On Mar. 12, 2005 Robert McNaught discovered a 15.5 mag comet with the 0.5m-Uppsala Schmidt-telescope at Siding Spring. Comet C/2005 E2 (McNaught), positioned in the southern parts of Sagittarius, showed a uniform 7" coma and a 13" tail in p.a. 250°. First orbit calculations implied a perihelion in spring 2006 in a rather great solar distance with the comet expected to reach magnitude 12-13. Additional astrometric observations then put the comet at a significantly smaller solar distance, with the comet now expected to reach magnitude 9 (IAUC 8494, MPEC 2005-F54). Alas, during the most interesting period (Dec. 2005 to April 2006) the comet will be at small solar elongations, moving from Capricornus into Pisces, not climbing higher than about 20° for mid-European locations.
This comet was under visual inspection since July 2005. Unfortunately it was then one magnitude fainter than expected, meaning that its maximum brightness would reach only 10 mag.
At the end of October 2005 it became visible for mid-European observers. However it was a difficult object at this time. The German Comet Section received 5 observations from 4 members. Additional 90 international observations were used for the analysis, which shows - neglecting some post-perihelion observations, which were done with the comet very low above the horizon - a steady brightness evolution according to the formula
m = 6.8m + 5×log D + 8.7×log r
indicating a maximum of 10.1 mag in mid-February 2006.
During the first months the coma diameter increased rather slowly from 0.5' (50.000 km) in July 2005 to about 1.0' (90.000 km) at the end of October. During the following 5 weeks, however, it rapidly grew to 3.0' (290.000 km), where it remained until the comet disappeared in the evening twilight. After perihelion the coma diameter was estimated at 1.5' (200.000 km) at most. During the first months of the apparition the degree of condensation showed a constant value of DC 5. Since the beginning of 2006 it is decreased slowly, reaching DC 3 at the beginning of March. Visually a tail was not reported.
Total Brightness and Coma diameter
Andreas Kammerer
FGK observations