Periodic comet 8P/Tuttle was recovered by C.W. Hergenrother on images taken on Apr. 22, 2007. Situated in Lacerta the star-like comet was of magnitude 20.3 (IAUC 8848). Comet Tuttle will pass earth at a distance of 0.253 AU on Jan. 1, 2008. Accordingly during December/January it will brighten rapidly to magnitude 5-6 and the coma diameter should swallow to about 20'. Additionally it will accelerate its movement, finally sweeping nearly 4° per day. Positioned near the border of the constellations Draco/Cepheus at the beginning of December (visible nearly the whole night) it will move through Cassiopeia, Andromeda and Pisces into Cetus and Fornax, where it will plunge below the evening horizon for mid-European observers during the last week of January. Maximum brightness is predicted a few days after perigee. Earth will cross the comet's plane on Dec. 22.
The comet was observed visually between October 2007 and May 2008. For the analysis 63 observations by 9 observers of the German Comet Section and additional 420 international observations could be used. Prior to perihelion (on Jan. 27, 2008) the brightness evolution was very constant. It was followed by a nearly two month long period in which the brightness evolution cannot be described in a satisfactorily manner by a standard formula. Then, 55 days after perihelion it can again be described rather well by a standard formula. The derived formulae follow (with a rather low correlation for the 55 day interim period)
t < 0d: m = 8.0 mag + 5×log D + 19.3×log r 0d < t < +55d: m = 7.6 mag + 5×log D t > +55d: m = 5.1 mag + 5×log D + 21×log rwhat means that the maximum brightness of 5.7 mag was reached around Jan. 5, 2008.
Total Brightness and Coma Diameter
During the first weeks the apparent coma diameter increased slowly from 2' at the beginning of November 2007 to 5' at the end of this month. Thereafter the increase accelerated, reaching 12' in mid-December and peaking at 23' at perigee (Jan. 1, 2008). After perihelion the coma diameter decreased, at first slowly, from 12' to 9' at the start of April. Thereafter it decreased faster, measuring 4' at the beginning of May.
The evolution of the absolute coma diameter was very different. Between the beginning of the apparition and mid-November it increased rapidly from 100.000 km to 175.000 km. Thereafter the increase slowed considerably, with the coma measuring 225.000 km at the end of 2007. After perihelion it increased considerably from 275.000 km to a maximum of 450.000 at the beginning of April. Thereafter it decreased, reaching 250.000 km at the start of May.
At the start of the apparition the coma was very diffuse (DC 2). Not until the beginning of December began it to condense, showing a maximum degree of condensation of DC 4 at the end of 2007. Suprisingly, this degree was kept until at least the end of March. A tail could not be observed visually.
Andreas Kammerer
