On images taken by the All-Sky Automated Survey (ASAS) on Sep. 3, 2004 a comet was discovered in the constellation Canis Major, close to Sirius. The used instrument is a CCD camera behind a 200 mm telephoto lens with 70 mm aperture. Alan Hale estimated the brightness of comet C/2004 R2 (ASAS) to be 10.9 mag on Sep. 9, while the 2.5' diameter coma was diffuse with no central condensation. John Drummond (New Zealand) observed the comet on the same day and estimated it at 9.8 mag with a diameter of the moderately condensed coma (DC 4) of 2.5'. The comet will pass the Sun on Oct. 7 at the very small distance of 0.11 A.U. (IAUC 8402). Since the comet has an absolute magnitude of only 10-11 mag it may dissolve before perihelion passage. From mid-Europe it was a difficult morning object for a few days after discovery. It rapidly disappeared in the dawn and may reappear above the evening horizon in mid-October - but this is rather unlikely due to its faint absolute magnitude - moving from the constellation Serpens towards the direction of Delphinus.
As expected, the comet did not survive its close proximity to the Sun. Its approach to the sun could be followed via the SOHO satellite, which showed a significant fading from day to day until it disappeared completely. Some optimists tried to explain this with the SOHO filter gradient, but the pictures told a different story. In fact, after perihelion not a single trace of the comet could be found. On Oct. 24 Dieter Schubert tried in vain to recover it with his 106quot; Schmidt-Cassegrain (comet altitude 24°) and estimated it as clearly fainter than 10 mag. On the same evening Michael Jäger did not find any diffuse object brighter than 14-15 mag at the expected position.
Due to the small number of observations (only 25 international ones) the evolution of this comet can only be derived crudely. The brightness evolution can be best described with the formula m = 6.8m + 5×log d - 0.105×|t-T|. At the time of its disappearnce into the twilight the comet had reached a brightness of 8 mag.
Total Brightness and Coma diameter
However, a brief look at the evolution of the heliocentric brightness over r shows clearly, that this comet was running short of reserves. The activity decreased continuously and the last observation even indicates a drop - generally a sure indicator for the start of disintegration of a comets nucleus.
Evolution of the heliocentric magnitude
Similar to the brightness increase from 10 mag to 8 mag between Sep. 10 and 28 the coma diameter increased from 2.5' (100.000 km) to 5' (190.000 km). Interestingly the coma became ever more condensed until short after Sep. 20, with the degree of condensation increasing from DC 2-3 to DC 6. Only during the very last days of its apparition it seemed to have become more diffuse (decrease to DC 3-4).
Andreas Kammerer
