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In the course of the Second Palomar Observatory Sky Survey, the 122-cm Oschin Schmidt Telescope at Palomar Observatory (California, USA) accidently photographed this comet on 1989 December 17.19. The faint trail near the center of the image was made by the comet as it moved during the exposure of the Kodak IIIaF (red) plate. The trail was originally found by Maik Meyer (Germany) while searching for prediscovery images of comets. This image was obtained through SkyMorph at the Goddard Space Flight Center.
The Lowell Observatory Near-Earth Object Search (LONEOS) survey program (Arizona, USA) discovered this apparently asteroidal object on 2003 October 16.40. The magnitude was given as 18.8. It was designated 2003 UD16. C. W. Hergenrother (Whipple Observatory, Mt. Hopkins, Arizona, USA) obtained images of the comet on November 30 with the 1.2- reflector and a CCD camera. The magnitude was given as 18.5. After co-adding 900-second R-band CCD exposures, Hergenrother noted a circular, condensed coma 11 arc seconds across. The comet was also accidentally detected on November 30 by the Lincoln Laboratory Near-Earth Asteroid Research (LINEAR) project. LINEAR gave the magnitude as 19.3-19.6.
Prediscovery images of this comet were found by M. Meyer (Germany) while examining Palomar Sky Survey plates. The discovery was announced on 2004 January 7 and the images were located on plates exposed on 1989 December 17 and 1991 February 19. Since the observations were from two separate nights, the link to 2003 UD16 was firmly established and this enabled the assignment of the permanent number, 159P.
The first orbit was actually published while this comet was still thought to be a minor planet. According to Minor Planet Electronic Circular 2003-U57 (issued 2003 Oct. 24) and Minor Planet Circulars Orbit (MPO) 53844 (issued 2003 Nov 09), the orbit was based on 14 positions obtained on October 16, 21, and 23. It indicated a perihelion date of 2004 April 27 and a period of 14.76 years. A new orbit was published on December 3, when the object was officially announced as a comet. The orbit was calculated by B. G. Marsden and used 31 positions obtained during the period spanning October 16 to November 30. It gave the perihelion date as 2004 March 3.63 and the period as 14.33 years. This orbit was essentially confirmed in the following weeks.
Several magnitude estimates were obtained during 2003 December. It was given as 19 by L. Buzzi (Schiaparelli Observatory, Mount Campo dei Fiori, Varese, Italy) on the 8th, 19.0 by M. E. Van Ness (Lowell Observatory, Arizona, USA) on the 17th, 18.9 by W. Hasubick (Buchloe, Germany) on the 23rd, and 19.5 by A. Nakamura (Kuma Kogen Observatory, Japan) on the 28th. Nakamura gave the magnitude as 19.6 on 2004 February 12 and Buzzi gave it as 19.7 on March 17. In both December and February, Nakamura gave the coma diameter as 0.25 arc minute.
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