Draconids 2005 - activity prediction
Draconids, also known as Giacobinids, are the meteor shower of the comet 21P Giacobini-Zinner. This comet was discovered in 1900, just after its orbit was moved by Jupiter close to the Earth orbit. In the second half of 1910s some astronomers pointed out that there is possibility of meteor action from this comet in the beginning of October and such activity was actually observed. The meteor shower had its radiant in the head of Draco, which was suitable for 21P orbit. During 20 century and up to the present days the orbit of 21P passed through some perturbations, but was remaining near the Earth orbit. In this period Draconids produced two strong storms in 1933 and 1946 and a number of non-stormy outbursts. The latest activity enhancements occured in 1998 and 1999, when the comet passed its previous perihelion. In 1998 ZHR reached 700 meteors and in 1999 - 15-20 meteors.
In July 2005 21P passed its perihelion again. Should we expect any activity from Draconids in 2005? As known, previous outbursts happened in the years around perihelions of the parent comet 21P, but far not every perihelion was accompained with an outburst. Below is the prediction if Draconid activity in 2005, obtained by the method of modelling meteor particles evolution.
The vast majority of Draconid outbursts is traced with the modelling very good (but there are some exceptions, first of all, the 1985 case). Particles ejected by the comet form lengthy trails. One of the reasons is radiation pressure force, which acts parallel with gravitational forces. Gravitational force is dependent on a particle mass, i.e. it is proportional to the third power of particle radius. The outcrying radiation pressuse is defined by the second power of particle radius. So far the influence of radiation pressure is the more the less is size of a particle. Its action 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 be 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 in the point of perihelion, but also during several months around it. However, comets are in perihelion part of their orbits 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 in the point of perihelion, it doesn't virtualy influence the results of computations.
Speaking of directions in which perticles 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 hundreds thousands 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, Draconid trails modelling allowed to prepare very good predictions of shower activity in the previous years, real maximums differed from predicted ones mostly no more than on several minutes - not very much considering that computations are made for dozens and hundreds years of particles movement. More serious problem is prediction of outburst intensity - how strong the maximum could be. For such predictions special empirical models were elaborated (the single way in this case) but as before for their improvment new observations are very necessary.
The results obtained by the Author for the Draconids 2005 using the modelling of particles ejected by the comet 21P Giacobini-Zinner are presented below. Main characteristics of computations are also described.
I wish to introduce the results of Draconid meteor stream simulation aimed to the prediction of shower activity in 2005. The simulation was made for the trails of latest 28 revolutions, i.e, from the 1817 trail. The Author used the program by S. Shanov and S. Dubrovsky to calculate orbital elements of ejected meteor particles. To estimate expected ZHRs for different encounters the model by E. Lyytinen and T. van Flandern given in their paper "Predicting the strength of Leonid outbursts" 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 Draconids. 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, 6-10 rev. trails - [-30;50] m/s, 10-20 rev. trails - [-20;30], older than 20 rev. trails - [-10;20] m/s.
The Fig. 1 below presents the distribution of 21P dust trails in the vicinity of the Earth's orbit within the period of 08.07.2005 - 10.01.2006. The vertical axis shows the minimal distance between trails particles and the Earth's orbit. So far, fig. 1 for verious trails and particles shows the moments of passing minimal distances to the Earth's orbit and these distances themselves. The central vertical line corresponds to 8 October 2005.
Fig. 1. Space-temporal projection of Draconid trails parts onto their minimal distance passages (for each trail the year of its formation and ejection velocities of particles, contained in depicted parts of trails)
It can be seen, that during the period the Earth will be near the descending node of Draconid stream (6-10 October) no one trail will be close enough to produce an outburst. The closest to the Earth will be 1953 trail (8 rev.), however, first, the distance will be about 0.0125 AU (very far on meteor scale, for an outburst it have to be at least 10 times lower), and, second, the given part of the trail, as shown on fig. 1, is severely stretched because of past approach to the Earth.
Such trails distribution doesn't allow to speak of any shower outbursts. Activity will be most likely on the zero or near zero (some separate meteors are possible) level.
However, considering the 1985 case, which is not perfectly described by this model, the Author would like to note, that a similar outburst (similar in the way of origin, but not in intensity) could occur within the interval of solar longitudes of 195°349 - 195°488, which corresponds to 8 October 14:56-18:19 UT. Nevertheless, probability of such activity seems to be very low, especialy due to the stretched part of 1953 trail.
The rising Moon will not disturb much during Draconids 2005 observations. I'd like to specially note, that even despite the negative prediction of stream activity, its observations are very important. As known, in science a negative result id still a result, which could confirm the prediction. And, on other hand, the prediction could not consider all the finest fiatures of stream dinamics, so there are still possible unexpected activity - from older, not computed trails or due to the possible imperfectness of the model.
Despite the perihelion of parent comet, there are no expectations of any unusual activity from the Draconids in 2005. Most probable is the total or nearly total (with separate meteors) lack of activity. Meanwhile, activity in analogy with 1985 case could occur in the interval 8 October 14:56-18:12 UT. However, the chances of such activity and its probable intencity are extremely low. In 2005 the Moon won't be serious disturbance for Draconids observation, so there are good conditions to confirm or disprove the negative prediction.
1. "Comet's dust 2.0" program by S. Shanov and S. Dubrovsky. [Used for orbital computations.]
2. Lyytinen E, van Flandern T. "Predicting the strength of Leonid outbursts", 2000, Icarus, P. 158-160.
3. Information from Gary W. Kronk's page http://www.maa.agleia.de