WHAT IS A KREUTZ COMET?
=====================
John Pazmino
Amateur Astronomers Association
New York
2002 March 17
Introduction
----------
You're seeing the term 'Kreutz comet' or 'Kreutz group of comets' more
frequently than ever. This is tied with the massive harvest of comets
hauled in by SOHO in the course of its solar studies. So, what is a Kreutz
comet?
Succinctly, a Kreutz comet is a member of a seemingly numberless
family of comets sharing the same orbit whose perihelion is one or so
solar radii from the Sun. Only their perihelion dates differ, causing them
to be strung out along this common orbit like cars on a highway.
Such a comet skims thru the middle or inner corona and sometimes
intersects the photosphere itself. As you may surmise, the close encounter
can incinerate the comet. Very few survive to ride away from the Sun on
the outbound arm of their orbit.
History
-----
Edmond Halley was the first to seriously inquire about a distinct
group of comets that buzzed the Sun. He speculated but did not
demonstrate, that the comets of 1106 and 1680 were the same comet on
repeat visits to the Sun's vicinty. Similarly, he connected the comet
of 44BC with the one of 531AD. These correlations have since been
disproved by modern orbit simulations.
Halley's work did lead ultimately to his discovery that comets can
have closed orbits allowing them to repeatedly return to perihelion.
He did correctly link several comet orbits into what later become
known as Halley's comet, periodic comet #1.
Because of the high status of Halley and Newton, who also worked on
comet motions, there are today occasional revivals of their work. All,
sadly, fail to consider modern data and orbit theory.
The next attempt seems to be that of Kirkwood in 1880, after he
studied comet 1880-I. He showed that it and comet 1843-I may be pieces of
the comet of -371, or 372BC.
The name 'Kreutz comet' comes from Heinrich Kreutz, who made the first
detailed study of them in 1888, 1891, and 1901. He found that the comets
1843-I, 1880-I, and 1882-II had similar orbits with close perihelia. (He
did not yet know of the Eclipse Comet of 1882.) He hypothesized that they
were debris from a single original comet which got dispersed along the
orbit.
Vela supernova
------------
Some astronomers link the Kreutz parent comet to the Vela supernova.
The explosion, about 12,000 years ago, jostled the Oort cloud and threw
the Kreutz parent to the Sun. This comet supposedly is described is
ancient scriptures of 10 millennia ago and broke into two parts.
A variation of this story has the comet coming from the supernova
ejecta, or the surrounding nebula, being propelled toward the Sun.
The supernova was seen in the Middle East lands as a humongously
bright star hanging near the southern horizon. If it was comparable to
SN1987A but only 1,200 lightyears away, it could have incandesced at
fully, yikes!, -8 magnitude. It is by some anthropologists called the
'star of civilization' on the speculation that it galvanized humans to
invent writing and record keeping.
So far, however, there is no firm evidence that the Kreutz comets are
really a spinoff of this supernova.
All in all, we plain do not know much about these comets at all!
Comets -371 and 1106
------------------
The large comets observed in 372BC (-371) and 1106 AD are quite likely
sungrazing comets, altho positional and temporal data are woefully
inadequate by modern standards. For the latter comet, Europe was deep in
the Dark Ages and in no shape or mind to contemplate comets. The reports
so far uncovered come from Asia Minor or Far East. Descriptions, including
some sightings of it next to the Sun by day, place the 1106 comet on a
path quite similar to a typical Kreutz comet. Its appearance may have
resembled comet Ikeya-Seki, with a long gently curved feathery tail.
The comet of 372BC is harder to document in as much as it was seen
during the Greek era, when comets were objects of fear rather than favor.
Ephorus, no astronomer, allegedly saw it split into two. This comet is
usually taken to be the parent of the Kreutz stream but there is no
definite proof. In fact, modern studies of the Kreutz family often
omit this comet.
Possible one-time sightings
-------------------------
A Kreutz comet is likely to be in the sky only briefly, like weeks.
Unless it is caught and tracked near the Sun, it is impossible to confirm
its membership in the sungrazing group. If the comet is faint and
brightens only next to the Sun we may have but a single or very few
sightings of it.
From time to time, mentions in observatory logs and the like refer to
a bright spot, cloud, cometlike puff, near the Sun on such and such a day.
The usual scenario is that attempts to confirm the object on the next day
ore even later in the same day were unsuccessful. The incident is written
up and forgotten.
It is possible that several visual sightings are overlooked. With so
scanty a record of them, they have to be left out in any study of Kreutz
comets.
Marsden 1966-1967
---------------
Brian Marsden in a 1966 abstract and a full paper in 1967 made a
detailed account of the known Kreutz comets including comet Ikeya-Seki.
This comet, possibly the most spectacular of all Kreutz comets in history,
likely galvanized him.
With only seven modern and one early example, Marsden milked out some
surprising features of the Kreutz group. He left out the eclipse comet
of 1882 for having no orbital data but threw in comet 1668. From these
he deduced that comets 1882-II and 1965-VIII were pieces of a one
parent comet; the split may have happened on the last perihelion, near
the time of the comet of 1106.
He established the perihelion point to be in ecliptic heliocentric
longitude 282.0 deg, latitude +35.3 deg. These correspond to RA 18h 42m,
decl +12d 20m. The aphelion point, opposite to the perihelion point,
is therefore at RA 6h 42m, decl -12d 20m. This is close to the one I
determine below, but with far more specimina to work with. Marsden
used the then current 1950 equinox; I banked off of that for 2000.
Marsden explored the effect of Jupiter on the comets. Not for the
direct gravitational force but for displacing the Sun from the barycenter
of the solar system. For a comet coming so close to the Sun it makes a big
difference if the Sun is on the perihelion side of the barycenter or the
aphelion side. In the former case, the comet may hit the Sun; in the
latter it may pass far enough away to survive. Jupiter, the only other
considerable mass in the solar system, outweighing all the other planets
together, tugs the Sun off of the barycenter by just about one solar
radius.
He also looked into the Einstein effects. A comet so close to the SUn
reasonable will suffer spacetime distortion of the sort associated with
large masses. As it turned out, even at perihelion and carrying out the
Einstein theory over a large part of the inner orbit, Marsden found the
deviation from a straight Newton calculation to be negligible. The
discrepancies were well within the errors of observation.
I, too, using a pigeon method applied only between the latera recta of
the orbit, find that for a model comet of 1,000 year period under Newton
has a 1002.3 year period under Einstein. This is an inconsequential
disparity easily accommodated within observational error.
Marsden did something which in today's physics is rather curious. he
tried a calculation under the Brams-Dicke theory. This in 1961 was put out
as an alternative to Einstein physics. The results Marsden found were
totally indistinguishable from Einstein. Other tests of the Brams-Dicke
theory led to its abandonment by 1980.
Marsden provided a search ephemeris based on an averaged set of
elements for the span 50 days before and after perihelion. This resembles
my table of the path below. Marsden cautioned that a Kreuts comet may be
up to 3 degrees from each tabulated position. The table leaves out the
summer months because a sungrazer comes from behind the Sun and would be,
in the 1960s, utterly unobservable. This is brought out in my table, where
the Sun is in the summer part of the ecliptic and the vanishing point is
far beyond it.
Marsden 1989
----------
There were several papers on Kreutz comets following the passage
of comet Ikeya-Seki in 1965. Most were observational reports about
this very comet itself. Marsden in 1989 reworked his 1967 paper to
include these new data and the comets newly found by the SOLWIND and
Solar Max Mission spaceprobes.
He expressed frustration that the astrometry from the satellites
was not up to snuff. These were not astrometric probes. They studied
the Sun, an easily aimed at target requiring no astrometry to find.
He, never the less, demonstrated that all the comets found by SOLWIND
and SMM are in fact Kreutz members by comparing their paths with those
from comets 1965-VIII (Ikeya-Seki) and 1963-V (Pereyra).
These two comets are, he believed, prototypes of Kreutz subgroups. He
alluded to thm in his 1967 paper and seemed to adhaere to the idea by
1989. In any case, he traced the comets 1843-I and 1880-I to a common
parent.
With the additional examples, Marsden noticed that they seem to come
in bunches, with longer gaps between them. This strengthened his belief
that the Kreutz comets are fragments of some progenitor object. Debris
from a plausible breakup would tend to collect in clumps and not be
uniformly dispersed over an orbit. He traced back some of the earlier
comets to a possible origin in the 1106 comet.
In this paper Marsden applied a barycentric computation of the orbits.
Comparison with the heliocentric orbits showed a cyclical variation of,
for instance, perihelion distance, according to the longitude of Jupiter.
I verify this by spot checks below.
Kreutz orbital elements
---------------------
Taking an eyeball mean of the Kreutz comets of 1999, all discovered by
SOHO, the orbital elements are:
perihelion AU = 0.0055 inclination = 144.0
excentricity = 1.0 long of asc node = 4.0
perihelion date = any date arg of perihelion = 83.0
By reference, the Sun's radius is 0.0046 AU.
This average is based on 65 Kreutz comets rounding perihelion in 1999
as known at 2001 July. As I explain later, SOHO 'discovers' comets
years after perihelion passage. Hence, when you read this, there can
very well be SOHO Kreutz comets newly found to have rounded perihelion
in 1999. However, the elements above remain quite typical of them all.
Bear in mind that all the SOHO comets were observed for only tens of
hours or, at most, a couple days. The orbit is calculated on just this
short span. Almost always only the part of the orbit before perihelion
actually exists because the comet poofs itself while so close to the Sun.
For this reason the elements of a Kreutz orbit are only approximate,
with no hope of refinement from later observations. Improvements can come
from reexamining the SOHO data. Specificly, we declare the excentricity to
be unity, for a true parabola, and not try to work it out.
Groundbased discoveries
---------------------
The list, for the era prior to the first spacebased finds, is laid out
here. I give the present designation, explained later, and the one
used when the comet flourished, so you can hunt up the comet in both
old and new litterature.
Comet Designations Perih date
------------------- ----------------- -----------
Great March 1843-D1, 1843-I 1843 Feb 27
Great Southern 1880-C1, 1880-I 1880 Jan 28
Eclipse 1882-K1, [none] 1882 May 17
Great September 1882-R1, 1882-II 1882 Sep 17
Great Southern 1887-B1, 1887-I 1887 Jan 11
du Toit 1945-X1, 1945-VII 1945 Dec 28
Pereyra 1963-R1, 1963-V 1963 Aug 24
Ikeya-Seki 1965-S1, 1965-VIII 1965 Oct 21
White-Ortiz-Bolelli 1970-K1, 1970-VI 1970 May 14
Besides these definite members of the Kreutz class, there are several
other candidates. The comets of 372BC and 1106 are two which are
usually included. The problem with comets earlier than about 1700 is
that observational techniques were too crude to work up solid orbits.
It doesn't take much of an error in position on the sky or in hour of
day to completely louse up a fit to a Kreutz orbit.
Subgroups
-------
Marsden tried valiantly in both of his papers to divide the Kreutz
comets into two subgroups. The division was based on subtile differences
in the orbital elements, mainly the longitude of the ascending node. With
so few specimina for the 1967 study, he did not push the idea of subgroups
too strongly.
In the 1989 paper, Marsden refined his thinking and applied it tot the
new crop of SOLWIND and SMM comets. On the whole, Subgroup I has longitude
of the ascending node centered on 5 degrees ecliptic longitude. Subgroup
II has it centered around 345 degrees.
On this basis, we have the following assignment of comets from the
pre-satellite era. The number of satellite comets is just too large for
easy tabulation. The longitude of the ascending node is from current
catalogs and precessed to the 2000 epoch.
Comet Designations Asc Node PeriAU SG
------------------- ----------------- -------- ------ --
Great March 1843-D1, 1843-I 3.5272 0.0055 I
Great Southern 1880-C1, 1880-I 7.7774 0.0055 I
Eclipse 1882-K1, [none] [no orbit] 0.005
Great September 1882-R1, 1882-II 347.6566 0.0077 II
Great Southern 1887-B1, 1887-I 4.5850 0.0048 I
du Toit 1945-X1, 1945-VII 351.2006 0.0075 II
Pereyra 1963-R1, 1963-V 7.9393 0.0051 I
Ikeya-Seki 1965-S1, 1965-VIII 346.9947 0.0078 II
White-Ortiz-Bolelli 1970-K1, 1970-VI 337.0148 0.0089 II
----------------------------------------------------
Average Long Asc Node for Subgroup I = 4.207 degrees
Average Long Asc Node for Subgroup II = 345.717 degrees
Average Perihelion for Subgroup I = 0.0052 AU
Average Perihelion for Subgroup II = 0.0080 AU
The usual explanation for such a segregation of these comets is that
each group derived from the first order breakup of the parent comet and
the members in each group are the supraordinal fragments. The particles
from the supposedly larger chunk are now subgroup I, which by yearend 2001
contains aboout 3/4 of all known Kreutz comets. Those from the smaller
part consitute subgroup II, with about 1/4 of all Kreutz comets.
Eclipse Comet of 1882
-------------------
This is one of the strangest comets ever witnessed. Altho widely and
reliably seen by astronomers of great repute and experience, this comet
is, from a dynamics perspective, about the lousiest one on record. It, in
fact, has no orbit of its own!
During totality of the solar eclipse of 1882 May 17 observers noticed
a curved bright streak next to the Sun. It was plainly visible by eye and
several eclipse artists depicted it. The streak was at the right of the
Sun and it extended from about 1 to 1-2/3 solar diameters out. Its greater
brilliance than and misalignment from the rest of the coronal structure
attracted special attention to it.
Photographs taken of the corona definitely identified it as a comet.
It had a nucleus, cometary structure in the tail, and the tail did arc
away from the Sun. And it resembled the comet of 1843, which was seen to
graze the Sun.
The comet was never seen again.
Modern analysis of the reports and pictures demonstrate conclusively
that this was a Kreutz comet that by stroke of luck was on approach to the
Sun during the eclipse. The lack of any means to study celestial targets
near an uneclipsed Sun and the comet's likely demise in the solar heat
prevented further observation on following days.
Because the arc of path was so short, merely the few minutes of
eclipse totality, no orbit could be calculated for it. For the record, we
adopt the perihelion to be an hour after the eclipse, because the comet
was on its incoming trip, and assign the other elements from the comet
1880-I.
In honor of the local native governor who was very hospitable to the
astronomers who went to Upper Egypt, the comet is sometimes called
Tewfik's comet; this name was never officiated. With a lack of a proper
orbit, the Eclipse Comet of 1882 never earned a contemporary designation.
However, it does have a newstyle designation of 1882-K1.
Comet Ikeya-Seki
--------------
The singular exception among the indiscernible sungrazers is comet
Ikeya-Seki. Ikeya and Seki independently and almost simultaneously found
it as an 8th magnitude body near alpha Hydrae on 1965 September 18. Soon
after discovery the comet was recognized as a sungrazer and the alarm went
out for a cosmic spectacle.
It brightened to 6th magnitude by October 1 and to 2nd magnitude by
October 20. Already it was the most showy comet for home astronomers since
comet Arend-Roland of 1957.
The modern designation for Ikeya-Seki is 1965-S1 but you may
remember it under its current name of 1965-VIII. For parts of the
world with clear weather, the comet was a magnificent sight with a
tail up to 60 degrees long! In the New York it was generally cloudy.
By fate, on a couple mornings before dawn, clouds in the City thinned
out in late October 1965, giving astronomers there a glimpse of the tail,
some 20 degrees of it. Thru the cloud!
My own one and only sighting of this comet came near the moment of
perihelion in daylight. I hid the Sun behind a roof edge shortly after
noontime and with binoculars spotted the lucid puffball sitting a degree
from the Sun! Later I learned from other reports that at this time the
comet was blazing at magnitude -10.
Large observatories found that the comet's nucleus broke apart into at
least three main pieces as the comet rounded the Sun. By mid November it
receded into invisibility for home instruments. It was tracked by large
telescopes until February of 1966.
Speculation, based on a period of 877 years, was that comet Ikeya-
Seki was a return of the 1106 comet. (1965)-(877) = 1087, a bit too
far from 1106, altho not impossibly so. Remember that a Kreutz comet
is steeply inclined to the ecliptic and suffers minimal perturbations
from the planets. Perhaps, just perhaps, jetting effects from a
previous breakup come have slowed the comet a little bit?
The thinking today is that Ikeya-Seki came from comet 1106 thru a
fragmentation, but is not itself the very comet. We can not award
Ikeya-Seki with a periodic comet designation.
Visibility of Kreutz comets
-------------------------
The first Kreutz comets, up until the advent of spacebased
discoveries, were all found by visual sighting in the sky. These are the
ones in the table above. In modern times there have been no readily
visible sungrazers for home astronomers. I mean none visible by
homebased equipment. As I show later, home astronomers are finding
sungrazing comets by purusing the records of a spacebased observatory.
When close up against the Sun they can brighten to 3rd or 2nd
magnitude, but then they are lost in daylight or the Sun's glare. Among
the ones found by satellite, many attained to 1st or negative magnitude in
the hours around perihelion. These are the rare exception, altho they do
get heavy mention among home astronomers.
Thus, we have the weird situation that a lot of 'bright' comets,
handily within range of home astronomers if they were many degrees from
the Sun, end up being totally unobservable!
With the SOHO roster of comets, essentially all, by sheer numbers,
Kreutz comets are miserably faint and small, even close against the Sun.
In fact, many are invisible on the wide-angle LASCO plate but discernible
only on the narrow-angle plate, which presents a larger image and darker
background sky.
Path on the sky
-------------
The Kreutz track's vanishing point is in northwest Canis Major, at RA
6h 33m, dec -14.6d, near Sirius. Assuming its orbit is a parabola, the
vanishing point is also the direction to the orbit's aphelion.
The comet spirals counterclockwise out of this homebase as if winding
up for the throw to the Sun. This spiral is nothing more than the
reflection of the Earth's motion around the Sun, a very close parallax
effect.
By the time the spiral spills beyond the frontiers of Canis Major, the
Kreutz comet is 12 AU from the Sun. All the comets cycle from Canis Major,
Lepus, Orion, and Monoceros, around where these four constellations meet.
Eventually the spiral unravels, when the comet is 9 to 10 AU from the
Sun and the path shoots off toward the Sun. I describe the path relative
to the constellation frontiers; the irregular size and shape of which
makes this description only approximate.
The paths were traced with a comet tracker for the typical elements
set out above. The perihelion date for each run was the first of each
month in the year. C, E, N, S, W mean central, east, north, south, and
west. Combos can be like SC for south-central and E-C for 'from east to
central'.
perih path in sky
----- -----------
Jan 1 NE Pup, C Pyx, C Vel, NE Car, NC Mus, S Cir, C TrA,
SE Ara, NW Tel, C CrA, C Sgr, Sun
Feb 1 C Pup, W Car, S Pic, SC Dor, C Ind, EC Tou, C Gru, WC PsA,
C Cap, Sun
Mar 1 W Pup, SE Col, N Ouc, S Cae, N Hiem SC Eri, SW For, NE Scl,
SW Cet, C Aqr, Sun
Apr 1 C Col, C Cae, SC Eri, NE For, W Eri, C Cet, S Psc, Sun
May 1 M Col, N Cae, C Eri, NE Cet, S Ari, Sun
Jun 1 E-N Eri, C Tau, Sun
Jul 1 E Eri, C-NE Ori, SC Gem, Sun
Aug 1 NE Eri, C-E Ori, N Mon, N CMi, SE Gem, C Cnc, Sun
Sep 1 S CMi, N Hya, SW Leo, Sun
Oct 1 E-NC Hya, C Sex, S Leo, W Vir, Sun
Nov 1 W-C Hya, C Crt, C Crv, S Vir, W Lib, Sun
Dec 1 NE Pup, SW-S Hya, N-E Cen, N Lup, NE Sco, Sun
A Kreutx comet with perhelia between mid May and mid August would
likely be missed from the ground. The comet comes to the Sun from beyond
it in local daylight.
If a comet survives the approach to the Sun it makes a beeline back to
central Orion, entering from the west. It loops thru Monoceros and Puppis,
and spirals into the vanishing point. Except for the whirl around the Sun
at perihelion, a Kreutz comet stays south of the ecliptic.
Planet Interactions
-----------------
Normally for comet tracking we use a two-body method of working out
the orbit. We consider the comet as a single particle under the gravity of
only the Sun; the other planets are neglected. This tactic works well for
almost all home astronomy purposes in as much as the error on the sky of
ignoring the planetary modulations of the path are well within the fuzzy
disc of the comet itself.
To further hedge against large deviations on the sky we seek the
newest orbital elements to plug into the comet tracker. If we want to
investigate periodic comets at several returns we employ elements proper
for each return.
A Kreutz comet has a high inclination on the ecliptic of about 144
degrees, or 36 degrees for the lesser angle. Thus, roughly, its distance
from the ecliptic plane is equal to 0.6 times its distance from the Sun.
By the time the comet is at the orbit of Jupiter, the bully of all comets,
it's some 3 AU south of the planet to escape major gravity influence from
it. Hence, a Kreutz orbit is a reasonably stable one.
This thinking is bolstered by the fact that with the enormous and
steady stream of Kruetz comets, there are always some near Jupiter's
orbit. Yet, so far, we have not detected diversions of the stream when
Jupiter itself passes closest to it. Granted, we barely accumulated one-
half Jupiter year of data, from 1996 thru 2001, but still there is no
perturbation trend in the data.
There is utterly no threat to Earth. From the orbit in the 'elements'
section above the closest a Kreutz comet can approach to Earth is 0.577
AU. I only checked the inbound arm since so few sungrazers survive the
ride around the Sun. This is for a comet whose perihelion is on 10 Jan of
a particular year and the event occurs on the preceding 21 Dec.
Barycentric Orbit
---------------
Jupiter, with quite 1/1,000th the mass of the Sun is essentially all
the other mass of the solar system in itself. It outweighs all the other
planets. We can model the solar system's gross dynamics as a binary system
of just Sun and Jupiter.
The nontrivial mass of Jupiter forces the center of mass of the solar
system, the barycenter, off of the geometric center of the Sun. The Sun
executes a tiny orbit around this barycenter in synchronism with Jupiter's
11.8 year orbital period.
This solar orbit is indeed small, its radius is only quite the radius
of the very disc of the Sun. For almost all ordinary comet work we can
ignore this wobble. For a Kreutz comet ignorance can be, ahem, fatal.
If a comet's two-body orbit is used, it could be that he Sun is
displaced, by its mutual orbital motion with Jupiter, a bit toward the
perihelion. With so close a perihelion, in the order of a solar radius,
the Sun could intercept the comet orbit and the comet is a kamikaze comet.
Or the Sun could be displaced away from the perihelion, letting the
comet pass far enough away to survive the ride.
Marsden did an assessment of barycentric orbits in his 1989 article. I
did a couple spot checks using sample SOHO comets, because of their better
astrometry, in each year of SOHO's operations. This period spans quite one
half Jupiter year so any solar wobble should show up.
I leanred quickly enough that using a program that calculates a two-
body orbit, where the Sun remains stationary at the origin on the
heliocentric coordinate system, doesn't bring out this difference of
perihelion. Why? The orbital elements I had to key in for this program
itself had the perihelion distance in them! So the comet was forced to
stand the proper distance from the Sun regardless of the bodily
displacement of the Sun from the solar system barycenter!
For an other strategy I calculated the angular separation of Jupiter
from the Kreutz perihelion, (Jupiter longitude) - (280.1), and from this
the displacement of the Sun toward or away from the perihelion. Then I
worked out the separation of the sungrazer from the Sun as if the
displacement were not factored in.
I used a Kreutz perihelion longitude of 280.1 degrees. This is my
aphelion point from the 'path' section flipped to ecliptic coords and
reversed 180 degrees. It is for epoch 2000. The Sun-Jupiter barycenter is
0.00494 AU or 1.058 solar radius from the Sun's center in the direction
opposite from Jupiter. In other words, the Sun is displaced off of the
coordinate origin toward 180 degrees from Jupiter's longitude. The Sun's
radius is 0.0046 AU.
comet peri date Jup-l del-l bar-AU Sun-AU del-AU
------- --------- ----- ----- ------ ------ ------
1996-A2 Jan 14.56 268.6 -11.5 0.0006 0.0054 -0.0048
1996-O1 Jul 22.09 284.2 4.1 -0.0004 0.0045 -0.0049
1997-B2 Jan 12.08 300.2 20.1 0.0010 0.0056 -0.0046
1997-P1 Aug 05.68 315.6 35.5 0.0027 0.0067 -0.0040
1998-A1 Jan 13.26 330.8 50.7 0.0020 0.0051 -0.0031
1998-M10 Jun 24.04 345.4 65.3 0.0043 0.0064 -0.0021
1999-C2 Feb 06.74 6.1 86.0 0.0065 0.0068 -0.0003
1999-O2 Jul 22.56 21.3 101.2 0.0089 0.0079 +0.0010
2000-A2 Jan 15.21 37.5 117.4 0.0087 0.0064 +0.0023
2000-N1 Jul 03.25 52.9 132.8 0.0083 0.0049 +0.0034
2001-A1 Jan 08.65 69.8 149.7 0.0112 0.0069 +0.0043
2001-N1 Jul 06.30 85.4 165.3 0.0100 0.0052 +0.0048
bar-AU is the distance the sungrazer would be from the Sun if the Sun
stayed on the origin of the heliocentric coordinate grid, the one by which
most comet trackers base their computations on. The Sun-AU is the actual
distance, measured by SOHO, from the Sun which includes its displacement
off of the heliocentric origin. del-AU is the component of the Sun's
displacement to or from the longitude of the perihelion.
The comets from 1996-A2 thru 1998-M10 would have, if the Sun stayed at
the heliocentric origin, be erroneously shown to intersect the Sun's
photosphere.
SOHO discoveries
--------------
From the nine comets of this group known in 1979, orbiting solar
observatories almost tripled this number by 1995. SOLWIND found 6 and SMM
captured 10. But it was SOHO that put the Kreutz name in front of the home
astronomer. It -- despite a many month breakdown in 1998 -- captured,
eeek!, 320 new Kreutz comets! That was in end of June 2001, with new ones
logged in twice or thrice per week!
In terms of sheer numbers of comets discovered, SOHO already accounts
for fully 1/4 of all comets known thruout human history.
This vast number of SOHO comets comes from some new features of this
spaceprobe. The previous solar observatories concentrated narrowly on the
Sun and their data were reserved only for the probe's own scientists.
SOHO, in addition to narrow field monitoring of the Sun, has a wide
field coronagraph, LASCO. This takes continuous pictures of a 14-degree
square centered on the Sun for studies of the outer corona. The Sun itself
is hidden behind an occulting paddle and, from the vacuum of space above
the atmosphere, stars around the Sun are plainly captured. In fact, the
number of stars in this field, right up to the edge of the paddle!, is
rather more than what you see on a clear night thru binoculars! I estimate
that the depth of stars about equals that of Tiron's Sky Atlas 2000.
Moreover, like other newer spaceprobes taking advantage of
Internet, the SOHO data are archived on websites open to anyone. Any
person may examine the LASCO plates and extract new, previously
unrecognized, information. This is why nowadays we read of 'new'
discoveries gleaned from data collected by spaceprobes many years ago.
Among the new features discovered by SOHO of the Kreutz family is
the tendency for the comets to arrive in bunches. Sevearl will come in
within hours, separated by gaps of days. This was presaged by Marsden
in his 1967 and 1989 articles, see above. This feature is consistent
with the Kreutz group being debris from some antecedent comet, which
naturally would collect in clumps.
SOHO found several nonsungrazers along the years, but far and away
the bulk of its harvest are the Kreutz members.
Home astronomers and Kreutz comets
--------------------------------
It is the home astronomer who's finding most of the new Kreutz comets.
Home astronomers download the LASCO pictures, vigorish them with image
processors, apply astrometric operations, and uncover the comets. Some
home astronomers by this method 'discovered' scores of comets. Because
they are using data collected by other parties, these home astronomers do
not qualify for CBAT's Edgar Wilson award. Only those finding comets in
their own data are eligible. Nor do they get their own name affixed to the
comet. The name comes from the data's own source, such as SOHO.
In the litterature you'll read of LASCO C2 and LASCO C3 cameras.
The C3 has the 14-degree field of view and is the one most of us look
at to see stars around the Sun. It is ideal for demonstrating planet
conjunctions with the Sun. The C2 field is about 5 degrees with a much
darker background sky. Most of SOHO's sungrazers are found on the C2
image and are too small and dim to show up on the C3 plates.
New York's first comet
---------------------
Despite the vigorous culture and pursuit of astronomy in New York City
no comet was discovered from within its praecincts until 2002. On 2002
february 14 the Minor Planet Center announced new comet 2002-C4, the 389th
in the SOHO harvest. This was found on LASCO images for 2002 February 10
by Tony Hoffman, from the Ridgewood section of Queens.
Hoffman, like many other home astronomers, examines LASCO C2 and C3
images downloaded from SOHO's website for possible new comets. During 2001
he reported a half dozen candidates, but they proved to 'noise' or
blemishes, like from cosmic-ray hits, that mimicked a comet's behavior.
For that February 10th set of pictures, Hoffman was first in time to
report a genuinely new comet.
Comet 2002-C4 is the first comet ever discovered from New York City,
despite that it was not captured in the City's very sky, but in a computer
inside Hoffman's home.
The comet is a classical Kreutz member:
Perihelion date 2002 Feb 11.66
Perihelion distance 0.0054 AU
Argument of perihelion 78.76 degrees
Longitude of ascending node 0.34 degrees
Excentricity 1.0
Inclination 144.46 degrees
The comet melted about 12 hours after perihelion, as is the fate of almost
all Kreutz comets.
Like for all the other astronomers who found comets in SOHO material,
Tony Hoffman does not get his name attached to 2002-C4 nor qualify for the
Edgar Wilson award for new comet discoveries. The 'discoverer' by the new
rules of naming comets, is the source of the data within which the comet
was found, in this case the SOHO project.
Tony Hoffman uses a low-tech system for ferretting out SOHO comets. He
opens several pictures in separate Windows panels, overlaps them, and
flips thru them quickly. This is a crude but effective and efficient
'blink comparator'. When he finds a comet, he measures its position
relative to the Sun with the geometry features of his image processing
software, not an astrometry program, and the known scale of the image.
He guesstimates, due to his uncertainty about the filtration and
spectral response of LASCO, that his comet reached 6th magnitude at
brightest.
Comet designation
---------------
The present scheme, in force since 1995 January 1, is to assign a
designation based on the year, halfmonth, and serial order. The halfmonths
are the 1st to 15th and from the 16th to the month end. The first
halfmonth for May has the letter 'J'; second, 'K'. Note that with 24
halfmonths and 26 English letters, 'I' and 'Z' are left out.
At first this looks like merely a small alteration of the older
scheme of lettering the comets in order of discovery within a year.
The eighth comet found in 1986 is 1986h. But there is a fundamental
difference. The older method, altho prevalently descriebd as a
sequence of comet discoveries, was in fact a sequence of comet
announcements. It can -- and did! -- happen that a comet discovery did
not rach the world for many weeks, months, or, during Wolrd War II,
years. As long as comets were promptly reported in realtime, the two
interpretatons were about congruent, yet the 'legislative' definition
of the lettering system was based on the temporal sequence of comet
announcement, and not discovery.
When the new system was adopted, SOHO wasn't launched yet and
almost all comets were discovered in realtime. That is, they were
found visually in the sky or on pictures taken only a few days
earlier. Hence, the logging of comet discoveries did capture them in
the sequence of discovery.
But the definition of the newstyle designation is fundamenetal
distinct from the old one. The designation is applied to the date of
the record from which the comet was found. The comet found in a ship's
logbook from the second halfmonth of March 1762, altho only revealed
by examining the papers in 2001, is 1762-D1. (There happens to be no
comet in this period, so this is the first of the '1762-Dx' comets.)
Examine this table of comets discovered in May 1999. They are arranged
according as the actual discovery date.
Comet 1999-JV127 was originally treated as an asteroid; this is an
asteroid name. When it was seen to really be a comet, the former
designation was kept. Stuff like this happens; got over it already.
disc'y date designat'n proper name
----------- ---------- -----------
1999 May 03 1999-J12 SOHO-176
1999 May 05 1999-J7 SOHO-125
1999 May 07 1999-J8 SOHO-122
1999 May 07 1999-J1 SOHO-062
1999 May 09 1999-J9 SOHO-126
1999 May 10 1999-J10 SOHO-124
1999 May 10 1999-JV127 1999-JV127
1999 May 10 1999-J6 SOHO-109
1999 May 12 1999-J3 LINEAR
1999 May 12 1999-J5 LINEAR
1999 May 13 1999-J2 Skiff
1999 May 14 1999-J11 SOHO-123
1999 May 15 1999-J4 LINEAR
1999 May 17 1999-K4 LINEAR
1999 May 19 1999-K11 SOHO-129
1999 May 19 1999-K2 Ferris
1999 May 20 1999-K3 LINEAR
1999 May 20 1999-K6 LINEAR
1999 May 20 1999-K5 LINEAR
1999 May 20 1999-K1 SOHO-063
1999 May 23 1999-K12 SOHO-131
1999 May 23 1999-K9 SOHO-064
1999 May 24 1999-K7 LINEAR
1999 May 24 1999-K13 SOHO-132
1999 May 26 1999-K8 LINEAR
1999 May 27 1999-K14 SOHO-133
1999 May 28 1999-K15 SOHO-134
1999 May 31 1999-K10 SOHO-065
Whoa! The designations are all jumbled up!
SOHO upsets the scheme
--------------------
It is claimed in some astronomy circles that SOHO overthrew this
neat system. When SOHO started to uncover comets in archival data, the
designation scheme 'broke down'. In fact, the method of the halfmonth
naming remains valid, as long as we remember its intent.
The newstyle designation is applied in serial order of discovery
within each halfmonth. It doesn't matter if that discovery occurs many
months or years later. If within a halfmonth the next comet is the
12th in line of discovery, it is comet, say, 1999-K12.
For May 1999 SOHO #62, #63, #64, and #65 were found in realtime as
images were sent down to the ground station. The others, by their much
higher numbers, were found in 2000 and 2001 on plates archived for the
year 1999. The sequence number of SOHO comets is not part of the official
proper name, which is plain 'SOHO'. SOHO keeps nice linear track of its
discoveries. LINEAR, among others, is soho-so.
I compiled the table from data on the books as at late July 2001. By
the time you read this other SOHO comets for May 1999 could well have
been uncovered in the archive. Literally, the books of record for a
year can no longer ever be finally closed!
SOHO isn't the only culprit. LINEAR, an asteroid hunting project, also
finds comets on plates examined a while after they were taken. The
delay for LINEAR is weeks at the most.
Future space or ground observatories may image the sky in wide
areas and archive the plates for easy access and analysis. You can
expect the ageold practice of logging new discoveries in realtime to
rapidly fade to a minority portion of all discoveries.
nonSOHO searches
--------------
As bonntiful as the SOHO data are, they trace a Kreutz comet only
within its LASCO field of view. Are these comets traceable far from the
Sun? Can specific hunts for Kreutz comets be done?
Brian Marsden of CBAT pondered this question since his 1967 treatise
on the Kreutz group. From the SOHO record, he finds that essentially all
these comets are extremely faint and small, right near their perihelia. At
any remoteness from the Sun they would be far fainter and tinier. The few
that were spotlighted in home astronomy circles as being of 3rd or 2nd
magnitude when next to the Sun are the rare exceptions.
Marsden notes, in discussion I had with him in August 2001, that
occasional searches for remote Kreutz members were made with no success.
These did, he recalled, turn up a few new unrelated comets.
Confining the search to the parallax spiral worsens the odds of
success. The comets are then farthest from the Sun and smallest and
dimmest. With present day search techniques, efforts spent in this project
would be wasted.
There is the outside chance that a Kreutz member would be spotted away
from the Sun during a solar eclipse. Kreutz comet 1882-K1 was found in
exactly this way during the solar eclipse of 17 May 1882. This comet never
had a current designation because its orbit was not worked at the moment.
The entirely of its path is the arc within the few minutes of totality.
Today, we assign it rounded elements from comet 1880-C1, which it
superficially resembled.
How many Kreutz comets?
----------------------
How many of these Kreutz comets are there in all? No one knows and we
may never know. Suppose the particles come from the disruption of one or
two large comets within the last two or three millenia. Could there be
enough time to spread the pieces out evenly over the orbit to generate the
constant stream we now see? Granted, we got only the SOHO data from 1996-
2001 as a continuous record of these comets, but within this span there
seems to be a generally uniform spread of comets.
If the pieces did come from a comet with a period of, say, one
thousand years, the order of period postulated for the Kreutz parent, then
we have a problem. Recall that essentially none of these comets survive
their perihelion passing. This means the supply of comets is depleting
with no obvious means of replenishment.
After a full period of the parent comet, the tail end of the stream
rounds the Sun and then the flow of comets shuts off. We could, just
could, with the supposed period for the Kreutz parent, be near the end of
this flow. On the other hand, the parent's period may be much longer, so
the flow may continue far into the future.
And then, plain because we really do not know where these comets come
from, there may in fact be an indefinitely huge reservoir of these things
out there. Perhaps they are like a stubborn faucet leak from the Oort
cloud? Can they be particles which never did coalesce into large comets at
all?
Other streams of comets?
----------------------
We are blessed, or cursed, to witness the Kreutz comets only because
they do graze the Sun and are then rendered visible. Far from the Sun they
are essentially beyond the grasp of present comet-hunting skills and arts.
It's conceivable that this stream of comets just happens to consist of
sungrazers, but there may be nothing that requires such a close approach
to the Sun. Suppose there were a second, third, nth, comet stream whose
perihelia are too far from the Sun to allow them to achieve visibility
with current methods?
By yearend 2001 enough SOHO comets were collected that Brian Marsden
and Maik Meyer suggest possible other families of sungrazing comets.
These have no proper names as yet. Being that I focus on Kreutz
comets, the new groups are described in general terms only.
So far, end of 2001, each group has only a few members, most with
perihelia in 2000. The orbits of each group are characterized as
follows, where the values are the artihmetic mean of the members:
element new group #1 new group #2 new group #3
--------- ------------ ------------ ------------
Members six four two
Perihelion 0.0367 AU 0.0490 AU 0.0249 AU
Excentricity 1.0000 1.0000 1.0000
Inclination 72.30 deg 25.88 deg 88.16 deg
Long asc node 73.44 deg 81.89 deg 229.20 deg
Arg of perih 76.55 deg 22.80 deg 88.58 deg
Earliest periT 97 Jun 10.87 99 May 11.59 00 Dec 20.85
Latest periT 01 Dec 12.86 00 Feb 05.17 00 Dec 20.85
The third group so far has only two members. They, 2000-Y6 and 2000-
Y7, arrived at perihelion virtually simultaneously as twin comets. For the
time being, in view of the emerging prospect of there being several
sungrazing streams, they are set aside in their own group.
The mystery persists
------------------
It is still a mystery where these comets come from or what
significance they have in the grand scheme of the solar system.
Despite the explosion of examples since the commissioning of SOHO,
there has been no substantial role for the Kreutx comets in the
mainstream scenarios of the solar system theory. There are side
remarks about these comets embedded in other works, like the one I
noted of the Vela supernova and an other called to my attention on
orbits of micrometeoroids.
In fairness to astronomers, by the time a SOHO comet is announced,
it's surely already good and gone. There just was no time to collect
spectrometry, longterm astrometry, or other data germane to comet studies.
Eventually, newer and better observatories, most likely in space
or on the Moon, will reveal longer arcs of the orbit, capture full
astronomy-relevant data, giving us fresh raw material to study. Until
then all we are doing now is logging in the steady stream of these
comets week after week in numbing monotony.
One awesome vision of Kreutz comets is the scene from the IMAX movie
about the Sun, 'Solar Max'. There you see an animation of LASCO plates.
You have to view it in an IMAX theater; I saw it in early 2001 at the
Liberty Science Center. There is no home video of it as far as I know. The
attention is called to the activity in the corona. But, from the bottom,
south, of the Sun is a steady stream of comets pouring upward to their
death in the inner corona!
Hearty thanks
-----------
This paper started out as a couple pages to answer some small
questions. It grew as I explored the history and recent work on these
sungrazing comets. In its gestation this paper passed thru many reviews,
notably by members of the AAA Recent Astronomy Seminar. Special thanks for
comments, clarifications, and corrections, presented on marked up copies
of the article go to Tony Hoffman, Bruce Kamiat, Bernie Kleinman, Brian
Marsden, and Stewart Rorer.
Dr Marsden took time from his most busy duties studying Kreutz --
and all other -- comets at the Centtral Bureau for Astronomical
Telegrams for a detailed commentary on this paper.
In addition to hardcopies distributed at the Seminar in fall of 2001.
The paper is hung in the file area of the NYSkies maillist in
Yahoogroups.com.