Astron. Astrophys. 317, 953-961 (1997)

4. Implications

These results allow new interpretations of historic accounts of outbursts and the forcasting of future events.

4.1. Historic Lyrid outbursts

There are many historic accounts associated with the Lyrid stream, dating back to 687 BC. The mere presence of the outbursts implies that the orbit has changed only marginally over time during the past six or so orbital revolutions of the parent comet, which may be on account of the orbit's high inclination (i = ). However, small changes are expected. A change in the barycentric position of the Sun during outbursts can give information about such subtle orbital changes.

The positions of Jupiter and Saturn at the time of historic Lyrid outbursts were derived from the geocentric longitude tables of Stahlman & Gingerich (1963) and are listed in Table 2. The table also gives the barycentric displacement of the Sun due to these two planets alone, which is 0.002 AU of the total.

Table 2. Historic outbursts that have been associated with the Lyrid stream. Solar longitudes () are in Equinox 1950.0. The barycentric displacement of the Sun () is in units of 0.001 AU. Note that before 1800 only Jupiter's and Saturn's influence is included (). Data by [1] Jenniskens 1995; [2] Guth 1947; [3] Biot 1846; Newton 1963, 1964; [4] Denning 1884; [5] Olivier 1925; and [6] Tian-shan 1977.

It is found that historic outbursts, too, correlate with planet positions. Three of the historic outbursts (in 582 AD, 464 AD and 686 BC) occurred when Jupiter was in conjunction with the node of the stream. Saturn was in all cases close to = 180 degree, opposite the position during the recent outbursts of 1803, 1922, and 1982. Two other events in 840 AD and 15 BC occurred when Jupiter was not in conjunction with either node of the stream. In both cases, however, Jupiter and Saturn were at a similar location and the times of peak activity were about 0.6 degree earlier in solar longitude than for the other events. The resulting barycentric displacement perpendicular to the Earth's orbit is similar for both cases and somewhat less than for modern outbursts. Hence, it seems that in historic times the gravitational perturbations needed to be slightly different for the dust to collide with the Earth. The data suggest an outward movement of the descending node of the orbit by +0.005 ( 0.002) AU in a period of two millennia. The motion parallel to the Earth's orbit in that time period was some +0.003 AU/1000 years, which compares to the predicted mean nodal motion of +0.010 AU over the past 1000 years from model calculations by Fox (1986).

A series of spectacular westward showers was seen in Europe in the Middle Ages in mid April between 1000 and 1204 AD, close to the date of Lyrid outbursts (Guth 1947, Dall'olmo 1978). The descriptions of these outbursts are inconsistent with other known far-comet type outbursts. Hence, these are not related to P/Thatcher. In stead, they were probably of near-comet type, due to a short period comet with a period of about P = 14 years, or a fraction thereof. A gradual shift in node prior to 1204 suggests that the parent comet orbit was perturbed at that time, moving away from the Earth's orbit.

4.2. Predicting future far-comet type outbursts

The previous findings allow the prediction of future outbursts by calculating the planetary perturbations on a stream of dust particles and by matching the resulting oscillating path of intersection points with the ecliptic near the Earth's orbit to that of observed meteor outbursts. Such calculations are beyond the scope of this paper. However, some prediction can be made by simply calculating the position of the planets (e.g. from Montenbruck & Pfleger 1994) and from that the barycentric displacement of the Sun, and by matching the Sun's displacement to that at times of other observed outbursts. Figure 10 shows the future barycentric path of the Sun between 1995 and 2050. The figure varies slightly for different times of the year. Table 3 summarizes the years when the Sun will be close to the same position as during the most recently reported previous meteor outburst.

Fig. 10. Diagram as Fig. 5, now showing the barycentric path of the Sun in the near future, between 1995 and 2050. Black dots show the position of the Sun when the meteor streams produced their last far-comet type outburst. Close encounters are listed in Table 3.

Table 3. Possible far-comet type outbursts from the catalog of Jenniskens (1995), the year that they were seen last, the date and time of the peak activity, and the next occasions when the Sun has a similar barycentric displacement in the period 1995-2050. Notes: *) Telescopic showers. Outbursts not listed before:

The alpha-Monocerotid event in 1995 is anticipated with most confidence (Jenniskens 1995a) . No outburst was reported in 1994. Other streams are less certain, notably when the identification "far-comet type" is in doubt. For example, no kappa-Pavonid outburst was observed in 1995 during a dedicated observation from Pretoria, South-Africa, covering solar longitude 113.96-114.26 and 114.36-114.38. The annual activity was found to be less than = 0.5. However, it is possible that the stream will return in 1996.

The best opportunity for detecting outbursts of far-comet type of yet unknown meteor streams is expected to be during superior and inferior opposition of Jupiter and Saturn, when the trail displacements are largest at a given time in the year, and especially in the years when Jupiter and Saturn are in conjunction. The next five years are promising and the newly discovered streams may be reobserved 24 years later.

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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