On Apr. 10, 2010 R.E. Hill discovered a 18.5 mag comet in the constellation Hercules, near the border to Ophiuchus, on Catalina Sky Survey images. Comet C/2010 G2 (Hill) showed a diffuse 6-8" coma and a 20" fan-shaped tail in p.a. 200°. It will reach perihelion at the start of September 2011, predicted to peak at 12 mag. It will be brighter than 14 mag during the period March 2011 and the end of 2011, during which it will move through Draco, Cepheus, Cassiopeia, Camelopardalis, Lynx, Auriga, Gemini/Taurus, Orion into Eridanus. On May 15/16 it will pass the North Pole at an distance of just 1.5° thereby passing Polaris at an distance of only 0.7°. Most of the time it will be observable in the morning sky, becoming an evening object near the end of the mid-European visibility. The comet rounds the sun with a period of about 950 years (IAUC 9134 / MPEC 2010-L69).
The comet evolved better than expected, but did not experience a standard evolution, as can be derived from 18 observations by 6 members of the German Comet Section and 160 international observations. According to these the brightness evolution can only be described satisfactorily by using time-dependent formulae:
pre-perihelion: m = 9.6 mag + 5×log D - 0.017×(t-T) post-perihelion: m = 9.3 mag + 5×log D + 0.002×(t-T)These yield a maximum brightness of 10.0 mag around Nov. 20, 2011.
Total Brightness and Coma Diameter
The coma diameter, too, showed an unusual evolution: At first, the apparent diameter increased very slowly from 0.5' at the beginning of the apparition to 1.5' in July 2011. Beginning in August it increased significantly, reaching the maximum of 5.5' at the end of September. This sudden increase cannot be explained by a change of instrumentation! Thereafter the coma diameter decreased, measuring 1.0' at the beginning of March 2012. In absolute terms the coma increased from 75.000 km at the start of the apparition to 150.000 km at the end of July 2011. Starting in August it swallowed rapidly, reaching 450.000 km in mid-September. Between October 2011 and January 2012 it measured 325.000 km. Thereafter it shrunk significantly, measuring 150.000 km at the beginning of March. During the whole apparition the coma grew ever more diffuse. Estimated as DC 6-7 at the start of the appariton, the degree of condensation steadily decreased to 4 until the end of June. Since the end of 2011 the degree of condensation was estimated as DC 3. Visual tail sightings were reported between September and November 2011, with the length not exceeding 7' (1.5 mio. km).Andreas Kammerer
