the comet s talejds/tail11.pdfmohamed ibrahem, andreas kammerer, heinz kerner, atilla kosa-kiss,...

BAA COMET SECTION NEWSLETTER

THE COMET’S TALENewsletter of the Comet Section of the British Astronomical Association

Volume 6, No 1 (Issue 11), 1999 April

INTERNATIONAL WORKSHOP ONCOMETARY ASTRONOMY

The BAA Comet Section is verypleased to be hosting the secondInternational Workshop onCometary Astronomy. This willbe held at New Hall Cambridgefrom August 14 to 16. Themeeting aims to bring togetherprofessional and amateurobservers and as many cometdiscoverers as possible.

The first IWCA was held atSelvino, Italy in February 1994and details appeared in theComet's Tale for 1994 May. Fourprevious American Workshops onCometary Astronomy had beenheld in the U.S.A., but had notattracted much attention inEurope.

The 1994 meeting marked the 15th

anniversary of the ICQ andattracted a good attendance ofactive observers. Presentations onthe work of various nationalobserving groups were given,along with talks by individuals ontheir own particular programs. Ofgreater importance were sessionswhich focussed on observingmethodologies and these lead tosignificant improvements in thequality of observations.

Since then we have had two verybright comets which presentednew problems in observation.There has been a huge growth inthe Internet, enabling rapiddissemination of observations andthe suspicion that this may biasobservers. Planetarium programsusing the Guide Star Cataloguenow give observers precise findercharts enabling them to observemuch fainter comets than in thepast.

These new problems need newsolutions and hopefully discussion

at the meeting will lead to someconclusions that further improveour observing standards.However, observers generallyhave very fixed views so expectsome heated arguments!

Although many amateurs observecomets for their own satisfaction,it can add enjoyment if you knowthat your observations arecontributing to the scientificunderstanding of comets. To thisend a number of leadingprofessionals will be at themeeting, explaining how ourobservations are used and whatadditional observations wouldhelp with their research.

Bearing all this in mind themeeting has three main goals: 1)improving the acquisition ofcometary information. 2)increasing understanding as towhat science can be gainedthrough observing comets by bothamateurs and professionals. 3)providing a forum in whichcometary astronomers can meetothers from distant geographicallocations and discuss variousissues.

The format of the meeting will beto have selected invited talks,open panel discussions,contributed talks and postersessions. Speakers scheduled toappear include Doug Biesecker(SOHO comets), Nicolas Biver(visual magnitudes and COoutgassing beyond 3 AU),Stephane Garro (French archivalobservations), Dan Green (cometphotometry), Eleanor Helin(comet searching/discovery atPalomar and with NEAT), GaryKronk (Cometography), BrianMarsden (amateur contributions),Herman Mikuz (CCD

photometry), Charles Morris(possibly on web issues),Mahendra Singh (cometspectroscopy).

The college bar will be open in theevenings for informal discussion.If it is clear there will beopportunity to use the two historicrefractors at the UniversityObservatory.

Cambridge has many sites ofastronomical interest and therewill be free time for participants toexplore at their leisure. Manyfamous scientists with cometconnections studied at Cambridgeincluding Newton, Herschel,Challis and more recentlyLyttleton.

To conclude the meeting I havearranged a trip to Stonehenge onthe Monday evening, when wewill be allowed in to the innercircle to view the stones close up.There have been some suggestionsthat Stonehenge was firstconstructed as either a comet ormeteor observatory! Numbers forthis visit have to be restricted andthose attending the meeting willhave priority.

A registration form is enclosed, sotake the time to fill it in now andcome and enjoy what should be avery interesting meeting. Thelatest details will be posted on theSection web page.

Contents

Comet Section contacts 2Section news 2Tales from the Past 3Professional Tales 4Review of observations 6Comet hunting notes 8Comet History (Part 1) 9

2 THE COMET’S TALE


Comet Section contacts

Director: Jonathan Shanklin, 11 City Road, CAMBRIDGE. CB1 1DP, England.Phone: (+44) (0)1223 571250 (H) or (+44) (0)1223 221400 (W)Fax: (+44) (0)1223 362616 (W)E-Mail: [email protected] or [email protected] page : http://www.ast.cam.ac.uk/~jds/

Assistant Director (Observations): Guy Hurst, 16 Westminster Close, Kempshott Rise, BASINGSTOKE, Hampshire.(and also Editor of RG22 4PP, England.The Astronomer magazine) Phone & Fax: (+44) (0)1256 471074

E-Mail: [email protected] or [email protected]

CCD Advisor: Nick James, 11 Tavistock Road, CHELMSFORD, Essex. CM1 5JL, England.Phone: (+44) (0)1245 354366E-mail: [email protected] or [email protected]

Photographic Advisor: Michael Hendrie, Overbury, 33 Lexden Road, West Bergholt, COLCHESTER,Essex, CO6 3BX, EnglandPhone: (+44) (0)1206 240021

Subscription to the Section newsletter costs £5 for two years, extended to three years for members who contribute to thework of the Section in any way. Renewals should be sent to the Director and cheques made payable to the BAA. Thosedue to renew should receive a reminder with this mailing.

Section news from the Director

Dear Section member,

I returned from my 11th visit to theAntarctic in March. It was a verybusy stay at Halley station where Iwas forecasting for our aircraftoperations in addition to myexpected tasks of installing a newautomatic weather station andcarrying out an inspection andcalibration of the instrumentwhich measures ozone. Theforecasting was quite successfuland we had no aborted flightsduring the 8 weeks of operations.

On the flight down to Antarctica Ihad a window seat (thanks to akindly RAF airman) and was ableto see the spectacular return of theLeonids. At its best we saw onefireball every 10 seconds and Ilogged over 300 meteors. Thepeak was much earlier thanpredicted by the experts so whoknows where the best location forthe 1999 return will be.

I saw a number of other interestingphenomena during my stay: abrilliant halo display whichincluded Hevelius’ parhelia, theparhelic circle and acircumzenithal arc withsupernumery colours, a fogbow,miraging, iridescence on clouds, a40 knot blizzard and a very weakaurora.

On passage I was able to completedrafts for papers on the comets of1995, 1996 and 1997 and anoutline of 1998 which will appearin the Journal in due course. Ialso entered another 2267observations into the Sectiondatabase, this time of the longperiod comets of the 1980s. Thistask is now almost completed andI only have to enter theobservations of these long periodcomets which solely appeared inTA. I have not typed up all theobservations in the files becausesome had already been sent to theICQ by Michael Hendrie and I didnot want to duplicate his typing. Isuspect, however, that I may nothold all the observation formsfrom some observers and that theymay have gone missing before Itook over as Director. If anyonewants a list of magnitudeobservations that I hold, please letme know.

The computer archive currentlyhas over 26,000 visual magnitudeobservations by 617 observers at338 cometary apparitions. 117observers have made only oneobservation, 376 fewer than 11and only 54 have made more than100. Major contributions include2433 from myself, 884 fromAlbert Jones, 549 from Roy

Panther, 504 from Tony Tanti,370 from Guy Hurst, 351 fromWerner Hasubick, 225 fromMelvyn Taylor, 224 from MichaelGainsford, 171 from GabrielOksa, 151 from Michael Hendrie,136 from Frank Ventura, 125 fromSally Beaumont, 104 from GeorgeAlcock and 98 from James Fraser.Over the years many non andformer BAA Members have alsocontributed observations, inparticular TA contributors and theAustralian comet section,including Andrew Pearce (1187),John Bortle (989), Martin Lehky(697), Herman Mikuz (625),Alexandr Baransky (621), BjornGranslo (481), David Seargent(475), Graham Keitch (460),Vittorio Zannotta (410), GrahamWolf (406), Jose Aguiar (375),Reinder Bouma (364), AtillaKosa-Kiss (334), Chris Spratt(316) and Paul Camilleri (281).For most observers these are notthe full total that they have madeas it excludes those sent to theICQ prior to the commencementof the archive. If any readerswould like to add to their totals bysending me observations in ICQformat either via email or onfloppy disc I’ll be very pleased toinclude them.

As described on the front page, weare hosting the International

1999 April 3


Workshop on CometaryAstronomy in August, and I hopeto see as many of you there aspossible. Don’t miss it, whoknows where the next one will be!

Visitors who have indicated thatthey will be coming include JoseAguiar (Brazil), AlexandrBaransky (Ukraine), DougBiesecker (USA/UK), GiuseppeCanonaco (Belgium), JoseCarvajal (Spain), Tim Cooper(South Africa), Stephane Garro(France), Dan Green (USA),Werner Hasubick (Germany),Michael Jager (Austria), GaryKronk (USA), William Liller(Chile), Don Machholz (USA),Brian Marsden (USA), HermanMikuz (Slovenia), Charles Morris(USA), Andrew Pearce(Australia), James Scotti (USA),David Seargent (Australia), ChrisSpratt (Canada), PatrickStonehouse (USA), KesaoTakamizawa (Japan).

Since the last newsletter (whichwas incorrectly numbered volume6 instead of 5) observations orcontributions have been received

from the following BAAmembers: Sally Beaumont, DavidBriggs, Denis Buczynski, DavidClegg, Werner Hasubick, GuyHurst, Nick James, John Lewis,John Mackey, Gabriel Oksa,Jonathan Shanklin, David Strange,Melvyn Taylor and Alex Vincent

and also from: AlexandrBaransky, Sandro Baroni, JohnBortle, Reinder Bouma, StephenGetliffe, Roberto Haver, AymenMohamed Ibrahem, AndreasKammerer, Heinz Kerner, AtillaKosa-Kiss, Martin Lehky, Jean-Claude Merlin, Herman Mikuz,Andrew Pearce and SeiichiYoshida (apologies for anyomissions or miss-classifications).

Comets under observation were:4P/Faye, 10P/Tempel 2,21P/Giacobini-Zinner,29P/Schwassmann-Wachmann 1,37P/Forbes, 52P/Harrington-Abell, 59P/Kearns-Kwee,60P/Tsuchinshan 2, 65P/Gunn,74P/Smirnova-Chernykh,88P/Howell, 93P/Lovas 1,95P/Chiron, 139P/Vaisala-Oterma, 140P/Bowell-Skiff, Hale-

Bopp (1995 O1), Spacewatch(1997 BA6), Meunier-Dupouy(1997 J2), Mueller (1998 K1),LINEAR (1998 K5), LINEAR(1998 M2), Larsen (1998 M3),LINEAR (1998 M5), Williams(1998 P1), LONEOS-Tucker(1998 QP54), P/LINEAR-Mueller(1998 S1), LINEAR (1998 T1),P/Mueller 6 (1998 U2), P/Jager(1998 U3), LINEAR (1998 U5),Spahr (1998 W1), LINEAR (1998W3), P/ODAS (1998 X1),P/LINEAR (1998 Y1), P/Li (1998Y2), Tilbrook (1999 A1),Hermann (1999 D1) and Li (1999E1).

Many of the fainter comets wereobserved only by Seiichi Yoshidawho is using a CCD camera on an18 cm reflector to very goodeffect. I hope to see many moreCCD observations from UKobservers in the next edition,though I can't set an example asmy camera has been 'borrowed' tohelp test equipment for Gemini!

Jonathan Shanklin

Tales from the Past

This section gives a few excerptsfrom past RAS Monthly Notices,BAA Journals and Sky &Telescope.

150 Years Ago: Francis De Vicodied on 1848 November 15. Hediscovered 6 comets and wasawarded the King of Denmark'sGold Medal four times in 1846 forhis telescopic discoveries. Hedied of typhus on a visit toEngland and was buried in thecemetery of the Roman Catholicchapel in Chelsea.

100 Years Ago: An interimreport on Brooks' comet 1898Iappears in the December Journal.It was conspicuous in earlyNovember and the reports suggestthat it was a little fainter thanM13. [only Messier objects wereused for comparisons, a practicethat is not recommended today.].In the January Journal MrCrommelin reported that platestaken to determine the Leonidradiant had also revealed a newcomet. Ten comets were observedin 1898, a record, beating that of1858 when eight were observed.In March a review of "RemarkableComets" by W T Lynn, noted thatcomets were probably attractedinto the orbit of meteor swarms,

rather than the meteors formingfrom comets. At theConversazione (an eveningexhibition meeting) MrCrommelin gave a lecture oncomets and recommended moreuse of photography in theobservation and discovery ofcomets. In the intervals betweenlectures, the Blue Anglo-Hungarian Band played music andthere was the chance to view theexhibits. A note on comet seekingrecommends the use of one eye forsolar observing and the other forcomets and nebulae, and neverlooking at strong light such as thesun or electric arcs.

50 Years Ago: The Novembermeeting had several reports on theeclipse comet, which was one ofthe brightest of the century(further reports appeared in Sky &Telescope). Next month, DrMerton gave a talk on comets. Inresponse to a question hecommented "We do need carefulobservations of its magnitude.Most of those who sent the firstreports appear to have been somoved at the splendid sight thiscomet made that they omitted torecord precise information ofphysical details." The JanuaryJournal contains an abstract of apaper on cometary statistics which

appeared in Bulletin No 283 of theNew South Wales branch. Detailsof this also appeared in Sky &Telescope. The March Journalhas a report of a lecture given byDr J G Porter to the Newcastle-on-Tyne Astronomical Society. Atthis time the 'sandbank' theory ofcomets was popular and hesuggested that Halley's cometmight consist of 3x1013 tons ofblocks of meteoric materialscattered over a space about 20miles in diameter. [Giotto showedthat the comet was a singlenucleus x miles long, weighingsome x tons]. Sky & Telescopewas running a series of articlesentitled 'Terminology Talks'.Several of these focussed oncomets. In December there was ashort article on the great comet of1882, including somerecollections from Americans whohad seen it. [Perhaps olderreaders would like to contributerecollections of the Eclipse comet,or other bright comets seen over25 years ago]. An explanation ofcomet terminology again likenedthe comet's nucleus to a swarm ofmeteorites extending over a fewthousand miles, with considerablespace between the individualparticles. Finally 1948 equaledthe record number of comets with14.



Professional Tales

Many of the scientific magazineshave articles about comets in themand this regular feature is intendedto help you find the ones you'vemissed. If you find others let meknow and I'll put them in the nextissue so that everyone can lookthem up.

Jonathan Shanklin

The following abstracts (someshortened further for publication)are taken from the CambridgeConference Network (CCNet),which is a scholarly electronicnetwork devoted to catastrophism,but which includes muchinformation on comets. Tosubscribe, contact the moderatorBenny J Peiser at<[email protected]>.Information circulated on thisnetwork is for scholarly andeducational use only. Theabstracts, taken from dailybulletins, may not be copied orreproduced for any other purposeswithout prior permission of thecopyright holders. The electronicarchive of the CCNet can be foundat http://abob.libs.uga.edu/bobk/cccmenu.html

M. Fulle, G. Cremonese, C.Bohm: The preperihelion dustenvironment of C/1995 O1 Hale-Bopp from 13 to 4 AU.ASTRONOMICAL JOURNAL,1998, Vol.116, No.3, pp.1470-1477

Two UK Schmidt plates of cometHale-Bopp dust tail taken in 1996May are analyzed by means of theinverse dust tail model. The dusttail fits are the only available toolsproviding estimates of the ejectionvelocity, the dust-loss rate, and thesize distribution of the dust grainsejected during years preceding thecomet discovery. These quantitiesdescribe the comet dustenvironment driven by COsublimation between 1993 and1996, when the comet approachedthe Sun from 13 to 4 AU. Theoutputs of the model are consistentwith the available comaphotometry, quantified by the Afrho quantity. The dust mass lossrate increases from 500 to 8000 kgs(-1), these values being inverselyproportional to the dust albedo,assumed here to be 10%.Therefore, the mass ratio betweenicy grains and CO results is atleast 5. Higher values of the dust-to-gas ratio are probable, because

the model infers the dust-loss rateover a limited size range, up to 1mm sized grains, and because thepower-law index of the differentialsize distribution ranges between -3.5 and -4.0, so that most of thedust mass was ejected in thelargest boulders that Hale-Boppwas able to eject. The dustejection velocity close to theobservations, between 7 and 4AU, was close to 100 m s(-1) forgrains 10 mu m in size, muchhigher than that predicted by R. F.Probstein's theory, thus confirmingprevious results of Neck-Linephotometry. This result is anindicator of CO superheating withrespect to a free sublimating COice, in agreement with the highobserved CO velocity. Thefundamental result of the paper isthat such a high dust velocityremained constant between 13 and4 AU, thus providing a strongconstraint to all models of the GO-driven activity of the comet duringits approach to the Sun: COsuperheating must have beenactive since 13 AU from the Sun.It might be provided by theabundant dust itself, or byseasonal effects heating thesubsurface layers, as wassuggested for comet29P/Schwassmann-Wachmann 1.Another similarity between thetwo comets is provided by thepower-law index of the time-averaged size distributions: -3.6+/- 0.1 for C/1995O1 and -3.3 +/-0.3 for 29P/SW1. However, othercharacteristics of the dustenvironments are very different, sothat, in general, it is impossible todistinguish a CO-driven cometfrom a typical water-driven one.Copyright 1998. Institute forScientific Information Inc.

P.L. Lamy, I. Toth, H.A. Weaver:Hubble Space Telescopeobservations of the nucleus andinner coma of comet19P/Borrelly. ASTRONOMYAND ASTROPHYSICS, 1998,Vol.337, No.3, pp.945-954

The nucleus of comet19P/Borrelly was detected usingthe Planetary Camera (WFPC2) ofthe Hubble Space Telescope(HST). During the time of ourobservations, the comet was 0.62AU from the Earth, 1.40 AU fromthe Sun, and had a solar phaseangle of 38 degrees. The highspatial resolution of the HSTimages allowed us to discriminate

clearly between the signal fromthe nucleus and that from thecoma. The lightcurve of thenucleus indicates that it is a highlyelongated body rotating with asynodic period of 25.0 +/- 0.5 hr.Assuming that the nucleus has ageometric albedo of 4% and is aprolate spheroid with a rotationalaxis pointing in the directiondetermined by Sekanina (1979),we derive that its semi-axes are4.4 +/- 0.3 km and 1.8 +/- 0.15km. The corresponding fractionalactive area of similar to 8%suggests a moderately activecomet. The highly anisotropiccoma is dominated by a strongsunward fan, and the dustproduction rate exhibited signs oftemporal variability throughoutour observations.Copyright 1998. Institute forScientific Information Inc.

W. Waniak, S. Zola: Dustemission for CometsShoemaker-Levy 1991a1 andMcNaught-Russell 1993v.ICARUS, 1998, Vol.136, No.2,pp.280-297

We present CCD photometricresults for the dust comae of thedynamically new CometShoemaker-Levy 1991a1, carriedout at heliocentric distances from1.2 to 0.8 AU pre-perihelion, andthe high-eccentricity, long-periodComet McNaught-Russell 1993vobtained at a heliocentric distanceclose to 1.0 AU post-perihelion.Maps of the directionaldistribution of the dust emissionrate from these cometary nucleiwere obtained using thedirectional deconvolution method(Waniak 1994, Icarus 111, 237-245). For Comet Shoemaker-Levy the prominent region ofenhanced dust production wassituated between the solarterminator and the nucleocentricmeridian opposite the subsolarpoint. Activity in this region onthe night side of the nucleus maybe explained both by the heatingof the nucleus' surface by scatteredvisible and reemitted infraredradiation, which is produced bythe dust coma, or by non-solarradiation sources of energy, suchas chemical reactions or phasetransitions. During the period ofobservations the dust emission ratefor this region decreased incomparison with that of anotherregion of enhanced dustproduction situated on the

1999 April 5


subsolar hemisphere. For CometMcNaught-Russell two activeregions were also visible, althoughthe subsolar region was muchmore active than that on the nightside of the nucleus. For bothcomets, dust was emitted from theentire surface of the nucleus at alevel no lower than 30% of themaximum value for the activeregions. The total (integrated overa 4 pi solid angle) dust emissionrate for Comet Shoemaker-Levychanged as r(h)(-2.3) for theobserved range of heliocentricdistance r(h). For both comets,the ejection velocity of submicrondust particles was of the order of0.1 km sec(-1) and the power-lawsize distribution of dust particles(a(-n)) had a mean value ofexponent n equal to 2.9. Thepower-law dependence of theejection velocity upon the betaparameter (nu similar to beta(k))was specified by a mean value of kclose to 0.18.(C) 1998 Academic Press.

P. Wiegert & S. Tremaine: Theevolution of long-period comets.ICARUS, 1999, Vol.137, No.1,pp.84-121

We study the evolution of long-period comets by numericalintegration of their orbits, a morerealistic dynamical approach thanthe Monte Carlo and analyticmethods previously used to studythis problem. We follow thecomets from their origin in theOort cloud until their final escapeor destruction, in a model solarsystem consisting of the Sun, thefour giant planets and the Galactictide. We also examine the effectsof nongravitational forces as wellas the gravitational forces from ahypothetical solar companion orcircumsolar disk. We confirm theconclusion of Oort and otherinvestigators that the observeddistribution of long-period cometorbits does not match the expectedsteady-state distribution unlessthere is fading or some similarphysical process that depletes thepopulation of older comets. Weinvestigate several simple fadinglaws, We can match the observedorbit distribution if the fraction ofcomets remaining observable afterm apparitions is proportional tom(-0.6+/-0.1) (close to the fadinglaw originally proposed byWhipple 1962); or ifapproximately 95% of comets livefor only a few (similar to 6)returns and the remainder lastindefinitely, Our results also yieldstatistics such as the expected

perihelion distribution,distribution of aphelion directions,frequency of encounters with thegiant planets and the rate ofproduction of Halley-type comets.(C) 1999 Academic Press.

P. Gronkowski & J. Smela: Thecometary outbursts at largeheliocentric distances.ASTRONOMY ANDASTROPHYSICS, 1998, Vol.338,No.2, pp.761-766

A model is presented explainingchanges in cometary brightnessduring an outburst at largeheliocentric distances. It is shownthat a combination of thefollowing effects can explain themain characteristics of outburst atlarge heliocentric distances: thespecific exothermic processes incometary nucleus (as the HCNpolymerisation and thecrystallization of the wateramorphous ice, connected with theejection of the large quantities ofdust) and the sublimation of CO orCO2 from the comet's nucleus.The obtained results are in goodagreement with observations.Copyright 1998. Institute forScientific Information Inc.

Z. Sekanina: A double nucleus ofcomet Evans-Drinkwater (1996J1). ASTRONOMY ANDASTROPHYSICS, 1998, Vol.339,No.1, pp.L25-L28

The nucleus of comet C/1996 J1,whose duplicity was first detectedin early May 1997, similar to 4months after perihelion, is foundto have split nontidally similar to70 days before perihelion at 1.65AU from the Sun. The secondarynucleus, discovered when inoutburst and subsequentlyobserved for 8-1/2 months, hadseparated from the primarynucleus at a rate of 1.7 m/s anddrifted away from it with a radialnongravitational deceleration ofsimilar to 31 x 10(-5) the Sun'sattraction, typical for the short-lived companions. At the time ofsplitting, this dynamically newcomet was near conjunction withthe Sun and thereforeunobservable from Earth. In late1997 and early 1998, when lastseen, the companion was greaterthan or similar to 100 times fainterrelative to the primary componentthan it had been when firstreported.Copyright 1998, Institute forScientific Information Inc.

A. Brunini & M.D. Melita: On theexistence of a primordialcometary belt between Uranusand Neptune. ICARUS, 1998,Vol.135, No.2, pp.408-414

The existence of stable orbits inthe interplanetary region betweenUranus and Neptune over thelifetime of the Solar System hasbeen reported by Holman (1997,Nature 387, 785-788). A long-term integration of test particles inthat zone resulted in a number ofsurviving bodies with semimajoraxis roughly between 24 and 27AU, and eccentricities andinclinations smaller than 10(-2)and 1 degrees, respectively.According to up-to-date surveys,the total mass of this putative belthas been estimated as less than orequal to 10(-3) M+ and it wouldbe composed of objects withradius smaller than 50 km. In thiswork we assess the plausibility ofthe existence of a real populationof objects in such a belt. Time-reversal arguments rule out thepossibility of capture in such long-term stable orbits, for example, ofobjects escaped from the Kuiperbelt. So if a real population existsnowadays, it should haveundergone the conditions ofplanetesimal accretion in theregion. Hence, we have studiedthe orbital evolution of testparticles under different kinds ofplausible primordial scenarios.The conditions considered weremutual collisions and gravitationalencounters, planetary migrationand the presence of an adjacentPluto-sized object. Under none ofthese conditions have we obtaineda surviving substantial populationon the reported belt region, fromwhich it is concluded that, atpresent time, it would be veryunlikely to find a substantialnumber of primordial objects inthose long-term stable orbits.(C) 1998 Academic Press.

Z. Sekanina: Multiplefragmentation of cometP/Machholz 2 (1994 P1).ASTRONOMY ANDASTROPHYSICS, 1999, Vol.342,No.1, pp.285-299

Discovered in August of 1994,periodic comet Machholz 2consisted of five condensations,A-E, of which D later becamedouble. They were lined up alongtheir common heliocentric orbit(with A being the leading andbrightest component) andconnected by a trail of material,



suggesting that the comet's nuclearfragmentation was accompaniedby a copious release of large dustparticles. The earliest breakup isfound to have occurred in late1987, similar to 600 days beforethe comet's 1989 perihelion,giving birth to fragment B and thegrand precursor of A. Theprecursors of A and D and

fragments A and C appear to haveoriginated, respectively, similar to5 days prior to and right atperihelion. The last breakupepisode during that same return tothe Sun was the separation of E,probably from the precursor of D,similar to 600 days afterperihelion. The division of D intoD-1 and D-2 is the only event

analysed in this paper thatoccurred one revolution later, in1994. The circumstances andimplications of this fragmentationsequence are examined in detailand predictions are presented for1999/2000.Copyright 1999, Institute forScientific Information Inc.

Review of comet observations for 1998 October - 1999 March

The information in this report is asynopsis of material gleaned fromIAU circulars 7030 – 7138 andThe Astronomer (1998 October –1999 March). Note that thefigures quoted here are roundedoff from their original publishedaccuracy. Lightcurves for thebrighter comets are fromobservations submitted to TheAstronomer and the Director. Afull report of the comets seenduring the year will be publishedin the Journal in due course.

4P/Faye was observed severaltimes over the winter as it slowlybrightened to around 13th mag.Perihelion is not until May, butthis is a poor return and furtherobservations are unlikely, thoughobservers with large apertures mayglimpse it in the autumn.

10P/Tempel 2 makes its 20thobserved return since its discoveryby William Tempel (Milan, Italy)as a 9th magnitude object in 1873.Several unfavourable returns weremissed in the earlier years. Theorbit is very stable, which is onereason why it is a favoured targetfor planned spacecraft missions.In 1983 the IRAS satellitedetected an extensive dust trailbehind the comet.

Traditionally the light curve isregarded as highly asymmetricwith a late turn on. There is arapid rise in brightness asperihelion approaches, whichcontinues more slowly for acouple more weeks afterperihelion, followed by a slowdecline until activity switches off.An alternative view is that thelight curve is linear with a peakabout a month after perihelion,which at this return occurs in earlySeptember.

With a 5.5 year period alternatereturns are favourable and this isone of them. A CCD observationby Seiichi Yoshida in late Marchput it at 18th mag. The comet maybe picked up in large telescopes in

April when it is in Ophiuchus andit should reach 10th magnitude inJune. It is closest to the Earth inJuly (0.65 AU) when it could be9th magnitude and UK observersshould be able to follow it untilAugust, but it then moves too farsouth. More southerly observersmay be able to observe it until theend of the year as it fades.

21P/Giacobini-Zinner, the parentcomet of the October Draconidmeteors, remained visible untilMarch, though most observers lostit in December. Observationsshow the comet brightened veryrapidly with distance from sun andearth changing in step. The 214observations give a light curve of8.3 + 5 log d + 11.4 log r.

Comet 21P/Giacobini-Zinner

1998 - 1999

Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May

Obs

erve

d m

agni

tude

8

9

10

11

12

13

14

15

16

29P/Schwassmann-Wachmann 1has frequent outbursts and overthe past couple of years seems tobe more often active than not,though it rarely gets brighter than12m. It is possible that its patternof behaviour is changing. Thisyear Andrew Pearce detected amajor outburst at the end ofMarch.

It is at opposition in May on theborders of Hydra and Libra andreaches solar conjunction inNovember. Observers areencouraged to check the comet atevery opportunity over theapparition, although it will remainat low altitude for UK observersfor several years to come. It is

worth monitoring with CCDcameras on a regular basis.

37P/Forbes will be 13m betweenApril and June. Andrew Pearcepicked up the comet at 13.5 in thesecond half of March.

52P/Harrington-Abell wasdiscovered in outburst at 12th

magnitude by Alain Maury,Observatoire de la Cote d'Azur, onCCD images taken on July 21.1.It slowly faded, but thenunderwent a second outburst inOctober that peaked some 29 daysbefore perihelion, which was inlate January. This is the seventhobserved return of the comet sinceits discovery in 1954 and it hasnever became brighter than 17thmagnitude at previous returns.Normally it would have beenexpected to get no brighter than15th magnitude at this return, butpeaked near 10th magnitude at theturn of the year.

Observations received so far (90)give an uncorrected preliminarylight curve for the second outburstof 10.9 + 5 log d + 0.0299 abs(t -T + 28.5)

Comet 52P/Harrington-Abell

1998 - 1999

Aug Sep Oct Nov Dec Jan Feb Mar Apr May

Obs

erve

d m

agni

tude

9

10

11

12

13

14

15

16

17

59P/Kearns-Kwee was observedby Werner Hasubick in January at15th mag. This is rather brighterthan expected and it could reach12th mag towards the end of theyear.

1999 April 7


60P/Tsuchinshan 2 is yet anotherfaint object imaged by SeiichiYoshida at 16th mag, which is asbright as it will get.

65P/Gunn remains on view toCCD observers, although it is only17th mag.

Another faint object is74P/Smirnova-Chernykh whichdoesn't reach perihelion untilJanuary 2001 but has already beenimaged by Seiichi Yoshida at 16th

mag.

88P/Howell reached a peakmagnitude of around 10 in theautumn, but was essentially asouthern hemisphere object. The33 observations received so fargive a preliminary light curve of8.8 + 5 log d + 0.0319 abs(t - T -27.8)

Comet 88P/Howell

1998 - 1999

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar

Obs

erve

d m

agni

tude

9

10

11

12

13

14

15

16

93P/Lovas 1 was quite wellobserved and faded from 13th magat the turn of the year. The 53observations give a preliminarylight curve of 9.3 + 5 log d + 13.8log r

Comet 93P/Lovas 1

1998 - 1999


Obs

erve

d m

agni

tude

12

13

14

15

16

17

18

Comet/Asteroid 95P/Chiron willbe 16m when at opposition in lateMay in Libra. Seiichi Yoshidaimaged it near 17th mag in the firstquarter of the year.

Hale-Bopp (1995 O1) is now atelescopic object, but still wellplaced for Southern Hemisphereobservers as it loops round theLarge Magellanic Cloud.Somewhat to my surprise I wasable to glimpse it in 20x80binoculars from the FalklandIslands in mid November at 10.5but it will reach 13th mag by theend of this year. The observed arcnow covers 1349 days withobservations made on 734 days.The equation -0.69 + 5 log d +7.66 log r fits daily means verywell, but there are long periodvariations about this mean lightcurve of around a magnitude,which are shown plotted with anoffset of -2. It is currently a littlebrighter than indicated by theequation.

Comet Hale-Bopp (1995 O1)

1995 - 1999

SepDecMarJunSepDecMarJunSepDecMarJunSepDecMar

Obs

erve

d m

agni

tude

(da

ily m

ean)

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

11

12

Spacewatch (1997 BA6). Theorbit of this comet is veryeccentric, with a period near 4500years and a semi-major axis ofseveral hundred AU. Currently itis 13th mag and is at high southerndeclination. It is heading forperihelion at 3.4 AU in 1999December when it may be around12th mag.

Comet Spacewatch (1997 BA6)

1998 - 1999

Dec Jan Feb Mar Apr May

Obs

erve

d m

agni

tude

12

13

14

15

16

17

18

Meunier-Dupouy (1997 J2)faded away and no observationswere received after December.The 359 observations received so

far suggest a preliminary lightcurve of 5.1 + 5 log d + 5.8 log r.

Comet Meunier-Dupouy (1997 J2)

1997 - 1998

MayJunJulAugSepOctNovDecJanFebMarAprMayJunJulAugSepOctNovDec

Obs

erve

d m

agni

tude

9

10

11

12

13

14

15

Mueller (1998 K1) was imagedby Seiichi Yoshida over thewinter, but is now fainter than 18th

mag.

LINEAR (1998 K5) was a littlefainter and a little more diffuse inmid October compared to the earlyautumn. By the end of October itwas still 14th mag, but becominglarger and more diffuse and fadedrapidly in November. The lightcurve is very unusual and thecomet's absolute magnitude seemsto have peaked some 73 days afterperihelion. The combination ofchanging distance from earth, withthe late peak in brightness kept themagnitude nearly constant fromJuly to October.

The 71 observations analysed givea preliminary light curve of 13.1 +5 log d + 0.0298 abs(t - T - 73.0).

Comet LINEAR (1998 K5)

1998 - 1999

Jun Jul Aug Sep Oct Nov Dec Jan

Obs

erve

d m

agni

tude

11

12

13

14

15

16

17

18

19

LINEAR (1998 M2) was imageda couple of times by SeiichiYoshida over the winter, but wastoo faint for most other observers.

Larsen (1998 M3) was alsoimaged by Seiichi Yoshida, thistime at 18th mag.



LINEAR (1998 M5) was atperihelion in January at around10th magnitude and moved northfrom Lyra, passing very close tothe pole in mid March. At thispoint its motion effectivelycancelled out the earth's rotationand imaging was possible with afixed mounting. Heading south itpasses through Camelopardalusand Lynx, reaching Cancer midyear when it will have faded to13th magnitude. It will then betoo faint and close to the sun forfurther observation.

Observations received so far (225)give a preliminary light curve of6.2 + 5 log d + 7.4 log r

Comet LINEAR (1998 M5)

1998 - 1999


Obs

erve

d m

agni

tude

9

10

11

12

13

14

15

Williams (1998 P1) wasrecovered after perihelion byAndrew Pearce in late Novemberbut its rapid fade had alreadybegun and it was nearly 10th mag.This fade continued, though someobservers reported surprisinglybright magnitudes as late asMarch.

The 82 observations received sofar give a light curve of 5.1 + 5log d + 15.5 log r.

Comet Williams (1998 P1)

1998 - 1999


Obs

erve

d m

agni

tude

6

7

8

9

10

11

12

13

14

15

LONEOS Tucker (1998 QP54)was another target for SeiichiYoshida, and was recorded at 16th

mag in December. Martin Lehkyand Werner Hasubick also

recorded it visually at near 15th

mag earlier in the autumn.

P/LINEAR-Mueller (1998 S1)faded from 14th mag in Novemberto 16th mag in February.

LINEAR (1998 T1) wasdiscovered during survey workwhilst still a long way fromperihelion. Several observersrecorded it over the winter whilstnear 14th mag. The light curve isnot very well defined yet, but itshould reach 9th magnitude when itis at perihelion in June, though itwill not be visible from the UK.

LINEAR (1998 U1) is yet anothercomet discovered by LINEAR.The object, discovered on October18, was reported as asteroidal andfast-moving, and a request forfurther observations was placed onThe NEO Confirmation Page.From the confirmatoryobservations, made at fourobservatories on October 20 and21, it quickly became clear thatthe object had a retrograde, nearlyparabolic orbit. Referring to theobservations made with the 0.65-m f/3.6 reflector at the OndrejovObservatory on October 20.9 UT,P. Pravec noted that a faint tailwas visible on individual 3-minexposures with a clear filter. Thetail, extending 16" in p.a. 60 deg,was confirmed on co-addition ofsix frames. [IAUC 7033, 1998October 21]. The comet is faintand distant and past perihelion.

P/Mueller 6 (1998 U2) wasdiscovered by Jean Mueller onplates taken by herself on Oct 21.3(and with K. Rykoski on Oct. 22)with the 1.2-m Oschin SchmidtTelescope in the course of thePalomar Outer Solar SystemEcliptic Survey. The comet had astrong condensation and a short,faint tail to the southeast. [IAUC7035, 1998 October 22].Prediscovery images fromSeptember 14 were found inLONEOS data and the comet is anintrinsically faint periodic objectfound at a favourable opposition.Seiichi Yoshida made a singleobservation in December when itwas 17th mag.

P/Jager (1998 U3) wasdiscovered by TA correspondentMichael Jager on 16- and 9-minTechnical Pan film exposures witha 0.25-m f/2.8 Schmidt camera.The comet had a 1'-2' coma withcondensation and a tail severalarcmin long in p.a. 275-281 degand was around 12th mag. [IAUC

7038, 1998 October 25]. NickJames imaged the comet onOctober 26. I observed it onOctober 29.1, making it around12.5 in the Northumberlandrefractor x105, DC s3 anddiameter around 1'. It reached avery broad peak of 10th magbetween December and February.

Observations received so far (94)give a preliminary light curve of9.8 + 5 log d + 0.0201 abs(t - T +27.8). This suggests that it willnow fade quite rapidly.

Comet Jager (1998 U3)

1998 - 1999

Nov Dec Jan Feb Mar Apr May

Obs

erve

d m

agni

tude

8

9

10

11

12

13

14

P/Spahr 1 (1998 U4) wasdiscovered by Timothy B Spahr ofthe University of Arizona onOctober 27.4 on CCD imagestaken with the Catalina SkySurvey 0.41-m f3 Schmidt. It was17th mag and was confirmed bySpacewatch Telescope images.[IAUC 7042, 1998 October 29].The preliminary orbit is a distantelliptical one and the comet didnot become any brighter.

LINEAR (1998 U5) was anotherobject discovered by LINEAR thatwas later identified as a comet.[IAUC 7044, 1998 October 31].On November 1.18 I estimated itat 12.3, rather brighter than 1998U3. It brightened explosively andreached 8th mag just over afortnight later.

Comet LINEAR (1998 U5)

1998 - 1999

Nov Dec Jan Feb Mar Apr

Obs

erve

d m

agni

tude

7

8

9

10

11

12

13

14

15

16

17

1999 April 9


Observations received so far (119)give a preliminary light curve of6.1 + 5 log d + 28.9 log r.Although it faded quite quickly,very few observations were madeafter December

P/LINEAR (1998 VS24). Onfurther examining the Nov. 10-11observations, published on MPS3154, of an apparentlyunremarkable asteroidal discoveryfrom the LINEAR program, G. V.Williams made identificationswith isolated sets of observationsfrom Visnjan on Oct. 24, 28 (thelatter having been published onMPS 2894 as one of two objectsthat were both erroneouslyidentified with 1998 UD19) andNov. 26. The resulting orbit wasvery cometary in form, with aperiod of 9.6 years. Furthermore,he recognized that the object hadmade an extremely close approach(< 0.01 AU) to Jupiter in Oct.1971, with additional approachesto 0.5-0.7 AU in both 1983 and1995. It is a distant intrinsicallyfaint object, so of no concern forthe moment.

P/Spahr 2 (1998 W1) Timothy BSpahr of the University of Arizonadiscovered another comet onNovember 16.4 on CCD imagestaken with the Catalina SkySurvey 0.41-m f3 Schmidt. It was16th mag but there was no visibletail, with the round coma 18" indiameter [IAUC 7052, 1998November 17]. It reachedperihelion in January and was nobrighter than 15th mag on CCDimages, but visual observers madeit as bright as 14th mag.

Comet P/Spahr 2 (1998 W1)

1998 - 1999

Dec Jan Feb Mar Apr

Obs

erve

d m

agni

tude

12

13

14

15

16

17

18

P/Hergenrother (1998 W2) C WHergenrother found a comet onCCD images obtained by TimothyB. Spahr on Nov. 22.10 in thecourse of the Catalina Sky Survey.The comet was 17th magnitudeand will fade. The comet was ashort period one, almost at

perihelion. [IAUC 7057, 1998November 23]

LINEAR (1998 W3) TheLINEAR Team discovered anobject that had unusual motion onNovember 25.3 and placed theinformation in The NEOConfirmation Page. On reportingastrometric follow-up, G. Hug,Farpoint Observatory, Eskridge,KS, noted the object's appearancewas probably cometary, a pointconfirmed by other observers onrequest from the Central Bureau,and by the near-parabolicretrograde orbit, which showed itto be a distant object. [IAUC7063, 1998 November 28].Seiichi Yoshida was able to imageit on several occasions and MartinLehky also recorded it visuallynear 15th mag.

139P/Vaisala-Oterma (1998WG22) Precise positions of 1939TN, an apparently asteroidalobject discovered by Y. Vaisala atTurku on 1939 Oct. 7 andobserved by him and L. Oterma onthree more nights over a 35-dayarc, were published in 1979 inTurku Obs. Report R10 and onMPC 4811. The orbitcomputation by Oterma suggestedto her that the object was a comet,and in a communication to theMinor Planet Center in 1981 sheremarked that a carefulreexamination of the platessuggested that the object wasperhaps somewhat diffuse. Theintroduction to the 1982 edition ofthe Catalogue of Cometary Orbitsquotes her conclusion that theobject was probably a comet, butthe object was not actually listedas one. On MPEC 1998-X19, theobject is identified by S. Nakano,Sumoto, with 1998 WG22, a 19thmag apparently asteroidal objectobserved by the LINEAR programon Nov. 18.26 and 21:

Fortuitously, this object waslocated only 4' from the result ofintegrating forward from 1939 theorbital elements by Brian Marsdenon MPC 6815. Further LINEARobservations were made on Nov.24. On observing the object withthe 1.8-m reflector at theDominion AstrophysicalObservatory on Dec. 6.3 UT, D.Balam noted (in FWHM 3".1seeing) an 8" coma and a tailextending 18" in p.a. 260 deg. OnDec. 7.1 W. Offutt, Cloudcroft,observing between clouds, alsoremarked on a tail (or antitail)extending 17" in p.a. 257 deg, butno coma was noted. M. Tichy,

observing in poor conditions atKlet on Dec. 7.9, remarked on apossible 6" coma.

The object's cometary natureseems now reasonably assured andit has a period of 9.55 years[IAUC 7064, 1998 December 7].Seiichi Yoshida was able toobserve it at 18th mag shortly afterdiscovery.

P/ODAS (1998 X1) TheObservatoire de la Cote d'Azur-Deutsches Zentrum fur Luft undRaumfahrt Asteroid Survey(ODAS) discovered an 18thmagnitude comet on December15.17 with the 0.90-m CaussolsSchmidt camera. The orbit is ofshort period (6.8 years), withperihelion having occurred in July.[IAUC 7067, 1998 December 17].Once again Seiichi Yoshidasuccessfully imaged it.

140P/Bowell-Skiff (1998 X2). G.V. Williams, Minor Planet Center,rediscovered comet P/1983 C1 (=1983c = 1983 II) in the course ofhis processing of two-night"asteroid" data obtained by theLINEAR program on Dec. 14 and17. The indicated correction tothe prediction on MPC 27081(ephemeris on MPC 32547) wasDelta T = +17.3 days. [IAUC7076, 1998 December 28].Seiichi Yoshida has again beensuccessful at imaging it, at around16th mag and it could brighten alittle further.

D/LINEAR (1998 Y1) LINEARdiscovered another faint periodiccomet on December 22.31 [IAUC7072, 1998 December 24].Perihelion occurred a monthearlier and it is not due to returnfor over 100 years. Because it is aone apparition comet with a periodwhich is a substantial fraction ofthe notional upper limit of 200years it has been given a D/designation. Seiichi Yoshida wasable to observe it on severaloccasions between discovery andearly February.

P/Li (1998 Y2) Weidong Li,Department of Astronomy,University of California atBerkeley, reported his discoveryof a comet in the course of theLick Observatory SupernovaSearch (cf. IAUC 6627; with theparticipation of M. Papenkova, E.Halderson, M. Modjaz, T. Shefler,J. Y. King, R. R. Treffers and A.V. Filippenko). The object wasfound automatically by theKatzman Automatic Imaging



Telescope in the field of NGC1041, but it was immediatelyrecognized as a comet by Li, whothen used the equipment to make adeliberate confirmatoryobservation, as well as follow-upobservations on the followingnight. [IAUC 7075, 1998December 28]. The comet isperiodic with a period of about15.2 years. Seiichi Yoshida hasimaged it on several occasions ataround 16th mag. Martin Lehkyobserving visually made it overtwo magnitudes brighter.

Tilbrook (1999 A1) JustinTilbrook (Clare, South Australia )discovered a second comet onJanuary 12.49 with his 0.2-m f6reflector x70. [IAUC 7084, 1999January 13]. Although it hadperihelion inside the earth's orbitand was relatively close, it wasintrinsically quite faint and fadedafter discovery. Andrew Pearcemade some early observations,reporting it at a little fainter than10th magnitude. Moving south, itwill not be visible from theNorthern Hemisphere.

SOHO (1999 C1). Having beenrestored to operation in October

SOHO was quickly disabled againby gyro failures. Groundcontrollers managed to get roundthe problems and it returned to thefray in early February when itquickly discovered another Kreutzgroup fragment.

Hermann (1999 D1) S MHermann of the LONEOS teamdiscovered a comet on imagestaken on February 20.4 [IAUC7111, 1999 February 20]. It is anintrinsically faint short periodcomet. It faded after discovery,but not before it had been imagedby Seiichi Yoshida.

Li (1999 E1) Weidong Li and M.Modjaz, Department ofAstronomy, University ofCalifornia at Berkeley discovereda 17th mag comet in the course ofthe Lick Observatory SupernovaSearch on March 13.18 [IAUC7126, 1999 March 16]. Theobject was found automatically bythe Katzman Automatic ImagingTelescope (KAIT), recorded byModjaz as a supernova candidate,and recognized by Li as a comet.Li then used the equipment tomake confirmatory observationsthree nights later. The comet

shows an apparent tail in p.a.about 120 deg. It is in a distantparabolic orbit and will fade,though Seiichi Yoshida imaged itat 15th mag just over a week afterdiscovery.

P/Machholz 2 has not yet beenrecovered, though recovery is notreally expected until late spring. Itmay reach 7th magnitude in thelate autumn. This comet split intoseveral fragments at its discoveryreturn in 1994 and both theephemeris and expectedmagnitude are a little uncertain.The date of perihelion is uncertainby up to a day and more accurateephemerides will be publishedwhen the comet is recovered. UKobservers should be able toobserve it from November at 10thmagnitude, though more southerlyobservers may find it a monthearlier.

For the latest information ondiscoveries and the brightness ofcomets see the Section www page:http://www.ast.cam.ac.uk/~jdsor the CBAT headlines page athttp://cfa-www.harvard.edu/cfa/ps/Headlines.html

Comet Hunting NotesDon Machholz

JULY 1998: At what declination(number of degrees north orsouth of the equator) doamateur astronomers visuallydiscover comets? The finds rangefrom +72 degrees to -62 degrees.From 1975 to the present, of the43 comets found in the NorthernHemisphere, seven were foundnorth of +45 degrees. Sixteenwere between +20 and +45degrees and the remaining 20 werefound between the celestialequator and +20 degrees. As forthe 34 southern comets, five werefound south of -45 degrees while16 more were between -20 and -45degrees. The remaining 13Southern Hemisphere comets werefound between the celestialequator and -20 degrees.

AUGUST 1998: The EdgarWilson Award has beenannounced for amateurs whodiscover comets. A cash award ofabout $20,000 will be distributedeach June 12 among those findingcomets during the previous year.The rules are few. The cometmust be named after you and youmust be using your ownequipment in an amateur capacity.

The discovery may be made byvisual, photographic, or electronicmeans. The amount an individualreceives depends upon the numberof comet finds during the year.For example, in the past twentyyears, any individual who found acomet would have receivedbetween $1500 and $20,000 forthat comet find.

SEPTEMBER 1998: While comethunting I've always recorded thenumber of meteors I happen toobserve passing through my fieldof view. On June 16 of this year Irecorded my 10,000 telescopicmeteor. The span of time was6314 hours over the course of 23years. The number of meteors Isee per hour in the morning skyaverages 1.9 while the evening skyaverages 0.9. I also record thenumber of artificial satellites I see.The hourly rates for these hasincreased, and now exceeds themeteor rates. I made my 10,000satellite sighting about a year ago.

OCTOBER 1998: Williams'comet discovery on August 10brings the total number ofAustralian visual comet

discoverers to five, this is nowhalf the number of Americans (10)who have visually found cometssince 1975. There are only twoother Southern Hemisphere cometdiscoverers: Austin of NewZealand and Campos of SouthAfrica. All 24 comets found bythese seven men were discoveredsouth of the celestial equator, 23being found by only onediscoverer. Peter Williams is thefirst person to qualify for theEdgar Wilson Award: a cash sumof about $20,000 to be divided upamong all the amateurs whodiscover comets before June 11,1999.

NOVEMBER 1998: As seen fromthe earth, how far are cometsfrom the sun when firstdiscovered? This angle, calledelongation, has been calculated forthe 78 comets found visually byamateurs since 1975. It rangesfrom 22 to 171 degrees. Over halfof the comets have been foundwithin 58 degrees of the sun.Seventy of the seventy-eight werefound within 92 degrees of thesun. Why are they found at suchsmall elongations? Not only do

1999 April 11


comet hunters concentrate theirsearches on areas near the sun, butcomets generally become brightestin those regions.

DECEMBER 1998: Williams,Jager and Tucker are all noweligible for the Wilson Cometaward. Each amateur used adifferent methods to find "their"comets: visual, photographic andCCD.

JANUARY 1999: Father LeoBoethin of the Philippines passedaway on Sept.15. He was thediscoverer of Periodic CometBoethin (85P/) on Jan. 4, 1975. Itorbits the sun every eleven years.

FEBRUARY 1999: In whichmonth do amateur astronomersvisually discover the mostcomets? Of the 79 comets foundin the past 24 years, 12 werefound in July while 10 were found

in January. In last place is bothFebruary and April with threefinds each.

MARCH 1999: Five of the lastseven visual discoveries (covering2.5 years) have been made bySouthern Hemisphere observers,all from Australia, and all fivecomets being found south of thecelestial equator. Two wereaccidental finds (by Williams andTilbrook), with Tilbrook thenfinding one more and Taburfinding two.

APRIL 1999: Of the 79 visualcomet discoveries since 1975, 30(38%) have been made in the firsthalf of the year, with 49 finds inthe second half. Subdividing theyear into quarters, the first quarterhas 18 discoveries, the second has12, the third has 26 and the lastthree months of the year yields 23finds.

Don's Comet Hunting Hours:

Comet Hunting Hours 1975-1998:6468.00

Hours January through February1999: 26.25

Total hours at last discovery (10-8-94): 5589.00

Least hours in any month since hebegan comet hunting on 1/1/75:4.00 (02/98), 4.50 (01/86), 5.50(02/80)

Most hours in any month since hebegan comet hunting: 69.25(05/76), 63.00 (05/78)

These notes are taken from CometComments by Don Machholz,which is published on the Internet.

A Brief History of Comets I (until 1950)

The following text is taken fromthe ESO web page and is adaptedfrom a major review on Comets,prepared by Michel C. Festou(Observatoire Midi-Pyrenees,Toulouse, France), Hans Rickman(Astronomiska Observatoriet,Uppsala, Sweden) and Richard M.West (European SouthernObservatory, Garching, Germany)and published in the reviewjournal Astronomy &Astrophysics Reviews (A&AR)(Part I, Vol. 4 pp. 363-447, 1993)

The present account deals with theperiod up to around 1950. Itincludes some references to majorpapers in this period (by author ofyear of publication), but theoriginal version of this review inAstronomy & AstrophysicsReviews must be consulted for thefull details about these.

Introduction The history ofcometary astronomy is naturallydivided into five major periods,the transitions being marked byimportant new insights. Before1600, comets were essentiallyconsidered to be heavenly omensand were not yet clearlyestablished as celestial(astronomical), rather thanmeteorological phenomena in theterrestrial atmosphere. Thenfollowed two centuries of mostlypositional measurements withemphasis on the motions and theorbits, lasting until the early 19thcentury, when the era of cometary

physics was inaugurated, inparticular by the passage ofP/Halley in 1835. The next majorstep forward occurred in 1950with the sudden emergence of themodern picture of comets as beingessentially very old solar systemobjects made of primordial ice anddust, generally in unstable orbitsand intensively interacting withthe solar electromagnetic andcorpuscular radiation. Finally, thespace missions to P/Giacobini-Zinner in 1985 and especially toP/Halley in 1986 provided the firstin situ observations of comets anddramatically widened ourscientific horizon, but also posedmany new questions which are yetto be answered.

Before 1950: The main eventsThe word comet, now used in allEuropean languages, comes fromGreek (kometes= `the hairy one'),but the earliest extant records ofcometary observations date fromaround -1000 in China andprobably from about the sametime in Chaldea (on the territory ofpresent-day Iraq). Ideas about thetrue nature of comets are availablefrom the time of the rise ofHellenistic natural philosophy atabout -550 when the Pythagoreansconsidered comets to be a kind of(wandering) planets that were seenrather infrequently and mostlynear the horizon in the morning orevening sky. Aristotle in hisMeteorology (ca. -330) relegatedcomets to the lowest, `sublunar'

sphere in his system of sphericalshells and described them as `dryand warm exhalations' in the upperatmosphere. There is no mentionof comets in Ptolemy's Almagest,presumably because they were notconsidered of celestial origin, buthe described them in astrologicalterms in his Tetrabiblos. TheAristotelian view on comets wasdogmatically upheld during thefollowing millennium; the firstdoubts seem to have beenexpressed by Thomas Aquinas andalso by Roger Bacon in his OpusTertium from 1267, but like theirpredecessors they stronglybelieved comets to be evil omens.

Comet 1664 W1 illustrated byStanislaw Lubieniecki in theTheatrum Cometicum, publishedin 1667. Observations of it leadto many advances in cometaryscience. It also presaged theplague of London.

Finally, Paolo Toscanelli observedP/Halley in 1456 and several othercomets between 1433 and 1472with improved accuracy,inaugurating the renaissance ofEuropean observational astronomy



after the long period of dormancy.The decisive demonstration wasdelivered by Tycho Brahe (andconfirmed by a few otherobservers, especially MichaelMästlin), on the basis of extensiveobservations of the bright cometwhich first appeared in late 1577.He showed that the horizontalparallax of this comet wascertainly smaller than 15 arcmin,corresponding to a distance inexcess of 230 Earth radii, or fourtimes the distance to the Moon.The question of how comets movearose as a natural consequence andin 1610, the amateur Sir WilliamLower proposed that they do so invery elongated ellipses, whileRobert Hooke and GiovanniBorelli suggested that cometaryorbits may be parabolic. GeorgDörffel was the first to specificallystate that the two bright cometsseen in 1680 and 1681 are one andthe same before and after itsperihelion passage, and that itmoved along a parabola with theSun in the focal point. IsaacNewton in Principia (1687),applied his new theory ofgravitation to show that the 1680comet moved in an elliptical,albeit very nearly parabolic orbitand that it passed only about0.0016 AU above the surface ofthe Sun. Edmond Halley (1705)computed the orbits of a dozenwell-observed comets anddemonstrated the periodical natureof the bright comet of 1682.`Halley's Comet', as it was fromnow on called, was telescopicallyrecovered in December 1758 byJohann Palitzsch; this provedconclusively the validity ofNewton's law of gravity out to thedistance of the aphelion at 35 AU,more than three times the distanceof Saturn, the outermost planetknown at that time.

18th century cometary astronomyis characterised by the gradualdevelopment of improved methodsfor orbital computations and at thebeginning of the 19th century, thishad become a straightforward, ifstill somewhat arduous task, inparticular when planetaryperturbations were taken intoaccount by means of iterativecorrections. Some basic featuresof the orbital distribution ofcomets were established, e.g. theextremely broad range of orbitalperiods, over which the differentobjects are scattered. While somecomets turned out to have orbitsvirtually indistinguishable fromparabolas, others were confined tothe inner solar system in the

vicinity of Jupiter's orbit or insideof it. As time passed, aconcentration of comets moving insimilar orbits with fairly lowinclinations and with aphelia closeto Jupiter's orbit became more andmore obvious; this concentrationbecame known as the Jupiterfamily. It either called for acontinuing ejection from Jupiter orfor a mechanism of dynamicalevolution, called `capture',whereby the comets wouldbecome concentrated into suchorbits. It was realized that cometsin general, and Jupiter familymembers in particular, suffer byfar the largest orbital perturbationsdue to the action by Jupiter, andthe restricted three-body problem(Sun-Jupiter-comet) thereforeoffered an interestingapproximation for the study oftheir dynamical behaviour.

After Halley, Johann F. Encke wasthe second to successfully predictthe return of a comet (in 1822)which as a consequence nowcarries his name. It turned out tohave, and still has, the shortestperiod of all known comets, 3.3years, and it was soon found toarrive systematically about 0.1days earlier at perihelion thanpredicted, even when taking allplanetary perturbations intoaccount. Inspired by hisobservations of an asymmetricdistribution of luminous matter inthe head of P/Halley in 1835,Friedrich W. Bessel interpretedthis as a Sun-oriented asymmetricoutflow and suggested that a non-gravitational effect might arise dueto the rocket-type impulseimparted by such an outflow. As aconsequence, such perihelionshifts as observed for P/Enckemight arise.

During the next decades,progressively more sophisticatedinstrumentation became availableand the road was opened for amore physical approach to thestudy of comets. Comet tails wereexplained by Heinrich W.M.Olbers (1812) and Bessel (1836)by assuming that they were madeof solid particles on which wasacting a repulsive force directedanti-sunward. The closeconnection between comets andmeteors was demonstrated byGiovanni Schiaparelli (1866,1867) who found that the orbits ofthe Perseid and the Leonid meteorstreams coincide with those ofcomets P/Swift-Tuttle (1862 III)and P/Tempel-Tuttle (1866 I),respectively. In 1835, P/Halley

became the first comet in whichdetailed structures wereextensively observed, in particularby John Herschel, Bessel andFriedrich G.W. Struve, whodescribed jets, cones andstreamers, cf. the Atlas by Donn etal. (1986). This led Bessel (1836)to postulate the ejection ofmaterial in the direction of the Sunwhich was then somehow forcedback in the opposite direction byan unknown repulsive force.Feodor A. Bredikhin (quoted byJaegermann, 1903) furtherdeveloped this interpretation intothe Bessel-Bredikhin' mechanicalmodel which remained in use untilthe late 1950's. Sir ArthurEddington (1910), introduced thefountain model of particle ejectionin which the parabolas representthe outer envelopes of particletrajectories emitted from the sunlithemisphere of the nucleus orsurfaces of high density of matter.One of the repulsive forces actingon the dust was identified bySvante Arrhenius (1900) as theradiation pressure by sunlight.The corresponding theory wasfurther developed by KarlSchwarzschild (1901) andextended to molecules by PeterDebye (1909).

The Section archives containmany reprints, some dating backto the early 19th century. Thisdrawing of comet Donati wasmade by G P Bond on 1858October 2 using the greatrefractor at Harvard Observatoryand is from the MathematicalMonthly of December 1858.

The first spectroscopicobservations of comets were madeby Giovanni Donati (1864) and bySir William Huggins (1868) whovisually compared the spectrum ofcomet Winnecke (1868 II) withflame spectra and found that thebands seen in the comet and in the

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flame, now known as the `carbon'or `Swan bands', were similar.Subsequent observations showedthat these bands were present in allcomet spectra and that carbon wastherefore an important constituentof comets. Spectroscopy soonbecame the standard technique forstudying the light of comets andnew emissions were discovered atan increasing rate; Baldet (1926)published a detailed description ofthe spectra of about 40 comets,obtained since 1864, together witha complete bibliography of allcomets observed until that time byspectroscopy. Schwarzschild andKron (1911) studied the intensitydistribution in P/Halley's straighttail during the 1910 passage andsuggested that the emission couldbe explained by the effect ofabsorption of solar light, followedby re-emission, i.e. fluorescence.Polydor Swings (1941) solved thelong-standing problem of why theviolet CN bands (3875 A) incometary spectra did not resembleCN laboratory spectra and variedin appearance: because of thecrowding of absorption lines in thesolar spectrum, the intensity at theexciting wavelengths criticallydepends on the doppler shiftcaused by the comet's motionrelative to the Sun and sodetermines the strength of thefluorescence emission lines in thecomet's spectrum; this is nowknown as the Swings effect.

1950 - 1951: Two crucial yearsA major revolution in cometaryscience took place in 1950-51,with the formulation within a shorttime span of three fundamentalideas: 1) the icy conglomerate(`dirty snowball') model of thecometary nucleus by FredWhipple (1950), 2) theidentification from kinematicstudies of the existence of a distantreservoir of comets, now known asthe Oort cloud, by Jan HendrikOort (1950), and 3) theexplanation of the motions incometary plasma tails as due tointeraction with the solar wind byLudwig Biermann (1951).Interestingly, none of these ideasresulted directly from newobservational evidence, andimportant parts of them had beenproposed earlier, but it was thefirst time that the known factswere effectively combined toreveal the new picture.

The icy conglomerate nucleusKarl Wurm, in a series ofenlightening papers publishedbetween 1932 and 1939,

suggested that, because theobserved cometary radicals andions are not chemically stable,these species must be created bypure photochemistry of morestable molecules residing insidethe nucleus, cf. for instance thereviews by Wurm (1943) andSwings (1943). In the 1940's,Swings contributed significantly tothe development of ideas alongWurm's line of thinking and hiskey role appears to have beenoverlooked in the later literature.The presence of CO, C2N2, CH4,CO2, N2 and NH3 was invokedon the basis that CO+, CN, CH,CO2+, N2+ and NH wereidentified in comet spectra,respectively. Swings proposedmany possible and reasonablecandidates as parent molecules,among others CH4 since CH2 washeld responsible for the emissionrecorded in the 4000-4100 Ainterval, as well as H2O, followingthe discovery of the OH 3090 Aultraviolet emission in 1941 bySwings, and despite the fact thatthe low vapor pressure of waterwas considered a serious problemwhen explaining the observedpresence of the OH emission farfrom the Sun. In 1948, he camevery close to actually proposing anicy model for the nucleus bysuggesting that the mentionedmolecules could exist in the solidstate in the nucleus.

Swings (1942) also suggested thatmolecules similar to those foundin meteorites were possibly storedin the nucleus by occlusion. Thisidea was quantitatively (and mostprobably, independently) exploredby Boris Yu. Levin (1943), whodeveloped the desorption theory ofoutgassing from the surface ofmeteoritic material to demonstratethat his sand bank model for thenucleus had a solid basis.However, although the averagedesorption heat, about 6000cal/mole, as deduced empiricallyfrom the observedbrightness/heliocentric distancerelation, was in agreement with thelaboratory values for the cometarymolecules mentioned above, theamount of material that could bedesorbed from a sand bank with anexpected cometary mass fell farshort of explaining the persistenceof comae over several months atsingle passages, or indeed, thesurvival of comets like P/Halley orP/Encke for many apparitions.

Since the mid-19th century, agreat deal of research hadconcentrated on understanding the

nature of the central source of gasand dust in comets. Transits ofcomets across the solar disk hadnever shown any dark silhouette,proving the absence of anyextended, optically thick object.Seeing-limited observations ofcomets passing near the Earthshowed a central, unresolved lightsource of dimensional upper limitsin the 10-100 km range (NicolausB. Richter, 1963). Upper limits tocometary masses had beenestimated for instance from theabsence of evidence for mutualgravitational attraction of thecomponents of P/Biela in 1846 orof any influence on the Earth'sorbit at very close passages likethat of P/Lexell in 1770; in theend, masses in the 1012 - 1017 kgrange were estimated (Whipple1961). Comets were obviouslysmall and light bodies, possiblyeven without a solid nucleus at thecenter. At the end of the 1940's,the nature of the nucleus was stilla subject of much speculation andno consensus had been reached. Inan attempt to put together allknown facts about the cometarynucleus, and with particularattention to the long-standingproblem of explaining the non-gravitational perihelion shifts,Whipple (1950, 1951) laid thefoundations for the model of anicy conglomerate, solid nucleus.Building on the idea dating backto Pierre S. de Laplace (1813) andBessel (1836), Whipple describedthe nucleus as a mixture of icesfrom which the gases in the comaare produced by sublimation inincreasing quantities as the cometapproaches the Sun and thenucleus surface temperature rises,and meteoritic dust that is releasedfrom the nucleus when these icesevaporate. This model had thevirtue of explaining at onceseveral observed features: 1) thelarge gas production rates, forwhich the desorption model wastotally inadequate, 2) the observedjet-like structures in the coma andthe erratic activity, impossible toproduce if the nucleus were acloud of particles, 3) the observednon-gravitational forces by meansof momentum transfer by theoutflow of gas from the nucleus,the net effect on the orbital motionbeing dependent, among others,on the sense of the nuclear spinand the direction of the spin axis,4) the fact that most comets whichpass extremely close to the Sun,e.g. the Kreutz sungrazing group,apparently may survive suchapproaches intact and with littlechange after perihelion, and 5) the



fact that comets are the sources ofmeteor streams. Items 2-4 gaveparticularly strong arguments for asolid nucleus rather than a sandbank structure.

Many of the late 19th centurycomets showed intricate structurenear to the nucleus. Theseillustrations of 1881 K1 and 1881N1 were made by Otto Boeddickerusing the 3 foot reflector of theEarl of Rosse at Birr Castle,which is perhaps better known fordeep sky observations. Thisreprint is from the ScientificTransactions of the Royal DublinSociety for August 1882.

The Whipple model quickly wongeneral acceptance and wasgradually refined during thefollowing decades. It had,however, some shortcomings; themain one was pinpointed byWhipple himself as being the largedifference between the latent heatsof vaporization of the various ices.As a consequence the highlyvolatile material should be rapidlyremoved from the surface layer ofthe nucleus long before perihelion,in contradiction to the observationof radicals and ions like CH andCH+ near the Sun. This objectionwas tentatively removed whenArmand Delsemme and Swings(1952) noticed that almost allparent molecules (except NH3)required to explain the observedradicals and ions in comets couldco-exist in the nucleus in the formof solid clathrate hydrates. In thisway, the highly volatile materialdoes not disappear too rapidly andis also freed together with lessvolatile molecules; this explainswhy the spectrum remains more orless similar throughout the cometapparition.

The Oort Cloud Many orbitalstudies of individual comets withparticular attention to theinfluence of planetary

perturbations were carried out atseveral observatories during thefirst decades of the 20th century.They were naturally followed bystatistical considerations about thedistribution and dynamical originof comets, including the questionof whether or not some cometshave `original' hyperbolic orbits(reciprocal semi-major axis1/aorig < 0) and are therefore ofinterstellar origin. The work at theCopenhagen Observatory by ElisStrömgren (beginning in about1910) and his associates is typicalof such studies and showed theabsence of originally hyperbolicorbits, all observed orbits of thistype having been caused byplanetary perturbations. Sinding(1948) produced a list with thevalues of 1/aorig for 21 long-period comets which together withthe work by van Woerkom (1948)formed the basis for Oort's famouspaper (1950) on the existence of acometary reservoir in the outerreaches of the solar system. Theidea of a cloud of distanthypothetical comets, stable againststellar perturbations, and itsnecessity in case many observedcomets would have 1/aorig 10000AU, had been expressed earlier byErnst J. Öpik (1932).

Based on van Woerkom's (1948)theory of the orbital diffusioncaused by planetary perturbations,Oort found that the number ofcomets with very small values of1/aorig is much larger than onewould expect, when comparingwith the neighbouring, long-periodelliptical orbits. This suggestedthat many of the comets becomeunobservable after their firstpassage through the inner solarsystem. In a subsequent study,Oort and Schmidt (1951)distinguished between `newcomets' (those coming directlyfrom the Oort cloud, making theirfirst visit near the Sun) and `oldcomets' (those returning on ellipticorbits). The former appeared to bedustier and brighten more slowlythan the latter. These tentativeconclusions have later beenrevisited and modified, and therole of stellar perturbations inproviding new comets has beenreconsidered. However, the basicconcept of the Oort cloud as anouter halo of the solar system hasbeen substantiated by later studies,based on improved samples ofcometary orbits.

The solar wind The tails ofcomets have been the objects ofmany investigations. It is exactly

these appendices that make cometsso impressive to the layman, andastronomers of all times have beenstruck by the fact that the tails mayvary so dramatically from oneobject to another. In the early 20thcentury, perhaps the strangestcharacteristic was the enormousrepulsive force found to act onstraight comet tails in the anti-solar direction.

Already in 1859, RichardCarrington (1859) suspected aphysical connection between themajor solar flare observed in themorning of Sept. 1, and enhancedmagnetic activity on the Earthsome hours thereafter. Ideas aboutthe possible existence of a streamof particles from the Sun, perhapselectrically charged, emergedtowards the end of the 19thcentury, in particular to explainthe excitation of molecules andions observed in cometary comae.It was also found that cometaryion tails (formerly Type I) developcloser to the Sun than dust tails(formerly Type II). However, itwas only 50 years later that CunoHoffmeister (1943) provided thecrucial observations of a gas tailaberration of about 6o, i.e. theangle between the observed tailand the anti-solar direction. Thiswas correctly interpreted byBiermann (1951) in terms ofinteraction between the cometaryions in the tail and the solar wind,a stream of electrically chargedparticles from the Sun withvelocities of several hundredkm/sec. His derived plasmadensities were unrealistically high,since electrons were thought toaccelerate the cometary ions, butHannes Alfven (1957) settled thisproblem by introducing the notionof an interplanetary magnetic fieldwhich is carried along with thesolar wind. Its existence was soonthereafter confirmed byexperiments onboard some of thefirst spacecraft launched after thespace age opened in late 1957(Lunik I and II, Explorer X,Mariner II, etc.). Still, for quitesome time, cometary ion tails werethe only well-distributed solarwind probes in interplanetaryspace and they remain so outsidethe ecliptic. Another importantresult of Alfven's study is that anion tail must be considered as partof the comet since it ismagnetically connected to thecometary head.

To be continued

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the comet s talejds/tail11.pdfmohamed ibrahem, andreas kammerer, heinz kerner, atilla kosa-kiss,...

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