Perseids 2006: prediction of activity
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Perseids are one of the most famous meteor showers. Perhaps the only other shower can rival them in popularity - Leonids in November known with their phenomenal storms. The reason is in regular and very high level of activity as well as summer time in northern hemisphere (where Perseids are best seen) when the shower is active. Comfortable weather allows very large number of observers watch it.
The parent comet of the shower is 109P Swift-Tuttle having orbital period about 135 years. The comet passed its last perihelion in December 1992, which caused considerable increase in Perseid activity in the 1st half of 1990s, up to 600 meteors per hour. Primary maximum, produced by the fresh comet material was being observed till the end of 1990s, gradually decreasing to 100-120 meteors per hour. It had ceased to appear since 2000. In general the future shower activity is to decline as the parent comet is now moving into the outer layers of the Solar system. It doesn't cancels however the possibility of bursts in separate years.
Principally when a comet came close to the Sun (and to the Earth so far), our planet meets more dust ejected by it and we see more meteors. But this picture is a very big simplification of the real situation as during each perihelion passage the comet ejects material which soon forms itself into a dust trail. These trails continue moving in space and can keep their structure during a long time. So far the stream consists of a number of trails with slightly differing orbits. This means that the shower activity can vary greatly from year to year.
Can we compute the movement of these trails and make a prediction of shower activity? We can if we have enough volume of information. Such computations are most frequntly done for the Leonids as this shower is studied much better than the Perseids, for which much less predictions were done. For example, Esko Lyytinen issued a prediction for the Perseids 2004, namely for the encounter with 1 rev. trail at 20:50 UT 11 August. Around this time Perseid activity increased up to about ZHR=170 (ZHR - see below), which was considerably higher than usual levels of maximum (ZHR=100-120).
As in case with other showers, evolutuon of Perseid stream is traced with the modelling very good. However, the Perseids have the following distinctive feature: the main part of trails passes too far outside of the Earth orbit and so far observed Perseid activity is almost totally caused by bachground material. Now we have only one more or less clearly fixed case of trail encounter - already mentioned Perseids 2004.
Particles ejected by the comet form lengthy trails. One of the reasons is radiation pressure force, which acts parallel with gravitational force. The latter is dependent on a particle mass, i.e. it is proportional to the third power of particle radius. The outcrying radiation pressure is defined by the second power of particle radius. So far the influence of radiation pressure is the more the less size of a particle is. Its influence is equivalent to the diminishing of gravitational constant G. So it increases the orbital period of particles, and the tinier a particle is, the more it is continuously retarded from larger particles after their ejection from the comet. This process therefore leads to the formation of lengthy comet trails.
Meteor modelling is done through computation of orbital evolution of particles ejected by the comet with different velocities in directions tangential to the comet trajectory at the moment of perihelion. In the reality, of course, particles are ejected not only at the point of perihelion, but also during several months around it. However, comets are close to the perihelion during quite a little time comparing to their overall orbital period and main perturbations happen around their aphelions, so when comets are closer to the Sun newly ejected particles are moving very close to them in a compact dust cloud. This is the reason we can take that cloud as completely ejected at the point of perihelion.
Speaking of directions in which particles are ejected we can say that, again, in the reality they are ejected far not only in tangential directions, but in all possible ones. However, ejection velocities (from 0 to 100 m/s, and the overwhelming majority of real ejections - from 0 to 20 m/s) are negligibly small comparing to the own comet velocity (from 30 to 40 km/s) near the Earth's orbit), ejected particles have only slightly changed orbits and don't "fly away in all directions". Radial part of ejection velocity defines only thickness of a trail, which usually reaches several hundred of thousand kilometers. The shape of the trail is defined by tangential part of ejection velocity.
And the last. Non-gravitational forces are often not taken into consideration in meteor calculations, as is in our case. However, some of them, say, radiation pressure, can be considered indirectly. As far as this kind of force works as diminishing of gravitational constant G, this is equivalent to increase of ejection velocity which could be easily accounted in the model. So this non-gravitational force, as many others doesn't change the configuration of trails, but leads to shifting of particles with different masses along them.
As spoken previously, Perseid trails modelling allowed to prepare very good predictions of shower activity in the previous years. More serious problem is prediction of outburst intensity - how strong the maximum could be. For such predictions special empirical models were elaborated (the single possible way in this case) but as before for their improvement new observations are very necessary.
The results obtained by the Author for the Perseids 2006 using the modelling of particles ejected by the comet 109P Swift-Tuttle are presented below. Main characteristics of computations are also described.
I wish to introduce the results of Perseids meteor stream simulation aimed to the prediction of shower activity in 2006. The simulation was made for trails of latest 7 revolutions, i.e, from the 1992 trail. The Author used the program "Comet's Dust 2.0" by S. Shanov and S. Dubrovsky to calculate orbital elements of ejected meteor particles. To estimate expected ZHRs for different encounters the model described in  was used with some Author's alterations made in order to adopt the model for ejection velocity (v) instead of da0 (difference in a-semimajor axis) and to turn the model from the Leonid stream (for which it was originally created) to the Perseids. The computation considered only gravitational forces, however, the results are on the whole in good accordance with these of other researchers. The prediction includes all encounters found within interval +/-0.007 a.u. The following parts of trails were computed: the first 5 rev. trails for ejection velocities [-50;100] m/s, 6-7 rev. trails - [-30;50] m/s.
The Fig. 1 below presents the distribution of 109P dust trails in the vicinity of the Earth's orbit within the period of 11.05.2006 - 12.11.2006. The vertical axis shows minimal distances between trails particles and the Earth's orbit. So far the Fig. 1 shows the moments of passing minimal distances to the Earth's orbit and these distances themselves for various trails and particles. The central vertical line corresponds to 12 August 2006.
Fig. 1. Space-temporal projection of Perseid trails parts onto their minimal distance passages (for each trail the year of its formation and ejection velocities of particles, contained in the depicted parts of trails)
In 2006 the single regular formation in the vicinity of the Earth orbit (but still quite far from it) will be a part of 1479 trail (3 rev.) with high particles ejection velocities. It means, that no activity enhancements from 109P trails encounters are expected.
Considering background activity we can say that the Earth orbit is now crossed by low-perturbed backgroung stream, so traditional peak is expected to be quite moderate, perhaps lower than in 2005. During the maximum ZHR most likely will not exceed 80-90 meteors.
Perseid maximum in 2006 will be moonlit as the Moon passes its full phase on 9 August. However, Perseid shower is strong enough to give up to 50 meteors an hour under good sky even in such conditions. So in 2006 Perseid observations can be very useful. Prediction could not consider all the finest features of stream dynamics, so unexpected activity is still possible - from older, not computed trails or due to the possible imperfections in the model. Shower radiant is in Perseus (RA=58°, Dec=+59). Its traditional maximum, according to the IMO data, is to occur at 23:00-1:30 UT 12-13 August. Meteors are swift (entry velocity is 58 km/s), the shares of bright and trailed meteors are quite high.
Perseids 2006 are expected to show a rather moderate background maximum with ZHR not exceeding 80-90 meteors. Full Moon on 9 August will create significant embarrassments for its observations but even in such conditions Perseids are able to give a spectacular display.
1. "Comet's dust 2.0" program by S. Shanov and S. Dubrovsky. [Used for orbital computations.]
2. Information from Gary W. Kronk's page http://www.maa.agleia.de
3. IMO Meteor Shower Calendar 2006 http://www.imo.net/calendar/russian/2006/spring.
4. Lyytinen E, van Flandern T. "Predicting the strength of Leonid outbursts", 2000, Icarus, P. 158-160.