“It’s this celestial bucket brigade, he notes, that allows a select group of comets to grace the skies above Earth, flaunting their dusty tails as they deliver key organic compounds into our atmosphere.”
Someone once said comets are like cats — they have tails and they do what they want. As of this writing (late Nov. 2013) it appears that Comet Ison has not survived its close passage around the Sun intact and will not, therefore, provide the hoped for celestial display that might inspire folks other than astronomers to take a deeper interest in cosmic events. This is not to say that important knowledge will not be gleaned from its demise, but it is disappointing nonetheless. What we have seen is a comet in the final stages of its life cycle, disintegrating into a stream of cosmic dust and boulders. Let me elaborate upon what I mean by the life cycle of a comet, from its genesis to its final termination as a celestial body.
The life of a comet falls naturally into five stages, or phases, of existence. Its birth, commensurate with that of the Solar System, goes back some 4 and a half billion years. This first stage in its existence involves the aggregation and condensation of a cometary nucleus out of the primordial cosmic material, fragments of the proto-planetary disk left over after the construction of the planets and their satellites, the ‘stone the builders rejected’, so to speak. This material, of which comets are composed, is a remarkable mix of exotic ingredients, including crystalline rock ranging from dust size, through the size of boulders, up to the size of mountains; various frozen gases such as carbon dioxide, carbon monoxide, methane and ammonia; organic compounds such as methanol, ethanol and formaldehyde; also hydrocarbons and amino acids— all frozen together in a matrix of ice and finally enclosed within a crust of cemented cosmic dust and rock several meters thick, like the shell of an egg. This first stage of the cometary life cycle I call Aggregation.
The second stage begins after ejection of these remnants of creation from the inner Solar System through gravitational interactions with the large outer planets, and their subsequent banishment to the far reaches of the outer Solar System into two vast reservoirs, the Kuiper belt and the Oort Cloud. The Kuiper belt, or disk, named after Dutch astronomer Gerard Kuiper, is a region of comets extending from just outside the orbit of Neptune at about 30 Astronomical Units (an AU is the distance from the Sun to the Earth, about 93 million miles) to about 55 AU. It lies more or less in the Plane of the Ecliptic, the plane of the planetary orbits. The number of comets residing in the Kuiper belt is estimated to be in the hundreds of billions! The Oort cloud, named after astronomer Jan Oort, who first predicted its existence in the 1950s, is a huge spherical shell of comets surrounding the Solar System at a distance of 5000 to 10,000 AU. (By contrast, it is over a quarter million AU’s to the nearest stars.) There may be as many several trillion comets residing in the Oort cloud! Both reservoirs slowly revolve about the Sun in huge circular orbits and are part of the Solar System in its entirety. Here, in these regions, comets lie in a state of dormant deep freeze, a sort of cosmic hibernation which represents the second stage of their life cycle. We will call this 2nd stage Hibernation. (Wikipedia, under the heading of Oort cloud has a good artists illustration showing both the cloud and the belt.)
The comets within both reservoirs orbit in a quasi-stable state, somewhere between unstable and metastable, that is, it requires only minimal force to dislodge them from their orbital position, but for most of the time there exists in the vastness of space no force sufficient to disturb their delicately balanced arrangement. However, from time to time something does happen that disturbs the deep sleep of these extraordinary bodies. In the realm of the Oort cloud it may be an errant star, a nearby supernova, or the passage of the Solar System across the galactic plane— each can potentially trigger the dislodging of comets from their place of cosmic slumber and send them on a long, slow, spiraling descent towards the Sun. Comets commencing their journey from the Oort cloud may take up to several million years before reaching the Sun and can approach from any direction. These are the long-period species of comet that travel into the inner Solar System on an open-ended parabolic orbit, rendezvous one time only with the Sun, and then journey back out into the great deep of space never to return, at least within the time of mankind upon the Earth.
Read more at SacredGeometryInternational.com
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