On Mar. 19, 2007 the LONEOS team discovered an asteroidal object of magnitude 19 in the constellation Leo. Scrutinizing the object showed it to be cometary. Comet C/2007 F1 (LONEOS) showed a diffuse 8" coma of magnitude 18 and a 12" narrow tail in p.a. PW=185° (IAUC 8823 / MPEC 2007-FG20). This comet will pass perihelion at the end of October at a solar distance of only 0.41 AU, with the comet probably reaching magnitude 5.5. Unfortunately during the most interesting weeks the comet will quickly move southward from the constellation Coma Berenices into Scorpius. During the first weeks it will be an evening object, but will be better situated in the morning sky for a short interval at the beginning of October. Thereafter it will be an evening object again, eventually disappearing above the southwestern horizon at the end of October - just at the time of maximum brightness. However, during October the altitudes will be less than 10° for mid-European observers.
Although the apparition was very unfavourable, a reasonable number of observations of this comet was published. The analysis - using 15 observations from 5 members of the German Comet Section and 120 international observations - shows that the activity parameter n was neither pre- nor post-perihelion steady. Thus the following formula correlates not very well with the actual brightness evolution:
m = 8.3m + 5×log D + 5.7×log r
resulting in a maximum brightness of 5.3 mag during the last days of October 2007.
The apparent coma diameter increased from 2.5' around Sep. 20 to 6' at Oct. 10. Thereafter it decreased to about 3' around perihelion, where it hovered until the end of the apparition. The absolute coma diameter increased from 175.000 km around Sep. 20 to the maximum of 300.000 km (around Oct. 10). Thereafter it shrank to 100.000 km around perihelion. Until the end of apparition it increased again to 175.000 km. However, it has to be expressed that the comet could only be seen at low altitudes and twilight skies through the whole apparition. The coma was always significantly condensed. At the beginning of the apparition it showed a coma of DC 5, but condensed to DC 7-8 at perihelion. At the end of the apparition it was estimated as DC 6-7. Visually a tail could be seen from the first week of October until mid-November with a maximum length of 0.75° (1.5 Mill. km).
Total Brightness and Coma Diameter
Andreas Kammerer
FGK observations