On July 22, 2002, a great moment for Sebastian Hönig arrived, when he - while leasurly observing with his 10" Schmidt-Cassegrain telescope - discovered a comet of magnitude 12 in the northern parts of Pegasus. Due to the unfavourable circumstances (telescope not aligned, missing star charts, moon) he had only a rough idea of the position and the direction of movement. Neither he nor Alan Hale were able to relocate the comet in a sky brightened by the full moon. Not until July 27 K. Kadota succeeded in photographing the comet. Three days later first orbital elements were published of Comet C/2002 O4 (Hönig), which showed that the comet would pass perihelion in early October at a distance of only 0.78 AU and would be a circumpolar object during August (IAUC 7939/41).
For the analysis 57 observations by 10 group members and 235 international observations were taken into account, which show an interesting brightness evolution. During the first 4 weeks the brightness increased rapidly: from 11.0m at discovery to 7.5m on August 19. If the comet would have continued this evolution it would have reached magnitude 4.5 at perihelion! But this high activity (probably the reason for the non-discovery by LINEAR and others) probably exhausted it with the evolution dramatically altered thereafter. During the following weeks the heliocentric brightness decreased in spite of the still decreasing solar distance. But the situation even became worse: shortly after Sep. 20, when the visual brightness had decreased to 9.0m, the comet began to dissolve. Until Sep. 30 the brightness had decreased to 10.0-10.5m, thereby getting more diffuse with the central condensation becoming less obvious. On the evening of Sep. 30 Michael Jäger photographed the comet with his 8" Schmidt camera: it showed a 2' coma without an obvious center, transfering smoothly into the 10' long tail towards p.a. 32°. On the evening of Oct. 10 K. Kadota estimated the brightness on a CCD image at only 13.3m, two days later Y. Ezaki couldn't even find it to 15m (IAUC 7995). Prior to the disintegration the brightness evolution can be described by the following formulae:
t < -43d : m = 6.8m + 5×log D + 25×log r
t > -43d : m = 8.4m + 5×log D - 5×log r
Thus comet Hönig reached its maximum brightness of 7.4m around Aug. 22. Thereafter - at that time surprisingly for all - it got fainter due to the negative(!) activity factor and the increasing distance to Earth.
Evolution of the heliocentric magnitude (pre-perihelion)
The apparent coma diameter increased from 4' at the beginning to a maximum of 10' around Aug. 20. Therafter it decreased equally rapidly and measured only 3' around Sep. 20. The absolute coma diameter measured 125.000 km at first and increased until the first days of September to 300.000 km. After that it decreased rapidly and measured 125.000 km again around Sep. 20. The coma was moderately condensed, the degree of condensation increased during the first weeks from DC 3 to DC 4, where it remained until the disintegration. Visual tail observations were reported between the beginning of August until September 10 with values of up to 0.4° (700.000 km).
Total Brightness and Coma diameter
Andreas Kammerer
