Between spring and fall 2010 comet 10P/Tempel (P=5.37a) could be observed in the morning sky - alas at rather low altitudes above the southeastern horizon for mid-European observers. The brightness was predicted to peak at magnitude 8-9.
Once again, this comet showed its quite specific brightness evolution. According to 10 observations from 4 observers of the German Comet Section and 205 international observations the brightness evolution can be divided in its typical three periods: a rapid brightness increase nearly to the perihelion followed by a period of constant heliocentric magnitude, which however was of short duration during this apparition. The third period is characterized by the fading of the comet, which is much slower than the pre-perihelion increase. The superposition of the heliocentric brightness evolution with the changing distance comet-Earth resulted in a very broad peak of 8.5 mag at the start of August. The appropriate formulae for the 2010 apparition are:
t < -10d: m = 8.7 mag + 5×log D - 0.060×(t-T) -10d < t < +30d: m = 9.4 mag + 5×log D t > +30d: m = 9.1 mag + 5×log D + 0.012×(t-T)During the first eight weeks the apparent coma diameter increased rapidly from about 1' to almost 8'. From mid-June until the end of September this value remained constant. The variations shown in the diagram nicely reflect the interference of the Moon. After this long period the apparent coma diameter began to decrease, reaching 2.5' at the beginning of November. This value remained constant until the end of the apparition. The absolute coma diameter increased from 60.000 km at the beginning of the apparition to 275.000 km in mid-June. Then it decreased slowly at first, so that it measured still 225.000 km at the beginning of October. Then the decrease grew faster with the coma diameter at 125.000 km at the end of the apparition. Typically for this comet the coma was very diffuse. The degree of condensation was at DC 2-3 for most of the time, between mid-July and mid-October it was at DC 3.
Total Brightness and Coma Diameter
In mid-July, when Earth crossed the comet's orbital plane, Francois Kugel was able to observe up to 15° long, almost one-dimensional dust trails along the comet's path.
Andreas Kammerer
