On Feb. 10, 2011 R.H. McNaught discovered a 16.5 mag comet near the border of the constellations Libra/Scorpius with the Uppsala-Schmidt-telescope at Siding Spring. Comet C/2011 C1 (McNaught) showed a strongly condensed 12" coma and a 0.3' tail towards west. It passes perihelion in mid-April and could reach 14 mag (IAUC 9197/98). Between February an the beginning of May it will move from Ophiuchus into Pegasus, being situated above the southeastern and eastern horizon. However, the altitude will decrease from 20° to less than 10° during this interval. The comet orbits the sun on a very elongated ellipse with a period of about 7.000 years (MPEC 2011-G58).

The comet became much brighter than expected, resulting in a respectable number of observations despite the unfavorable circumstances. Based on 3 observations by 2 members of the German Comet Section and 110 international observations the comet peaked at 8.8 mag in mid-April. Astonishingly the comet was still estimated between 11 mag and 12.5 mag in summer 2011. The diagram indicates that this could be the result of an unexpected brightening of 2.5 mag during the second half of June. The brightness evolution can be described by the following formulae:

During the first weeks of the apparition the coma diameter increased rapidly from short of 3' (125.000 km) to the maximum of 5' (225.000 km) in mid-April. Thereafter it decreased equally fast, reaching 1.5' (110.000 km) at the start of June. This was followed by a much slower decrease with the coma measuring 0.5' (40.000 km) at the beginning of August. Most of the apparition the coma was medium condensed (DC 3-4). Starting in July the coma grew more diffuse with the degree of condensation at DC 1 at the beginning of August. Visually a tail could not be detected.

Total Brightness and Coma Diameter

Andreas Kammerer