On May 13, 2004 X.-m. Zhou, K. Cernis und M. Mattiazzo independently reported a possible comet in the constellation Eridanus, found in the SWAN data of April 29 and May 2, 5 and 8, respectively. Because false 'SWAN Comet discoveries' have been reported rather often during recent years, the CBAT did not forward the report, especially because confirmations by other observations failed (the comet was then very low above the bright morning horizon). Finally, on May 21 several observers independently succeeded in confirming the comet. The comet was 7.5 mag bright at the beginning of June and presented a 2' large, diffuse coma with a slight concentration (IAUC 8346/47). The comet should remain at this brightness for several weeks although it receeds from the Sun, but approaches Earth at the same time until the end of July. From mid-Europe it should become observable in the second week of July in the eastern morning sky.
The comet did not live up to the general expectations, as is clearly shown by the 5 observations of 2 members of the German Comet Section and 40 international ones. The first estimates, made under unfavorable conditions, were probably too faint. The brightness evolution can be simulated either by two dt-formulae or one r-formula (comparable to Encke). The following diagram shows both versions which converge after June 20.
t < +35d : m = 6.8m + 5×log D + 0.018×|t-T|
t > +35d : m = 6.8m + 5×log D + 0.075×|t-T|
alternatively: m = 2.2m + 5×log D + 5.4×r
Total Brightness and Coma diameter
The apparent coma diameter was estimated at about 2.5' at first. Thereafter it increased, reaching a maximum of 8' in mid-July. Then it started to decrease, reaching 2' on Aug. 20. The absolute coma diameter measured 150.000 km at first, increasing slowly to 230.000 km in mid-July. After that date it decreased rapidly to only 80.000 km on Aug. 20. The coma was strongly condensed at first, becoming more and more diffuse during the apparition, decreasing from DC 7 to DC 0!
A more detailed look at the evolution of brightness, coma and DC-value hints towards an outburst shortly before discovery, which diminished during the apparition.
Andreas Kammerer
