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(meteorobs) The November Leonids: Will They Roar?
The November Leonids: Will They Roar?
Donald K. Yeomans
Jet Propulsion Laboratory/California Institute of Technology
August 7, 1998
Each November when the Earth runs into the dusty debris from periodic comet
55P/Tempel-Tuttle, some Leonid meteor shower activity is noted. These annual
displays of meteors, or shooting stars, seem to originate in the
constellation Leo so they are termed Leonid meteors. Normally, the observed
rate of the Leonid meteors is about 15 per hour under ideal observing
conditions. However, every 33 years or so when the parent comet
Tempel-Tuttle returns to the Earth's neighborhood, there is a possibility
that the Leonid meteors rates can get substantially higher. In some years
such as 1799, 1833, and 1966, when the Earth passed particularly close to
the tube of debris following in the comet's wake, there were Leonid meteor
"storms" noted of up to 150,000 meteors per hour. Periodic comet
Tempel-Tuttle passed closest to the sun (reached perihelion) most recently
on Feb. 28, 1998 and a month later on March 5, the comet passed through the
plane of the Earth's orbit about the sun.
Another way of saying the same thing is to note that the comet passed
through the ecliptic plane from north to south or it passed through its
descending node. We can expect the maximum Leonid meteor shower activity
when the Earth arrives close to this nodal crossing point on November 17,
1998 at 19 hours 43 minutes Universal Time (UT). The peak Leonid meteor
shower activity takes place within one hour but some activity can be
observed for a few hours on either side of this peak. Unfortunately for
observers located in the United States, the Nov. 1998 shower maximum will
occur during daylight hours (2:43 pm local time on the east coast and 11:43
am on the west coast). While some enhanced 1998 Leonid activity may be
visible just before dawn for U.S. observers, the Leonid shower maximum
should be best observed by those located near the regions of Japan and
eastern Asia. In November 1999, the Leonid shower will be best observed from
the regions near Europe and North Africa.
Table 1. Predicted Leonid Shower Circumstances.
Although slightly enhanced meteor shower activity was evident in 1996 - 97,
impressive meteor showers are most likely in 1998 and/or 1999.
Predicted time of Observed time
Leonid shower peak of shower peak ZHR Good observing
Date (UTC) HH:MM (Hours) meteors/hr Locations
------------------ -------------- ----------- --------------------
1996-Nov-17 07:20 05 - 10 60 Eastern U.S.
1997-Nov-17 13:34 12 - 14 40 Western U.S., Hawaii
1998-Nov-17 19:43 200 - 5000? Japan, Asia
1999-Nov-18 01:48 200 - 5000? Europe, North Africa
As noted in Table 1, the predictions for the times of the 1996 and 1997
maximum shower events were rather accurate and there is no obvious reason to
doubt that the 1998 and 1999 predictions will be seriously in error. What
sort of Leonid meteor rates can we expect in 1998 and 1999? Meteor shower
rates are often expressed in terms of the so-called zenith hourly rate (ZHR)
or the hourly rate of meteors an observer would witness under ideal
conditions with the meteors appearing directly overhead (at the zenith). The
geometric circumstances between the comet's orbit and that of the Earth for
1998 and 1999 are most similar to those circumstances during the Leonid
showers in 1866-67 and 1931-32. Since the observed Leonid meteor rates in
1866-67 and 1931-32 were approximately 5000 and 200 per hour respectively,
we might anticipate a zenith hourly rate in 1998 and 1999 bounded by the
rates witnessed in the earlier events - between 200 and 5000 meteors per
hour.
Like the weather, it is extremely difficult to predict the hourly rates of
meteor showers. Table 1 is meant only as a rough guide. Peter Brown, a
respected researcher of the Leonid meteor phenomena, has suggested a more
optimistic prediction of between 1000 and 9000 meteors per hour in 1998
(zenith hourly rate). In any case, it is well worth the effort to observe
the upcoming Leonid meteors since it will be another century after the
1998-1999 events before significant Leonid meteor displays are once again
likely.
Suggestions for further reading:
* Meteor Streams (http://ssd.jpl.nasadot gov/meteor_streams.html)
* Kronk, G.W. 1988. Meteor showers, a descriptive catalog. Enslow
Publishers, Hillside, N.J.
* Mason, J.W. 1995. "The Leonid meteors and comet 55P/Tempel-Tuttle".
Journal of the British Astronomical Association 105(5):219-235.
* Rao, J. 1995. "The Leonids: king of the meteor showers". Sky and
Telescope 90:24-31.
* Yeomans, D.K. 1991. Comets: A chronological history of observation,
science, myth, and folklore. John Wiley and Sons, N.Y.
* Yeomans, D.K., K, K. Yau, and P.R. Weissman 1996. "The impending
appearance of comet Tempel-Tuttle and the Leonid meteors". Icarus
124:407-413.