Anomalous activity is probable first indicator
By David Morse
The Leonids are coming! The Leonids are coming! No, it's not the remake of a forgettable Alan Arkin film of the 1960s. Rather, it portends the arrival of a visual display of potentially much greater promise.
Ames scientist/astronomer and former National Research Council fellow Dr. Peter Jenniskens recently predicted the coming of a massive Leonid meteor shower. The projected level of activity may well rival that of any meteor storm in the past 100 years. Jenniskens based his prediction on recent observations which, he reports, showed the first indications of anomalous Leonid activity in November 1994.
"On rare occasions," Jenniskens said, "meteors appear in such large numbers that it seems our spaceship Earth is going in warp. Such meteor storms are brief and extremely rare, with only four such nights in the past century that might qualify. In November 1998, we may well see the occurrence of just such a night again."
According to Jenniskens, for the first time in 25 years, the Leonid shower was seen to flare up in November 1994 from a weak annual shower to an impressive display rivaling the best meteor showers in August. Last November, rates were up again and Jenniskens predicts that Leonid rates will continue to increase until a peak is reached on Nov. 17, 1998.
"On that day meteors may fall at the rate of three per second with occasional flares of up to 40 a second," Jenniskens said. "But the storm will last only an hour. The meteors will be very fast and will radiate from a point in the constellation of Leo. Unfortunately, not everybody will be able to see the event. At present, the odds seem best for observers in eastern Asia, where it will be deep in the night when the Earth is expected to hit the stream."
Meteor showers are not uncommon, according to Jenniskens, but ones of this magnitude are rare. Perseid meteor showers, which come annually in August, are the ones most familiar to non-scientific observers. They provide an interesting show with about the same relatively low-level of activity from year to year. It takes the Earth about a month to cross the Perseid stream, but the shower is good for a few days around Aug. 13.
"The new Leonid meteor showers are completely different," Jenniskens said. "It takes the Earth only one day to cross the stream during which many bright meteo rs appear. They are leading up to an intense Leonid storm of the type projected for 1998 that will be rich in faint meteors. That stream will be crossed in only about an hour," he said.
These showers typically are associated with the return to the inner solar system of the comet Tempel-Tuttle on its path around the sun every 33 years, according to Jenniskens. What causes the meteor shower is the existence of a trail of dust particles that both precede and follow the comet. These particles were shed during previous passages through the inner solar system. They range in size from sand grains to pebbles, and flare and burn up when colliding with the Earth's atmosphere.
The last major Leonid activity was observed in 1966. According to Bob Hillenbrand of Ames' Stratospheric Observatory for Infrared Astronomy project office, who observed the activity on that occasion, "meteors fell like leaves from a tree in a storm." Observers in Indiana estimated that up to 200,000 meteors were falling per hour at the height of that storm. Hawaiian observers estimates ran as high as 400,000 meteors per hour.
Jenniskens believes that those estimates are a little on the high side. However, he finds that the Leonid showers thus far are following a similar pattern to that experienced prior to 1966, and he is confident that the events of 1998 will provide excellent viewing. While it is too early to tell the exact time of the predicted event, current estimates of 1 p.m. PST suggest that observers in the darkened time zones much further to the east will have the best seats for the upcoming spectacle.
Jenniskens work is funded in part by the Director's Discretionary Fund. He reported his findings alerting the scientific community to the coming meteor shower in the March issue of the peer-reviewed journal `Meteoritics and Planetary Science' (formerly known as `Meteoritics' prior to January 1996).
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