An apparently asteroidal object of magnitude 18.0 discovered in the course of the Siding Spring Survey on Aug. 25, 2007 near the border of the constellations Horologium/Eridanus by D.M. Burton showed its cometary nature at closer inspection. Comet C/2007 Q3 (Siding Spring) displayed a 6" coma and a 12" tail in p.a. 290°. The comet with a rather high absolute magnitude will pass perihelion in fall 2009 - unfortunately at the rather far distance of 2.3 AU (IAUC 8865 / MPEC 2007-R59). Assuming an average brightness evolution it could reach 10 mag. For mid-European observers it will appear - nearly at maximum brightness - above the eastern morning horizon at the beginning of October 2009. Until May 2010 it should be brighter than 12 mag, thereby moving from the southern parts of Leo through Coma Berenices and Bootes into Draco, becoming an object of the whole night.
At the start of February 2010 the evolution of this comet can be based on only 4 observations by members of the German Comet Section and 50 international observations. They show a brightness evolution below average, according to the formula
m = 6.2 mag + 5×log D + 4.6×log ryielding a maximum brightness of 9.8 mag in mid-January 2010.
The coma diameter increased from less than 1' (150.000 km) in February to 4' (525.000 km) in June, peaking at 4.5' (525.000 km) in November 2009. Thereafter it shrunk, measuring 3' (325.000 km) in mid-February 2010. The coma showed a medium degree of condensation (nearly constant at DC 4-5). Visual tail sightings were reported between October and December 2009, reaching 10' (3.5 Mio. km), with the tail pointing towards west.
Total Brightness and Coma Diameter
Andreas Kammerer
