The Oort Cloud is a spherical halo of comet nuclei gravitationally bound and surrounding the Sun to a distance of 2.000 / 5.000 up to 100.000 / 200.000 AU (a third of the way to the next closest star), proposed by the Dutch astronomer Jan Oort in 1950.
While direct evidence for the existence of the Oort cloud is currently impossible to obtain, the idea is widely accepted as an explanation for the observed frequency and orbital characteristics of new long-period comets.
The Oort Cloud is believed to contain a population of up to 1012 comet nuclei!
Gravitational perturbations by passing stars may dislodge Oort Cloud nuclei, causing them to fall sunwards and perhaps to pass through the inner Solar System.
A comet of this nature may return to the far depths of space on a long-period orbit, for example C/1996 B2 HYAKUTAKE, which is not expected to return for 17.000 years !!!
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| http://www.hvezdarnacb.cz/de/2/79/wallpapers |
Refinements of the model from the 1950s onwards led to the suggestion that the Oort Cloud may become more concentrated towards the ecliptic plane at distances of 10.000 to 20.000 AU from the Sun, extending inwards to join the edgeworth-Kuiper Belt.
Interesting sites:
http://abyss.uoregon.edu/~js/ast121/lectures/lec24.html
Reference:
PHILIP’S ASTRONOMY ENCYCLOPEDIA
First published in Great Britain in 1987 by Mitchell Beazley
under the title The Astronomy Encyclopedia (General Editor
Patrick Moore)
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