Fine Leonid Meteor Displays Predicted Through To 2002
From Jacqueline MittonFINE LEONID METEOR DISPLAYS PREDICTED THROUGH TO 2002
November's Leonid meteor shower will produce good displays this year and next, and strong storms of meteors in 2001 and 2002, according to new research by Dr David Asher, of Armagh Observatory, and Dr Rob McNaught of the Australian National University. Writing in the Monthly Notices of the Royal Astronomical Society (21 August 1999 issue), they show how the times when Earth passes through the dense streams of matter in space that produce meteor showers can now be predicted with remarkable accuracy.
In the early hours of 17th November last year (1998), meteor watchers awaiting the Leonid shower were taken by surprise when a spectacular display of bright meteors occurred 16 hours before the predicted time for the maximum of the shower. The explanation for this phenomenon was discovered by Dr Asher and his colleagues Professor Mark Bailey of Armagh Observatory, and Professor Vacheslav Emel'yanenko of South Ural University, Chelyabinsk, Russia, and was published in April (see RAS Press Notice 99/09). They showed that the bright meteors were seen when Earth passed through a dense arc-shaped cloud of particles shed from Comet Tempel-Tuttle in the year 1333 and they proved for the first time that meteoroid streams can have complex braid-like structures within them. This work pointed the way to more precise predictions of the timing and intensity of meteor showers, such as those Asher and McNaught are now making for the Leonids.
The latest analysis, covering 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. The technique involves mapping the fine `braided' structure of the dense dust trails within the Leonid meteoroid stream. Although comet Tempel-Tuttle, the 'parent' of the Leonid stream, passed close to the Earth in 1998, Asher and McNaught predict strong meteor storms in both 2001 and 2002. 1999 and 2000 will be less spectacular, but good. In 1999, observers at European longitudes are favoured, and may see up to 20 meteors a minute (in ideal conditions under a clear, dark sky) at around 2 a.m. on the morning of November 18th.
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. Most meteors arrive in 'showers' 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 a storm, meteors appear at astonishing rates, sometimes several per second. The most famous example, the incredible Leonid display of 1833, is credited with starting the serious scientific study of meteors.
Good news for meteor observers can be a concern for satellite operators. A satellite can be disabled by the impact of even a small dust grain. While the hazard from man-made space debris is well known, the danger from meteoroids has been more difficult to assess. Prior 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. With this new work, McNaught and Asher have defined the structure of the Leonid dust trails more accurately than ever before.
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?
Although 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 over periods 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 the time is 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 (defined for an individual observer in 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 in both 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.