Analysis of past comet apparitions

C/2002 C1 (Ikeya-Zhang)

On Feb. 1, 2002, two amateurs could report success after many months! The famous Japanese comet discoverer Kaoru Ikeya, aged 58 (he discovered five comets between 1963 and 1967, including the daytime comet of 1965, Ikeya-Seki) and the Chinese amateur Daqing Zhang independently discovered a comet of magnitude 8.5-9 with a moderately condensed coma of diameter 2' in southwestern Cetus. A preliminary orbit could already be calculated the day after, since many astrometric data were received. The comet passed perihelion in mid-March at a solar distance of only 0.5 AU. Assuming an average brightness evolution it would reach 4.5m around that time (IAUC 7812/13).

Comet Ikeya-Zhang showed an even more positive evolution, becoming a nice object, observable with the unaided eyes in the evening sky of March and April. Exactly five years after Hale-Bopp (and in the same sky region) another naked-eye comet was visible. However, the comparision with Hale-Bopp is a bit exaggerated, since the former unmistakably stood in the sky, whereas one had to look more closely not to miss Ikeya-Zhang - except under very dark skies. This was not only due to the lower brightness but also because comet Ikeya-Zhang hang very low in the sky, barely exceeding altitudes of 10° around perihelion. In binoculars it sometimes resembled Hyakutake "en miniature", with a dominant central condensation and a tail that sometimes presented internal structure. Contrary, in a telescope it presented rather disappointing views as it showed, beside the extremely bright false nucleus, only little internal structure (perhaps also a result of the low altitude?).

In mid-February the number of astrometric positonal data had grown large enough to allow the calculation of an elliptical orbit with a revolution period of 400 to 500 years. A comparison of the orbital elements by S. Nakano led to the assumption that Ikeya-Zhang could be identical with the comet of the year 1532 (Fracastor) (MPEC 2002-C111). Further astrometric observations until early April led to a period of 365 years and a previous perihelion in 1660.2 ± 0.1, resulting in the identification with comet 1661 (Hevelius) (IAUC 7843 / MPEC 2002-G38). Spectroscopic observations showed the Sodium emmission line which was first found in comet Hale-Bopp (IAUC 7851). Observations with the 1.5m-Catalina-Reflektor on Mar. 3, 4, 5, 9, 10 and 26 (at around 2:30 UT) and with a 28cm-reflector on Mar. 9 (19:45 UT) showed dust shells within 30" of the nucleus of the comet on the sunward side. The shells displayed evidence of two discrete jets (IAUC 7862).A 1- to 12-micron photometry of this comet on May 22.37 UT at the Mt. Lemmon Observing Facility 1.52-m telescope showed no evidence for strong silicate emission; the observed spectral energy distribution at longer wavelengths yielded a blackbody color temperature of 270 +/- 15 K (IAUC 7921).

John Bortle investigated the apparition of comet 1661 (and a comet of 1273, which may as well have been Ikeya-Zhang) more closely and found that the comet remained visible for a prolonged time after perihelion. He expressed the assumption that the comet may show an asymmetric lightcurve with a maximum between T+10d and T+15d, followed by a very slow decrease. Under these circumstances a maximum brightness between 2.0m and 1.5m would result (The Astronomer, March 2002).

The following analysis is based on a well established data base. Until mid-September 256 observations by 19 group members were received and further 1080 international observations included. These clearly indicate a significant difference between the pre- and post-perihelion brightness evolution, according to the formulae:

pre-perihelion: m = 6.9m + 5×log D + 9.8×log r

post-perihelion: m = 6.4m + 5×log D + 8.3×log r

Evolution of the heliocentric magnitude (post-perihelion)

Thus the brightness evolved more rapidly pre- than post-perihelion, not uncommon for an active comet. The formulae yield a maximum brightness of 3.4m around Mar. 27; however values up to 3.2m are plausible. During the summer months the brightness evolved in an interesting way as shown in the accompanying diagram. About 65 days after perihelion the comet grew up to 1m too bright compared to the post-perihelion evolution before. Based on the available data it is not possible (by now) to decide if this was the result of a slower decrease in activity or of a limited phase of stronger activity. I personally tend to favour the latter scenario, expressed in the above brightness formula.

Comparing these parameters with that indirectly determined (and therefore relatively uncertain) from the ancient estimates (m0=4.6m/n=4), a difference of 2 magnitudes near maximum brightness results. Such a difference between two subsequent perihelia is quite surprising considering the long period of this comet, but not too uncommon for "old" periodic comets with many perihelion passages. If comet Ikeya-Zhang is an old periodic comet or the interpretation of the sources was wrong is not yet decided. Considering the number of split comets discovered during the last ten years one may speculate if comet Ikeya-Zhang may be a preceeding fragment of an even larger comet.

Total Brightness and Coma diameter

During the first weeks the coma diameter was not easy to estimate, first because of the low altitude of the comet and second because of the difficulty to discern it from the tail. At times when the comet showed a bright tail the transverse diameter had to be used. Estimates showed a stable diameter around 5' for a long period. Starting at the end of March it increased rapidly, reaching 15' by the end of April. Maximum occurred on May 10 (ten days after closest approach to the earth) with a diameter of 17'. Thereafter it has decreased slowly. However, during this period the estimates showed significant scatter again since the coma rapidly became diffuse during July. While in telescopes only a few arcminutes could be discerned, observers with binoculars estimated up to 10' under dark skies. Thus the apparent coma diameter actually decreased from 11' in mid June to 2.0' in mid August. The absolute coma diameter was about 340.000 km at discovery, but decreased continually, reaching 180.000 km at the end of April. Thereafter it increased considerably, reaching about 450.000 km around June 20, only to decrease to 220.000 km until mid-August. This swallowing could be interpreted as a further hint for a still increasing activity well after perihelion. The coma was moderately condensed (DC 3-4) at first, getting ever more compact until the beginning of March when it reached DC 8. It hovered around DC 8 until end of March, but thereafter got more diffuse rather rapidly, reaching DC 3-4 around May 20, which was retained until end of June. However, at the start of July it rapidly became extremely diffuse, reaching DC 1-2 in mid-August.

Tail observations were reported from mid-February onwards. At the beginning of March the tail was 1.5° long as seen in binoculars. At the time of perihelion it had increased to 4-5°. As with most comets the tail length still increased a while after perihelion, reaching its maximum of 7° in the first week of April (11 million km), with few observers estimating even 10°. Thereafter a rapid decline followed: on Apr. 20 the tail length was reported as being 3°, on May 10 only 2° and in mid-June few observers reported a tail for the last time (at 0.5° length). The tail pointed eastward at first, then rotating clockwise until it pointed due north during the first week of April. During the following weeks it pointed in westward directions and at the end of May due south. Finally the tail will point towards east again.

Visual tail length

Position angle of the ion tail

Andreas Kammerer

FG observations