Comet orbital distribution

Figure 2 Distribution of l/a in units of 10 AU the inverse of the semimajor axes (half length of the orbital ellipse) of 264 LP comets. The peak near zero contains dynamically new comets coming directly from the Oort cloud for the first time. Comets with larger values of l/a have previously entered the planetary region of solar system and have been perturbed into smaller lower-energy orbits (source Marsden, 1989b).

Originally it was thought that SP comets were Oort cloud comets that had been captured into short-period orbits following a close encounter with Jupiter. This process can occur and even explain why SP comets usually have prograde orbits with relatively low inchnations. However, Fernandez (1980) pointed out that this source could not produce the number of SP comets that are observed, and suggested that the SP comets were derived from a disk-hke distribution of bodies beyond Neptune. It was shown by numerical simulations that this trans-Neptunian distribution of comets could quantitatively supply the SP flux and explain their inclinations (Duncan et ok, 1988). 

Distributions in the solar system. More data on volatiles throughout the solar system are clearly required to confidently describe the volatile acquisition history of the terrestrial planets in the proper context. There are several unknown values for the solar composition, including the nitrogen-and carbon-isotope compositions. The compositions of comets from different orbital distances are needed to assess the extent of radial transport of volatiles late in accretion history. In addition, the causes of carbon- and nitrogen-isotope variations in chondrites must be better understood. While it is clear that the Earth cannot be constructed simply by mixing of different meteorite classes, it is not yet possible to unambiguously extrapolate to the volatile compositions of protoplanetary materials. 

The individual dust grains (technically meteoroids once they have left the comet) move along orbits that are similar to that of the parent comet. Gradually, over the course of several hundreds of years, the meteoroids form a diffuse shell of material around the whole orbit of the parent comet. Provided that the stream meteoroids are distributed in a reasonably uniform manner, a meteor shower will be seen each year when the Earth passes through the stream (Eig. 1). The shower occurs at the same time each year because the position at which the meteoroid stream intersects Earth s orbit does not vary much from one year to the next. There are long-term variations, however, and the days during which a shower is active will change eventually. 

Following another path of bifurcation in the hierarchy of complexity, one may now proceed from matter particles to the Earth and the Solar System, the Milky Way (that contains about as many stars - a hundred billion - as there are cells in a human brain), and the whole Universe. This latter involves regular stars (some with planets, satellites, asteroids and comets), neutron stars and dust clouds, which make up galaxies and clusters, and such strange objects as quasars, pulsars or black holes. The regularity of the distribution of planets in our solar system has inspired to Greek philosophers a correspondence with the musical scale that Kepler (who set up the laws of planetary motion) called the "harmony of the spheres". By transposing the orbital velocities of... 

The group of small solar system bodies, SSSBs, comprises asteroids, comets and smaller objects. The most part of the smaller objects is concentrated in belts. The orbits of the objects of the Main Asteroid Belt between Mars and Jupiter and the Kuiper Belt outside the orbit of Neptune are concentrated near the ecliptic (plane of the solar system), the objects of the Oort cloud are distributed spherically and extend as far as to 50 000 AU from the Sun. 

Already in 1932 E. Opik assumed that long-period comets originate in a cloud at very large distance from the inner solar system. Since the long-period comets have random orbits, such a cloud must be distributed spherically around the solar system. Oort, 1950 [255] postulated its existence from the volatile composition of comets and their orbits (see Fig. 5.2). He found a peak in the numbers of nearly isotropic distributed comets with apheUa of roughly 20 (X)0 AU. 

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