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.