Astronomy Now’s Nick Howes today took an image of the presently 11th magnitude comet C/2007 Siding Spring that suggests the nucleus of the thing might be breaking up. Images he took with the Faulkes Telescope North suggest a dense clump has formed behind the main nucleus. The clump is moving with the comet, suggesting it isn’t something in the background creeping into an image. The image below is a composite of 6 minute long exposures taken at 15:00 UTC:
Astronomers hope this could be the onset of an outburst such as that of Comet 17P/Holmes, which saw parts of the nucleus blasted off and the brightness of the comet flare dramatically.
The comet is one of four listed by the website Heavens Above as being above twelfth magnitude (and so worth looking out for in moderate telescopes). The finder charts for these are located at: C/2007 Q3 Siding Spring in the constellation of Draco (Mag 11.0 according to the website); C/2009 K5 McNaught (Mag 10.4) in the constellation of Aquila; 81 P Wild2 (mag 9.4) between Libra and Virgo; C/2009 O2 Catalina (mag 9.2) in Lacerta somewhere near Pegasus.