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C/2013 A1 (Siding Spring)


On Jan. 3, 2013 R.H. McNaught discovered a comet on images taken with the 0.5m-Uppsala-Teleskop on Siding Spring in the constellation Lepus. Categorized as an asteroid at first R.H. McNaught and other observers noticed the cometary nature in the course of follow-up observations. Comet C/2013 A1 (Siding Spring) showed a strongly condensed 10" coma of total magnitude 18.5 and a hint of a faint tail pointing towards north. It will pass perihelion not until October 2014, when it could be as bright as 7-8 mag. However, at this time it will be positioned in the far southern sky (CBET 3368). It will be brighter than 15 mag between November 2013 and August 2015, when it will move through the constellations Eridanus, Fornax, Horologium, heading for the far southern constellations. Thereafter it will move through Scorpius, Ophiuchus, Hercules, Draco, Ursa Major into Canes Venatici. Mid-European observers could follow the comet only between mid-October and the start of December 2014, less than 10° above the southwestern horiozon at dusk, expected to be of magnitude 8-9. Beginning in January 2015 it should become observable in the morning sky as an object of about magnitude 10.5. During the following weeks it will quickly gain altitude, but will fade continuously.

In spring/summer 2014 the evolution of this comet was followed with great interest by observers in the Southern hemisphere, especially because this comet passed Mars very close, reaching a minimum distance of only 140.000 km on Oct. 19, 2014 (no known comet has come this close to Earth). Starting in October, the comet could also be observed from the Northern hemisphere.

The extremely close passage of Mars by the comet could not only be observed by the Mars probes and professional astronomers, but also by amateurs. However, it remained quite unspectacular since the comet, which did not live up to the expectations, was vastly outshone by the planet. According to 180 observations, the comet showed a complex brightness evolution. Until August 2014 it could be well described by the formula

m = 8.0m + 5×log D + 6.2×log r

Also the estimates between December 2014 and February 2015 are compliant with it. In the time between the comet showed an brightness decrease, slightly at first, but faster from October onwards, resulting in a brightness 1.5 mag fainter than expected according to the above formula in mid-October. Thereafter the brightness increased slowly, then rapidly from the second week of November on, resulting in a brightness 1 mag above the values the formula above predicts in mid-November. This outburst was only of short duration. Between the start of December 2014 and the end of February 2015 the brightness again followed the formula. The maximum brightness of 9.6 mag was reached in mid-September as well as in mid-November 2014. Starting in March 2015 the comet faded much more rapidly, reaching 15 mag in April. This could hint towards an exhaustion of the gas/dust supply due to the increased activity in November 2014. Another explanation would be that the comet has only one major active region, which experienced unfavorable illumination since March.

Total Brightness and Coma Diameter

The apparent coma diameter increased from June 2014 onwards from 1.0' to its maximum value of 4.0' at the beginning of August 2014. This value remained constant until the end of September, with a decrease following until early November to 1.0'. Until mid-November the apparent diameter increased for another time to 2.0', only to decrease again until the end of January 2015 to 1.0'. The absolute coma diameter was 135.000 km in early June and increased until early August to its maximum value of 260.000 km. One week later it decreased significantly to 160.000 km where it remained constant until mid-October, thereafter decreasing further to only 100.000 km in early November. Following a brief increase to 210.000 km it shrunk again ultimately to 110.000 km at the end of January 2015. During the first months the degree of condensation was at DC 2-3, then increased until August to DC 5 where it remained constant until the beginning of 2015, thereafter decreasing to DC 3 until the end of January. Visual tail observations were reported in September/October 2014 with a maximum length of 7' (500.000 km).

Andreas Kammerer


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