The November Leonids
By Wes Stone
LEONID STORM! My Leonid Report is now Available HERE!
Active Dates: November 14-21
Maximum: Several peaks predicted, all on November 18:
9:55 UT (1:55am PST), 17:24 UT (9:24am PST), 18:13 UT (10:13am PST)
(by McNaught-Asher)
10:28 UT (2:28am PST), 18:15 UT (10:15am PST)
(by Lyytinen-Van Flandern) 14h UT (6am PST) (by Brown/Cooke)
ZHR at Maximum: Storm (>1000) ?????????
Radiant Position on Date of Maximum: RA 10h 15m; Dec. +22 (radiant drift map)
Radiant Elevation Reaches 20 degrees at About 1am PST
No shower contains as much intrigue or potential as the Leonids. Ever since the parent comet Tempel-Tuttle passed by in early 1998, hopes have been high that the Leonids would produce a rare and spectacular meteor storm. At times, the hype machine of the mass media got into the act, usually for the worse. During the past three years, we have learned a lot about meteors, and discovered that we have much more to learn. This year's Leonid shower should provide a few more answers.
First, a bit of history. The Leonids have produced some incredible storms in the past, most famously in 1833, 1866 and 1966. Research has uncovered other observations of very high rates. A meteor storm is defined as a shower with a ZHR >1000. In contrast, the Perseids and Geminids generally peak at ZHR ~100. Past storms accompanied ore closely followed the return of Comet Tempel-Tuttle to the inner solar system every ~33.3 years. However, there were no meteor storms in 1899 or 1933, so the Leonids were clearly not a clockwork event. Nevertheless, simplistic predictions were the norm.
It was widely held that 1998 would produce a storm, since Tempel-Tuttle had just passed by the Earth. Huge expeditions, including the news media, traveled to frigid Mongolia where the highest rates were predicted. Given the high expectations, disappointment was probably inevitable. The Leonid maximum occurred 16 hours ahead of schedule, and favored Europe and the Middle East. The ZHR was about 340, well short of a storm but that didn't matter to observers who saw the event in its full glory. Bright meteors predominated, with a good number of truly awesome fireballs.
The 1998 events led to a reexamination of the Leonids. It was found that the Leonid fireball shower resulted from material cast off by Tempel-Tuttle in 1333. Normally, such an old stream would have dispersed, but these meteoroids were held in formation by a resonance with Jupiter. Rob McNaught and David Asher, seizing on a theory that had successfully predicted a Draconid outburst in October 1998, sought to map the orbits of more recently ejected particles. These particles would form "dust trails," and a storm would result when the Earth met a trail.
The first test of McNaught-Asher came in 1999, when an outburst possibly reaching storm levels was predicted. The storm came right on schedule, although a bit stronger than expected. Observers in Europe and Africa saw a brief display of 30-50 Leonids per minute. The dust trail theory seemed to be vindicated, but other models gave the same result for 1999. More confirmation was needed.
In 2000, the Earth would barely graze three of the McNaught-Asher dust trails. The two experts predicted times of maximum activity, but felt that rates would not exceed 100/hour. Several other modellers also tried their hand. Ignacio Ferrin predicted a major storm for the Americas. Esko Lyytinen and Tom Van Flandern agreed with the times of McNaught and Asher, but predicted much higher rates of up to 700/hour. Each time the Earth passed a McNaught-Asher dust trail, observed activity increased. Eastern North America caught two outbursts, Europe a third. ZHRs were 150, 300, and 500, although observed rates were lower due to moonlight interference. No storm occurred at the time of Ferrin's prediction. The rates seemed to be intermediate between McNaught-Asher and Lyytinen-Van Flandern, and again the basic dust trail theory seemed stronger than ever.
What do the cards hold for 2001? Both McNaught-Asher and Lyytinen predict multiple meteor storms. The strongest (ZHR ~8000) would occur over parts of Asia and Australia, but a secondary peak (ZHR 800-2000) favors the Americas. Not to be outdone, competing modellers have proposed alternate scenarios. Peter Brown and Bill Cooke argue for a different placement of dust trails, with a broad peak of ZHR 1300 for the American West Coast and Hawaii. Brown-Cooke do not expect a true storm over Australia and Asia. Peter Jenniskens agrees with the McNaught-Asher timing, but predicts that the storm over the Americas will be the strongest, with a ZHR up to 4200.
So, what is the poor observer to do with all of this sometimes contradictory information? Why, go out and observe, of course! Keep the models in mind, but mainly try to find clear and dark skies on the morning of November 18 (a Sunday)! Try to watch from midnight or 1am until morning twilight, and keep a quantitative record of what you see. Remember that you can't see Leonids until about 11pm (the radiant hasn't risen yet), and that rates will be low for a couple of hours thereafter. Adjacent mornings will probably offer low rates, but you never know. The Earth passes nearest the comet's orbit on November 17 at 5am PST. This will likely be the time of highest background activity, with rates anywhere from 10-100/hour feasible. Adventurous observers should also monitor the sky between 9am and 11am PST on November 18, for the chance to catch a daylight fireball associated with the storms over Australia and Asia.
Leonids are very fast, and rather bright on average. The brightness profile during outbursts does depend on the age of the dust trail; younger streams, while richer overall, contain a higher proportion of small meteoroids that produce faint meteors. Many Leonids leave persistent trains. The train of the magnitude -10 bolide I saw in 1999 lasted for about 50 minutes, but that was certainly exceptional.
Like the Orionids, the Leonids are accompanied by both sporadics and Taurids. The Taurid radiants are actually in Taurus in mid-November (map). Taurid rates may be 3-5 per hour in the evening and early morning, but the radiants get pretty low as morning twilight approaches. Sporadic rates are quite high on November mornings. One particular concentration of sporadics comes from the diffuse Northern Apex radiant (RA: 9h 48m; Dec. +28). This radiant is rather close to that of the Leonids, but its rates of only a few per hour will likely be overwhelmed by the Leonids.
The generally poor weather prospects for the Pacific Northwest in November may have you thinking about travel plans. It is generally agreed that the area around Tucson, Arizona has the best chance for clear weather (~95%). On the other hand, long-term weather models are about as dependable as Leonid rate models, and Oregon has seen some clear skies during the last two Leonid showers. It could happen again...
More Leonid Info.
