Comets – I

 

Before starting, one of the best places to go for information about comets is http://cometobservation.com/ . Another site that many astronomers use (even pasting images from it into research proposals) is Seiichi YoshidaÕs web site : http://www.aerith.net/index.html

 

 

Nomenclature

 

Names

 

Traditionally, comets have been named for their discoverers. In the case of co-discovery, up to 3 names were listed (recently reduced to 2). Example: Comet Hale-Bopp. The exceptions were Halley and Encke, which were named for the people who first computed orbits for these two comets. An interesting case was Comet IRAS-Araki-Alcock. Here one of the co-discoverers was the team off scientists operating the Infrared Astronomical Satellite (IRAS), which was making the first IR map of the sky, beginning in 1983.

 

Some were also given names such as ÒThe Great Comet of (year)Ó.

 

Numbers

 

Gradually, a numbering system based on the year of discovery or recovery was used, using the year and a roman numeral for the order in that year: i.e. Comet West 1976 VI.

 

This was eventually replaced with a system like that used for newly-discovered asteroids: the year followed by a Roman letter for the half-month, and an Arabic number for the order within the half-month. Comet Hale-Bopp is also designated as Comet 1995 O1, being the first comet discovered in the first half of August 1995. Keeping the naming the same as that of asteroids is useful, as the identity (comet vs. asteroid) may not be obvious immediately, or even after a number of years! In the past few years, the ÒasteroidÓ Chiron has develop cometary structure!

 

One also sees a designation that indicates the order of perihelion passage: 1986a would be the first comet to go through perihelion in 1986. This name can only be made once the order of all the comets of that year has been established.

 

P and C

 

 Once the orbit of a comet is firmly established after more than one orbit, it may receive a designation prefix P/ as in 1P/Halley, 3P/Encke, 69P/Taylor. A nice listing of numbered periodic comets can be found at Gary Kronk's Comets and meteor Showers web site. Those that havenÕt reached this status (usually because their periods are too long to have been observed for more than one perihelion passage) get the asteroid-like designation, usually prefixed with a C/. Thus Hale-Bopp is actually C/1995 O1.Most comets with periods less than 200 years are specifically called ÒperiodicÓ and will usually have the P/ in from of their names. If they have longer periods they will get the C/, at least for a while! Strictly speaking, comets like Hale-Bopp are periodic, even if their periods are thousands of years. For this reason, we often refer to those with P < 200 yrs as Short Period (SP) comets, and those with longer periods as Long Period (LP) comets.

 

The official names are given by the folks at the Minor Planet Center http://cfa-www.harvard.edu/iau/mpc.html  at the Smithsonian Astrophysical Observatory. Here you will also find a more complete description of the Cometary Designation System http://cfa-www.harvard.edu/iau/lists/CometResolution.html . A converter page exists for going between old & new designations http://cfa-www.harvard.edu/iau/CometDes.html

 

 

The Oort Cloud, Kuiper Belt, and Comet Families

 

The majority of bright comets have exceedingly high orbital eccentricities, and a random distribution of orbital inclinations. This means that:

 

1.     The come from far away

2.     They come from all directions

 

Around 1952, Jan Oort realized that this could only be true if the source of these comets was a huge quasi-spherical cloud stretching to tens of thousands of AU from the Sun. We now refer to this as the Oort Cloud. Due to gravitational tidal effects of the Milky way galaxy as a whole, and some individual close stellar passages, these objects get stirred up gravitationaly, and occasionally one finds itself heading inward toward the Sun.

 

 

 

Most of the short-period comets have low inclinations (less than 7 degrees) and prograde orbits, if they were retrograde, their inclinations are numerically negative. Halley is an example of a comet with a highly inclined and even retrograde orbit. But the majority are of the first kind. About the same time Oort made his suggestion for the origin of the Oort Cloud comets, Gerard Kuiper suggested that the SP comets had their origin in a disk beyond the orbit of Neptune. The objects in this Kuiper Belt (or more accurately Kuiper Edgeworth Belt since Edgeworth seems to have suggested it independently) should be the remnant of the SunÕs propostellar disk, and still have all those planesimals/cometesimals in it.

 

We have not seen the Oort Cloud directly (with one possible exception, below), but hundreds of Kuiper Belt Objects (KBOs) are now known.

 

CREATOR: XV NOAO Version 3.10a Rev: 12/29/94 Quality = 75, Smoothing = 0

Relative sizes of some KBOs (Pluto is one, also)

 

ArtistÕs rendition of the Kuiper Belt – but they wonÕt be this densely-packed

 

 

Sedna (2003 VB12) is the record-holder for distant objects in the solar system so far. With an orbit that carries it from under 90 AU to over 800 AU.

SOFTWARE: Adobe Photoshop CS Macintosh DATE: 2004:03:12 13:40:20 Ignored Tags: $0152, $02BC, $A001, $A002, $A003

 

You can see a plot of the locations of KBOs & Centaurs at: http://cfa-www.harvard.edu/iau/lists/OuterPlot.html and David Jewitt has a wonderful web site on KBOs & Centaurs at http://www.ifa.hawaii.edu/faculty/jewitt/kb.html

 

In summary, the comets are objects from the Oort Cloud and Kuiper Belt that have been perturbed into orbits that bring them to the inner solar system.

 

Comets - Dynamical Groups:

Dynamically New - DN: 1/a < 50x10-6 AU-1

Young Long Period - YLP: 50x10-6 < 1/a < 2000x10-6 AU-1

Old Long Period - OLP: 2000x10-6 < 1/a < 2.9x10-2 AU-1(P > 200 yr)

Halley Family - HF: P < 200 yr and TJ < 2  (TJ is a dynamic variable we wonÕt get into).

Jupiter Family - JF: P < 200 yr and TJ ³ 2

 

Adobe ImageReady

 

In addition, there is a special group of comets, the Kreutz sungrazers, that follow similar orbits, and many come very close to the Sun. The great comet C/1965 S1 Ikeya-Seki was a bright member of this family.

The ÒstandardÓ picture of Ikeya-Seki (Roger Lynds)

 

Coronagraphic image (Moriyama & Hirayama)

 

It was said that one could see the comet in the daytime by just blocking the sun out with your hand! It probably reached a magnitude of –10 to –11 at its brightest. Coronagraphic observations showed it may have broken into 3 part just 30 minutes before perihelion.

 

 

 

 

Numerous comets found by the SOHO coronagraphs ate also Kreutz comets. It is believed that they originate from the breakup of a large object long ago.

 

Movies made by SOHO of two recent comets:

 

http://www.physics.uc.edu/~sitko/AdvancedAstro/26-Comets-I/CometMovies/KF_SOHOmovies/C2comet_all_4

 

http://www.physics.uc.edu/~sitko/AdvancedAstro/26-Comets-I/CometMovies/KF_SOHOmovies/C3comet_zm

 

http://www.physics.uc.edu/~sitko/AdvancedAstro/26-Comets-I/CometMovies/NEAT_SOHOmovies/NEAT1 - my favorite!

 

http://www.physics.uc.edu/~sitko/AdvancedAstro/26-Comets-I/CometMovies/McN_SOHOmovies/sohoMcNaught_sm.mov note that the detector is saturatingÉ

 

 

 

 

 

 

 

 

Although it was not a Kreutz group comet, C/2006 P1 McNaught was the brightest comet since Ikeya-Seki, and also visible in the daytime. Unfortunately for observers in the northern hemisphere, it was very difficult to observe at its brightest, and some of the most spectacular images came from observers south of the equator.

 

 

Origin of the Oort Cloud

 

While the origin of the Kuiper-Edgeworth is relatively ÒstraightforwardÓ, that of the Oort Cloud is not. Where did it come from?

 

It is now thought that the Oort Cloud resulted from objects in (roughly) the Uranus-Neptune region being gravitationally scattered outward by interactions with these two planets which respond by their own migration.

 

The orbital evolution of one test body to gravitational perturbations by a two-planet system.

 

 

 

 

 

 

Such interactions ought to produce a true ÒouterÓ classical Oort Cloud, but also an inner Oort Cloud, of which Sedna might be a member.

 

Objects in the Jupiter-Saturn zone would be given a strong enough kick to leave the solar system entirely, of even (in some cases) be sent toward a solar destruction.

 

The bottom line: the currently more distant (Oort Cloud) comets were originally formed closer to the sun than the currently closer (Kuiper Belt) ones!