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.
The comet experienced a very different evolution pre- and post-perihelion, based on 8 observations by 3 members of the German Comet Section and 85 international observations (until the start of June 2010). Pre-perihelion the comet showed a slow evolution, peaking at 9.6 mag in mid-November 2009. After perihelion the comet faded surprisingly fast. The appropriate formulae are:
pre-perihelion: m = 4.9 mag + 5×log D + 7×log r post-perihelion: m = 0.3 mag + 5×log D + 19.8×log rThe coma diameter increased from 1.5' (175.000 km) in February 2009 to a maximum of 4.5' (475.000 km) between mid-November 2009 and mid-January 2010. Thereafter it shrunk, measuring 1' (150.000 km) at the beginning of June. The coma showed a medium degree of condensation (constant at DC 4-5). Visual tail sightings were reported between the end of October 2009 and the start of March 2010, reaching 10' (3.5 Mio. km), with the tail pointing towards west.
Total Brightness and Coma Diameter
On Mar. 13. 2010 F. Colas (Paris Observatory) discovered a fragment of magnitude 18 at a distance of 6" from the primary. On Mar. 17 the distance between the primary and fragment B had shrunk to 3". On Apr. 12 it could not be detected, suggesting its disintegration (IAUC 9135).
Andreas Kammerer
