Leonid Dust Trail Theories

At the end of the 19th century, Irish astronomers Johnstone Stoney and A.M.W. Downing showed in principle how to predict meteor storms. Their work and later developments applying similar ideas are reviewed here. The theories essentially involve determining the dynamical behavior of dust trails, which are narrow, dense structures within a broader stream. The formation of dust trails and the main features of their evolution are explained. Some Leonid dust trails are perturbed by the planets so as to result in meteor storms at the start of the 21st century. 1 A hundred years: from Stoney and Downing to the 1999 Leonid storm 1.1 IRAS trails and meteor storms In 1983 the Infra-Red Astronomical Satellite (IRAS) discovered narrow, dense trails of me-teoroids and dust near the orbits of some known short-period comets. It was clear that there are similar trails associated with many periodic comets, those detected by IRAS generally being in cases where the comet was near perihelion during the IRAS mission. It was recognized that the Earth's passages through such dense trails correspond to the occurrence of meteor storms (see Sykes & Walker 1992, Kress ak 1993). As the trails are narrow, it follows that at any given time most trails will not intersect the Earth's orbit in 3-dimensional space. However, the gravity of the planets continuously perturbs the orbits of dust and meteoroid particles in the trails, and from time to time, a trail (or a subsection of a trail) is brought to Earth intersection. A meteor outburst results, provided that the Earth and the particles in that subsection of the trail reach their intersection point at the same time. Therefore by calculating the gravitational perturbations on sections of trails that might come near the Earth, meteor storms can be predicted. This idea has been considered at various times, albeit not always using the term`dust trail' to describe the theory involved. Indeed, some of the studies in question date back to before IRAS detected the cometary dust trails. The orbit of the Leonid parent comet, 55P/Tempel-Tuttle, comes closer than most comet orbits to the Earth's orbit. The total perturbation required to bring Leonid dust trails to Earth intersection is therefore smaller than for trails associated with most other comets, and the Leonid stream provides a primary candidate for meteor storms.