There are two populations of comets - high and low inclination Original 2/23/2005 Rev. 7/21/2006

Note: This paper was originally written in 2005 during the flyby of Comet Machholz C2004/Q2.

Knowledge of the two populations of comets, that can be separated by their inclination to the orbital plane, can increase amateur's enjoyment of these celestial visitors.

Common experience does suggest that comets would orbit nearly in the plane of the solar system. Planets that we can see with the naked-eye nearly follow the ecliptic. So why comets not be expected to follow this same pattern? This common experience does not extend to comets because their are two populations of comets with different sources in the Kupier Belt and Ort cloud.

Figure 1-c (in conjunction with 1-a and 1-b) in the following thesis paper, available online, is helpful in understanding the distribution of the inclination of comets.

There are two populations of comets, when their distribution is measured by the characteristics of eccentricity, inclination and semi-major orbital axis size. The first large population of comets have nearly circular orbits (low eccentricity), short orbital periods (< 200 years) and relatively small orbits. The second population - those with higher eccentric orbits - are evenly distributed in inclination, are more distant from the Sun and have long orbital periods ( > 200 years).

These populations correspond to Kupier Belt and Ort Cloud. See Figure 2 in Willman, Jr. (1995), above.

The natural distribution of the inclination of comets varies greatly. It is not unusual that a comet like 2005's Comet Machholz C2004/Q2 has a higher inclination to the plane of the solar system - an observation that is counter-intuitive to one's common experience gained from watching planets.

Comet Machholz appears to be in the long-period Oort cloud category.

The following 100 year animation of comets at the Harvard Minor Planet Center may also give you a better feel for the Kuiper Belt and Oort cloud groups of comets.

The inclination of some individual long and short period comets can also be graphically viewed through other Minor Planet Center animations at:

Compare a long-period high-inclination comet Hyakutake (C/1996 B2):

with short-period low-inclination comet Tempel-Tuttle 55P:

These general categories are statistical and not absolute. You may find a short or long period comet that has inclination characteristics that are statistical exceptions.

The absolute magnitude of comets, asteriods and planets is not recorded on the same scale used for stellar magnitudes.

Prepared by K. Fisher 7/2006