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J. Brit. Astron. Assoc., 108, 5, 1998, p.239-240
Following a lean spell resulting from unfavourable moonlight conditions and poor weather, meteor observers have rather more to look forward to in the closing months of 1998. Prospects for the Orionids, active in the second fortnight of October, are excellent; a New Moon on Oct 20 means that skies will be dark for the shower's maximum which extends for a couple of days around Oct 21. The Orionids require late-night watches. The radiant, roughly midway between Betelgeuse and Gamma Geminorum, does not rise until about 22h local time, and is at its highest in the pre-dawn hours. Orionid meteors are very fast, and often leave persistent trains. Observed rates up to 15-20 per hour may be found on occasion. Watching the Orionids will be good practice for what many see as the main event of the autumn - the Leonids.
The parent of the Leonid meteor stream, Comet 55P/Tempel-Tuttle, returned to perihelion in February. Earth passes the descending node of the comet's orbit on November 17-18, some 258 days behind the comet, bringing the possibility of a meteor storm. Leonid activity has been building since 1994. From quiet-time levels with sky- and radiant altitude-corrected Zenithal Hourly Rate (ZHR) around 10-15 at maximum through the 1980s and early 1990s up to and including 1993, the shower stepped up markedly in 1994 when, despite moonlight interference, observed rates were in the 20s. The 1995 return, in darker skies, brought substantial activity for observers in the British Isles on November 17-18, with peak ZHR about 40 for a couple of hours in early morning, and many bright Leonids. In 1996, the peak had broadened out - as happened previously in the early 1960s - to give a 12-hour span with ZHR above 40, and as much as 70, though no single obvious 'storm' peak was apparent. Leap year adjustments took the 1996 peak back to Nov 16-17, with best activity occurring up to about 13h UT.
Moonlight was again a hindrance in 1997, as was the weather for British observers. Data from the western US and Japan indicate that the Leonid peak on Nov 17 was again broad, and lacked a single very high-activity interval: the 1997 return offered no obvious clues as to where any storm peak might lie. It would appear that no storm occurred in 1997 ahead of the comet's node-passage. This is not a bad sign; historical analyses by John Mason[1] and Joe Rao[2] show that Leonid storms most often occur when Earth follows 55P/Tempel-Tuttle to the node. In the absence of a clear indication from the 1997 data, forecasts of when (and, from the point of view of being under dark skies with the radiant above the horizon, where) a storm peak might occur in 1998 are, at best, pure guesswork. The most likely outcome is that Earth will encounter the densest cloud of recently-ejected Leonid meteoroids close to the time of node passage, giving rise to an outburst which will be at its most intense for perhaps only 30-40 minutes. The node is currently encountered at Solar longitude = 235°.26, which is reached at 19h UT on 1998 November 17-18. At this time, the radiant will be far below the horizon for UK-based observers. Several groups are preparing to travel eastwards in longitude to improve their chances of seeing any outburst. Favoured locations may include India and the extreme east of Europe.
Past storms appear to have occurred an hour or two after the time of node passage. A delay of four hours or so in 1998 might just allow observers in the British Isles a glimpse of any extremely high activity as the radiant, in the 'Sickle' of Leo rises (around 22h30m local time). There is an outside possibility that the late evening of November 17-18 will be marked by unusually high numbers of long, near grazing-incidence Leonids streaking up from the eastern sky. Realistically, however, the chances must be considered better for observers at more easterly longitudes. The intensity of the major peak is also uncertain. Probably the greatest of all Leonid storms was that seen in 1833 over the Americas, when - on the night of Nov 12 - Earth passed very close to the node of 55P/Tempel-Tuttle's orbit about 300 days behind the comet. The 1966 storm occurred with a greater miss-distance, but was still intense for 30-40 minutes, with reported rates of the order of 100,000 Leonids/hr in this interval.
In 1998, the distance between Earth and the node of the comet's orbit at closest approach is much greater, and it is possible that we could miss the densest parts of the meteoroid cloud. The display in this instance might be substantial, but not reach full-blown storm proportions (what some have described as a grand display).
Whatever the nature of any intense, sharp peak, it is fairly certain to be surrounded by a 12-hour (or longer) period of enhanced activity, the equal of that from the Perseids at peak in an average year. Some of this will undoubtedly be visible from the British Isles: we cannot depend on seeing any storm activity from our longitudes, but rates of perhaps a Leonid per minute are far from improbable in the hours after midnight on Nov 17-18. Storm or no storm, the Leonids will be well worth observing, whatever your location.
Reports of any Leonid observations should be sent to the Meteor Section as soon as possible after the event. Important details to record are the stellar limiting magnitude (an indication of sky transparency) and the start and end times of the watch. Ideally, watches are carried out for intervals of an hour, or several hours at a time. For each meteor, give time of appearance (UT), type (Leonid or sporadic) and magnitude relative to background stars. The presence and duration of any persistent trains left behind after meteors should be noted (Leonids sometimes produce spectacular, long-duration trains). Under conditions of very high activity, it will be more sensible simply to count Leonid numbers in intervals of five minutes or less, provided the times of these intervals are also recorded. Report forms and more detailed observing instructions can be obtained from the Meteor Section Director at the address on the inside front cover of the Journal.
Best current predictions suggest that the 1999 return could produce a grand display or a storm for observers in western Europe, in the early morning hours of November 18. The results from 1998 should help refine this forecast.
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