On June 4, 2006 G.J. Garradd discovered a 17.5 mag comet in Antlia in the course of the Siding Spring Survey. Detailled observations on June 6 showed comet C/2006 L1 (Garradd) with a diffuse coma and a 15" tail in p.a. 150°. It will pass perihelion in fall 2006 and could reach magnitude 13-14 in December (IAUC 8719). Between October and mid-January it could be brighter than 16 mag, moving from Sextans through Leo, Cancer, Auriga, Perseus and Andromeda.
In fall 2006 the comet was much brighter than expected, as is demonstrated by 12 observations of 7 members of the German Comet Section and 100 international observations. The brightness peaked at 9.4 mag (instead of 13 mag) around Dec. 8. Due to the rather large scatter in the estimates the very continuous evolution of the brightness could be recognized only after several weeks of surveillance. It can be well described by the formula
m = 6.5m + 5×log D + 17×log r
The apparent coma diameter measured 2' at the beginning of the apparition, then increased to 6' until mid-December. Thereafter it decreased in a very rapid manner, reaching 1.5' in mid-January 2007, where it rested until the end of the apparition. The absolute coma diameter measured 200.000 km until the beginning of 2007, when it rather suddenly dropped to 125.000 km. The degree of condensation was DC 3 at the beginning of the apparition and DC 2-3 at the end.
Total Brightness and Coma diameter
Andreas Kammerer
