Several persons, notably H. Sato M. Suzuki, M. Mattiazzo, M. Jäger and V. Bezugly, reported a possible comet in the UV data collected by the SWAN satellite beginning on Aug. 4 in the constellations Sextant/Leo. Following a CBAT request Alan Hale reported a visual observation on Aug. 25: Comet C/2005 P3 (SWAN) showed a moderately condensed coma of diameter 3' and magnitude 9.5 (IAUC 8587/88). Assuming an average brightness evolution the comet's brightness did not exceed 8.5 mag at perihelion. Taking into account the then low altitudes, no assumption of an outburst is necessary to explain the late discovery date. According to the most recent elements the comet passed its perihelion on Aug. 9 and orbits the sun with a period of about 270 years.
The brightness of the comet decreased throughout the apparition, based on 5 observations by 3 members of the German Comet Section and 43 international observations. The brightness evolution can be described by the formula
m = 11.0m + 5×log D + 9.3×log r
yielding a maximum brightness of 10.0 mag at the end of August. Until the end of October it had decreased to 13.0 mag. In parallel, the coma diameter decreased continuously from about 3' (175.000 km) to 0.8' (50.000 km). During the apparition the coma grew ever more diffuse; the degree of condensation decreased from DC 3-4 at first to DC 0.
Total Brightness and Coma diameter
According to CCD observations between the last days of October and mid-November 2005 the comet probably experienced a dramatic fading. Still of magnitude 13.0 mag on Oct. 25 its brightness dropped to about 15.5 mag on Oct. 31. Peter Birtwhistle and Richard Miles could not even find any object brighter than 20.0-20.5 mag at the comet's position on Nov. 16 (Comet's Mailing List).Andreas Kammerer
