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Sun, 31 Jul 2005 - SPA Comet News, July 2005

Comet 2004 Q2 (Machholz) put on a good show in the early spring, but comets 2003 T4 (LINEAR) and comet 9P/Tempel rather failed to live up to expectations as far as visual observation was concerned. Many members had a look at comet Machholz, particularly when it tracked through the Pleiades and it looks as if it will get into the top five of visually observed comets. Prospects for the next six months are not particularly good, though comet 2005 E2 (McNaught) may reach 9th magnitude after Christmas.

On July 4th, comet 9P smashed into the Deep Impact target, which took spectacular images before its demise. The images are still being analysed, but should shed much new light on the origin, structure and evolution of comets. As far as visual observations were concerned, the comet became more condensed immediately after the impact, but its total magnitude did not increase significantly. This was roughly what I predicted on the basis that the impact would not significantly increase the total area of the comet’s surface that was active. Further news will be published in the astronomical press over the coming months.

The asteroid search programmes have thrown up a number of interesting objects that may or may not be comets. Asteroid 2003 WY25 may be a fragment of lost comet D/Blanpain, which has not been seen since 1815 and to the Phoenicid meteor shower, which put on an intense display in 1956. There may be another display from the shower this autumn, but the radiant is too far south for observation from the UK. It is possible that the fragmentation occurred just before the 1815 apparition of the comet. Asteroid 2005 NA56 may be linked to comet 41P/Tuttle-Giacobini-Kresak, but if so the fragmentation must have occurred over 500 years ago. The comet experienced a series of encounters with Jupiter in the sixteenth century, which reduced its perihelion distance from around 1.5 AU to 1.0 AU. Asteroid 2005 NA82 is in a short period retrograde orbit and can approach quite closely to Jupiter, suggesting that it could be an extinct comet. Asteroid 2005 OE is in a highly inclined long period orbit, with a period of 260 years and perihelion at 2.8 AU.

The SOHO spacecraft is closing in on its 1000th comet, which is likely to be found in the next month. The vast majority of these are from the Kreutz group of sungrazing comets, which can produce spectacular naked eye objects, but none this bright have been seen since 1970. Two other groups of SOHO comets known as the Marsden and Kracht groups of sunskirting comets have tentatively been linked together in an evolutionary hierarchy of fragmentation. It seems that the Marsden fragments are in a periodic orbit of about 5.5 years, and the groups are also linked to comet 96P/Machholz and the Quadrantid and daytime Arietid meteor showers. Michael Oates has been overtaken as the leading discoverer by German amateur Rainer Kracht who now has 169 comets compared to Michael’s 145. French amateur Xavier Leprette has 131.

Jonathan Shanklin

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