During August 2013 owners of large instruments should succeed in glimpsing comet 2P/Encke (P=3.30a) for the first time during this apparition. How bright it will be and in which way it will develop is debated. According to ICQ it should evolve according to the brightness parameters m₀=11.5 mag / n=6. Thus it should become brighter than 16 mag around Aug. 20 and will brighten to 10.0 mag at the end of the first week of October. According to my analysis comet Encke shows an extraordinary evolution which may be described by the formula m = 5.6^m + 5×log D + 4.5×r. Thus it should reach 16 mag at the start of August and be of magnitude 9.0 at the end of the first week of October. It has to be expected that the comet will show a coma of very low surface brightness during the first weeks. Its fast brightening will be partly compensated by its growing coma, which is a result of its approach to Earth. The comet moves from Aries through Lynx into Virgo, where it will disappear above the eastern morning horizon in mid-November.

Again, this comet evolved in the typical manner, based on 25 observations by 6 members of the German Comet Section and 90 international observations. Its brightness evolution can be best described by an unusual formula, which for this apparition had the form

This yields a maximum brightness of 7.5 mag on Nov. 15. Alternatively the evolution can be fitted with the following classical formula, which however does the job not as well:

This formula yields a maximum brightness of 6.3 mag on perihelion date.

Total Brightness and Coma Diameter

Interestingly the diagram indicates a significant drop in brightness after Nov. 11. In those days several observers reported the comet as very inconspicuous and nearly stellar. It seems that the coma of high surface brightness in the days before evolved in a very diffuse coma from which only the inner part could have been discerned against the bright sky background due to the twilight. This is supported by the fact that the coma estimates dropped in parallel in those days.

At the start of the apparition the coma measured 2.5' (125.000 km). With the comet approaching Earth (minimum distance of 0.478 AU occurred on Nov. 17) the coma diameter increased, reaching 9' (225.000 km) around Oct. 25. Thereafter the increasing distance Earth-comet combined with the increasing strength of the solar wind resulted in the shrinking of the coma to 1.5' (65.000 km) when the comet disappeared in the twilight. The coma was extremely diffuse (DC 0-1) until the end of September. This was followed by a period of slow condensation to DC 3-4 (until Oct. 25). Thereafter the coma condensed more rapidly, reaching DC 8 around Nov. 10. At the opening of November several observers could visually glimpse the tail (length up to 0.3°), despite the brightened sky - due to the twilight - and the low altitude of the comet.

FGK observations