An asteroidal object of magnitude 19, discovered by the Catalina project on Oct. 31, 2013 in the constellation Aquarius, showed a miniscule coma in PanSTARRS images, which was confirmed by other observers. Comet C/2013 US10 (Catalina) was still 8 AU from the Sun at discovery, but will pass perihelion at a distance of only 0.83 AU in mid-November 2015, when it could reach 5 mag (CBET 3688). For mid-European observers the comet will appear above the southeastern morning horizon at the beginning of December 2015 (maximum brightness), positioned in the constellation Virgo. During the following weeks it will move northward, reaching Draco and Camelopardalis in January/February 2016, when it should still be an object of magnitude 7.
This comet was - without doubt - the most interesting cometary object in winter 2015/16. Thus it is no surprise that 102 observations from 11 members of the German Comet Section and 685 international observations could be used for the analysis. Pre-perihelion the brightness estimates can't be well represented by one standard formula. On the other hand, the width of the confidence band allows only marginally a division in distinct phases. Nevertheless, it is obvious, that the estimates are consistently too faint - in comparison with the formula - between early May and early July (difference in early July: +0.5 mag), but too bright thereafter, with the difference reaching -0.6 mag in early September. During the last weeks before conjunction with the Sun the estimates are too faint again (early October: +0.6 mag), which can only partly be explained with the then deteriorating observing conditions. Furthermore the post-perihelion brightness estimates are consistently fainter, compared to the pre-perihelion formula. So far, post-perihelion the comet shows a steady brightness evolution. The maximum brightness of 6.2 mag was reached around Jan. 12, 2016. At the end of the first week of April 2016 the comet had faded to 11.0 mag. The corresponding brightness formulae are as follows:pre-perihelion: m = 4.9 mag + 5×log D + 8.8×log r post-perihelion: m = 6.0 mag + 5×log D + 7.0×log r
The apparent coma diameter started to increase rapidly in May 2015. Then only 1' it reached a first maximum of 14' in mid-August, with this value remaining constant until mid-September. Nearing its conjunction with the Sun it decreased systematically, reaching 7' in mid-October. At its reoccurrence it showed a compact coma of diameter 4'. Approaching Earth the apparent coma diameter increased rapidly, reaching a maximum of 17' when closest to Earth (0.725 AU on Jan. 17, 2016), only to decrease immediately thereafter, measuring short of 4' in mid-March and 1.5' at the start of May. In absolute dimensions the coma began to expand significantly in May 2015, swallowing from 160.000 km to 800.000 km at the end of September (rapid increase until August, getting slower thereafter). After the reappearance it measured only 200.000 km (a result of the then strong solar wind), expanding rapidly during the following weeks, reaching a second maximum of 550.000 km in mid-January 2016. This value remained constant until mid-February, thereafter decreasing to 350.000 km until mid-March and to 250.000 km in early May. Between April and July 2015 the degree of condensation increased from DC 4 to DC 5-6. Then, surprisingly, a longer period followed during which the coma got steadily more diffuse, reaching DC 3-4 in September (despite a higher-than-average activity). Thereafter the coma got more condensed again, reaching DC 5 when it disappeared. Post-perihelion the degree of condensation was DC 5-6 at first, decreasing constantly to DC 1-2 until the beginning of May. Post-perihelion the coma showed a false nucleus, which was prominent at first, but still pronounced thereafter, reaching a brightness of about 11.0-11.5 mag.
Total Brightness and Coma Diameter
Whereas the comet showed only a gas tail pre-perihelion, it presented a very interesting tail morphology post-perihelion, which, however, could only be seen clearly under a dark sky. The gas tail was reported visually from July 2015 onwards, with lengths not exceeding 20'. Post-perihelion the gas tail could be seen visually with a length of up to 1.5° (7 Mio. km) - however, only for observers under a very transparent sky (only the first 40' could be seen under a moderate sky). The southward oriented dust tail could be seen up to 0.5°. During the first weeks after perihelion both tails were linear and showed a high surface brightness - a result of the comet's passing of the ecliptic plane on Nov. 26 and Earth passing the comet's orbital plane on Oct. 1, resulting in a larger number of dust particles within the line of sight. The angle between both tails was 145° at the end of November, but decreased in the following weeks to 90° in mid-January and to only 30° in early February. Until mid-December the gas tail displayed itself very dynamic in images: beside multitude rays several tail clouds and turbulences were recorded. From mid-December onwards, photographically rays alone have been recorded. From mid-January onwards the gas tail became inconspicuous whereas the dust tail now became dominant. It then displayed a more diffuse appearance and got curved. The visual tail sightings ended at the end of March 2016. While the orientation of the gas tail changed from West over North to SE pre-perihelion, it pointed quite constantly to WNW until the end of January when it started to turn to ENE quite rapidly. Between end of November and mid-January the dust tail's orientation changed only from SE to South, but then rather rapidly towards East until early February.