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Comet 103P/Hartley (P=6.47a) will approach Earth on Oct. 20, 2010 to within 0.121 AU, remaining closer than 0.15 AU for almost four weeks. According to predictions it should peak at about 4.5 mag (m0=8.5 mag / n=8). It is comfortably placed for mid-European observers after midnight, moving from Cassiopeia through Perseus, Auriga, Gemini, Monoceros into Canis Major.

Comet 103P/Hartley became the most interesting comet in fall 2010. According to 63 observations by 9 members of the German Comet Section and 345 international observations the comet reached a maximum brightness of 5.0 mag on Oct. 21, 2010. Similar to the past apparitions the brightness evolution was very different pre- and post-perihelion: pre-perihelion the comet brightens very rapidly, fading much slower post-perihelion. The evolution can be described by the following formulae (which, however, do not represent the maximum very well):

pre-perihelion: m = 9.2 mag + 5×log D + 28.0×log r
post-perihelion: m = 9.4 mag + 5×log D + 9.4×log r

Total Brightness and Coma Diameter

The apparent coma diameter measured 1' at the start of the apparition, increasing to only 3' until the beginning of September. This was followed by a rapid increase to 18' until the first days of October (which was not due to a change of instrumentation) and to 35' at perigee. Thereafter it decreased - rapidly during the first weeks, than more slowly - reaching 25' in mid-November, 10' at Year's End, 4' in mid-January 2011, 2' at the end of March and 1' at the end of April. The absolute coma diameter measured 25.000 km at first, increasing to 150.000 km in mid-September and 200.000 km in mid-October. Between mid-November and the end of December the coma showed its maximum extent of 210.000 km. During the following weeks it shrunk rapidly, measuring just 180.000 km at the beginning of January 2011 and 130.000 km at its end. Until the end of April it shrunk slowly to 100.000 km. The degree of condensation was DC 5-6 at the start of the apparition, constantly decreasing to DC 3 until mid-September. Thereafter this degree of condensation remained nearly constant, showing only a very small decrease to DC 2-3 until the end of 2010 and to DC 2 until the end of April 2011. Visually a tail could be observed between September and November 2010, reaching a maximum of 6' (0.1 Mill. km) and pointing towards west.

Andreas Kammerer

FGK observations