The interest in C/2002 C1 (Ikeya-Zhang) has been intense and shows no sign of diminishing. In the March TA three articles appeared on the comet plus a full-page editorial and more than 3 pages of data. Although it is still necessary to dot the "i"s and cross the "t"s, it is evident that C/2002 C1 is a return of C/1661 C1. In the accompanying article Graeme Waddington offers some solid evidence that it may also be a return of C/1273 F1, although this connection is still highly speculative at present. It is interesting to note that the linkage between 2002 and 1661 probably requires a non-gravitational term in the solution as a purely gravitational solution, as presented on MPEC-G08 now gives a perihelion date for the previous return of Spring 1660. Although this discrepancy appears small, it is in fact nominally 7 sigma (Marsden, B.G.: 2002, MPEC-G08) and thus well beyond the known errors in the gravitational solution. The effects of the presence of this term is described in Graeme's text and leads to an uncertainty of several years in the date of the possible 1273 return and more than a century by the 5^th Century (2 returns earlier), where we suspect that the splitting of the precursor comet may have occurred.

John Bortle (Bortle, J.: 2002, TA, 38, 455, 298) also argues convincingly that the best solution for the apparent discrepancy between the brightness of the 1661 and the 2002 returns of Comet Ikeya-Zhang may be a strong perihelion asymmetry in the light curve of the comet. Such an asymmetry would have important dynamical implications for the comet's orbit too and would significantly affect extrapolations of the orbit into the past. Even if we invoke the enduring photometric effects of a splitting event at a previous perihelion passage that would gradually diminish with time and could account for the comet having a significantly brighter absolute magnitude at a previous return, Bortle shows that the comet should have been easily detected in the evening sky in January 1661.

At present the data archive of the "Cometas_Obs" mailing list has a total of 93 total visual magnitude estimates (m₁) and 107 CCD measures that are divided into 64 measures with a 10" aperture (treated as m₂, although not genuine m₂ measures) and 43 measures through other with other CCD apertures up to 3.6'. This light curve is shown below as Figure 1. The observed relationship between aperture size and measure integrated magnitude can be found at http://www.iac.es/galeria/mrk/Light_2002c3.gif. Two eye fits to the data are shown for m₁ and m₂. The very large dispersion after perihelion is mainly caused by the very poor observing conditions. Note that the m₂ curve is much steeper than m₁, consistent with the considerable increase in the degree of central condensation noted by visual observers. A series of points from several observers immediately after perihelion seems to suggest that the comet was brightening further, but further observations suggest that this was simply a statistical artefact and that the comet is continuing to show virtually the same trend post-perihelion to its pre-perihelion behaviour.

When the data are plotted as m₁-5 log Delta against log r (Figure 2) with pre-perihelion data as solid points and post-perihelion data as hollow points we see that there is evidence that visually the comet is brighter post-perihelion than pre, although only by about half a magnitude. Interestingly, the m₂ data does not show this effect.

Finally, a plot is shown of the magnitude, corrected for geocentric distance against the number of days from perihelion (Figure 3). Note that the peak visual brightness appears to be slightly shifted with respect to perihelion, although only by about 2 days. This effect appears insufficient to explain the light curve asymmetry required in 1661. Curiously, we find that the peak brightness of m2 was attained some 8 days before perihelion. This explains why the post-perihelion data falls mainly below the fitted pre-perihelion curve for m₂.

In conclusion, at least based on this limited, but hopefully significant data set, the mystery of the 1661 apparition remains. A very small light curve asymmetry is seen, but nowhere near large enough to explain the inconsistencies in the data for 1661 found by John Bortle. It is possible that the full TA archive will give a different result and thus resolve this problem. This study is pending and is expected to be prepared for the May edition of TA.

Figure 1: The raw light curve of C/2002 C1 (Ikeya-Zhang) from the "Cometas_Obs" archive.

Figure 2: Magnitude corrected for geocentric distance against log r for C/2002 C1 (Ikeya-Zhang) from the data in Figure 1.

Figure 3: Magnitude corrected for geocentric distance against days from perihelion for C/2002 C1 (Ikeya-Zhang) from the data in Figure 1.

Ó mrk@ll.iac.es