There is considerable interest in Comet C/2002 C1 (Ikeya-Zhang) which appears to be the first confirmed return of a comet with a period greater than 155 years (the current record holder is Comet Herschel-Rigollet, last seen in 1939). Suntoro Nakano suggested initially that the comet might be identical to C/1532 R1 but, more recently, showed that C/1661 C1 offers an even better linkage. This linkage is now close to definitive.

Although C/1532 R1 was observed from September 2nd to December 30th. C/1661 C1 was less well observed. It was discovered on 1661 Feb. 3 in the dawn sky, just after passing perihelion, with a tail already 6º long. The comet faded rapidly and was last seen on March 28^th. The orbit used for the former in the IAU/CBAT/MPC "Catalogue of Cometary Orbits" is that of Olbers, calculated in 1787. The orbit is not completely determined, despite the long visibility of the comet and a 1785 solution by Méchain gave a rather different solution, with an inclination of 42º, and an Ascending Node of 126º. For C/1661 C1 the orbit used is the one calculated in 1785 by Pierre Méchain. These orbits are compared below to that of Comet Ikeya-Zhang. As we can see, the similarity with the Olbers orbit of C/1532 R1 is quite impressive. The similarity with C/1661 C1 is less so; its longitude of perihelion is very close to the corresponding value for Ikeya-Zhang, although other parameters are not quite so close.

On seeing this similarity, the overwhelming impression is that the three objects may all be related and that both C/1532 R1 and C/1661 C1 may be fragments of a single object that split in the past.

C/1532 R1 was evidently an exceptional object. David Hughes's 1987 catalogue of cometary absolute magnitudes from 568 - 1978 assigns it an absolute magnitude of +1.8, one of just 12 comets that has an absolute magnitude of +2 or brighter, putting it into the "giant comet" class, almost 100 times intrinsically brighter than the average long-period comet. In contrast, C/1661 C1 is a more normal object. David Hughes lists its absolute magnitude as +4.6, much closer to that of Ikeya-Zhang, particularly as the observations suggest that the comet became diffuse and faded out rapidly. Light curve fits to C/2002 C1 (Ikeya-Zhang) suggest an absolute magnitude of 6.8, slightly fainter than average for a "new" comet.

The data archive from the Spanish "Cometas_obs" mailing list is shown below. It shows a sustained and rapid rate of brightening in all three data sets, although the knee in the magnitudes during mid-February appears to be genuine. The brightening seems not to be as fast has been suggested. Seiichi Yoshida finds a good fit with m1 = 6.8 + 5 log D + 10 log r. If we take the archetypal active evolved object, 1P/Halley, the brightening rate at r < 1.7AU was found by Daniel Fischer to be 9.1 log r. C/1995 O1 (Hale-Bopp), also an active multiple-return object, brightened at 7.5 log r for most of its inbound passage. C/2002 C1 is brightening slightly faster than these comets, but not spectacularly so. Even so, it seems to be an unusually gassy object.