MASTER ROSTER OF COMETS
---------------------
John Pazmino
NYSkies Astronomy Inc
nyskies@nyskies.org
www.nyskies.org
1996 November 1 initial
2020 November 13 current
Inroduction
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My interest in comet orbits, aroused with my first comet Arend-
Roland in 1957, ebbed and flowed over the years. Now it's on a high
from the recent flurry of comet activity -- deVico, Hyakutake, Hale-
Bopp. Maybe I should get my comet stuff onto a proper computer
database? There are, of course, all kinds of lists in both the paper-
based and digital literature. These I drew on for my own consolidated
database.
Comet number scheme
-----------------
The first thing I needed was a cross-index of the several
numbering schemes for comets. Until 1995 a comet had two designations.
The first was a discovery number. This is the year of discovery (or
recovery for a periodic comet) plus a serial letter. Comet 1982-i is
the ninth comet found in 1982. It happens to be Halley's Comet.
Later the comet gets a perihelion number. This is the year of
perihelion plus a Roman serial number. 198-6III is the third comet to
round perihelion in 1986. Yes, it's Halley again. This perihelion
number replaces the discovery number in the comet's record. (Note that
Halley was found four years before perihelion, an incredible feat
which brought major attention to the then-new CCOgraphy.)
New-style scheme
--------------
From 1995 an all-new system went into effect. Each new comet gets
a year-letter-number designation, much like the old discovery number.
But this new-style number is also the comet's archival number.
A new comrt gets a year of discovery, a lwtttr for the half-month
of discovery, and a sequence number within the half-month. A half-
month is the days 1-15 or days 16-end of each calendar month. There
being 24 half-months and 26 letters, 'I' and 'Z' are not used.
1995-O1 is the first comet found in half-month 'O', end July, in
1995. This is comet Haale-Bopp and its new-stule number is its
permanent ddesignation.
Under the new-style rules comets are no longer named for its
discoverers, but the custom is so deeply seated in astronomy that IAU
continues to refer to comets by proper name. The name is after the
new-style number in parens. It did drop, with the 1986 return of
Halley's comet, the practice of attaching the recoverer's name to the
comet on each retUrn.
Periodic comets are treated a bit inconsistently in the new-style
system. A returning comet may get a new-style number or it may be
logged in under its sequence number. The latter is assigned in the
order a comet is proven to be a periodic one. So Comet d'Arrest, altho
it was picked up on its return in 1995, gets no new-style number. It
is merely Comet 6P/d'Arrest, the sixth comet to make it into the ranks
of periodic. comets.
A major wrinkle is the retrofitting of new-style numbers to all
the old comets. Halley's Comet is given the designation 1982-U1,
replacing the former 1982i or 1986111. Hence, depending on the vintage
of the literature you are reading, a comet may have one of three
nUmbers, in addition to its proper name.
Keeping track
-----------
One problem with the new-style system is that there is now no
simple way to catch 'missing' comets. In the old method you spot the
missed out letter or Roman number. For my own sake I continue the
perihelion and discovery designations by using these columns, vacant
in the new-style system, for the actual month and day of perihelion or
discovery. This ensures that any rogue comets are spotted by the
absence of one or the other date. '
Now with all the designations cross-indexed, no comet can escape
my notice. I can find any comet knowing any of its numbers or name. If
it be a periodic comet, I turn up a list for all of its returns. This
also catches perioic comets with misse return.,Comet de Vico after a
couple returns was lost, until just recently recovered.
coemts versus asteroids
---------------------
In the discovery of comets, it happened from time to time that a
'comet', given a comet number, is later recognized as an asteroid in
the stead. 1996-N2, Elst-Pizarro, ls a very new instance. It turned
out to be a recovery of the poorly observed asteroid l9710W7.
Or an 'asteroid' is later found to be a comet. The most notable
example is the asteroid Chiron found in 1971. After several years of
study it was seen to have a coma and other comet features. It was
reclassed as a comet, 95P/Chiron. It reached perihelion in February
1996 and has a period of some 50 years. Because these are periodic
comets, in my database I inserted a dummy entry, along with the other
similar case, 107P/Wilson-Harrington.
Oebit of record
-------------
My next chore was to compile a set ot orbits: When a new comet is
announced a preliminary orbit is worked up. It MAy be revised as more
positions are accumulated. However, there's no declaration that, OK,
guys, this here orbit is the final one for the books.
Once the comet recedes from observation the orbit is no longer
revised. In this sense, the archival orbit is that which the comet
rode into invisibility. Not a tight definition, yet that orbit
reasonably best describes the entire instant apparition of the comet.
There is the occasional review of an old orbit between
apparitions of a comet. These are normally circulated in the
literature outside of the usual comet news channels. These prospective
orbits are supercedd by the observed orbit when the comet returns
The trick is to know when a comet is gone, so I can accept its
orbit without much fear of later doctoring. Comets still within sight
can undergo orbit updates at any time. These I flag in my database.
Some time after its perihelion passage, I check up on what the comet
is doing.
For recent examples, Comet Perrine-Mrkos never turned up in 1995,
nor did Comet Kohoutek (not THAT Kohoutek) in 1994. Their prospective
orbits linger on the books to fool the unwary cometeer.
Oebit epoch
---------
One problem I faced is the epoch of the orbit. The elements are
cited for some equinox, which shifts over the ages. While it is
virtually always stipulated, l was not eager to undergo the one-by-one
normalizing of the elements. to the 2000 epoch, even tho I threw
together a BASIC routine to do so. In the course of my project I was
relieved to learn that this chore was already done elsewhere and I was
given data massaged into the 2000 epoch.
There's another problem I came onto. An orbit can be issued for the
expected return of a periodic comet, then the comet doesn't show up!
This happens for many reasons, including a loosely determined orbit
in the first place. It was a detective job to close out these
comets!
Xomrt parameters
--------------
At first I had columns for the year, month, and day of perihelion,
but this made it clumsy to manipulate the data. I adopted a false
decimal notation in use by some comet programs. This has the year, a
decimal point, two digits for the month, two digits for the day, and
then whatever extra digits are needed for the decimal part of a day.
Halley's Comet in 1986 has perihelion date 1986.02095. This parses to
1986 Feb 09.5.
In accumulating the orbit data I was at first amused by the
outlandish accuracy the elements can be given. Halley's 1986
perihelion distance is 0.587104 AU, precise to 150km. That's about the
radius of two-hour commyting ring around New York! I had to learn
later the hard way that such tight elements are necessary for the
reenactment of comet motions thru the solar system.
Parameters I thought I could include are the H and G in the comet
magnitude formula.These are not at all routinely announced. Often all
that's in the literature ls a column for magnitudes in an ephemeris.
More serious is the innate behavior of comets. The H and G numbers
change during a comet's visit so that no one set reasonably
characterizes the comet.
An other feature I thought would be helpful is the maximum tail
length. The data are just too crude and erratic to make any sense
out of tail length. In any case, the reported length is overwhelmingly
dependent on the local sky conditions, as dramaticly demonstrated by
the recent COmet Hyakutake.
You do want these pithy details on comets, but, please!, don't
cry. In my work I met, in cyberspace, Gary Kronk. He runs a website
with deep letterpress about comets. This is part off his magnum opus, a
true cometography, to issue in 1997. It is a massive. rework of his
legendary 'Comets, a descriptive catalog' of 1984. Keep your eye out
for it.
Database features
---------------
All of my comet data are in dBase form in two grand files. One is
the master roster of designations; the other, orbit elements. The
master roster I can sort or query by any of the numbers a comet may
have or its proper name.
The elements database has one field for the comet number and it
may hold the perihelion, periodic sequence, or new-style number. With
no uniform entry in this field, I sorted this file by perihelion date.
I also split off a secondary file for just the periodic comets.
Now a periodic comet is any comet whose orbit is closed, with
excentricity less than unity. There are scores of such comets. Only a
select few are formally recognized as periodic and are awarded
sequence numbers. The others are probably lost, demised, or ot very
long period. And, possibly, the original positions may generate a
spurious elliptical orbit. I keep just the sequenced comets in this
file. The others, of course, remain in the elements file.
Do appreciate that my database is for the archival data on comets.
I will not hustle to catch every new comet as it comes along. Such new
comet info you can far better get from BBSs, websites, and
email. Coming to me to check on new comets will get you stale news.
Comet Schwassmann-/Wachmann-2
---------------------------
I'm now starting to explore thru the database and play with the
comets. I relate here two fascinating examples of what I found. There
is the established periodic comet Schwassmann-Wachmann-2. (No, I do
not go and make this stuff up) Since its discovery in 1928 it followed
a stable circuit. My elements database calculates the period for each
return ranging from 6.38 to 6.53 years. An inspection of the
data indicates that the next return should be in June of 2000.n rout
years from now.
I cranked up Dance and fed it the orbit of SW2. The computer
groaned while it dutifully plotted SW2 round and round the Sun. It
duplicated quite closely the tabulated motion, perihelion after
perihelion, right thru the January 1994 return. Things looked good
thruout 1994 and 1995 and most of 1996.
Then!, Jupiter sneaked up behind Schwassmann-Wachmann-2 and
started to tug it outward from its 1994 orbit. Steadily it pulls the
comet away, farther from the Sun, circularizing the orbit and
increasing its perihelion distance. In March of 1997 the comet's
closest approach to Jupiter is only 36,300,000km, or 0.24AU. This
seems like a long way off. Do remember that Jupiter is over 300 times
more massive than the Earth, so its gravity has a long reach.
I let the computer clock up the months. SW2 gradually dips back
toward the Sun and finally hits its -- new -- perihelion on 27
January 2002. That's 1-1/2 years later than by its historical orbit.
The new perihelion distance is now 3.44AU, substantially greater
than its historical 2.07AU.
I checked around and found that in the late 1970s Dr Balyaev of
the Soviet Union computed almost the exact same behavior for SW2. It
took him days and days with slide ruler and function tables. This
computation also showed that the present orbit is the result of a
close approach to Jupiter in 1926, before SW2's discovery. I didn't
yet try and explore this encounter.
Comets Liller and Tabur
---------------------
The second example is a true and genuine comet discovery. Marsden
had already caught it when I called about it, so I am not famous yet.
CBAT did welcome my effort, noting that only two or three astronomers
in the world beat me to the find.
I sifted thru the elements database after I got a newer set of
elements for Comet Tabur. The October 1996 EYEPIECE, newsletter of
amateur Astronomers Association, article for Tabur was based on a
first-look set soon after discovery. You may want to rework the
ephemeris and map with the newer set below. The perihelion date is
1996 November 03.50783.
Boom!, my computer kicked out an other comet for having very
nearly the same orbital elements as Tabur! It sure as hell looks like
Tabur is the same comet as Liller, which was found in 1988 as 1988-a,
or 1988-A1 under the new-style system.
Lo here the comparison:
-----------------------------------------------------------------
Comet Peri-AU Excenty Inclinat Asc-Node Arg-Peri Ang-Mom
------------ -------- -------- -------- -------- -------- -------
1996Q1 Tabur 0.840196 1.000000 73.34764 31.42215 57.36127 1.68039
1988a Liller 0.841333 0.996565 73.3224 31.5154 57.3876 1.67978
-----------------------------------------------------------------
The last column is the square of the angular momentum per unit
mass. It's not part of the elements database. I figured it by hand
from the perihelion distance and excentricity.
Altho Tabur is, by the latest orbit assessment, in a parabolic
path, while Liller was in an elliptic one, their angular momenta are
quite the same. With the other elements also being nearly the same,
the chances of a random match are essentially ziltch.
There is a problem.
The period of Liller, from its elements, is some 3800 years, yet
it is only 8 years between the perihelia of 1988 and 1996. Could the
orbit elements be in error such that the real period is in fact 8ish
years?
Well, allowing the perihelion distance to be well determined, what
excentricity for Liller yields an 8 year period? It turns out to be
0.78... . This is just too deviant from 0.99... for modern orbital
mechanics.
Evidently, Tabur's body now in the sky is not the very body that
visited the Sun in 1988. The history of Liller is spotty because it
was a minor comet only sparsely observed. Never the less I did find
passing reference that the nucleus split soon after the 1988
perihelion. Is Tabur one of the pieces?
I think not. If Liller broke up in 1988 and Tabur is a returning
piece, this piece's orbit must have that 0.7S-something excentricity.
Else it plain would not have come back so soon. A reasonable
hypothesis is that Tabur already was separated from Liller and follows
Liller 8 years behind. The disruption occurred on some previous
visitation of Liller.
This correlation between Comets Tabur and Liller, found thru a
home computer exercise during a rainy afternoon, is substantially that
which Marsden's office sussed out. Should I hope for more rain?
conclusion
--------
With all this work I was immensely assisted by Dr Brian MarSden's
crew at Central Bureau for Astronomical Telegrams and the authors of
Dance of the Planets at ARC Scientific Simulations. They patiently
discussed all manner of inquiries from me by phone over the weeks of
my project. I was also aided by curators of the many websites I
posted queries to about their comet data. Furthermore, these good folk
routinely referred me to digital data which could be computer edited
to fit into my database.
Dance of the Planets enters my work because of my interest in
simUlating comet motions. It is the only program for home computers
that employs the full gravitational influence of the planets in moving
the comet around the Sun. The proper orbit to give Dance is the one
laid down by the sum of effects from all of the planets. An average
or typical orbit is not merely miSleading but actually dangerous.
Many astroprograms give files of comet data, but in my discussions
I found that too often the orbits are preliminary or approximate. They
were good enough for moving the comet along a 'trolley track' path
among the planets, but totally useless for any dynamical work.