On Oct. 14, 2012 T.H. Bressi discovered an 18.5 mag comet in the constellation Taurus with the 0.9m Spacewatch telescope. Comet C/2012 T5 (Bressi) showed a diffuse coma and a faint 9" tail in p.a. 260°. Additional observations showed a 12x16" coma of magnitude 17.5 and a broad 45" tail in p.a. 275°. This comet will approach the Sun to within 0.32 AU at the end of February 2013. Alas, it is an intrinsically faint comet, thus expected to reach only magnitude 7.0 at perihelion (CBET 3261 / MPEC 2012-U109). According to my empiric formulae it should then display a 4' coma and a 0.7° tail. However, it is very uncertain if it will survive perihelion because its absolute magnitude is well below the Bortle limit. The comet should reach magnitude 15 in mid-November 2012, disappearing above the southern evening sky for mid-European observers in mid-January 2013, when it is expected to be of magnitude 12. Within this time span it will move from the extreme northeastern to the extreme southwestern corner of the constellation Cetus. In the case it should pass perihelion safe it should reappear as a 9-10 mag object in the morning sky at the start of March, being situated in the western parts of Pegasus. Thereafter it should fade quickly, reaching magnitude 15 at the end of April, when it has moved into Lacerta.

As predicted the comet disintegrated prior to its perihelion passage. Based on only 30 international observations the apparition can be described only approximately. The brightness evolution is best represented by a time-dependent formula, which is not atypical for a disintegrating comet. In addition, the short-term brightness outburst at the end of January and the start of February, with the comet reaching 9.0 mag is typical for a fragile comet. With the exception of the outburst the brightness evolution until early February can be approximately described by the formula

The last visual observations seem to indicate the onset of the fading, caused by the beginning disintegration, but at that time the comet had already plunged into twilight. Post-perihelion it could not be detected visually. A deep CCD-image on Mar. 20 shows the comet as a diffuse 18 mag blob. The coma diameter seems to have increased slightly from 1.5' (65.000 km) to 2.0' (90.000 km), while the coma grew more diffuse (DC 5 towards DC 3).

Total Brightness and Coma Diameter

Andreas Kammerer