Comets – where are they ?

 

Updated 2017 January 14

 

Contents

 

There have been a number of schemes classifying cometary orbits and this presentation outlined them and gave examples of the various classes of comets.

 

Historical classification schemes

 

Historically comets were classified as short, intermediate or long period – the upper limit of short period comets varied from 13 to 39 years and the intermediate class sometimes included long period comets (periods > 200 years) and sometimes did not. Figure 1 shows the accepted classification immediately prior to 1996.

 

Figure 1

 

Figure 1. Pre-1996 classification of cometary orbits

 

Long period comets had orbits with random inclinations and short period comets orbited closer to the plane of the ecliptic. Long period comets fell into two categories – new; those with semi-major axis > 10,000 AU entering the inner Solar System for the first time and returning; comets with semi-major axis < 10,000 AU which had made a previous pass. Short period comets were divided into Halley type with periods between 20 and 200 years and Jupiter family comets with periods less than 20 years.

 

Harold Levison’s 1996 classification

 

In his 1996 paper, Comet Taxonomy, Harold F. Levison proposed the classification scheme shown in Figure 2.

 

Figure 2

 

Figure 2. Harold Levison’s Comet Taxonomy

 

The Tisserand parameter, TJ – a measure of the influence of Jupiter on cometary orbits, is used to define the various classes of comets in this scheme. The Tisserand parameter is calculated from the orbital elements of Jupiter and the comet in question and can also be used to relate comets seen at different apparitions i.e. same value of TJ indicates that the comets may be one and the same.

 

Nearly Isotropic comets can enter the inner Solar System at any inclination and most have semi-major axis between 10 and 100,000 AU.  New comets are those visiting us for the first time whereas, as their name suggests, Returning comets have been here before and will have evolved, by planetary encounters, from the New category. Returning comets are sub-divided into two categories - Halley type comets which have less random .inclinations, semi-major axis < 40 AU and are trapped in Mean Motion Resonances (MMRs) with Jupiter. MMRs occur when the orbital period of a comet and a planet are close to the ratio of two small integers. Those comets with semi-major axis >40 AU (that of Pluto) are classified as External as such a large semi-major axis means they are unlikely to be trapped in an MMR.

 

How do we know whether or not a comet is of the New or Returning variety ? Figure 3 shows the distribution of the inverse of the semi-major axis, a, for comets in Brian Marsden’s 1992 catalogue. The peak includes those comets which are passing through the inner solar system for the first time and have semi-major axis of approximately 20,000 AU - i.e. New comets. On making this first pass their values of 1/a will, on average, be altered, due to planetary encounters, by the amount indicated by ‘Size of the average planetary kick’. Thus, on subsequent visits, their orbits will fall outside of the peak and they will be identified as Returning comets.

 

Figure 3

Figure 3. Distribution of the inverse of the semi-major axis for long-period comets

 

Comet C/2010 X1 (Elenin), Figure 4, is an example of a New comet. This has failed to live up to expectations and may be breaking up (most recent images show what appears to be an elongated debris cloud).

 

Figure 4

 

Figure 4. New comet C/2010 X1 Elenin

 

Ecliptic comets, as their name suggests, have low inclination, between 0 and 30 degrees, and semi-major axis between 2 and 8 AU. Comets with TJ >3 do not cross the orbit of Jupiter and fall into two groups, Enke type (named after comet 2P/Encke – Figure 5)

with semi-major axis <2.6 AU and Chiron types with semi-major axis >2.6 AU. Jupiter family comets are generally on orbits which cross that of Jupiter – most Ecliptic comets are in this category.

 

Figure 5

 

Figure 5. Comet 2P/Enke. STEREO spacecraft image

 

A diversion into orbits

 

In a paper ‘On hyperbolic comets’ published in the Journal of the British Astronomical Association in 1991 Professor David Hughes divided comets into two types – elliptical comets bound to the Solar System and hyperbolic comets not belonging to the Solar System.

 

As can be seen in Figure 6 orbits (elliptical or parabolic) of comets bound to the Solar system have values of eccentricity, e, between 0 and 1.  Hyperbolic orbits have values of e>1 and comets in such orbits may be extra-solar in origin or have been injected into such an orbit by passing close to Jupiter. At 1 AU from the Sun objects in various orbits move at the speeds shown in Figure 5 and thus knowing the speed of a comet helps to determine the shape of its orbit and its origin.

 

Figure 6

 

Figure 6. Cometary orbits

 

What else ?

 

So what else is out there and how do they fit into the groups previously described ?

 

Sungrazers and sunskirters, as their names suggest, pass very close to the Sun at perihelion. Sungrazers close to within 2 solar radii and don’t survive their perihelion passage. These comets are comprised of Kreutz I and Kreutz II sub-groups and their orbital elements put them into the Nearly Isotropic/New category.. Sunskirters with perihelion distances between 6 and 12 solar radii do, in general. survive their close approaches to the Sun. Kracht, Kracht II, Marsden and Meyer sub-groups make up this category – Kracht and Marsden comets being Halley types and Meyer Nearly Isotropic/Returning. Many of these comets have been discovered by amateurs searching images obtained by the Solar and Heliospheric Observatory (SOHO). Figure 7 shows one of the brighter of these.

 

Figure 7

 

Figure 7. SOHO image of a Sungrazing comet

 

Within the asteroid Main Belt there are several objects which display cometary activity  - for example a coma indicating outgassing of some kind. To date five are known; 7968 (Elst-Pizarro), 118401 (LINEAR), 238P/Read, P/2008 J2 (Belshore) and P/2008 R1 (Garradd). These fall into the category of Enke type Ecliptic comets.

 

The populations of comet-like bodies in the Solar System

 

This is the title of a paper published by Horner, Evans, Bailey and Asher in 2003. Comets are classified according to the planets which control them at perihelion and aphelion. One Hill radius marks the largest distance at which a moon may orbit a planet and, if a comet passes within three Hill radii of a planet, then that planet will, as the case may be, control the perihelion or aphelion of that comet. Table 1 lists the object classification for the region of  the Solar System beyond Jupiter.

 

Object

Perihelion

Aphelion

Object

Perihelion

Aphelion

S

6.6 ≤ q ≤ 12.0

Q ≤ 12.0

UE

12.0 ≤ q ≤ 22.5

33.5 ≤ Q ≤ 60.0

SU

6.6 ≤ q ≤ 12.0

12.0 ≤ Q ≤ 22.5

UT

12.0 ≤ q ≤ 22.5

Q ≥ 60.0

SN

6.6 ≤ q ≤ 12.0

22.5 ≤ Q ≤ 33.5

N

22.5 ≤ q ≤ 33.5

Q ≤ 33.5

SE

6.6 ≤ q ≤ 12.0

33.5 ≤ Q ≤ 60.0

NE

22.5 ≤ q ≤ 33.5

33.5 ≤ Q ≤ 60.0

ST

6.6 ≤ q ≤ 12.0

Q ≥ 60.0

NT

22.5 ≤ q ≤ 33.5

Q ≥ 60.0

U

12.0 ≤ q ≤ 22.5

Q ≤ 22.5

EK

33.5 ≤ q ≤ 60.0

Q ≤ 60.0

UN

12.0 ≤ q ≤ 22.5

22.5 ≤ Q ≤ 33.5

T

33.5 ≤ q ≤ 60.0

Q ≥ 60.0

 

Table 1. Object classes for the Solar system beyond Jupiter

 

The first letter in the Object column designates the planet controlling perihelion and the second, aphelion. EK objects are close to or within the Classical Edgeworth-Kuiper Belt and T objects include Scattered Disk Objects and have aphelia beyond the outer edge of the EKB (60 AU). Figure 8 shows the orbit of (5145) Pholus – a Centaur asteroid and thus considered to be a comet-like body by this classification. Pholus has a perihelion distance of 8.7 AU and an aphelion at 32.1 AU and is therefore an SN object.

 

Figure 8

 

Figure 8. Orbit of 5145 Pholus. NASA/JPL

 

Conclusion

 

Not being sure as to which of the classification schemes described here was the accepted one I contacted Paul R. Weissman. Paul works at NASA JPL and is a member of International Astronomical Union (IAU) Commission 20, Positions and Motions of Minor Planets, Comets and Satellites and is thus well qualified to answer such a question. His response was ‘…it (Harold Levison’s 1996 classification scheme) is pretty widely accepted though some people cling to the old SPC (Short Period Comet)/LPC (long Period Comet) classification scheme’.

 

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