On Mar. 23, 2004 the most famous australian comet hunter, W.A. Bradfield (at age 76!) found a comet of magnitude 7-8 in the constellation Cetus in the course of his search for sungrazing comets. It was his 18th comet discovery! The following night he could observe it for the second time, but thereafter no additional confirmation was reported (mainly because the comet was already situated in deeper twilight at very low altitudes). Then, on Apr. 8, Bradfield saw the comet again and on the following night, R.H. McNaught and T.M. Smith eventually confirmed the comet. In 12x120-binoculars they estimated the brightness of the strongly condensed blue 0.5' coma as 5m, and could detect a hint of a tail. Finally T. Lovejoy succeeded on Apr. 11 in taking a CCD image, which showed the comet as a 3.5m object with a 0.5° tail. Due to these very unfavorable circumstances only a very preliminary orbit could be calculated (IAUC 8319/20). Not until Apr. 15, when comet Bradfield moved for four days through the SOHO field, could this orbit be refined. Comet Bradfield passed perihelion on Apr. 17 at a distance of only 0.17 AU. Expected to be around 2m the SOHO images rather point to an object of magnitude 0-1. This would mean that comet Bradfield has an absolute magnitude above the line of disintegration (according to John Bortle's formula).
The comet outshone the long awaited comet C/2001 Q4 (NEAT) for a few days after April 22. It appeared as a 3.5 mag bright and highly condensed object in the bright morning twilight. During the following days it gained altitude but, as expected, faded rapidly. Most impressive was the long thin tail (similar to that of comet Ikeya-Seki), which was discernible over several degrees and showed a high surface brightness close to the comet's head. But as the brightness the tail length decreased during the following days. The brightness evolution is well represented by the formula
m = 8.3m + 5×log D + 8.3×log r
resulting in a maximum brightness of about 2.0 mag at perihelion. This formula is derived by using 5 visual observations by 3 members of the German Comet Section and 100 international reports (until mid-September 2004).
Total Brightness and Coma diameter
The evolution of the coma diameter can be described only crudely. The first post-perihelion observations, which were strongly influenced by the twilight conditions, noted a coma diameter of about 1-2'. Thereafter it increased and reached a maximum of about 7' in mid-May. An increase of the coma diameter shortly after perihelion is the rule, but in the case of comet Bradfield it is nearly impossible to define the amount. After mid-May the coma started to decrease, rapidly during the first days, with a moderate pace after May 20, measuring only 1.5' in mid-June. According to these estimates the absolute coma diameter measured 40.000 - 80.000 km when the comet emerged in the morning sky, reaching 400.000 km at maximum, decreasing to 150.000 km at the end of June. The degree of condensation was extremely high at first (DC 8), decreasing steadily during the following weeks (end of June: DC 1). Thus the extremely diffuse coma could be the main reason that after the end of June no further visual sightings have been reported.
The visual tail length estimates were equally influenced by the unfavorable conditions. The maximum tail length was reached on Apr. 28 with 8° (22 Mio. km), decreasing rapidly during the following days. Last observations (about 0.3°) could be obtained around May 20. The tail constantly pointed towards p.a. 300°.
Andreas Kammerer
