On Aug. 1, 2002, M. Suzuki (Japan) reported the discovery of a comet in the constellation Eridanus on publicly accessible images of the SOHO instrument SWAN taken on July 25 and 27. Additional searches showed the object on images back to July 13. Shortly after the discovery Alan Hale succeeded in visually observing the object under rather unfavourable conditions (twilight, clouds and interfering moon). He reported the comet to be of magnitude 9.5 with a 4' coma (IAUC 7944). The orbital elements calculated shortly after discovery showed that this comet would pass Earth on Aug. 9 at a minimum distance of only 0.26 AU with perihelion taking place at the beginning of September at a distance of 0.49 AU. Further visual estimates on Aug. 2 and 3 gave brightnesses between 7m and 8m (IAUC 7948). In early September astrometric observations showed that this comet rounds the sun on an elliptical orbit with a period of 6.700 years, indicating that this is not its first visit to the Sun.
Ten days after discovery D.A.J. Seargent pointed out via the Comet Mailing List, that this comet probably will not survive its perihelion passage, based on an empiric formula by J. Bortle (ICQ 79, July 1991). Bortle's research indicates that the survival of a comet is dependant on the absolute brightness and its perihelion distance. With appropriate values of 10m and 0.5 AU this comet is right on the borderline for survival. It could also be possible that comet C/2002 O6 experiences a rapid decrease in brightness following perihelion.
Altogether 12 observations made by 10 group members were received. For the analysis 85 international observations were also taken into account. These indicate that the comet started to dissolve around Sep. 10. During the time preceding the disruption the brightness evolution can be described with the following formulae:
t < -20d : m = 9.7m + 5×log D + 10×log r
t > -20d : m = 6.0m + 5×log D - 10×log r
Evolution of the heliocentric magnitude (pre-perihelion)
This indicates, that the comet got noticeably fainter from Aug. 23 onwards, despite its decreasing solar distance - a clear indicator for its depleted gas and dust supplies. The maximum brightness was reached around Aug. 15 at 6.0m. The evolution of the apparent coma diameter was rapid. Measuring around 8' at first, it increased until mid-August to 11', decreasing rapidly again (measuring 2' on Sep. 10). The absolute coma diameter measured 100.000 km at first, increasing to 200.000 km during the last week of August, decreasing again to 90.000 km on Sep. 12. The coma was well condensed for a rather long time: the DC value increased from DC 4 to DC 5 where it remained until the end of August, only to decrease rapidly thereafter. Visual tail sightings were rare, probably partly due to the rather unfavourable observing conditions.
Total Brightness and Coma diameter
Andreas Kammerer
