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C/2004 U1 (LINEAR)

Added to the NEO Confirmation Page as a LINEAR discovery late on 2004 Oct 19 this magnitude +18 object in the morning sky was first confirmed by Andre Knofel at the Volkssternwarte Drebach Observatory early on Oct 20. James McGaha at Sabino Canyon Observatory, Tucson recorded it later the same day. Positions obtained from Great Shefford early on Oct 21st were sent though to the Minor Planet Center with a note that there was a suspicion that the object was diffuse and with a possible short tail to the North-West.

A weather system had passed through earlier that day and the seeing was very poor. The first attempt at recording the object was with the CCD set to 2x2 binning (with a pixel scale of 3"/pixel) and a total of 10.5 minutes of exposure with the object 38 above the horizon. It looked possibly diffuse and there were hints of a tail but not definite enough to be absolutely sure.

Another 8 minutes of exposure were taken starting about 45 minutes after the first run was completed. The object was higher in the sky, but still the appearance was possibly diffuse but not certainly so.

40 minutes after the second run had completed a third run was started, but this time with the CCD set to 1x1 binning (with a pixel scale of 1.5"/pixel) and by this time the object had risen to an altitude of 52. The exposures were kept short (16 seconds) to minimise any blurring of the image due to imperfections in the telescope mount and a total of 10mins 40sec exposure  was taken. The resulting image (above) shows a 9" diameter core with some brightness extending to the North-West (upper right). The diameter was slightly larger than nearby stars of similar brightness and the PSF profile was less concentrated than nearby stars (see P/2003 O2 PSF example).

At this point it was decided that it was worth informing the Minor Planet Center that the object might be a comet. IAUC 8421 announced the comet at 18:09 UT on 2004 Oct 21.

 In a retrograde orbit with an inclination of 130, the comet was due at perihelion during the first week of December 2004 at a distance of 2.7 AU from the Sun. In the two months after discovery the distance from Earth would reduce from 2.8 AU at discovery to 1.8 AU in mid December 2004, both effects causing the comet to brighten significantly. During the December dark of the Moon it was being reported by visual observers as bright as mag +13.2.


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