar System are the asteroids, meteoroids, and smaller moons of all the major planets.  In the outer solar system, beyond the greater heat of the sun, original planetesimals remain, unstripped of their lighter components but too sparse to have accumulated into planets.  These are the comets.  They only survive if either their orbit carries them only seldom to our vicinity, or not at all.  We see those that appear seldom, or for the first time.








Importance:





It is this origin that makes comets interesting to astronomers (beyond the shear magnificence of a great comet).   It is becoming more and more accepted that the oceans and the atmosphere - and the material of which most living things on Earth are made - had their origin in comets that rained down on the Earth early in its history.  To see one or a piece of one is to see tho original stuff of which the planets are made, and everything on them, including us. To see one blown into coma and tail is to see the process in microcosm that ocurred en masse when the Sun first ignited.  We who study this know that the proto-Earth's "first dawns", if we can call them such, were lit by the light of the newborn sun and of myriads of comets.







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