Leonid Meteor Shower
A particle in space which collides with the Earth's atmosphere is termed a meteoroid. A typical meteoroid is the size of a grain of sand. Friction generated by the passage of a meteoroid through the Earth's upper atmosphere can cause it to become so hot that it glows incandescently and becomes briefly visible before it vapourises; when it becomes visible in this way it is termed a meteor. When the Earth encounters a trail of dust and debris left behind by a comet, the relatively high density of particles can result in a correspondingly elevated rate of meteors becoming visible: up to one a minute or exceptionally much more. The term meteor storm is used when the meteor rate is many thousands per hour. The comet responsible for creating a meteor storm is termed the parent comet.
The Leonid meteor shower occurs around 17-18 November each year when the Earth passes trough the trail of dust and larger particles left by its parent comet, Tempel-Tuttle (1866I). The cometary nucleus ejects the dust when it is close to the Sun. Most of the dust travels in orbit around the Sun following very closely behind the comet itself: thus when the comet is near aphelion (furthest from the Sun) meteor rates are very low and when the comet is close to perihelion (closest to the Sun) rates are generally very much higher. Incidentally, the fact that so much dust is close to the comet in its orbit indicates that the comet has not been in orbit around the Sun for very long; eventually, the dust will become smeared out along the entire path of the comet and meteor rates will be constant year after year.
The orbital period of Tempel-Tuttle is 33.2 years and peak Leonid meteor rates occur with the same average periodicity. Spectacular Leonid meteor storms were witnessed in 1799, 1833 and 1866. In 1899 and 1933 hopes were high but disappointingly low rates were observed. In 1966, fortunate observers in North America saw perhaps the most spectacular Leonid meteor storm ever, when the rate peaked at about 15,000 meteors in 20 minutes! Meteor storms do not usually last all night, and indeed the main peak of activity in 1966 lasted for only about 40 minutes.
Tempel-Tuttle is in retrograde orbit around the Sun (i.e. in the opposite direction to the orbits of the planets and other major bodies in the Solar System). The dust in the comet's path is also in retrograde orbit and therefore collides with the Earth's atmosphere at high speed: this results in meteors which are bright and often leave trains of ionised gas, some produced from the air and some from the disintegrating meteoroid. Leonid meteors sometimes fragment and sometimes explode in a terminal flash which can have a distinct colouration and can (if the meteoroid is large) produce a sonic boom.
Observing reports by members of OASI: