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P. Jenniskens (SETI Institute, at NASA Ames Research Center), E. Lyytinen (Finland)
Dust trails of Halley-type comets are composed of a large number of narrow dust trailets that result from ejecta during individual returns. The meteoroids remain spatially correlated by librating around mean motion resonances. These dust trailets can be identified from their encounter with Earth during a meteor shower. We calculated comet 8P/Tuttle's dust trail encounters in the same way as for the Leonid showers. We discovered that it takes 6 centuries to change the orbit enough to bring the meteoroids to Earth's orbit. During that time, the meteoroids and comet separate in mean anomaly by 6 years, thus explaining the unusual aphelion occurrences of Ursid outbursts. We predicted enhanced activity on December 22, 2000, at around 7:29 and 8:35 UT from dust trails dating to the 1405 and 1392 return, respectively. This event was observed from California using video and photographic techniques. At the same time, five Global-MS-Net stations in Finland, Japan and Belgium counted meteors using forward meteor scatter. The observations confirm the occurrence of an Ursid outburst, with a maximum at 8:06±07 UT, December 22nd, when activity peaked at ZHR ~ 90. The Ursid rates were above half peak intensity during 4.2 hours. This is only the second time that the meteoroids of a meteor shower could be dated to a specific epoch of ejection from a parent comet. The relative contribution from both dust trails to the outburst is discussed. Implications for past and future Leonid showers are discussed as well, with some emphasis on upcoming Leonid storms in Nov. 2001 and 2002. Part of this work is supported by the Planetary Atmospheres program.