Comet 78P/Gehrels (P=7.23a) will pass perihelion in mid-January 2012. At this time the distance comet-earth will already increase, thus the maximum brightness (according to ICQ: 12.5 mag) is expected to take place in November 2011. Throughout the apparition the comet will be placed in the constellation Pisces, thus being best obervable in the morning sky during the first part of the apparition, in the evening sky during the second part.

The comet evolved much more positive than expected, based on 22 observations by 6 members of the German Comet Section as well as 165 international observations. However, the time-dependent formulae, with which the brightness evolution of the last apparitions could be rather well described, do not work for this apparition. Instead the brightness evolution can be best described with a standard log r - formulae, however with extreme values of the parameters:

Based on this, a maximum brightness of 10.6 mag around Dec. 10, 2011 can be derived. However the estimates in March/April deviate significantly from the formula above. Thus an alternative approach for the post-perihelion period was followed, which yields the formula m = 9.5 mag + 5×log D. This approach represents the mentioned estimates better, but it does not yield a more plausible physical explanation.

Total Brightness and Coma Diameter

The apparent coma diameter increased steadily from 0.8' at the start of the apparition to a maximum of 3' at the start of December 2011. This value was constant until the end of January 2012. Thereafter it decreased, being estimated at about 2' at the end of March 2012. The absolute coma diameter increased from 65.000 km at the start of the apparition to 260.000 km at the end of January 2012. This diameter remained constant during the following weeks (at least until end of March). During the whole apparition the coma grew ever more diffuse. Estimated at DC 5-6 at the start, the observers reported a degree of condensation of only DC 3-4 since the end of 2011. Visual tail sightings were rare with the length not exceeding 5'.

FGK observations