SPA logo 

Comets      A to Z


Comet McNaught



























Comet C/2006 P1 (McNaught), Credit S. Deiries/European Southern Observatory


Last updated 2017 February 14










If you have been observing or imaging or just interested in comets for a while you will have absorbed much of the related jargon. However those who are just venturing in to this field may well be a little mystified so this publication may be of some help.


The objective of this A to Z is to assist those who have a basic grounding in matters astronomical, but are new to comets, to better understand the terminology used, for example, in

- papers appearing in the Journal of the British Astronomical Association

- the BAA Comet Section’s Observing Guide

- the Comet Section and Project Alcock websites

- articles in Popular Astronomy


If you re looking for a more in-depth explanation of any of the topics listed in the A-Z the book and website listings may help.


This publication is not intended to be an observing guide – for that please refer to the;

- British Astronomical Association Comet Section’s ‘Observing Guide to Comets’

- BAA Comet Section’s website at and

- Project Alcock website at

- Handbook of the BAA


In the manner of early manuscripts this document has been ‘illustrated’ by including images of comets adjacent to each letter of the alphabet. The originals can be found in the BAA website Galleries. Where possible a link to Gary W. Kronk’s Cometography has been included.


Suggestions for additional entries will always be welcome.


Roger Dymock

Assistant to the BAA Comet Section Director (Outreach and Mentoring)









C/1957R1 (Arend-Roland)C/1956 R1 (Arend-Roland), R.L. Waterfield, 1957 April 29
















a (semi-major axis)


Half the length of the long axis of the orbit. See Orbital elements for further details.




The prefix A is used in comet designations where the object was initially classified as a comet but was subsequently found to be an asteroid. An example is the unusual asteroid A/2010 AJ [Steward] which was discovered at the Steward Observatory, Kitt Peak on 2010 January 6. It has a period of 5.5 years and passed through perihelion at 1.14 AU from the Sun in 2009 October. In its current orbit, typical of a Jupiter family comet, it can approach to around 0.3 AU of Jupiter and 0.15 AU of the Earth. 




The parameter Afρ (pronounced Af-rho) was introduced in 1984 by Michael A'Hearn et. al. to describe the brightness of the cometary dust comae [1]. This quantity widely used to compare measurements of the dust produced by and surrounding a comet made under different geometric circumstances, at different times and using different instruments. It is useful for long-term studies of the behavior of the dust comae because it removes variations introduced by changing geocentric and heliocentric distances and aperture sizes.


Afρ is defined as the product of the albedo of the dust grains (A) within the coma, a filling factor (f) of the grains within the photometric aperture and the radius (ρ) of that aperture projected to the distance of the comet. The filling factor is the total cross section of the dust grains divided by the area within the aperture. The quantity Afρ defines the height, usually measured in cm, of a cylinder of base area equivalent to the projection of the aperture used for photometry at the distance of the comet from Earth completely filled with dust particles – Figure A1. An Afρ measurement of 100 centimeters equates to about 100 kilograms of dust produced per second.


Figure 1 Afrho parameters


Figure A1. Parameters used to calculate Afρ (based on a similar diagram by M. Müller and E. Grün)


Derivation of the equation for Afρ


The total cometary flux, F, as shown in Figure A2, is given by the equation;


F = where;

A = the albedo of the dust grains

N = number of dust grains within the aperture

σ = cross sectional area of a single grain

Fsun = solar flux at 1 AU

r = sun – comet distance in AU


The flux falling on the Earth per unit area, Fcomet, is given by the equation;

Fcomet =  where F is the total cometary flux and 4πΔ2 is the surface area of a sphere of radius Δ (the Earth – comet distance).


Figure 2 Cometary flux


Figure A2. Cometary flux


Substituting for F we get;


Fcomet = which can be rearranged as;


A =  


The filling factor, f, is the total cross section of the dust grains within the aperture divided by the area of the aperture i.e. f =  Therefore;

Af = x =  and

Afρ =


Example calculation


This example uses data from observations of comet 29P/ Schwassmann-Wachmann with the Sierra Stars Observatory Network 0.61m reflector sited on the east side of the Sierra Mountains in Alpine County, California, USA The report below was generated using Astrometrica and then FoCAs as previously described.


MPC report



OBS R.Dymock

MEA R.Dymock

TEL 0.61-m f/10 reflector + CCD


ACK G68_2010_03_22-1

COM USNO-A.2 used for photometry




0029P         C2010 03 22.18161 09 19 27.48 +13 17 35.0          15.88N      G68

0029P         C2010 03 22.20221 09 19 27.14 +13 17 36.2          15.90N      G68

0029P         C2010 03 22.22304 09 19 26.82 +13 17 37.0          15.89N      G68

0029P         C2010 03 22.24388 09 19 26.47 +13 17 38.1          15.88N      G68

0029P         C2010 03 22.26471 09 19 26.13 +13 17 39.1          15.88N      G68


Multibox report



OBS R.Dymock



                            10x10  20x20  30x30  40x40  50x50  60x60   SNR   SB   COD

OBJECT    DATE      TIME     +/-    +/-    +/-    +/-    +/-    +/-     N   FWHM  CAT

------ ---------- --------  -----  -----  -----  -----  -----  -----  ----  ----  ---

29P    22/03/2010 05:21:15  15.89  14.99  14.39  13.95  13.60  13.34  23.2  19.9  G68

29P    22/03/2010 05:21:15*  0.01   0.01   0.02   0.02   0.02   0.02     5   3.5  USN


FoCAs II - 17/03/2010


At the time of the observations;

Earth – comet distance (Δ) = 5.4377 AU = 5.4377 x 149,597,870 kms = 8.1347 x 108 km

Sun – comet distance (r) = 6.2010 AU

ρ – aperture size (radius) at the distance of the comet. Originally FoCAs used a square 10x10 arc sec aperture (as previously described) giving an area of 100 arcsecs2 but now use a circular aperture. The angle, α, subtended by the radius of a circle of the same area is given by the equation;


α = = 5.64 arc secs. Therefore ρ =  8.1347 x 108 x tan 5.64 = 22243 km


The magnitude of the comet used in this example = 15.89 which is the average of the five observations and shown in the Multibox report above under the 10x10 heading.

The sun’s apparent visual magnitude =  -26.7.


To convert magnitudes to flux we must seek the help of N. R. Pogson who proposed that a difference of 5 magnitudes should correspond to a difference in brightness of 100 between the two objects concerned.  If the brightness of two stars is B1 and B2, the difference in their magnitudes, m1 and m2, is given by the equation;


 which can be rewritten; m1 – m2 = - 2.5 log10

Brightness is a measure of the energy received, luminous flux or the rate of flow of photons, and is measured in Lumens (Watts in old money) so the above equations can also be written;

m1 – m2 = - 2.5 log10 and therefore  so;

  = 9.1868 x 10-18


Using the above values;

Af ρ =  =  = 0.04203 km = 4203 cms


Log Afρ =  Log 4203 = 3.623


As mentioned previously, this calculation of Afρ is based on an observational method developed by a group of Spanish amateur astronomers and the data, an example of which is shown in Figure A3, can be viewed on-line at 


A cautionary note – close in to the Sun active comets show very strong Swan band emissions (named after the Scottish physicist William Swan), e.g. C2, C3, CN and CO, and the use of unfiltered photometry in such instances can lead to AFRho values that are too large. This problem could be overcome by imaging bright comets using an R filter but for the vast majority of comets observed by amateurs Swan emission is not a problem since it is not strong enough to affect photometry obtained from unfiltered images.


Figure 3 Mag and Afrho plots


Figure A3. Magnitude and AfRho measurements for comet 29P/Schwassmann-Wachmann


The actual dust production rate can be calculated knowing Afρ and the velocity, bulk density, geometric albedo and scattering function of the dust grains. Comets not only throw off dust but all manner of molecules and even quite large pieces of their surface material. Some cease such activity temporarily or permanently and become dormant or dead comets and some, for example sun-grazers, disintegrate completely but all of that is another story.


Apparent asteroidal object


A comet which, when first observed, appears to be an asteroid as it lacks a coma or a tail. An example of this is C/2001 (A2 (LINEAR)


Argument of perihelion (ω)


Defines how the major axis of the orbit is oriented in the orbital plane and is the angle between the ascending node and the perihelion point measured in the plane of the orbit in the direction of motion. See Orbital elements for further details.


ASAS - All Sky Automated Survey


The All Sky Automated Survey is a low cost project dedicated to constant photometric monitoring of the whole available sky which includes approximately 107 stars brighter than 14th magnitude. The project’s ultimate goal is detection and investigation of any kind of photometric variability. Although one of the main objectives of ASAS is to find and catalogue variable stars two comets have been discovered; C/2004 R2 (ASAS) and C/2006 A1 (Pojmanski).




The point in a comet’s orbit furthest from the Sun. See Orbital elements for further details.




The period of time during which a Solar System object is visible from Earth and therefore a term that is applied to moving objects and not for example stars or galaxies.




It used to be so simple. There were the large: Sun, planets; the small: asteroids, comets; and the very small: dust, meteoroids, solar wind, cosmic rays and the like. Pluto, with its eccentric and highly inclined orbit (relative to the other planets), was something of an oddity but nobody really questioned whether or not it was a planet, at least not until the discovery of a large Edgeworth-Kuiper Belt Object (EKBO) in 2005 July. Subsequently numbered and named (136199) Eris, 2003 UB313 proved to be slightly larger than Pluto (now numbered 134340). Should this object, informally named ‘Xena’ at the time of discovery, be considered as the 10th planet? The astronomical world was divided – some wanted it defined as a planet proper while others were not so sure. There was much debate as to what such a non-planet should be called or indeed how planets and asteroids should be categorised.


The matter was resolved at the XXVIth General Assembly of the International Astronomical Union (IAU) which was held in Prague, Austria during August 2006. Two resolutions, 5 and 6, were passed, but not without considerable discussion, relating to planets, asteroids and comets. The outcome of these resolutions is that the solar system is now made up of Planets, Dwarf Planets and Small Solar System Bodies (i.e. asteroids and comets). The formal definitions are:


Resolution 5


A planet is a celestial body that:

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

(c) has cleared the neighbourhood around its orbit.


A dwarf planet is a celestial body that:

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

(c) has not cleared the neighbourhood around its orbit,

(d) is not a satellite.


All other objects, except (natural) satellites, orbiting the Sun shall be referred to as ‘Small Solar System Bodies’.


Resolution 6


Pluto is a dwarf planet and is recognised as the prototype of a new category of Trans-Neptunian Object (TNO). An IAU process will be established to select a name for this category.


In summary a planet is a large round object and a dwarf planet is a small round object. In practice the term ‘Small Solar System Bodies’ appears to have been still-born. These mostly irregularly-shaped bodies are still known, and will probably always be known, as asteroids and comets.


At this time asteroids in orbits similar to that of Pluto were known, informally, as Plutinos. The first of these, 1993 RO, was discovered by Dave Jewitt and Jane Luu in 1993. Such objects make 2 orbits for every 3 made by Neptune and are thus said to be in a 3:2 resonance with that planet. 152 of these objects were discovered up to 2004 and it is estimated that there could be 1400 with a diameter greater than 100 km.


In 2008 June the IAU introduced the term plutoid – the formal announcement being:

‘Plutoids are celestial bodies in orbit around the Sun at a semi-major axis greater than that of Neptune’s that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity’. The three known and named plutoids are (134340) Pluto, (136199) Eris, and (136742) Makemake. There are many more large asteroids waiting in the wings to be ‘upgraded’ to dwarf planet status and, almost certainly, many more orbiting beyond Neptune waiting to be discovered.


You will note, in the IAU announcement concerning plutoids, that there is no mention of 3:2 resonance with Neptune, merely that the semi-major axis of a plutoid should be greater than that of Neptune. So all plutinos are plutoids but not all plutoids are plutinos!

EKBOs, TNOs, plutinos and plutoids are explained in more detail in Chapter 3.


Earlier I used the term ‘resolved’ but that is perhaps a little too definitive at the present time! In 2008 August a conference ‘The Great Planet Debate: Science as Process’ was held at the Johns Hopkins University Applied Physics Laboratory, Maryland, USA. The post-conference press release stated ‘Different positions were advocated, ranging from reworking the IAU definition (but yielding the same outcome of eight planets), replacing it with a geophysical-based definition (that would increase the number of planets well beyond eight), and rescinding the definition for planet altogether and focusing on defining subcategories for serving different purposes. No consensus was reached’.


One of the most sensible proposals I have come across suggests that in the same way we have various classes of stars, we should have various classes of planets, but that they should all be planets, e.g. Jovian, Terrestrial and Dwarf.


For now, we can define planets and dwarf planets but what of asteroids? The Encyclopedia of the Solar System, second edition published by Academic Press in 2007, defines an asteroid as ‘A rocky, carbonaceous or metallic body, smaller than a planet and orbiting the Sun’.


Asteroids are by no means all solid bodies. Those less than 100 to 150 metres in diameter can be considered as solid while larger ones, between 100 and 300 metres or so, are frequently rubble piles, for example (25143) Itokawa, Figure A4, visited by the Japanese spacecraft Hayabusa in 2005 September and shown below. These are the result of the parent bodies being disrupted by impact and then reforming under the influence of gravity – much as planetesimals formed in the early Solar System.




Figure A4. (25143) Itokawa – an example of a rubble-pile asteroid. Credit JAXA


Itokawa shows no outward signs of such an impact but (2867) Steins certainly does! Figure A5 shows a series of images obtained by the Rosetta spacecraft in 2008 September while on its way to comet 67P/Churyumov-Gerasimenko. The asteroid is approximately 5 km in diameter and was obviously involved in a mighty collision – the crater at the top being of the order of 2 km in diameter. The crater chain running from top to bottom in the image shows that it suffered further after the main impact. The evidence of the sequence of impacts is that the topmost small crater overlaps the rim of the large one, showing that it occurred after the major impact.


(2867) Steins


Figure A5. (2867) Steins showing multiple impact craters. Credit ESA 2008 MPS for OSIRIS team


The distinction between asteroids and comets is somewhat fuzzy – pun intended. If an object shows no signs of a coma or tail then it is usually classed as an asteroid. However some objects, initially classed as asteroids, have later shown evidence of cometary activity. One such example, which I imaged in 2005 August, is 2005 EX12, Figure A6, This was reclassified as a periodic comet, 169/P. Its faint tail can be seen in the image below.




Figure A6. 2002 EX12, initially classified as an asteroid but later defined as a comet,169/P.


This image is actually a number of images stacked to allow for the motion of the object – the stars therefore appearing as lines of dots. The software that makes this possible is Astrometrica.


On the other hand comets, after many orbits around the Sun, eventually outgas all of their volatiles and become extinct. 2003 PG3 may be just such an object. Just to complicate matters further (5154) Pholus, a Centaur, is most likely a comet nucleus that has never have been active.




Astrometry in its broadest sense is the measurement of positions, parallaxes and proper motions of an astronomical body on the sky. In the next two chapters I will use a narrower definition – the measurement of the position of an asteroid in terms of its Right Ascension (RA) and Declination (Dec) known as equatorial coordinates. For nearby objects such as asteroids and in particular Near Earth Objects (asteroids and comets), these coordinates can vary slightly depending on your location on the Earth. Hence the importance, when obtaining an ephemeris from the MPC for example, of specifying one’s exact position which includes latitude, longitude and height. If you have an Observatory Code then inputting that will supply these parameters.


Accurate astrometry is necessary to:

– determine the orbit of a newly discovered comet or asteroid,

– refine that orbit leading to that comet or asteroid being numbered,


In the days of astrophotography measurement of position was a complicated and time consuming task involving the use of a mechanical plate measuring engine – a single measurement could take several hours to complete. Nowadays it requires a CCD image, an accurate star catalogue, an astrometry software package and a PC – measurement time is reduced to minutes if not seconds. The accuracy of the position so determined will, to a large extent, only be as good as the accuracy of the reference stars used. Astrometrica software allows you to select from several catalogues which it accesses via the internet – USNO-B1.0, NOMAD and UCAC-4 are recommended for astrometry.




Comet Bennet 1969iComet C/1969 Y1 (Bennett), R.L. Waterfield, 1970 April 4



















The centre of mass of a system of orbiting bodies – the system orbits around its barycentre. For two bodies the barycentre may be positioned between the bodies as in the Pluto-Charon system shown in Figure B1 (animation at


Where one body is considerably more massive than the other the barycentre may lie within the larger body. This is the case of the Earth-Moon system where the barycentre lies 1600 km below the Earth’s surface.


File:Pluto-Charon System.gif


Figure B1. The Pluto-Charon system


Bisei Asteroid Tracking Telescope for Rapid Survey (BATTeRS)


A Japanese project to find asteroids located at the Bisei Spaceguard Center. Telescopes are operated by the Japan Spaceguard Association. Comet C/2001 W2 (BATTERS), later classified as periodic, and therefore renamed P/2001 W2, was discovered by this facility on 2001 November 21.




32P Comas SolaComet 32P/Comas Sola, A Baransky, 2013 October 9















Comet designation prefix indicating a long period (>200 years) comet for example C/1995 Y1 (Hyakutake)


CSS = Catalina Sky Survey


The mission of the Catalina Sky Survey is to contribute to the inventory of near-earth objects (NEOs), or more specifically, the potentially hazardous asteroids (PHAs) that pose an impact risk to Earth and it's inhabitants. The identification of the iridium anomaly at the Cretaceous-Tertiary boundary (Alvarez et al., 1980), associated Chicxulub impact crater (Hildebrand et al., 1991) and the Permian- Triassic "great dying" possibly being associated with Australian Bedout crater (Becker et al., 2004) strongly suggest that impacts by minor planets play an important role in the evolution of life.


The Catalina Sky Survey (CSS), Mt. Lemmon Survey and Siding Spring Survey (SSS) working together under the name of the first survey, are carrying out sustained, highly productive searches for NEOs, contributing to the Congressional mandates of obtaining an inventory of better than 90% of the previously mentioned NASA goal. Further, these surveys are operated in such a manner that same night follow up on newly discovered objects can usually be done facilitating the rapid determination of orbits and thus the specific hazard posed by the newly found objects.


The "Catalina Sky Survey" currently consists of a consortium of 2 cooperating surveys: the original Catalina Sky Survey (CSS) and the Mt. Lemmon Survey (MLS). These are also known by their MPC codes of 703 and G96 respectively. Though united by the common mission of meeting the Congressionally mandated goal in obtaining an inventory of better than 90% of the 140-meter or larger Near Earth Objects (NEOs) they have separate development histories and facilities. In late 2013 a 1-meter robotic telescope (MPC code I52) will be added to this team to handle follow up and arc-extension work, relieving the burden on CSS and MLS freeing up more of their time for discovery. 


CBAT - Central Bureau for Astronomical Telegrams


CBAT operates under the auspices of Commission 6 of the International Astronomical Union (IAU) and is a nonprofit organization, with principal funding coming from subscriptions to the various services offered by the Bureau, and (during 2008-2010) also from the U.S. National Science Foundation. The Central Bureau has operated for more than a decade on computers generously provided by the Tamkin Foundation, first in collaboration with the Minor Planet Center at SAO and now also on new computers at EPS/Harvard.


The CBAT is responsible for the dissemination of information on transient astronomical events and various IAU news including the announcement of designations and names of various celestial objects -- via the IAU Circulars (IAUCs), a series of printed-postcard-sized announcements issued at irregular intervals as necessary in both printed and electronic form and via the electronic-only Central Bureau Electronic Telegrams (CBETs). The CBAT is the official worldwide clearinghouse for new discoveries of comets, solar-system satellites, novae, supernovae, and other transient astronomical events.




Centaurs are a class of small bodies which have characteristics of both asteroids and comets. Their orbits place them amongst the gas giant planets (and are therefore unstable). A study of observations obtained by NASA’s Wide-field Infrared Survey Explorer (WISE) found that the majority of them were comets and probably originated further out in the Solar System.




Links to entries under names of various classifications


Harold Levison’ 1996 classification


The populations of comet-like bodies in the Solar System – Horner, Evans, Bailey and Asher




In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. As a comet approaches the inner Solar System solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the coma. Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflecting sunlight directly and the gases glowing from ionisation.


The coma is generally made of water and dust with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun. The water parent molecule is destroyed primarily through photo dissociation, and to a much smaller extent photo ionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. 


Although the solid nucleus of comets is generally less than 60 kilometres (37 mls) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun. For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun – Figure C5. 


File:17P-Holmes Auvergne 2007 11 02.jpg

Figure C5. Comet 17P/Holmes, 2007 November 2. Credit, Gil-Estel




There is masses of descriptive material on the internet and in books so there is little point in duplicating that. Here is a link to get you started try Wikipedia, Comet -


As far as books are concerned try;

‘Comet Science’ by Crovisier and Encrebaz, published by Cambridge University Press

‘Introduction to Comets’ by Brandt and Chapman, published by Cambridge University Press


The drawing below shows the make-up of a typical comet.




Comet designations


The formal definition of the cometary designation system is given at


Periodic comets that have been observed at more than one return are assigned numbers e.g. 1P/Halley. 2P/Encke. A complete list of periodic comet numbers is at;


List of designations;

A/        a comet designation that actually refers to a minor planet (or asteroid)

C/       for a comet that is not periodic (as described for P/ comets) e.g. C/1995 Y1 (Hyakutake)

D/        for a periodic comet that no longer exists or is deemed to have disappeared

P/         Periodic (defined to have a revolution period of less than 200 years or confirmed observations at more than one perihelion passage) e.g. P/1996 A1 (Jedicke)

X/        comet for which a meaningful orbit can not be computed


In the case of a comet that has separated into discrete components, those components should be distinguished by appending -A, -B, etc., to the designation (or to the P/ or D/ periodic comet number)


D family comet 33P/Daniel




















Designation prefix for a comet that no longer exists for example; Figure D1, comet D/1993 F2 (Shoemaker-Levy) which collided with Jupiter in 1994 July.


File:Shoemaker-Levy 9 on 1994-05-17.png


Figure D1. Fragments of comet D/1993 F2 (Shoemaker-Levy). Credit STSci




Damocloids are inactive nuclei of Halley Family comets. 5335 (Damocles), also known as 1991 DA, was discovered by Robert h. McNaught in 1991. Its orbit reached from inside the  Mars to as far out as Uranus and it has a period of 40.7 years. It appears to be in transition from a near-circular outer Solar System orbit to an eccentric orbit taking it into the inner Solar System. There is a good probability that it will become an Earth-crosser asteroid

Orbit of 5335 (Damocles). Credit NASA/JPL


DC - Degree of condensation


The Degree of condensation is a measure of how condensed the coma is. It provides a visual description of the coma intensity across its diameter and runs on a scale from 0 to 9.

0 = Diffuse coma of uniform brightness
1 = Diffuse coma with slight brightening towards center
2 = Diffuse coma with definite brightening towards center
3 = Centre of coma much brighter than edges, though still diffuse
4 = Diffuse condensation at centre of coma
5 = Condensation appears as a diffuse spot at centre of coma – described as moderately condensed.
6 = Condensation appears as a bright diffuse spot at centre of coma
7 = Condensation appears like a star that cannot be focused – described as strongly condensed
8 = Coma virtually invisible
9 = Stellar or disk like in appearance.


A diagram of the various DCs can be found in the British Astronomical Association Comet Section’s ‘Observing Guide to Comets’.




See under relevant letter;







Dirty snowball


A term first used by American astronomer Fred Lawrence Whipple in the early 1950s to describe the nucleus of a comet. The basic features of his dirty snowball or  icy conglomerate hypothesis were later confirmed.




The words disintegration and fragmentation seem to be interchangeable when it comes to comets.


Comets, being composed of a mixture of rock and various ices, are quite fragile objects. They are therefore quite susceptible to being broken up by the gravitational influence of Jupiter or the Sun for example or by outgassing when close to perihelion.


In this A-Z I will classify a disintegrated comet as one that has literally turned to dust as was the case with C/2010 X1 (Elenin) - before, Figure D2, and after, Figure D3.


Figure 1


Figure D2. Comet C/2010 X1 (Elenin), 2011 Apr 27, 0507UT, 20x20 arc mins, 4x60 sec exp, SSON OMI 0.61m f/10 Cassegrain, FLI Proline PLO9000 CCD, unfiltered, R Dymock, MPC G68


Figure 2


Figure D3. Comet C/2010 X1 (Elenin), 2011 October 22, 116x87 arc mins, 6x300 sec exp, GRAS Takashashi FSQ 106ED refractor, SBIG STL-8300-C CCD, Rolando Ligustri.


Dormant comet


Such comets have their volatiles sealed inside by an inactive crust. An example is 60558 (Echeclus) which was discovered in 2000 and showed no signs of cometary activity – hence the asteroid designation. However in 2005 a coma was detected and it was subsequently given the comet type designation 174P/Echelus.


DSS - Digitised Sky Survey


The DSS, originally published in 1994, is a digitised version of several photographic atlases of the night sky. The ESO Online Digitized Sky Survey can be accessed here


Dust tails – go to here


Dust shells


Dust emitted by a comet may form concentric layers or shells around the comet as in Figures D4 and D5. These shells are formed by dust emanating from the rotating nucleus. See


comet diagram


Figure D4. Comet structure. Credit Stephen James O’Meara.


Hale-Bopp nucleus and dust


Figure D5. Shells around the nucleus of Comet Hale-Bopp. Credit Brad D. Wallis and Sky and Telescope.




2p enckeComet 2P/Encke




























e (eccentricity) – go to here


Ecliptic comets


Harold Levison’s 1996 classification divided ecliptic comets into 3 groups;

- Encke type

- Chiron type

- Jupiter family


As there name suggests they move in or close to the plane of the ecliptic and are defined by the relationship of the semi-major axis (a) of their orbits with respect to that of Jupiter – Figure E1.



Figure E1. Ecliptic comets


Comets 2P/Encke and 95P/Chiron give their names to two of the groups. Chiron was originally classified as an asteroid so also carries the designation 2060 Chiron. To complicate matters further Chiron is also a member of the Centaur class of Small Solar system Bodies.


Edgar Wilson Award


An award for amateur comet discoverers. Each is composed of a monetary award from the Edgar Wilson Charitable Trust Fund and an Award plaque (Figure E2) The Award is allocated annually among the amateur astronomers who, using amateur equipment, have discovered one or more new comets. The award was announced in International Astronomical Union circular IAUC 6936 which describes in detail the circumstances in which the award may be given.

Figure E2. An example of the Edgar Wilson award plaque


Edgeworth-Kuiper Belt


The EKB , Figure E 3, is a region of the Solar System beyond the outer gas-giant planets extending from the orbit of Neptune (30 AU from the Sun) out to 50 AU. Three dwarf planets reside there; Pluto, Haumea an Makemake plus approximately 100,000 objects with diameters of greater than 100 km. Periodic comets were once thought to originate here but it is now believed that they come from the Scattered Disk.


Figure E3. The Edgeworth-Kuiper Belt. Credit NASA


Emission lines


Cometary spectra exhibit mainly molecular and radical (fragments of molecules) emission lines. Such lines are caused when an electron in an atom jumps from a higher to a lower energy level and emits a photon of light and can thus indicate the atom, molecule or radical from which the emission emanates. An example of amateur work can be found here.




A table giving the predicted positions of an object at given times. For comets and asteroids ephemeredes can be obtained form the Minor Planet Center’s Minor Planet and Comet Ephemeris Service An example is shown below.

C/2013 A1 (Siding Spring)

Epoch 2014 Dec. 9.0 TT = JDT 2457000.5
T 2014 Oct. 25.3014 TT                                  MPC
q   1.398716             (2000.0)            P               Q
z  -0.000312       Peri.    2.4224      +0.4913979      -0.5613358              T = 2456955.80143 JDT
 +/-0.000000       Node   300.9764      -0.8115577      -0.5726014              q =     1.3987159
e   1.000436       Incl.  129.0428      -0.3160730      +0.5975197

1/a(orig) = +0.000034 AU**-1, 1/a(fut) = +0.000115 AU**-1.

Perturbed ephemeris below based on elements from MPEC 2014-T44.

Date       UT      R.A. (J2000) Decl.    Delta     r     El.    Ph.   m1     Sky Motion        Object    Sun   Moon
            h m s                                                            "/min    P.A.    Azi. Alt.  Alt.  Phase Dist. Alt.
2014 10 16 000000 17 38 10.1 -27 09 35   1.545   1.405   62.9  39.1  10.0    1.63    355.8    093  -39   -48   0.48   149  +11
2014 10 17 000000 17 37 57.6 -26 31 11   1.567   1.404   61.8  38.7  10.1    1.59    356.4    095  -39   -48   0.39   138  +03
2014 10 18 000000 17 37 47.1 -25 53 52   1.589   1.403   60.7  38.3  10.1    1.54    357.0    096  -39   -49   0.30   126  -06
2014 10 19 000000 17 37 38.7 -25 17 35   1.611   1.402   59.6  37.8  10.1    1.50    357.6    097  -39   -49   0.22   114  -14
2014 10 20 000000 17 37 32.0 -24 42 17   1.633   1.401   58.6  37.4  10.1    1.46    358.2    099  -40   -49   0.14   103  -23




The date and time at which an astronomical observation was made or the date for which positions and orbital elements of celestial objects are calculated. Because of precession and nutation celestial coordinates change with time so that positions and orbital elements must be referred to a given date.


ESO – European Southern Observatory


The European Southern Observatory operates three observing sites in the Atacama Desert in Chile; La Silla, Paranal and Chajnantor.


F 4P/Faye Armagh Observatory
















False nucleus


The false nucleus is a bright spot at the centre of the head of the comet - Figure F3. In this image the head is approximately 30 arcsecs in diameter so at a distance of 0.4 au this is an actual diameter of 9000 km. The bright false nucleus in the centre is 1/10th of this – 900 km whereas the diameter of the real, more or less solid, nucleus is estimated to be 6.8 km. Comet nuclei cannot be observed from Earth being embedded in the bright gaseous coma.

Figure F1, Comet C/2013 R1 (Lovejoy). Credit Damian Peach


FITS images


The Flexible Image Transport System is the standard data format used in astronomy. Included with a FITS image is data relating to the image, location, telescope, target, etc. as in the example below. What is included is dictated by the imaging software and the user. All you could possibly want to know about the subject can be found at


SIMPLE  =                    T / Conforms to FITS standard (NOST 100-2.0)

BITPIX  =                   16 / Number of bits per pixel

NAXIS   =                    2 / Number of axes

NAXIS1  =                 1528 / Width of image

NAXIS2  =                 1528 / Height of image

CREATOR = 'Astrometrica' / License: Dymock Roger

DATE    = '2014-04-17'         / File creation date

BZERO   = +3.2768000000000E+04 / Offset to unsigned integer

BSCALE  = +1.0000000000000E+00 / Scaling factor

DATE-OBS= '2014-04-10T07:54:00' / Start of exposure

TIMESYS = 'UTC'                / Time Scale

EXPTIME =                70.00 / Exposure time (seconds)

OBJCTRA = '12 27 39.593'       / RA of center of the image

OBJCTDEC= '-10 55 02.386'      / DEC of center of the image

EQUINOX =               2000.0 / Equinox for coordinates

CTYPE1  = 'RA---TAN'           / Coordinate type

CRPIX1  = +7.6400000000000E+02 / Pixel coordinate of x-reference

CRVAL1  = +1.8691475971855E+02 / Coordinate at x-reference

CDELT1  = +2.2191789684059E-04 / Image scale on x-axis, deg per pixel

CROTA1  = +3.5789385722817E+00 / Rotation of coordinate

CUNIT1  = 'deg     '           / Unit of coordinate

CTYPE2  = 'DEC--TAN'           / Coordinate type

CRPIX2  = +7.6400000000000E+02 / Pixel coordinate of y-reference

CRVAL2  = -1.0917564840669E+01 / Coordinate at y-reference

CDELT2  = +2.2191789684059E-04 / Image scale on y-axis, deg per pixel

CROTA2  = +3.5789385722817E+00 / Rotation of coordinate

CUNIT2  = 'deg     '           / Unit of coordinate

MZERO   = +2.7334562668434E+01 / Magnitude Zero Point

OFFSET1 =                    0 / Camera upper left frame x

OFFSET2 =                    0 / Camera upper left frame y

XFACTOR =                    2 / Camera x binning factor

YFACTOR =                    2 / Camera y binning factor

ORIGIN  = 'Sierra Stars Observatory'

TELESCOP= 'OMI 0.61-meter F/10 Cassegrain'

COMMENT Outburst comets

OBSERVER= 'Roger Dymock'       / Investigator(s)

OBJECT  = 'C/2006 S3 (LO'      / Object name

PRIORITY=                    5 / Scheduling priority

INSTRUME= 'FLI ProLine PL09000 Rev 1.36'

JD      =    2456757.829171814 / Julian Date, start of exposure

LST     = '13:08:48'           / Local sidereal time at exposure start

POSANGLE= '  10:26:58'         / Position angle, degrees, +W

LATITUDE= ' 38:48:42'          / Site Latitude, degrees +N

LONGITUD= '-119:46:30'         / Site Longitude, degrees +E

ELEVATIO= ' 39:16:11'          / Degrees above horizon

AZIMUTH = '193:11:31'          / Degrees E of N

HA      = '  0:41:29.29'       / Local Hour Angle

AIRMASS =          1.577392579 / Kasten-Young airmass computation

MOONANGL=               60.277 / Angular separation to Moon, Degrees

MOONPHAS=                76.86 / Percentage of full moon

RAEOD   = ' 12:28:28.84'       / Nominal center Apparent RA

DECEOD  = '-11:00:27.7'        / Nominal center Apparent Dec

RA      = ' 12:27:42.66'       / Nominal center J2000 RA

DEC     = '-10:55:33.2'        / Nominal center J2000 Dec

OBJRA   = ' 12:27:42.7'        / Target center J2000 RA

OBJDEC  = '-10:55:33.2'        / Target center J2000 Dec

EPOCH   =                 2000 / RA/Dec epoch, years (obsolete)

FILTER  = 'C       '           / Filter code

CAMTEMP =                  -35 / Camera temp, C

RAWHENC =          -0.21436123 / HA Encoder, rads from home

RAWDENC =          -0.18218588 / Dec Encoder, rads from home

RAWOSTP =            180.07766 / Focus Motor, rads from home

FOCUSPOS=            9099.9365 / Focus pos from home, um

RAWISTP =                    0 / Filter Motor, rads from home

COMMENT Weather at UT Thu Apr 10 07:55:10 2014

WXTEMP  =                  5.8 / Ambient air temp, C

WXPRES  =               1016.9 / Atm pressure, mB

WXWNDSPD=                   16 / Wind speed, kph

WXWNDDIR=                  202 / Wind dir, degs E of N

WXHUMID =                   60 / Outdoor humidity, percent

PIXDC0  =              9122.92 / Residual bias

BIASCOR = 'cbs40600.fts'       / Bias file used

THERMCOR= 'cth40600.fts'       / Thermal file used

FLATCOR = 'cfc40600.fts'       / Flat field file used

BADPXCOR= ''       / Bad Column Map file applied

AMDX0   =          3.262279451 / astrometric parameter alpha_c (radians)

AMDY0   =        -0.1905475262 / astrometric parameter delta_c (radians)

AMDX1   =      3.873532456E-06 / astrometric parameter a1

AMDY1   =      3.869471849E-06 / astrometric parameter b1

AMDX2   =     -2.412247631E-07 / astrometric parameter a2

AMDY2   =      2.420412488E-07 / astrometric parameter b2

AMDX3   =      -0.002765709037 / astrometric parameter a3

AMDY3   =      -0.003138231774 / astrometric parameter b3

AMDX4   =     -7.764098039E-12 / astrometric parameter a4

AMDY4   =      3.559547956E-12 / astrometric parameter b4

AMDX5   =      6.277706246E-12 / astrometric parameter a5

AMDY5   =     -5.413774204E-12 / astrometric parameter b5

AMDX6   =     -4.069434208E-12 / astrometric parameter a6

AMDY6   =      1.920613435E-12 / astrometric parameter b6

AMDX7   =                    0 / astrometric parameter a7

AMDY7   =                    0 / astrometric parameter b7

AMDX8   =                    0 / astrometric parameter a8

AMDY8   =                    0 / astrometric parameter b8

AMDX9   =                    0 / astrometric parameter a9

AMDY9   =                    0 / astrometric parameter b9

AMDX10  =                    0 / astrometric parameter a10

AMDY10  =                    0 / astrometric parameter b10

AMDX11  =                    0 / astrometric parameter a11

AMDY11  =                    0 / astrometric parameter b11

AMDX12  =                    0 / astrometric parameter a12

AMDY12  =                    0 / astrometric parameter b12

AMDX13  =                    0 / astrometric parameter a13

AMDY13  =                    0 / astrometric parameter b13

FWHMH   =               6.6845 / Horizontal FWHM median, pixels

FWHMHS  =               1.2669 / Horizontal FWHM std dev, pixels

FWHMV   =               6.5774 / Vertical FWHM median, pixels

FWHMVS  =               1.1115 / Vertical FWHM std dev, pixels





The words disintegration and fragmentation seem to be interchangeable when it comes to comets.


Comets, being composed of a mixture of rock and various ices, are quite fragile objects. They are therefore quite susceptible to being broken up by the gravitational influence of Jupiter or the Sun for example or by outgassing when close to perihelion.


In this A-Z I will classify a fragmented comet as one that has broken up but still exists as several discrete parts, for example comet D/1993 F2 (Shoemaker-Levy) before its various parts impacted Jupiter.




Garradd passing Collinder 399 by Martin MobberleyComet C/2009 P1. Martin Mobberley 2011 September 3

























Gravitational effects


The matter composing the Oort cloud most likely formed closer to the Sun and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System's evolution. Although no confirmed direct observations of the Oort cloud have been made, astronomers argue that it is the source of all long-period and Halley-type comets entering the inner Solar System and many of the centaurs and Jupiter-family comets as well. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System.


Great comets


There is no formal definition of a great comet but, typically, they may exhibit some of the following aspects;

- easily visible to the unaided eye e.g. magnitude 0 or brighter

- remains visible to the unaided eye for several weeks or even months

- brilliant and/or long tail e.g. 5 to 20 degrees

- multiple tails

- visible in bright twilight or daylight

- structure around the nucleus easily visible through a telescope



X/1106 (Great Comet of 1106)

C/1264 N1 (Great Comet of 1264)

Great comet of 1402

C/1556 D1 (Great Comet of 1556)

C/1577 V1 (Great Comet of 1577)

C/1680 V1 (Great Comet of 1680)

C/1743 X1 (Great Comet of 1744)

D/1770 L1 (Lexell)

C/1811 F1 (Great Comet of 1811)

C/1843 D1 (Great March comet or Great Comet of 1843)

C/1858 L1 (Donati's Comet0

C/1861 J1 (Great Comet of 1861)

C/1874 H1 (Coggia)

C/1880 C1 (Great Southern Comet)

C/1881 K1 (Great Comet)

C/1882 F1 (Wells)

C/1882 R1 (Great September Comet)

C/1887 B1 (Great Southern Comet)

C/1901 G1 (Great Comet)

C/1910 A1 (Great January Comet of 1910 or the Daylight Comet)

C/1911 O1 (Brooks)

C/1911 S3 (Beljawsky)

C/1927 X1 (Skjellerup-Maristanny)

C/1941 B2 (De Kock-Paraskevopoulos)

C/1947 X1 (Southern Comet)

C/1948 V1 The Eclipse Comet of 1948

C/1956 R1 (Arend-Roland)

C/1957 P1 (Mrkos)

C/1961 O1 (Wilson-Hubbard)

C/1962 C1 (Seki-Lines)

C/1965 S1 (Ikeya-Seki)

C/1969 Y1 (Bennett)

C/1970 K1 (White-Ortiz-Bolelli)


File:Comet Hale-Bopp 1995O1.jpgC/1995 O1 Hale-Bopp).  Image obtained on 1997 April 04, with a 225mm f/2.0 Schmidt Camera (focal length 450mm) on Kodak Panther 400 colour slide film with an exposure time of 10 minutes. The field shown is about 6.5°x6.5°. At full resolution, the stars in the image appear slightly elongated, as the camera tracked the comet during the exposure. Credit E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria




















 B2 (Hyakutake). Faint stars near the constellation Ursa Minor (the Little Dipper) shine through comet's long, graceful tail. Credit: NASA/Rick Scott and Joe Orman






















File:C-west-1976-ps.jpgC/1975 V1 (West) was discovered in photographs by Richard West on August 10, 1975. It reached peak brightness in March 1976. During its peak brightness, observers reported that it was bright enough to study during full daylight. The comet has an estimated orbital period of 558,000 years. Credit J. Linder/ESO




















File:Comet P1 McNaught02 - 23-01-07.jpgC/2006 P1 (McNaught or Great Comet of 2007). Image taken from Swifts Creek, Victoria, Australia at approx 10:10 pm,  f/4, ISO 800, 20 seconds and ~ 24mm with post processing in Photoshop to bring out details. Credit Fir0002/Flagstaffotos

















 W3 (Lovejoy). The comet is visible near Earth's horizon in this nighttime image photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on Dec. 22, 2011. Credit NASA
















































Websites listing great comets;


Fall of a thousand suns





Guest star


A term used In Chinese astronomy to indicate a star (cataclysmic variable, nova or supernova) or comet which has suddenly appeared in the sky where no object was previously visible.




103P HartleyComet 103P/Hartley, M Butcher, 2010 October 7











Harold Levison’s 1996 classification


In his 1996 paper, Comet Taxonomy, Harold F. Levison proposed the classification scheme shown in Figure H1.



Figure 2


Figure H1. 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 H2 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 H2. Distribution of the inverse of the semi-major axis for long-period comets


Comet C/2010 X1 (Elenin), Figure H3 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 H3. 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, Encke type (named after comet 2P/Encke – Figure H4

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 H4. Comet 2P/Enke. STEREO spacecraft image


Horner, Evans, Bailey and Asher, 2003. 'The populations of comet-like bodies in the Solar system’


This scheme classifies comets according to the planets predominantly controlling them at perihelion and aphelion.


Hyperbolic orbit


See here




Comet C/1965 S1 (Ikeya-Seki), James Young (TMO/JPL/NASA), 1965 October 30


i (inclination)


See here


IAU - International Astronomical Union


The International Astronomical Union (IAU) was founded in 1919. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. Its individual members — structured into Divisions, Commissions, and Working groups — are professional astronomers from all over the world, at the Ph.D. level and beyond, who are active in professional research and education in astronomy. 


IAUC - International Astronomical Union Circular


The International Astronomical Union Circulars (IAUCs) are a series of postcard notices giving information about astronomical phenomena requiring prompt dissemination, particularly the discovery and follow-up of novae, supernovae and comets. The IAUCs are also available in electronic form via e-mail or through the CBAT/MPC Computer Service. On-line IAUCs are available, the newer ones by paid subscription only, and many older ones freely available from the IAUC webpage.


ICQ - International Comet Quarterly


This is a key place to begin looking for useful and accurate information regarding news, observations, orbital data, designations and names, and good links regarding comets and related topics. Updates have been a little sporadic of late. 




The total amount of solar radiation energy received on a given surface area during a given time. See here


ISON – International Scientific Optical Network


The International Scientific Optical Network is an international project, currently consisting of about 30 telescopes at about 20 observatories in about ten countries (Russia, Ukraine (Andrushivka), Georgia (Abastumani), Uzbekistan, Tajikistan, Moldova, Spain (Teide), Switzerland (Zimmerwald), Bolivia (Tarija), USA (Mayhill), Italy (Collepardo)) which have organized to detect, monitor and track objects in space. ISON is managed by the Keldysh Institute of Applied Mathematics, part of the Russian Academy of Sciences. It was credited for the discovery of comets C/2010 X1 (Elenin) and C/2012 S1 (ISON). This comet, disappointingly, did not survive its perihelion passage.


Comet ISON comes in from the bottom right and moves out toward the upper right, getting fainter and fainter.

Figure I1. Comet ISON comes in from the bottom right and moves out toward the upper right, getting fainter and fainter, in this time-lapse image from the ESA/NASA Solar and Heliospheric Observatory. The image of the sun at the center is from NASA's Solar Dynamics Observatory. Credit ESA/NASA/SOHO/SDO/GSFC




Comet 290P/Jager, Nick James, 1998 October 6




Jets consist of gases sublimed from nuclear ices plus the dust carried away with them. Images of the nucleus of comet 67P/Churyumov-Gerisimenko obtained by OSIRIS (Rosetta's Optical, Spectroscopic, and Infrared Remote Imaging System) in the summer of 2014 showed jets of dust and gas emanating from the comet. Jets become more active as a comet nears the Sun.


This image was taken by the Optical, Spectroscopic, and Infrared Remote Imaging System, Rosetta's main onboard scientific imaging system, on Sept. 10, 2014. Jets of cometary activity can be seen along almost the entire body of the comet.

Figure J1. Jets of gas and dust emanating from comet 67P. Credit: ESA/Rosetta/MPS/UPD/LAM/IAA/SSO/ INTA/UPM/DASP/IDA 


JPL - Jet Propulsion Laboratory


JPL was originally tasked by the US Army to develop rocket technology. Following the foundation of the National Aeronautics and Space Administration (NASA) JPL was transferred to that organisation. In the 1970s, 1980s, and early 1990s, NASA focused JPL's expertise on large, complex, one-of-a-kind space missions. This era produced the Voyagers to the outer planets, the Vikings to Mars (in partnership with NASA's Langley Research Center), the Galileo mission to the Jupiter system (in partnership with NASA's Ames Research Center), and Cassini-Huygens to the Saturn system (in partnership with the European Space Agency and the Italian Space Agency).


Useful websites are;

- Near Earth Object Program for producing orbit diagrams and orbital elements of asteroids and comets;

- Horizons web interface for generating ephemeredes of Solar System bodies;


Jupiter family comets


See Ecliptic comets and Harold Levison’s 1996 classification A list of Jupiter family comets can be found here (note that this list defines a Jupiter family comet having a Tisserand parameter >2 and not lying between 2 and 3 – not everyone agrees on how comet should be classified).





Comet C/1973 E1 (Kohoutek), Credit NASA JSC


KLENOT – KLEt’ Observatory Near Earth and Other unusual observations Team and Telescope

The KLENOT project is a project of the KLEť observatory Near earth and Other unusual objects observations Team (and Telescope), concentrating particularly on fainter objects, up to a limiting magnitude of mV=22.0mag..

The Kleť Observatory is a research institution belonging to few natural science centers of Southern Bohemia in the Czech Republic. The observatory, founded in 1957, is situated south of the top of Klet mountain (at altitude of 1070 m), southwest from the town of České Budějovice. The number of clear nights, averaging 150 per year is significantly higher than the average for the Czech Republic, which results from two main factors - altitude of the observatory (1070 m), about two hundred meters above the average top of stratus clouds that typically form above the Czech basin during inversion periods (frequent in autumn and winter), and the dissolving of clouds by the foehn winds in the lee of Alp mountains.


Kracht Group


See here


Kreutz Group


See here




Comet C/2012 X1 (LINEAR), Roger Dymock, 2014 June 30


Large Angle and Spectrometric Coronagraph Experiment  (LASCO)

The SOHO spacecraft is equipped with a number of instruments for solar observation, one of which is LASCO. This instrument monitors the solar corona above the Sun's limb and produces images of the corona in the visible spectrum and with distance off the Sun's center ranging from 1.1 to 32 solar radii. In the innermost range from 1.1 to 3 solar radii observations are made at five different wavelengths. The C1 telescope (no longer operational) was designed at the Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany and was produced in a combined effort with the Naval Research Laboratory, Washington, DC, USA.

The C2 telescope, covering the distance range of 1.5 to 6 solar radii, and C3 (3 to 32 solar radii) were constructed by the Laboratoire d'Astronomie Spatiale, Marseille, France and Naval Research Laboratory, Washington, DC, USA, respectively. The instrument container was built by the Department of Physics and Space Research, University of Birmingham, Birmingham, England. This multinational cooperation was supervised and headed by the instrument's principal investigator Dr. Guenther Brueckner from NRL.



A plot of magnitude versus time of a variable brightness object e.g. comet, asteroid, variable star, nova, super nova. Comet lightcurves allow the absolute magnitude to be calculated and estimates of future brightness made.

Figure L1. Light curve of comet C/2011 L4 (PANSTARRS) from the COBS database


Please note that the CCD magnitudes fit well with the visual mags and allow the light curve to be extended by several months before and after perihelion. The method of generating such magnitudes is explained here. Imagers are encouraged to analyse their images in this way and submit their results to both the COBS database and the BAA Comet Section.


Lincoln Near Earth Asteroid Research (LINEAR)


Lincoln Near Earth Asteroid Research (LINEAR) is an MIT Lincoln Laboratory program funded by the United States Air Force and NASA. The goal of LINEAR is to demonstrate the application of technology, originally developed for the surveillance of Earth orbiting satellites, to the problem of detecting and cataloging near-Earth asteroids/objects (NEOs) that threaten the Earth. LINEAR has discovered 279 comets (to 2015 January 13).


Line of variation


A comet may arrive at a given point in its orbit a little earlier or later than predicted. Its orbit may, for example, have been perturbed (slightly altered) by it passing close to one of the giant planets (usually Jupiter). This may make it more difficult for observers to locate the comet but the Guide planetarium, program can help by plotting the line of variation (the possible location of the comet for a given number of days early or late). Figure L2 shows such a plot for comet 58P assuming the comet may be 3 days early or late. The rectangles are the fields of view for a Starlight Xpress SXV-H9 camera attached to a 10” Newtonian reflector and show how you might cover all eventualities.


Figure L2. Line of variation for comet 58P plotted using Guide




The orbital elements of two or more comets may contain similarities (linkages) which could indicate that they are actually one and the same object. The values of B and L derived from the orbital elements can indicate that two objects observed on different occasions may be one and the same.  These are defined as;

L = longitude of ascending node + arctan(tan(argument of perihelion) x cos(inclination))

B = arcsin(sin(argument of perihelion) x sin(inclination))


The Tisserand parameter is also an indicator of potential linkages.


Longitude of the ascending node (Ω)


See here


Long period comets


Historically (top line in figure L3 below) comets were divided into short period (< 200 yrs) and long period (> 200 yrs). short period comets were those that records showed had made more than one pass through the inner solar system in the past 200 years. Subsequently three classes were recognised – short, intermediate and long. The short period upper limit has varied but is somewhere between 13 and 39 years and intermediate has had two different definitions – including or not including the longer period comets.


Figure L3. Historical comet classification by period


Prior to 1996 the accepted classification was as shown in Figure L4 below. Harold Levison’s classification uses different terminology – and so do many astromomers !!!


Figure L4. Comet classification prior to 1996.


Lowell Observatory Near Earth Object Search (LONEOS)


LONEOS was a project designed to discover asteroids and comet which orbit near to the Earth The project, funded by NASA, was directed by Dr. Ted Bowell of the Lowell Observatory in Flagstaff, Arizona. It began in 1993 and ran until the end of February 2008.





Comet 96P/Machholz 1 SOHO, 2002 January 6-9


M (Mean anomaly)


The angle between the periapsis of an orbit and the position of an imaginary body that orbits in the same period as the real one but at a constant angular speed. The angular speed assigned to the imaginary body is the average angular velocity (or mean motion) of the real orbiting body and is measured from the body being orbited in the direction of orbital motion. It is included in data, for asteroids but not for comets, returned by the MPC Ephemeris Service.


MACE – Meteors, Asteroids and Comets in Europe


A series of meetings for amateur astronomers held in various European locations. See;

2002 -

2006 -

2010 -


More information is available here.



The term "m1" is used to represent the total or integrated brightness of the comet's coma, while "m2" represents the brightness of the nucleus. Visual observers estimate "m1" by memorizing the comet's appearance and then defocusing the surrounding stars to a size equivalent to the comet's coma diameter. The memorized comet's appearance is then compared to the defocused stars to determine the comet's brightness. CCD imagers can calculate the Visual Equivalent or total magnitude, m1, and the nuclear magnitude, m2, using the procedure CCD Astrometry and Photometry on this website.

The total magnitude of a comet is given by the equation;


m1 = H1 + 5.0 log(Δ) + K1log(r)


Where H1 and K1 are constants, Δ is the comet’s distance from the Earth and r its distance from the Sun. If few observations have been made it may not be possible to calculate a value for K1 in which case a value of 10 is assumed and the first constant becomes H10. For short period comets, those with a period of less than 200 years, a value of 15 is assumed for K1 and the first constant becomes H15.


A full explanation can be found in the December 1995 issue of the Journal of the British Astronomical Association – ‘Comet analysis’ by Jonathan Shanklin.


Main Belt comets


Main belt comets are objects that display cometary activity, e.g. signs of a coma, outgassing , yet have orbits indistinguishable from those of Main Belt Asteroids.  Comets classified as such are;

- 311P/PANSTARRS aka P/2013 P5

- 596 (Schella)

- 7698 (Elst-Pizarro) also known as P/1996 N2 and 133P

- 118401 (LINEAR) aka 1999 RE70 and 176P/LINEAR

- 300163 aka 2006 VW139

- P/2005 U1 aka 238P/Read

- P/2008 J2 (Belshore)

- P/2008 R1 (Garradd) aka 259P/Garrard

- P/2010 A2 (LINEAR)

- P/2010 R2 (La Sagra)

- P/2012 F5 (Gibbs)

- P/2012 T1 (PANSTARRS)

- P/2013 R3 (Catalina-PANSTARRS)


The orbit of 118401 (LINEAR) is shown in Figure M1 showing it would be classified as an Encke Type Ecliptic comet. An indication of its activity is shown in Figure M2 where it can be seen that the comet is only active close to perihelion.



Figure M1. Orbit of comet 118401 (LINEAR). Credit JPL NEO Program


118401 orbit

Figure M2. An indication of when comet 118401 (LINEAR) is active


Marsden Group


A category of Sunskirting comets with inclinations of 25º


Meteor showers


Comets are the parent bodies of most, but not all, meteor showers as listed in the table below. This table was extracted from




Parent object


Early January

The same as the parent object of minor planet 2003 EH1, and perhaps comets C/1490 Y1 and C/1385 U1 


late April

Comet Thatcher

Pi Puppids (periodic)

late April

Comet 26P/Grigg-Skjellerup

Eta Aquariids

early May

Comet 1P/Halley


mid June

Comet 96P/Machholz, Marsden and Kracht comet groups complex 

June Bootids (periodic)

late June

Comet 7P/Pons-Winnecke

Southern Delta Aquariids

late July

Comet 96P/Machholz, Marsden and Kracht comet groups complex 

Alpha Capricornids

late July

Comet 169P/NEAT[



Comet 109P/Swift-Tuttle

Kappa Cygnids


Minor planet 2008 ED69

Aurigids (periodic)

early September

Comet C/1911 N1 (Kiess)

Giacobinids (periodic)

early October

Comet 21P/Giacobini-Zinner


late October

Comet 1P/Halley

Southern Taurids

early November

Comet 2P/Encke

Northern Taurids


Minor planet 2004 TG10 and others

Andromedids (periodic)


Comet 3D/Biela

Alpha Monocerotids (periodic)





Comet 55P/Tempel-Tuttle

Phoenicids (periodic)


Comet D/1819 W1 (Blanpain)



Minor planet 3200 Phaethon  (possibly a dead comet –RD)


late December

Comet 8P/Tuttle


The Meteor Section of the British Astronomical Association can be found at


Meyer Group


A category of Sunskirting comets with inclinations of 72º


MOID - Minimum Orbit Intersection Distance


The MOID is the distance between the closest points of the osculating orbits of two bodies. Of greatest interest is the risk of a collision between e.g. a comet or asteroid and the Earth. The distance between the Earth and such a body is known as the Earth MOID. A list can be found here. If maths is your thing then you might like to read ‘Fast Geometric Method for Calculating Accurate Minimum Orbit Intersection Distances’ by T. Wisniowski and H. Rickman.


MPC - Minor Planet Center


The Minor Planet Center is the single worldwide location for receipt and distribution of positional measurements of minor planets, comets and outer irregular natural satellites of the major planets. The MPC is responsible for the identification, designation and orbit computation for all of these objects. This involves maintaining the master files of observations and orbits, keeping track of the discoverer of each object, and announcing discoveries to the rest of the world via electronic circulars and an extensive website. The MPC operates at the Smithsonian Astrophysical Observatory, under the auspices of Division F of the International Astronomical Union (IAU).


Useful webpages on this site which can be accessed from here;

- CMTChecker                                                    checks possible new comet suspects against known objects

- MPChecker - Minor Planet Checker                checks possible new minor planet suspects against known objects

- On-line MPECs                                                lists latest confirmed discoveries plus recent observations

- NEOChecker                                                    checks possible new NEO suspects against known objects

- NEOCMTChecker                                            checks possible new NEO and comet suspects against known objects

- NEO Confirmation Page                                  lists recently discovered objects requiring confirmation


MOTESS - Moving Object and Transient Event Search System


MOTESS was designed and constructed by Roy A Tucker.  It is an array of three thermally compensated telescopes, each with a thermoelectrically cooled CCD camera rigidly mounted at the focus and operated in continuous scan mode. 





Comet C/2002 V1 (NEAT), SOHO, 2012 February 18


Nearly Isotropic comets

- New

- Returning

               - External

               - Halley Type


NEAT - Near Earth Asteroid Tracking


NEAT was a program run by NASA and Jet Propulsion Laboratory to discover Near-Earth Objects. The NEAT project began in December 1995 and ran until April 2007.


Non-gravitational forces


The accuracies of the orbits and ephemerides for active comets are most often limited by imperfectly modeled rocket-like accelerations experienced by active comets as a result of the outgassing cometary nucleus near perihelion. A paper by D.K.Yeomans and P.W.Codas can be viewed here.


Non-group SOHO comets


Sungrazing or Sunskirting comets which do not belong to any of the recognised groups. Figure N1 shows one such comet.

Figure N1. SOHO comet 2875, 2015 February 20. Credit SOHO




The nucleus is the solid, central part of a comet, popularly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases. A typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust. Figure N2 shows the nucleus of comet 9P/Tempel approximately 5 minutes before Deep Impact's probe smashed into its surface.

Figure N2. Nucleus of comet 9P/Tempel. Credit NASA/JPL-Caltech/UMD


Numbered comets


When a comet has been observed over two perihelion passages it is given a permanent number e.g. 293P/Spacewatch. The assignment of periodic comet numbers, is the responsibility of the Minor Planet Center and a list can be found here




Oort Cloud


The Oort Cloud is a giant shell of icy bodies which encircles the solar system at a distance of approximately 100000 AU. In 1932 Ernst Opik, an Estonian astronomer, proposed that comets originate in an orbiting cloud. In 1950 the idea was revived by Dutch astronomer Jan Hendrick Oort hence the name but perhaps it should properly be called the Opik-Oort cloud. When its inhabitants interact with passing stars, molecular clouds, and gravity from the galaxy, they may find themselves spiraling inward toward the sun, or cast completely out of the solar system. Long period comets such as C/1995 O1 (Hale-Bopp) are thought to originate in the Oort Cloud


Kuiper Belt Oort Cloud

Figure O1. The Oort Cloud. Credit NASA


Orbital elements


The path followed by a comet around the Sun is defined by seven numbers (the orbital or Keplerian elements, after Johannes Keplar) plus the epoch (date) for which those numbers are valid, as described in Table O.2 and shown in Figures O2 and O3. The column headed ‘MPC notation’ lists the abbreviations used by that organisation – an example for comet 2P/Encke follows.




MPC notation





The date for which the values of the elements listed below are valid

Argument of perihelion



Defines how the major axis of the orbit is oriented in the orbital plane and is the angle between the ascending node and the perihelion point measured in the plane of the orbit in the direction of motion.




A measure of the deviation of the orbit from a circle, e = c/a. and for a circle e = 0. Many comet orbits tend to be highly elliptical with e approaching 1




The angle between the plane of the orbit of a comet and the ecliptic. If  the inclination is > 90º then the motion of the object is considered to be retrograde.

Longitude of the ascending node


The direction in space of the line where the orbital plane intersects the plane of the ecliptic. It is measured eastward (increasing RA) in the plane of the ecliptic from the Vernal Equinox (First Point of Aries).

Perihelion distance



The shortest distance between the comet and the Sun during its orbit

Semi-major axis



Half the length of the long axis of the orbit.

Time of perihelion



The date and time at which the comet is closest to the Sun


Table O.1. Orbital elements




Figure O2. Orbital elements.


The closest point of the orbit to the Sun, Perihelion, can be calculated using the formula:

q = a(1-e) and the furthest point, Aphelion, is given by Q = a(1+e).


2P orbit


Figure O.3. Orbit diagram of comet 2P/Encke. Courtesy NASA/JPL-Caltech with additional data and graphics by Roger Dymock


Orbital elements are freely available from the MPC and orbit diagrams from the Jet Propulsion Laboratory (JPL). The data below was obtained from the MPC’s Minor Planet Ephemeris Service website.


Epoch 2013 Nov. 4.0 TT = JDT 2456600.5
T 2013 Nov. 21.6945 TT 
Peri.  186.5356               
e   0.848232 
Incl.   11.7790
Node   334.5731
q   0.336127 
a   2.214743  
T = 2456618.19453 JDT




All objects move around their parent planet, star or our Sun in orbits which, depending on their shape may be described as elliptical (0<e<1), parabolic (e=1) or hyperbolic (e>1) - figure O4. Objects in hyperbolic orbits around the Sun will have arrived from interstellar space and will leave the Solar System (unless their orbit is modified by a close pass of one of the giant planets). Orbits are described by their orbital elements.




Figure O4. Orbits


Osculating elements/orbit


The osculating orbit of an object in space at a given moment in time is the orbit it would have about its central body if perturbations were not present.  An osculating orbit and the object's position upon it can be fully described by the six standard Keplerian orbital elements (osculating elements). The osculating elements would remain constant in the absence of perturbations. However, real astronomical orbits experience perturbations that cause the osculating elements to change over time. This is particularly true for Small Solar System Bodies, asteroids and comets, the orbits of which can be significantly altered when they pass close to the much larger planets.




Major cometary outbursts are due to the melting of cometary ices contained within the microporous gaps in the body of the comet. (e.g. water, methanol and hydrocarbons).The ices sublimate (turn directly from solid to gas) and carry dust away from the surface of the comet to form the coma. Comet 29P is prone to such outbursts as shown in Figure O5.


Figure O5. Comet 29P before and during outburst


Richard Miles explained this phenomenon in two talks to the British Astronomical Association. See;



Comet 12P/Pons-Brooks, George Alcock, 1954 March 31




The Panoramic Survey Telescope & Rapid Response System -- is a wide-field imaging facility developed at the University of Hawaii's Institute for Astronomy. The combination of relatively small mirrors with very large digital cameras results in an economical observing system that can observe the entire available sky several times each month. A major goal of Pan-STARRS is to discover and characterize Earth-approaching objects, both asteroids & comets, that might pose a danger to our planet. Images of comets discover by PanSTARRS can be found here.


Palomar Sky Survey


The National Geographic Society – Palomar Sky Survey (NGS-POSS) was a photographic survey of the night sky completed at the Palomar Observatory in 1958. The photographs were taken with the 48-inch Samuel Oschin telescope at the Palomar Observatory and the Survey was funded by a grant from the National Geographic Society to the California Institute of Technology. 


Parabolic orbit


See here


Perihelion (q)


The point in a comet's orbit (refer to Orbital elements and Orbits) and when it is closest to the Sun. Equally applies to any celestial body orbiting the Sun


Periodic comets


Periodic comets are comets whose orbital periods are well defined. They receive a permanent number after they have been observed during two perihelion passages. Examples are P/2005 T5 (Broughton) which has yet to make its second perihelion passage and is thus unnumbered and 2P/Encke which was discovered in 1786 and, with a 3.3 year period, has made numerous turns around the Sun.




The orbit of a comet (or asteroid) may be (sometimes quite significantly) perturbed (altered) by the effect of gravity due to close passes of more massive bodies - typically Jupiter. Such perturbations can cause periodic variations in the comet's orbital elements, as when one orbit is a multiple of the other, whereas others lead to variations which are chaotic (irregular in nature). A fuller explanation can be found here. Non-gravitational forces such as outgassing also cause changes in a comet's orbit.




The measurement of the brightness of a celestial object. For comets the brightness is expressed in terms of the nuclear and total magnitude. 


Populations of comet-like bodies in the Solar System – Horner, Evans, Bailey and Asher


This classification scheme covers the traditional comets plus Centaurs and Edgeworth-Kuiper Belt Objects. Comets are grouped according to the planets controlling them at perihelion and aphelion. For example a SN comet is controlled at perihelion by Saturn and at aphelion by Neptune. Table P1 shows the classification for the region of the Solar System beyond Jupiter. See the referenced paper for a complete description of all groups









6.6 ≤ q ≤ 12.0

Q ≤ 12.0


12.0 ≤ q ≤ 22.5

33.5 ≤ Q ≤ 60.0


6.6 ≤ q ≤ 12.0

12.0 ≤ Q ≤ 22.5


12.0 ≤ q ≤ 22.5

Q ≥ 60.0


6.6 ≤ q ≤ 12.0

22.5 ≤ Q ≤ 33.5


22.5 ≤ q ≤ 33.5

Q ≤ 33.5


6.6 ≤ q ≤ 12.0

33.5 ≤ Q ≤ 60.0


22.5 ≤ q ≤ 33.5

33.5 ≤ Q ≤ 60.0


6.6 ≤ q ≤ 12.0

Q ≥ 60.0


22.5 ≤ q ≤ 33.5

Q ≥ 60.0


12.0 ≤ q ≤ 22.5

Q ≤ 22.5


33.5 ≤ q ≤ 60.0

Q ≤ 60.0


12.0 ≤ q ≤ 22.5

22.5 ≤ Q ≤ 33.5


33.5 ≤ q ≤ 60.0

Q ≥ 60.0

Table 1. Classification of comets beyond Jupiter


Position angle


A comet's tail is measured in terms of length and position angle (PA). The position angle is measured from north through east - in Figure P.1 the PA is approximately 225°




Figure P.1. Comet's tail length and position angle


Prograde orbit


The inclination, i, of a comet's orbit, the angle between that and the ecliptic, defines whether it is prograde or retrograde. A value of i between 0° and 90° degrees indicates a prograde orbit whereas a value between 90° and 180° indicates a retrograde orbit.


Proper elements


The proper orbital elements of an orbit are constants of motion of an object in space that remain practically unchanged over an astronomically long timescale. The term is usually used to describe the three quantities: proper semimajor axis, proper eccentricity and proper inclination. A paper on the subject (as it relates to asteroids) can be found here.




q – perihelion distance


The distance of a comet from the Sun when it is closest to the Sun. See also here and here


Q – aphelion distance


The distance of a comet from the Sun when it is furthest from the Sun




R magnitude


The magnitude of a celestial object measured using a Johnson-Cousins red filter. The Johnson-Cousins UBVRI system, Figure R.1, which was described in the nineteen-fifties by Harold Johnson and modified a few decades later by A.W.J. Cousins, is one of the most widely used photometric systems.

Figure R.1. Astrodon Johnson-Cousins filter passbands


Retrograde orbit


The inclination, i, of a comet's orbit, the angle between that and the ecliptic, defines whether it is prograde or retrograde. A value of i between 0° and 90° degrees indicates a prograde orbit whereas a value between 90° and 180° indicates a retrograde orbit.


Robotic telescopes

- BAA Robotic Telescope Project


The Robotic Telescope Project was set up in 2008 and allows BAA members access to remote telescopes and imaging systems at attractive rates. Members are able to use the service at half the commercial rate up to a limit, then at full rate, and are provided with access to a wide range of equipment beyond a private budget. It also allows users to benefit from observing from a location with a better climate than Britain's. Although individual members are able to use the service for their own purposes, the project enables groups within the BAA – perhaps organised via the Observing Sections – to undertake collaborative projects, which can be educational or more research-oriented.


The Association uses the telescopes of the Sierra Stars Observatory Network (SSON) – a 61-cm f/10 Cassegrain research-grade telescope located on the eastern side of the Sierra Mountains in California (G68), the University of Iowa’s 37-cm f/14 Rigel Telescope in Sonoita, Arizona (857) and the Mt. Lemmon 81cm f/7 Ritchey-Chrétien Telescope in Tucson, Arizona (G84). The CCD images are around 1,500 x 1,500 pixels, covering about 20 x 20 arcmin, and with dark frame and flat field already applied. This reasonably large image size is ideally suited for deep sky objects, comets, variable stars, novae, supernovae, and asteroids. The 3-Mb zip file containing the image in FITS format is usually available for FTP download a few hours after acquisition.


Roche limit


The Roche limit is the distance within which a smaller secondary celestial body, held together only by its own gravity, will disintegrate due to the larger primary celestial body's tidal forces exceeding the first body's gravitational self-attraction. Inside the Roche limit, orbiting material disperses and forms rings whereas outside the limit material tends to coalesce. The term is named after Édouard Roche, the French astronomer who first calculated this theoretical limit in 1848.

Comet C/2012 S1 (ISON) passed inside the Roche limit during its perihelion passage around the Sun which may have been the (partial) cause of its demise, Figure R2.


 Figure R2, Roche Limit example


Comet 29P/ ,Damian Peach, 2013 July 31

Images obtained during June-July detailing comet’s recent outburst. The expanding tenuous coma and fading towards the most recent images is clearly apparent.


Saturn family comets


Such comets have a node and/or aphelion near Saturn. According to a paper The orbital history of two periodic comets encountering Saturn by Lagerkvist, Hahn, Karlsson and Carsenty the existence of a Saturn family still remains to be proven. Possible family members are; P/1997 T3 (Lagerkvist-Carsenty) and P/1998 U3 (Jager).


Scattered Disk


The Scattered Disk, a subset of Trans-Neptunian Objects, is a sparsely populated region of icy bodies extending from 30 AU to well beyond 100AU from the Sun. Figure S2 shows the semi-major axes and inclinations of all known scattered-disc objects (in blue) up to 100 AU together with Kuiper-belt objects (in grey) and resonant objects (in green). The eccentricity of the orbits is represented by segments (extending from the perihelion to the aphelion) with the inclination represented on Y axis.

Figure S2. Wikipedia image plotted by a program written by the User Eurocommuter


Short period comets


Historically comets were divided into short period (< 200 yrs) and long period (> 200 yrs). Short period comets were those that records showed had made more than one pass through the inner solar system in the past 200 years. Subsequently three classes were recognised – short, intermediate and long – Figures S3 and S4. The short period upper limit has varied but is somewhere between 13 and 39 years and intermediate has had two different definitions – including or not including the longer period comets.


Figure S3. Historical comet classification by period


Figure S4. Comet classification prior to 1996


Small Solar System Body


IAU Resolution B5 defined the various Solar System objects. i.e.



Definition of a Planet in the Solar System


Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.


The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the

following way:


(1) A planet1 is a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit.


(2) A "dwarf planet" is a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2,

(c) has not cleared the neighbourhood around its orbit, and

(d)is not a satellite.


(3) All other objects except satellites, orbiting the Sun shall be referred to

collectively as "Small Solar System Bodies".


I should point out that since this resolution was adopted I have never seen a reference to ‘Small Solar system Bodies’  - they are still known as asteroids and comets


Solar and Heliospheric Observatory (SOHO)


SOHO is a project of international collaboration between ESA and NASA to study the Sun from its deep core to the outer corona and the solar wind. SOHO was launched on December 2, 1995. The SOHO spacecraft was built in Europe by an industry team led by prime contractor Matra Marconi Space (now EADS Astrium) under overall management by ESA. The twelve instruments on board SOHO were provided by European and American scientists. As shown in Figure S5 the spacecraft is located at the First Lagrangian Point (L1) approximately 1.5 million km from Earth in the direction of the Sun.



Figure S5 Location of the SOHO spacecraft


SOHO Comets


Many comets have been discovered by amateur astronomers/citizen scientists examining images from the Large Angle and Spectrometric Coronograph Experiment (LASCO) cameras. A tutorial Finding comets on SOHO images can be found on this website. Figure S6 shows a very bright (for SOHO) comet.


Figure S6. The Christmas Comet, C/1996 Y1, discovered on 1996 December 23


Solar conjunction


Solar conjunction occurs when a comet, or any other Solar System body, is on the opposite side of the Sun from the Earth.




SPACEWATCH® is the name of a group at the University of Arizona's Lunar and Planetary Laboratory founded by Prof. Tom Gehrels and Dr. Robert S. McMillan in 1980.  Today, Spacewatch is led by Dr. Robert S. McMillan.  The original goal of Spacewatch was to explore the various populations of small objects in the solar system, and study the statistics of asteroids and comets in order to investigate the dynamical evolution of the solar system.  CCD scanning studies the Centaur, Trojan, Main-Belt, Trans-Neptunian, and Earth-approaching asteroid populations.  Spacewatch also found potential targets for interplanetary spacecraft missions. Spacewatch currently focuses primarily on follow-up astrometry of such targets, and especially follows up objects that might present a hazard to the Earth.




Sublimation is the process by which a solid changes to a gas (without going through a liquid phase). When a comet approaches the Sun ices inside the nucleus escape in gaseous form leading to the formation of a coma and/or a tail.


Sungrazer and Sunskirters


As the name suggests these comets pass close to the Sun and many of them may have originated from a single object. They can be subdivided in to two categories;

- sungrazers which have perihelion distances generally =< 2 solar radii (they don’t survive their perihelion passage)

- sunskirters which have perihelion distances between 6 and 12 solar radii (their greater distance form the Sun means that they do survive perihelion passage)


Sungrazers can be further subdivided in to two groups;

- Kreutz I  

- Kreutz II  



Figure S1. Kreutz group I comet C/1996 Y1 (SOHO) imaged by the LASCO C2 camera. Credit SOHO/LASCO


Sunskirters consist of the following groups;

- Kracht, 

- Kracht II

- Marsden

- Meyer


Note inclinations may vary by a few degrees for specific comets in the above groups. A full list of sungrazing comets can be found on the BAA Comet Section website here and a detailed description here


Many sungrazing comets have been discovered by amateur astronomers searching images obtained by the SOHO and STEREO spacecraft. A presentation on the Project Alcock website explains how this is done. A number of websites which may be of help are listed on the last slide but one of this presentation.


Solar Wind ANisotropies (SWAN)


The SWAN Instrument is designed to observe the solar Lyman alpha photons (121.6 nm) backscattered by the neutral hydrogen atoms present in the interplanetary medium. The background images show the distribution pattern of the backscattered Lyman alpha photons as observed from SOHO. This image, obtained in approximately 24 hours, is then processed to reveal spatial variations of the solar illuminating flux. These spatial variations are correlated to the actual activity on the solar disk. Because SWAN observes backscattered photons, it is actually possible to 'see' those which are originating from the far side of the Sun. Activity on the Sun's far side (Figure S2 left) is revealed by the glow of hydrogen gas in space, lit by ultraviolet rays from the Sun and detected by SWAN on SOHO. It is charted as if observed from a position beyond the Sun. This view joins with a corresponding impression of near-side activity (Figure S2 right) to cover the whole sky. The Sun's rotation carries the activity from left to right in both images. Black areas are regions not observed by SWAN.

Figure S2. SWAN images of far side and nearside of the Sun


Swan bands


Swan bands are a characteristic of the spectra of carbon stars, comets (Figure S3) and of burning hydrocarbon fuels. They are named for the Scottish physicist William Swan who first studied the spectral analysis of radical Diatomic carbon C2 in 1856. Swan bands consist of several sequences of vibrational bands scattered throughout the visible spectrum.


Image result for swan bands in comets

Figure S3. Comet spectrum showing diatomic carbon bands






- Anti


A comet’s tail that, as viewed from Earth, points towards the Sun rather than away from it – Figures T1, T2 and T3.


Comet PANSTARRS' orbital plane slices (marked by gray lines) slices right through the plane of the planets. Earth crosses that orbital plane on May 27. As we look up into space at the comet (blue arrow), all the dust it shed along its path - including a fine sheet of particles - stacks up to create a narrow, streak-like tail pointing toward the sun. The shorter, active dust tail sticks up and away (top). Credit: NASA with my own additions


Figure T1. Relative positions of Earth, comet and Sun necessary to produce an anti-tail.


Comet PANSTARRS’ orbital plane slices (marked by gray lines) right through the plane of the planets. Earth crosses that orbital plane on 2013 May 27. As we look up into space at the comet (blue arrow), all the dust it shed along its path – including a fine sheet of particles – stacks up to create a narrow, streak-like tail pointing toward the sun. The shorter, active dust tail sticks up and away (top). Credit: NASA

anti tail


Figure T2.Comet C/2011 L4 PANSTARRS on 2013 May 2013 when its anti-tail (right) had grown to more than 6º in length. Credit: Damian Peach

There is another explanation as shown in Figure A6 below.


File:Anti tail.gif


Figure T3. Relative positions of Earth, comet and Sun necessary to produce an anti-tail.


- Disconnection


- Dust (Type II or III)


In Figure T4 the dust tail is the broad vertically oriented tail. Such tails are formed by dust grains released from the nucleus, primarily on the sunward side, as sublimation of ices by solar heating occurs. The shape of the tail is defined by the positions of dust particles previously emitted by the comet which will be influenced by the speed at which they leave the nucleus and solar radiation pressure.


File:Comet Hale-Bopp 1995O1.jpg


Figure T4.Comet C/1995 O1 (Hale-Bopp)


- Ion, Plasma or Type I


In Figure D1 the ion tail is the blue tail to the left of the broader dust tail. It is formed by the ionisation of particles in the coma by solar ultra-violet radiation. The ions are propelled radially away from the comet by the solar wind. Ion tails may be as long as 150 million km.


- Position angle


See here


- Sodium


Observations of comet C/1995 O1 (Hale-Bopp) showed that the comet had a third type of tail. In addition to the more common dust and ion tails, this comet also exhibited a faint sodium tail (Figure T5) only visible with powerful instruments with dedicated filters. Sodium emission had been previously observed in other comets, but had not been shown to come from a tail. Hale–Bopp's sodium tail consisted of neutral atoms (not ions), and extended to some 50 million kilometres in length.

Figure T5. Comet Hale-Bopp’s sodium tail – the straight tail from bottom centre to upper left


- Solar wind, effect of


- Steamers


- Striae


- Sun’s magnetic field, effect of


Tisserand parameter


The Tisserand parameter or Tisserand's invariant is a value calculated from several orbital elements (semi-major axis, orbital eccentricity and inclination) of a relatively small object and a larger "perturbing body". It is used to distinguish different kinds of orbits. It is named after French astronomer Félix Tisserand.


TJ, the Tisserand parameter with respect to Jupiter as perturbing body, is frequently used to distinguish the various families of ecliptic comets.


Trans Neptunian Object (TNO)







V magnitude




Water production


WISE (Wide field Infrared Survey Satellite)






Comets designated with an X prefix are those whose orbits are undefined (typically those defined in historic literature). The ICQ Comet names and designations page lists such comets.