The Leonids and the Comet:- History and Theory

Extract

The recorded history of the appearances of spectacular Leonid displays have been discussed for example in Yeomans (1981) and also in other papers at this meeting. The similarity in the orbits and that of comet 55P/Tempel-Tuttle is a strong indication that they are related. Further proof of this comes from the fact that the most spectacular displays associated with this stream are seen at time intervals that are multiples of the orbital period of the comet. The explanation for the occurrence of storms is thus that a well populated group of meteoroids are located close to the parent comet. Such a grouping of meteoroids would be formed if the comet were still active, not an unreasonable assumption. However, this raises an other question, namely why have the older of these meteoroids ejected perhaps several thousand years ago, not spread about the orbit. Standard models (eg Williams 1992) suggest that in 20 or so orbits meteoroids will have spread all the way from being close to the comet to being at the opposite point of the orbit. Hence, meteors in similar numbers should be seen every November coming from the Leonid stream. This is not the case, the average zenithal hourly rate of meteors in non-storm years being less than 10 as opposed to several thousand during a storm.