On Nov. 18, 2006 E.J. Christensen discovered a comet near the border of the constellations Lynx/Auriga on Catalina Sky Survey images. Comet C/2006 W3 (Christensen) showed a medium condensed 15" coma of magnitude 18.0 and a 25" tail in p.a. 80°. According to the first orbit calculations it would have reached 8 mag in fall 2008. However, according to the actual orbit it will pass perihelion in summer 2009 at an expected magnitude of about 12 (IAUC 8777 / MPEC 2006-W117). During the most interesting months it will move from northern Pegasus into Aquila, being observable the whole night.

The following analysis incorporates 116 observations of 13 members of the German Comet Section and 560 international observations. These show a very continuous brightness evolution with extraordinarily high parameter values according to the formula

Thus the comet peaked at 8.3 mag in mid-August 2009.

The apparent coma diameter remained below 1' until the late summer months of 2008, increasing until November to 4' and remaining there quite constantly. In April 2009 it began to increase further, peaking at 7' in mid-August. Thereafter it shrunk, measuring only 3.5' at the end of November and short of 1' at the end of July 2010. The absolute coma diameter increased slowly until late summer 2008 from 110.000 km to 175.000 km, followed by a phase of rapid increase to the maximum value of 800.000 km in April 2009. During summer it decreased slightly to 725.000 km. Thereafter it shrunk more rapidly, still measuring 500.000 km at the end of November 2009 and 150.000 km at the end of July 2010. The coma was significantly condensed throughout the apparition. The degree of condensation was nearly constant near DC 5. Visual tail observations were reported between July 2008 and January 2009 and again between May and October 2009. The maximum observed tail length did not surpass 6' (3 Mio. km). It pointed NW at first, rotating to SE in summer 2009.

Total Brightness and Coma Diameter

Andreas Kammerer

FGK observations