On Dec. 13, 2002 the Japanese amateur Tetuo Kudo discovered a comet with his 20x120-binoculars near the border of the constellations Bootes/Hercules. Unaware ot this discovery Shigehisa Fujikawa found the same object on the following day. T. Kudo estimated the brightness of the comet as 9.5m with a 2' coma, which displayed a central condensation. S. Fujikawa reported a brightness of 9m and a 4' coma. The comet was confirmed shortly thereafter by a number of observers. The Japanese amateur Kushida reported the comet to be of magnitude 7.5 on Nov. 14; CCD-observations showed a 5.5' and a 18' tail in p.a. 331°. The first orbital elements showed that comet C/2002 X5 (Kudo-Fujikawa) would pass the sun on Jan. 24, 2003 at a distance of only 0.1 AU (IAUC 8032). Additional astrometric observations increased the perihelion distance, but the comet should nevertheless become visible with the unaided eye. The derived absolute magnitude of 6.5-7.0m indicates a medium-sized nucleus (IAUC 8033), which should survive the small solar distance. According to the current orbital elements the comet will pass perihelion on Jan. 29, 2003 at a distance of 0.19 AU. Dependant on the assumed evolution the comet should gain a maximum brightness between 2.0m (m0=7.0m/n=3) and -0.5m (m0=6.5m/n=4).
Thus the chances stood good for a pre-perihelion morning apparition lasting until Jan. 20 for mid-European observers, when the brightness was expected to be between 4.0m and 2.5m, the tail length between 5° and 10°. During the weeks following perihelion passage it should only be observable from southern latitudes (moving through Microscopium, Grus, Phoenix and Fornax), just to reappear in the evening sky during the first days of March for mid-European observers. Supposing a continuous evoluton the brightness was predicted to be about 6.5m and the tail length about 1° (independent of the value n!) during those days. The maximum tail length of 6° to 15° should be reached briefly after perihelion, with the comet unobservable from ground. Northern hemisphere observers would be better placed before, Southern hemisphere observers after perihelion.
However, the comet showed an evolution below average, as documented by 18 observations from 5 members of the German Comet Section and 190 additional reports from international observers. These indicate, that the brightness evolution of comet Kudo-Fujikawa was very smooth prior to perihelion, while it showed a significant change about 35 days (Mar. 5) after perihelion. During the first post-perihelion phase the brightness evolution was - similar to the pre-perihelion apparition - rather slow, accelerating significantly afterwards.
pre-perihelion
m = 7.2m + 5 ×log D + 5.5 × log r
post-perihelion
t < +35d: m = 7.9m + 5 ×log D + 5 × log r
t > +35d: m = 8.0m + 5 ×log D + 12 × log r
Evolution of the heliocentric magnitude prior perihelion
Evolution of the heliocentric magnitude after perihelion
Accordingly the brightness around Jan. 20 (with the comet already in deeper twilight) reached only 5.2m (which would give a visual tail length of only 2°, according to my empiric formula). At perihelion the formulae yield a plausible maximum brightness of 4.3m (assuming a smooth transition between the formulae) and a tail length of 0.7°, in good accordance with the SOHO-pictures during those days. At the time of reapparance on the mid-European sky the comet actually was of magnitude 8.0, fading quickly thereafter.
Total Brightness and Coma diameter
At the start of the apparition the coma diameter measured 5' (250.000 km), increasing to 8' (340.000 km) until the first days of January 2003. Thereafter the coma shrunk significantly, due to the solar wind getting stronger, measuring only 2.5' (100.000 km) in mid-January. These dimensions were reported again during the days of reappearance at the southern sky. During the first days of March a second maximum of 6' (240.000 km) was reached. Since then the coma diameter decreased steadily, measuring a mere 1' (70.000 km) at the beginning of April. Shortly after discovery the coma was only moderately condensed (DC 3-4). Starting around Christmas the degree of condensation increased quickly, reaching DC 7-8 at the time the comet disappeared in twilight. The first observations after perihelion reported a similarly condensed coma. However, during the following weeks the degree of condensation decreased rapidly, measuring DC 4 at the beginning of March and DC 2 in the first days of April.
Pre-perihelion visual tail sightings span the period Christmas 2002 to mid-January. After perihelion visual tail sightings were reported between mid-February and beginning of March. The reported tail lengths reached a maximum of 0.5° (1.5 Mio. km) pre- and post-perihelion, respectively.
Andreas Kammerer
