On Aug. 13, 2009 G.J. Garradd discovered a 19.0 mag comet in the constellation Phoenix. Comet C/2009 P1 (Garradd) showed a 15" Koma, but no tail. At discovery it was at an distance of nearly 9 AU from the sun! The comet will pass perihelion in December 2011 and could reach 7.0 mag in February 2012 (IAUC 9062 / MPEC 2009-R18). It should be brighter than 12 mag between May 2011 and July 2012, brighter than 9 mag between July 2011 and April 2012. During this period it crosses the constellations Aquarius, Pegasus, Delphinus, Sagitta, Hercules, Draco, Ursa Minor, Draco, Ursa Major, Lynx and Cancer. During the most interesting months it will be an evening object until November 2011 and again starting in March 2012. Between December 2011 and February 2012 it will be better placed in the morning sky. The maximum altitude is in the order of 80°.

Starting in summer 2011 comet C/2009 P1 (Garradd) was visible in binoculars for more than half a year and was therefore widely observed. The analysis can be based on 179 observations by 9 members of the German Comet Section and 885 international observations. Neglecting some of the observations made during the first months the brightness evolution can be well described using three different periods. During the first period the brightness evolution was rather average. It ended on Oct. 10 (75 days prior to perihelion) and was followed by a period during which the heliocentric magnitude brightened slower than a purely reflecting body. After perihelion the heliocentric magnitude faded - however with a slower pace than it brightened prior to perihelion. The overall brightness evolution can be described by the following formulae:

The brightness peaked at 6.7 mag at the end of February 2012. The comet was brighter than 7.0 mag between the start of December 2011 and mid-March 2012. At the end of May 2012 the brightness had dropped to 9.5 mag. In November 2012 few additional observations could be made, estimating the comet at magnitude 13.0-13.5 - about one magnitude fainter than the formula above suggests.

Total Brightness and Coma Diameter

During the first months of the apparition the apparent coma diameter was rather constant, measuring 1.5'. At the end of June 2011 it started to increase, reaching a first maximum of 12' in mid-September. Thereafter it decreased, measuring 10' in November/December. Then a second increase took place towards the maximum of 15' in mid-February 2012. Thereafter it eventually decreased, measuring 11' at the start of April, just 2.5' at the end of May and about 1.0' in November. The absolute coma diameter increased slowly from 250.000 km 400.000 km during the first months of the apparition. Starting at the end of June it increased considerably more rapid, reaching 875.000 km in mid-October. This dimension was constant during the following weeks, after which it swallowed further. The maximum diameter of 1 mio. km was reached at the end of January 2012. Thereafter the coma shrunk, measuring 775.000 km at the start of April and just 300.000 km at the end of May. Probably the fast decline at the end of the apparition may be at least partially attributed to the deteriorating observing conditions. The observations in November 2012 indicate a coma diameter of about 150.000 km.

Apart from the last weeks of the apparition the coma was considerably condensed. Whereas the degree of condensation was rather constant at DC 5 prior to perihelion, it showed a steady decline after perihelion, reaching DC 2-3 at the end of May 2012.

The comet showed an interesting tail morphology: In June and July 2011 it displayed a dust tail, which fanned for 45° and had brighter edges (mimicking two dust tails for visual observers). Contrary, in August only one broad dust tail was present. At the end of September the broad dust fan with the brighter edges reappeared, plus a faint gas tail near the position of the western edge of the dust tail. In mid-November the dust tail fanned over an angle of 45°, with the now more conspicuous gas tail still at the western edge. At the end of December the dust fan stretched over 70° with the eastern edge of the dust tail at an right angle with the gas tail. At the beginning of February 2012 the dust tail stretched over 150° with the brightest (eastern) part of it at an angle of 160° with the gas tail. At the time earth crossed the orbital plane of the comet (Feb. 15) the two brightest parts of the dust tail pointed in opposite directions - as expected. Thus the comet showed a conspicuous anti-tail, which pointed also in the opposite direction of the ion tail. Thereafter the dust tail fan diminished, measuring 90° at the end of March.

Visual observations of the dust tail and the two dust edges, respectively, have been reported since June 2011. The maximum length of 0.5° was reached in summer 2011 and at the time earth crossed the orbital plane of the comet. Assuming a straight tail pointing away from the sun the absolute tail length was 7 mio. km between August and October 2011, but only 4 mio. km at the time of the plane crossing. However, for this period the assumption is not applicable due to the then significant bending of the dust tail. Thus it is more plausible to assume that the length of the dust tail was rather constant at about 7 mio. km for many weeks. In June 2011 the dust tail pointed towards SW, but rotated during the following months, pointing towards south, east, north and west only to point towards SW again in March 2012.

The gas tail was first recognized at the end of September 2011. During the following weeks it became more prominent, peaking in brightness an length (0.5° = 4 mio. km) in January 2012. However, at the end of February 2012 it had already disappeared. For its whole apparition it was superimposed with the brighter part of the dust tail, which pointed away from the sun. Thus it rotated during its apparition from east over north towards west.

FGK observations