Overview Of Meteor Showers

Meteors can be seen at any time of the year, appearing on any part of the sky and moving in any direction. Such meteors are called sporadic and the mean sporadic rate is very low, no more than about ten per hour. Nevertheless, the flux of sporadics, averaged over a reasonable time span, is greater than the flux from any major stream averaged over the same time span. The major streams appear at well-determined times each year with the meteor rate climbing by two or three orders of magnitude. For example around 12 August meteors are seen at a rate of one or two per minute all apparently radiating from a fixed well determined point on the sky, called the radiant. This is the Perseid meteor shower, so named because the radiant of this shower lies in the Constellation of Persius. This behaviour is generally interpreted in terms of the Earth passing through a stream of meteoroids at the same siderial time each year. Olmstead [6] and Twining [7] are credited with first recognizing the existence of a radiant. Many of the well-known showers are rather consistent from year to year, but other are not. The best-known of these latter is the Leonids, where truly awesome displays are sometimes seen. For example, in 1966, tens of meteors per second were seen. Records show that such displays may be seen at intervals of about 33 years, with the displays of 1799, 1833 and 1966 being truly awesome, but good displays were also seen for example in 1866 and 1999. These early spectacular displays helped Adams [8], LeVerrier [9] and Schiaparelli [10], all in 1867, to conclude that the mean orbit of the Leonid stream was very similar to that of comet 55P/Tempel — Tuttlea,nd that 33 years were very close to the orbital period of this comet. Since then comet-meteor stream pairs have been identified for virtually all recognizable significant stream.

These simple facts allow a model of meteor showers and associated meteoroid streams to be constructed. Solid particles (meteoroids) are lost from a comet as part of the normal dust ejection process. Small particles are driven outwards by radiation pressure but the larger grains have small relative speed, much less than the orbital speed. Hence these meteoroids will move on orbits that are only slightly perturbed from the cometary orbit, hence in effect generating a meteoroid cloud about the comet which is very close to co-moving with the comet. As the semi-major axis of each meteoroid will be slightly different, each will have a slightly different orbital period, resulting in a drift in the epoch of return to perihelion. After many orbits this results in meteoroids effectively being located at all points around the orbit. With each perihelion passage a new family of meteoroids is generated, but, unless the parent comet is heavily perturbed, the new set of meteoroids will be moving on orbits that are almost indistinguishable from the preexisting set. Various effects, drag, collisions, sputtering, will remove meteoroids from the stream, changing them to be part of the general interplanetery dust complex and seen on Earth as Sporadic meteors.

An annual stream is thus middle-ages, with meteoroids having spread all around the orbit so that a shower is seen every year. In a very old stream where the parent comet may not still be very active, the stream is never very noticeable, but again constant each year. A very young stream on the other hand will only show high activity in certain years only since the cloud of meteoroids has had insufficient time to spread about the orbit.

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