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BAA electronic bulletin No. 00443
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ORIONID METEORS: ENHANCED ACTIVITY THIS YEAR? The Orionid meteor shower will be active throughout the coming week and until the end of October. One of two annual showers associated with Comet 1P/Halley (the other being the Eta Aquarids of early May), the Orionids are a moderately active shower, usually producing observed rates in excess of 10 meteors/hr around their maximum. In the years from 2006-2008, observers in North America, particularly, reported higher-than-normal Orionid meteor rates, and this month there is chance that enhanced activity may be observed again. With New Moon on October 18 and First Quarter on October 26, there will be no interference from moonlight this year. In a recent IAU electronic telegram (CBET 1976: 20091017) Peter Jenniskens, SETI Institute, reports that the strong Orionid meteor shower activity may well repeat this year, according to M. Sato and J.-I. Watanabe (2007, PASJ 59, L21). They ascribed the enhanced activity in the period 2006-2008 activity to dust trails of comet 1P/Halley that were formed by meteoroids ejected in the years -1400 and -11. The orbital evolution of the dust is affected by the 1:5 to 1:8 mean-motion resonances with Jupiter. This so-called "filament" component is expected to be in the Earth's path again around Oct. 18-24 in 2009, giving rise to a higher-than-normal Orionid-shower activity that is relatively rich in bright meteors. Thanks to the filamentary nature of the debris stream laid down by the parent comet, activity can vary markedly from one year to another: good rates can be experienced if Earth encounters a rich meteoroid filament, but at other times activity might seem disappointing. Several sub-peaks are usually seen between October 20-22, and intervals of slightly increased activity can be found even as late as October 27-28. Like 1P/Halley, stream meteoroids have a retrograde orbit around the Sun, meaning that they enter the upper atmosphere on 'head on' collisions at the high velocity of 66 km/sec. Orionid meteors are very swift, and the brighter ones, particularly, often leave behind brief persistent ionisation trains. Having been laid down over numerous returns of 1P/Halley, the Orionid meteor stream is quite spread out, and this is reflected in the shower's diffuse (probably multiple) radiant. Orionid meteors emanate from a region of sky midway between Betelgeuse (Orion's eastern 'shoulder') and the second-magnitude star Gamma Geminorum. The radiant doesn't rise until 22h local time, and best rates are generally found in the early morning hours once it has gained somewhat in altitude.
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