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This page was last modified on September 22, 1998 by Casper ter Kuile and Marco Langbroek |
Draconids 1998: Prospects
History, encounter- and observational conditions (by Marco Langbroek)
Introduction
Due to all attention and high expectations for the upcoming Leonid return, it might easily be forgotten that yet another stream could provide the opportunity of observing impressive rates in 1998, though not comparable to the rates expected for the Leonids. In November 1998, periodic comet Giacobini-Zinner, the parent comet of the October Draconids, will pass through perihelion, and this might produce a conspicuous shortlived revival of the currently dormant Draconid stream.
A slight touch of history
On December 20, 1900, M. Giacobini at the observatory of Nice discovered a faint comet. Only a limited amount of observations was gathered following the discovery, resulting in an orbit with rather large uncertainties. On October 23, 1913, Ernst Zinner in Bamberg rediscovered the comet and its orbital period was determined at only 6˝ years. The comet proved to be under serious perturbations by Jupiter. In the first half of this century, perturbations brought the orbit of the comet extremely close to that of the Earth, resulting in conspicuous meteor activity in the years of perihelion passage caused by the encounter with the debris of the comet. In 1926, Prentice observed the stream, now known as the Draconids or Giacobinids, with a ZHR ~14, the first well-recorded (modest) outburst of the stream.
In 1933, the distance between Earth and comet orbit was only 0.0054 AU. The Earth passed the cometary node only some 80 days after the comet went through its node. In the early local evening of October 9, thousands of Europeans, among them the late and now legendary J.H. Oort, witnessed a spectacular meteor storm rivalling with the famous Leonid storms. Two orbital periods later, in 1946, European and American observers again witnessed an intense display -moonlit, but not less impressive because of that unfavourable circumstance. That year, the distance between Earth and cometary orbit had decreased to only 0.0015 AU, and the Earth encountered the debris at the cometary node only 15 days after the comet had passed this point.
In the best traditions of this field of research, the absolute as well as relative strenghts of the 1933 and 1946 peaks is debatted. A summary of different opinions is given by Jenniskens [1], who himself provides the figures of 10 000 ± 2000 for 1933 and 12 000 ± 3000 for 1946 based on a reduction of original raw data.
In 1952, the cometary orbit had shifted such that the node became located inside the orbit of the Earth, but the distance remained small: 0.0057 AU. The Earth crossed the cometary node 196 days before the comet encountered this point. Radar observations from Jodrell Bank (UK) showed that conspicuous activity was present in the late afternoon, though evidently the rates remained rather modest compared to the incredible 1933 and 1946 events. In 1953, no activity was detected.
The interplanetary play of pinball between Jupiter and the comet then took a negative twist. The next years, the distance between Earth and comet orbit increased considerably and the Draconids vanished from the scene untill a new round of push and pull hurled the comet to an extremely close encounter in 1972: 0.0007 AU, with the Earth passing outside the cometary orbit, only 58 days after the comet passed its node. Surprisingly, and disappointingly, only a very modest activity was detected that year.
By 1985, the orbit had changed again. This time, the cometary orbit remained rather far from that of the Earth at a distance of 0.0329 AU, but the node had shifted outside the Earth orbit again. On October 8, only 27 days after the comet had passed its node, Japanese observers witnessed an impressive outburst with a peak ZHR of 700 ± 100 [1]: nothing compared to the 1933 and 1946 events, but still a rather remarkable event. Besides the strong main peak, a very low level background activity with a ZHR in the order of 2-4 seems to have been present for several hours before and after the main peak that year. In the following years, little or no activity of the stream was detected. The stream seems to be absent except for a short period around perihelion passage of the parent comet [1]. In non-perihelion years, the stream is virtually undetectable [1-3]. Yet, low but clear activity was observed last year by Japanese observers.
The 1998 encounter conditions
At the occasion of the 1998 perihelion passage of P/Giacobini-Zinner, the Earth and the comet will pass the cometary node with a time difference of only 50 days according to the latest orbital predictions [4-5]. The distance between the cometary orbit and the Earth orbit at the cometary node is only slightly larger than in 1985: 0.0383 AU (against 0.0329 AU in 1985). At first glance, this would suggest that, judging from the 1985 experience, the conditions for 1998 are favourable and we could expect a conspicuous 1985-like event again.
Yet, there is some reason for caution. In 1985, the comet passed its node 27 days before the Earth passed near this point. In 1998, the comet will pass 50 days after the Earth. It is very difficult to asses what effect this difference will have, but it might well be the difference between all or nothing. Usually, there is more dust well behind a comet than in front of a comet. On the other hand, during past returns the stream featured activity in front of the comet too (but that was with a much closer Earth-cometary orbit distance).
Observational conditions
As difficult as to asses if there will be an outburst, it is to asses when it will occur. The node of the 1998 orbit of P/Giacobini-Zinner, predicted at 195˙.39847 (J2000) [5], will be passed in the evening of October 8, 1998, around 21h UT. Kresák has predicted 17h UT as the possible peak location some years ago [6]. If the time difference between peak location and passing of the node is similar to 1985, we might expect it near 17:45 UT. Give or take a few hours, the event might be expected to occur in the early evening for Europe. The most favourable locations to observe the possible event might be the eastern medditeranean, the Balkans and west Asia. Though the actual peak might drown in evening twilight, observers in western Europe might still be able to catch a considerable part of the descending slope of the outburst from late twilight onwards if the event roughly mimics the 1985 event in general strength and ˙˙-t of the peak and the B-value [1] of the slopes (the last condition is very likely in case of a positive event [1]).
The possible 1998 reoccurence is not so favourable with regard to interference of the moon. Full moon will occur on October 5, 1998, only three days before the possible event: the moon has a fase of 0.87 on October 8. Yet, during the first hour (depending a little bit on the latitude) after the end of astronomical twilight the moon will still be low in the eastern sky and not too prominent, and it is during this early part of the evening that the radiant is located highest in the sky and observing conditions are perfect for eastern Europe and the Balkans.
References:
- P. Jenniskens, 1995: Meteor stream activity II. Meteor outbursts. Astron. Astroph. 295 (1995), p. 206-235
- P. Jenniskens; DMS Visueel Handboek, p. 120-123. Leiden, 1988
- Rendtel J., Arlt R. and McBeath A. (eds.), 1995: Handbook for visual meteor observers, p. 210-216.
- P. Bus; Priv. com.
- MPC 25182, May 1995.
- L. Kresák; Meteoroids and their parent bodies (eds. J. Stohl and I.P. Williams), p. 147-156. Bratislava, 1993.
Table 1: Encounter geometry of historic Draconid events and the 1998 perihelion passage of P/Giacobini-Zinner. Listed are (from left to right) date, approximate peak ZHR, the peak location (B1950), the difference in time (in degrees solar longitude) between the peak location and the passing of the node, the distance between Earth and comet orbit (in AU: cometary node inside (-) or outside (-) the orbit of the Earth) and the number of days that the Earth leads (+) or follows (-) the comet at passing the node. All data from ref. [1], except for the 1998 data (ref. [4-5]).
date ZHRmax peak +/- days +/- 9 Oct 1933 10000 ± 2000 196.302 +0.059 +0.0054 +80 9 Oct 1946 12000 ± 3000 196.292 +0.001 +0.0015 +15 9 Oct 1952 (~250?) 196.241 +0.001 -0.0057 -196 8 Oct 1985 700 ± 100 194.565 -0.147 +0.0329 +27 8 Oct 1998 +0.0383 -50
Table 2: Stream data
Radiant Velocity Cometary node date (1950.0) (B1950) RA 262 +/- 56 23 km/s (slow) 194 +/- 700 October 8/9
This is a slightly edited version of an article that appeared in WGN .... (1997).
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This page was last modified on September 22, 1998 by Casper ter Kuile and Marco Langbroek |