General Meeting Report,
11 July at the National Herbarium.

Predicting the Leonids...
Speaker Rob McNaught

Just two centuries ago everyone still assumed all atmospheric phenomena, wind, rain, hail, meteors and meteorites, were of terrestrial origin. "Stones don’t just fall from the sky"! Those who thought otherwise were considered heretics. – Rob McNaught’s syllabus started innocently enough, but it got technical very quickly with tables, graphs and formulas:
What is a meteor? A meteor results when a small particle (typically millimetres in size for a visual meteor) of dust orbiting the Sun collides with the Earth's atmosphere. The velocities of collision may range between 11 and 72 km/sec (40,000 - 260,000 km/hr), depending on the angle of impact and on the acceleration due to the Earth's gravity. At these immense speeds the particles and the air surrounding them start to glow. This phenomenon is known as a meteor. Meteors typically vaporise (burn up) at heights around 90 km above the Earth. Random meteors are encountered by the Earth continuously and can be observed from any one site at an average rate of 20 per hour.
Spectacular rises in this average rate have been noted throughout recorded history, but the first scientific account of a so-called meteor-storm was by Alexander von Humboldt in 1799 during his South America explorations; and on November 13, 1833 many Americans thought the world might be coming to an end when just before sunrise 100,000 shooting stars per hour lit up the sky. At Yale College, Dennison Olmstead studied the paths of the meteors and announced that they had arrived from deep space, centred somewhere in the constellation Leo. In a neighbouring office, Hubert Newton calculated that the swarm would return in 1866. And it did. But since then gravitational Ping-Pong in the Solar System has kept everyone guessing about when and how extensive the next appearances would be. (As Isaac Newton had already discovered, orbital calculations work accurately only with two gravitationally interacting masses) In 1899 and 1933 the Leonids were a bust. Then 1996 was again “ of the greatest shows on Earth”. There is obviously more to predicting meteor-showers than meets the eyes.

Modern study of meteors has shown that the Leonid meteors are fragments of the comet 1866 I (rediscovered as Tempel-Tuttle); little bits of debris that flake off during surface thawing and tidal disruptions as the 33.2-year elliptical orbit swings it around the back of the sun. Each November the Earth crosses Tempel-Tuttle's orbit and encounters some of the dust distributed there, but three times a century we come close to the comet itself and may plow through a dense cloud of debris expelled from the comet on a previous pass. And that is when fireworks really light-up the sky. Overcoming the difficulty in predicting where and when one of these clouds and the Earth cross tracks is the challenge Rob McNaught at the Australian National University and David Asher of Armagh Observatory in Northern Ireland have taken on. As we have seen prediction of Leonid activity has had a rather chequered history. What Rob and David now postulate is that the cloud of debris released from the comet on each pass of the sun breaks (by tidal disruption) into two sections, the bulge closer to the sun will have a fractionally smaller orbit and will consequently arrive ahead of the comet next time, conversely the bulge away from the sun will lag a predictable time behind the comet in its orbit. In their calculations they took account of a number of the dust clouds thus ejected by Comet Tempel-Tuttle on past approaches to the Sun. (Historic records of Leonid storms have been found back to 902 AD). Although the comet itself has passed around the sun two years ago they predict the Earth will encounter a cloud of 4 revolutions ago, on the 19 November this year, which should also coincide with the 9-rev trail.
Given that there is only a three hour window to observe the Leonids from Australia (from radiant [Leo] rise to astronomical twilight), there is not much leeway for error. But, Rob says, I believe the evidence is strong enough for the time of maximum to fall well within this three hour window making Australia a suitable location for 2001. Anyone contemplating a trip to watch the 2001 Leonids is well advised to visit Peter Anderson's “Leonids Australia 2001" website.

Rob finished with an entertaining video show of a recent meteor watch in Iran, which proves that language is no barrier to astronomy.