The most famous statistics of the original orbit set was presented by Oort in his fundamental paper on the existence of the cloud of comets surrounding the Solar System (Oort 1950). Those statistics consisted of only 19 orbits, and a high concentration of the reciprocal original semimajor axes of those comets towards a small, positive value was the main argument for the existence of the cometary cloud. Oort repeated this work lately, in a paper published as a chapter in the book on the physics of comets, edited by W. F. Huebner (Oort 1990). He accumulated data for 66 comets, restricted to those with a perihelion distance greater than 1.5 AU to be sure that he excluded any comets that may be influenced by non-gravitational effects. In our set (last updated in September 1999) we have almost doubled this number and we present a comparison of those three distributions in Table 2.
It has been well known since the publication of the Oort (1950)
paper that dynamically "new'' comets, entering the planetary region
for the first time, should have the smallest 1/a values. A widely-used
definition of dynamically "new'' comets, (e.g. Oort & Schmid 1951;
Kresák 1977, 1994;
Marsden et al. 1978; Oort 1990;
Levison 1996)
states that their
AU-1, which is
equivalent to
AU. Table 3 shows the division
of the whole set of 327 original orbits into dynamically "new'' and "old''
comets according to the above definition, with additional division with
respect to the orbit quality class
(Marsden et al. 1978). The statistics for the restricted
(q>1.5 AU) sample is also shown separately.
class 1 | class 2 | classes 1 + 2 | ||||
q [AU] | >1.5 | ![]() |
>1.5 | ![]() |
>1.5 | ![]() |
"new'' | 60 | 31 | 6 | 16 | 66 | 47 |
"old'' | 56 | 48 | 23 | 87 | 79 | 135 |
all | 116 | 79 | 29 | 103 | 145 | 182 |
Total | 195 | 132 | 327 |
However, treating all comets with AU as new is an obvious
oversimplification. Some of those comets may have passed near the Sun
during earlier returns. One of the perturbing agents acting on those
comets, namely the galactic disk tide, causes periodic changes of the
perihelion distances with periods of order of
104-105years. Depending on the semimajor axis and geometrical configuration
with respect to the galactic plane, the perihelion distance can be
significantly changed even during one orbital period, so we decided to
follow backward the motion of those comets to their previous perihelion
passage.
Copyright ESO 2001