Earth in a Twist: Predicting the Storm

Armagh Observatory press release, 1999 August 24th.

Research by David Asher, of Armagh Observatory, and Rob McNaught, of the Research School of Astronomy and Astrophysics, Australian National University, shows that passages of the Earth through dense swarms of matter in space can be predicted to a remarkable accuracy. The technique, described in a paper in the 21 August 1999 issue of the Monthly Notices of the Royal Astronomical Society, is applied to the forthcoming Leonid meteor storm, expected during the night of November 17/18.

Meteors, popularly known as `shooting stars', can be seen on any night, given a sufficiently clear, dark sky. They are produced by the impact on the Earth's atmosphere of small dust grains released from comets. Meteors usually arrive at fixed times of the year when the Earth passes close to the orbit of the parent comet, but occasionally, just a few times a century, a phenomenon known as a meteor storm occurs. During such a storm, meteors appear at astonishing rates, sometimes several per second. The most famous example, the incredible Leonid display of 1833 (JPEG image, 338k), is regarded as provoking the development of modern meteor astronomy. The predictions of David Asher and Rob McNaught involve mapping the fine `braided' structure of the dense dust trails within the Leonid stream.

The new analysis of Leonid meteor storms over the past two hundred years shows that the peak times of the strongest storms and sharpest outbursts are predictable to within about five minutes. Although comet Tempel-Tuttle, the parent of the Leonid meteoroid stream, passed close to the Earth in 1998, David Asher and Rob McNaught now predict strong storms in both 2001 and 2002. Before then, 1999 and 2000 will be less spectacular, but nevertheless still good. In 1999, observers at European longitudes should see up to 20 meteors a minute (given a clear, dark sky) at around 2 a.m. on the morning of November 18th. Since European longitudes are favoured this year, it should be worth staying up on the night of the 17/18th if the sky looks as though it may be clear.

Good news for meteor observers can be a concern for satellite operators. A satellite can be disabled by the impact of even quite a small dust grain. While there is an awareness of the hazard from man-made space debris, advance knowledge of the detailed structure of the Leonid stream is potentially of immense value. Satellite operators could use this information to take appropriate avoiding action and minimise the risk from the Leonid dust trails. McNaught and Asher have defined the structure of these trails more accurately than ever before.

For further details, contact the Armagh Observatory Public Relations Officer, Mr John McFarland, Dr David Asher or Professor Mark Bailey, at Armagh Observatory, College Hill, Armagh, BT61 9DG.
Tel: 028-3752-2928. Fax: 028-3752-7174.
Rob McNaught can be contacted at the Anglo-Australian Observatory at

Further Information

What are the Leonid meteoroid stream and the Leonid meteor shower?
The Leonid meteor display is associated with the Earth's passage through the Leonid stream. This stream consists of the debris of Tempel-Tuttle, a comet that orbits the Sun about every 33 years.

When do the most intense outbursts occur?
Whereas the Earth goes through the Leonid stream every November, in most years the Leonid meteor shower is unspectacular. However, there is fine structure within the stream, and meteor storms occur when the Earth runs through the highest density regions. The new technique for mapping out the structure involves precise calculations of the effect of the gravity of the planets on the dense dust trails, covering many revolutions of the dust grains about the Sun and times of a century or two.

Why are some longitudes favoured?
The meteors in any given shower come from a particular direction in space. You need to be on the hemisphere facing that direction to see the meteors. It also has to be night-time, except for incredibly bright fireballs. In the case of the Leonids, an approximate rule is to observe after midnight. Background Leonids (a few meteors per hour) appear for a few days, and so all parts of the Earth have a chance to catch them. But some outbursts are of high intensity for less than an hour, and you have to be at a longitude where it's between midnight and dawn. The next few years will provide various excellent Leonid opportunities, of which 2001 from East Asian longitudes will be best, especially as the moon will be absent from the sky. Most immediately, 1999 should produce a good display, although rates will not match the most spectacular ones: the Zenithal Hourly Rate (a rate normalised to near-ideal observing conditions) is estimated to peak at 1,200 per hour at 02:08 GMT on November 18th.

Can damage to satellites occur?
Very high speed impacts of tiny dust grains on satellites can cause plasma to be generated, which can lead to electrical failure. There is evidence that the Olympus communications satellite was disabled owing to the impact of a meteoroid from the Perseid stream in 1993.

History of this work
The famous Leonid storms of 1833, 1866 and 1966 were known to relate to the roughly 33 year period of the comet. But it was only when McNaught examined the details of those and other Leonid outbursts of the past two hundred years that the full predictive power of the `dust trail' technique became apparent. Whereas theories that considered the comet alone, rather than the dust trail structure in the stream, would sometimes match observed timings of storms within hours (but occasionally fail completely), the dust trail theory allows an accuracy that many astronomers never suspected possible. Further refinements to the theory, including a topographic correction, have reduced the uncertainty to around five minutes. A few months after developing the technique, McNaught and Asher extended their work to permit estimates of meteor rates (in addition to predicting storm timings), and applied it to forthcoming encounters of the Earth with Leonid dust trails. There is no doubt that 2001 and 2002 will provide opportunities to witness exceptional Leonid meteor storms. The fact that something out of the ordinary is expected both in 2001 and 2002 had in fact been published more than a decade ago, by two researchers, Kondrat'eva and Reznikov, in Kazan, Russia. The English translation of their paper did not come to the notice of many western researchers.

Further Leonid work at Armagh
Some separate but related work on the longer term dynamics of the Leonids, and the explanation of the 1998 fireball storm, was published by David Asher and Mark Bailey, of Armagh Observatory, and Vacheslav Emel'yanenko, of South Ural University, Russia (21 April press release).

Go to Armagh Observatory Leonid page.

Last Revised: 24th August 1999
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