On Mar. 13, 2007 G.J. Garradd discovered a comet on images taken in the course of the Siding Spring Survey in the southern parts of the constellation Libra. Comet C/2007 E1 (Garradd) presented a 20" coma of magnitude 15, displaying a central condensation, which was elongated towards northeast. The comet will reach its perihelion at the end of May (then passing through the constellation of Gemini, which will by this time be situated already low in the evening sky). During the weeks before it will (has) passe/d the constellations of Virgo, Corvus, Crater, Leo Sextant and Cancer. A maximum brightness of about 13 mag was expected at the beginning of April. According to the current orbital data it orbits the Sun in about 475 years (IAUC 8818 / MPEC 2007-G14).
To everyones surprise, images taken at the beginning of April showed a 15' coma of magnitude about 9. Visual observations with large instruments during the Easter days resulted in brightness estimates of 11 to 12 mag with coma diameters of a few arc minutes. However, observations taken with binoculars at the same time showed a 10' coma of very low surface brightness of magnitude 8 to 9. Thus this comet resembles comet 177P/Barnard during summer 2006.
Due to the extremely diffuse coma, comet C/2007 E1 (Garradd) was a most difficult object during the whole apparition. Therefore it is not surprising that, although it was visible at convenient times, only 19 reports from 6 members of the German Comet Section were received. For the analysis additional 50 international observations could be used. Although an outburst cannot fully be ruled out during the first days, it is rather unlikely. Favouring this suggestion is the extremely high activity parameter. However, it is more likely that the observers did not realize the whole extent of the extremely diffuse coma during the first days, resulting in an underestimate of the brightness. Excluding this very first estimates the brightness evolution can be described rather well by the formula
m = 6.3m + 5×log D + 27.5×log r
The rather faint estimates at the end of the apparition may be explained by the less favourable observing conditions at this time. The maximum brightness of 9.3 mag was reached on Apr. 10 - about one week after perigee (0.496 AU on Apr. 4).
Total Brightness and Coma Diameter
The apparent coma diameter measured - neglecting the very first estimates - about 9' at first, but decreased then continuously to 5' around Apr. 20 and to 1.5' at the end of May. Again it may be assumed that the last value is too small due to the then unfavourable observing conditions. Neglecting the first and last estimates the absolute coma diameter increased rapidly during the first days from 150.000 km to 200.000 km, where it remained. The extremely diffuse coma showed a constant degree of condensation of DC 1-2. At times a small knot of material could be seen in the centre. No visual tail sightings were published.
Andreas Kammerer
FGK observations