The Leonids: The Lion King of Meteor Showers

The night of November 12-13, 1833, sparked awareness of the Leonids meteor shower as well as the birth of meteor astronomy: from much of North America that night, a rain of shooting stars, a shower of flashing light, spread over the entire sky. More than one superstitious person on that spectacular night was certain that the end of the world had come. People kept repeating that the meteors were falling "like snowflakes". In the aftermath of the display, it was realized that meteors could be produced by an extraterrestrial source: streams or swarms of particle that travel around the Sun in more or less well-defined orbits, grazing, at least at one point, the orbit of our Earth. In 1866, G. Schiaparelli established the orbit of the stream of particles that produce the Leonids, and soon others independently noted a striking resemblance of the Leonids with the orbit of periodic comet Tempel-Tuttle. The comet and meteor stream were subsequently found to be following nearly identical orbits with periods of roughly 33 years. A few years earlier (in 1863) it was discovered similarly spectacular Leonid meteor displays had occured prior to 1833, with accounts of the Leonids traceable as far back as A.D. 902. Based solely on the 33-year cycle, a prediction for a meteor storm in the year 1866 verified. In 1899 a re-enactment of the 1833 storm was confidently expected, despite calculations that demonstrated that the orbit of P/Tempel-Tuttle (and probably the associated Leonid particles) were likely perturbed by the planets Jupiter and Saturn. The failure of a storm to materialize seriously damaged the credibility of astronomers in the eyes of the general public. Since 1899, the Leonids have been following a rather erratic and unpredictable schedule: meteor storms unexpectedly occurred in 1900 and 1901; no storm was noted in 1931 and 1932, leading many to believe that Leonid activity had significantly declined. But during the 1960s, they again revived, capped by a short-lived display in 1966 that possibly rivaled even the 1833 display. Radar observations of this 1966 display showed the densest part of the Leonid stream to be just 35000 km wide; the Earth swept through this filament of debris in just one hour. With the impending return of P/Tempel-Tuttle due in February 1998, prospect for another Leonid storm have begun to increase again. D.K. Yeomans' definitive study (1981) concerning the orbit of P/Tempel-Tuttle and its implications on future Leonid activity is examined. Yeomans takes into account the distribution of dust surrounding P/Tempel-Tuttle, determining that the majority of dust ejected from the comet evolves to a position lagging behind the comet and outside of its orbit. This is likely an artifact of solar radiation pressure and planetary perturbations on the Leonid particles. In 1994, Yeomans re-calculated the orbit of P/Tempel-Tuttle and re-computed future Leonid shower maxima. Yeomans notes that the conditions in 1998-1999 are optimum for a significant Leonid shower, but cautions that such an event is not certain because the dust particle distribution near the comet is far from uniform. The author concurs with Yeomans on this final point and concludes, based on the previous six Leonid epochs, that there is a possibility of a storm in any year from 1997 through 2000. He also believes that no reliable prediction as to the time of Leonid maximum for any given year can be made because we would be trying to anticipate interception not just a single stream along the orbit of P/Tempel-Tuttle, but possibly one of several: each stream having evolved from the solid debris spewed by the comet at previous perihelion passages. To get a storm, Earth must somehow interact with another dense, yet narrow filament of meteoric material which, unfortunately, cannot be anticipated or seen until it impacts with Earth's atmosphere. Still, the upcoming years hold the potential of some truly exciting observing with the prospects of much-better than normal Leonid activity. All night observing sessions worldwide, which would offer the best hope of catching sight of any unexpectedly strong meteor activity, is strongly urged in the coming years on the night of November 17-18.