In images, taken with the 0.4m-MASTER-telescope of the South African Astronomical Observatory on Apr. 7, 2015, a comet was discovered near the border of the constellations Capricornus/Aquarius. Comet C/2015 G2 (MASTER) showed a 1.5' coma, elongated towards West, of total magnitude 10.5. Additional CCD observers reported a coma diameter of 3' and a 15' tail in p.a. 253°. Visual observers estimated the comet to be of magnitude 10.0, showing a coma about 2' in diameter. The comet will pass its perihelion (at solar distance 0.78 AU) in the final days of May, but prior to this it will approach Earth to within 0.47 AU on May 13 (CBET 4092). Assuming an average brightness evolution the comet should reach 6.0 mag during perigee and 6.5 mag around perihelion. Alas, it will move through Sculptor, Fornax, Eridanus and Lepus during the most interesting weeks, being not visible from mid-European locations. For mid-European observers the comet will appear above the easterly morning horizon not until the start of September, then expected to be of magnitude 13.
This comet, during spring and summer observable only from the Southern Hemisphere, reached a maximum brightness of 5.9 mag around May, 15. Its activity parameter is slightly below average, as indicated by 135 international observations. Based on these the comet evolved according to the formula
m = 8.4 mag + 5×log D + 8.8×log r
Total Brightness and Coma Diameter
The coma diameter was 2.5' at the start of the apparition. During the following weeks it increased considerably, reaching a maximum of slightly more than 10' in mid-May. Thereafter it decreased equal rapidly, measuring short of 3' at the opening of June. This diameter was hold until the comet disappeared in the twilight. Thus the comet showed a constant absolute coma diameter of about 250.000 km during the whole apparition. Equally constant was the degree of the condensation, which was estimated as DC 5 during the whole apparition. Thus this comet did not condense despite the proximity to the Sun during its perihelion. During May a tail could be glimpsed visually, reaching a length of 0.4°.
Andreas Kammerer
