On May 25, 2012 the LINEAR project discovered an asteroidal object of magnitude 18.5 near the borders of the constellations Vulpecula/Cygnus/Lyra, which was recognized as a comet in the course of additional observations. Comet C/2012 K5 (LINEAR) showed a diffuse 16" coma of total magnitude 17.5 and a 25" tail in p.a. 215°. It will pass perihelion at the end of November 2012 and could peak at magnitude 11.5. During the following weeks it approaches Earth, reaching a distance of only 0.3 A.U. on Year's Eve, when it could be as bright as 9.5 mag. Thereafter the comet will quickly fade (CBET 3127 / MPEC 2012-L04). The comet should be brighter than 15 mag between September 2012 and February 2013. During this interval it will move through the constellations Bootes, Ursa Major, Lynx, Auriga, Taurus and Eridanus. Thus it will be positioned at high northern declinations during the most interesting weeks (December/January) - very comfortable for mid-European observers.

This comet became the brightest and most interesting during the winter months 2012/13. Its evolution can be determined by taking into account 8 observations from 4 members of the German Comet Section and 110 international observations. The rather small number of reports from members of the German Comet Section is most likely due to the unfavourable weather conditions in December and January, driving the comet observers almost crazy. The comet showed an unusual brightness evolution, which has to be divided into three periods and must be described by time-dependent formulae. On Nov. 1, 2012 - 27 days before perihelion - the rapid increase in activity stopped abruptly, followed by a 8 week long period (until 30 days post-perihelion) during which the activity decreased comparably fast. Thereafter the decrease of the activity stopped, with the heliocentric brightness remaining constant. This period may have ended in February or March, but no reports for this time are available. The maximum brightness was reached on New Year's Day at 8.3 mag. The three periods may be described by the following formulae:

Pre-perihelion the apparent coma diameter increased steadily from 0.3' to 4.0'. During the following 4 weeks it increased more rapidly - due to perigee – reaching a maximum of almost 6' at year’s turn. Then the apparent coma diameter decreased rapidly to 3.0' until mid-January. Thereafter it shrunk more slowly, reaching 1.5' to 2.0' at the beginning of February. The absolute coma diameter increased steadily until the activity stopped abruptly from 25.000 km to 240.000 km. During the period of decreasing activity the coma shrunk as well to only 80.000 km at the beginning of January. The coma retained this diameter until the end of the apparition. Thus the evolution of the absolute coma diameter was in parallel to the activity of the comet. The coma was significantly condensed (DC 5) until Nov. 20. Interestingly the degree of condensation increased thereafter, reaching its maximum of DC 6-7 around Dec. 20. During the following weeks the coma rapidly grew more diffuse, reaching DC 3 on Jan. 15, where it remained until the comet disappeared.

Total Brightness and Coma Diameter

Visually a dust tail was observed between the beginning of November and mid-January, showing a maximum length of 15'. During that period the position angle of the brightest part of the tail decreased from p.a. 210° to p.a. 130° (Dec. 10), returning to p.a. 220° thereafter. Prior to Dec. 30 the tail bent and fanned-out towards east, thereafter it bent and fanned-out toward west. During the days the Earth crossed the comet's orbital plane (Dec. 30) a faint, short anti-tail was observed photographically.

FGK observations