SURPRISE 1998 LEONID DISPLAY WAS A LARGE BLAST FROM THE PAST
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 has now been uncovered as a result of research by Dr David Asher and Professor Mark Bailey, of Armagh Observatory, and Professor Vacheslav Emel'yanenko, of South Ural University, Chelyabinsk, Russia.They have shown 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. By matching theory and observation, Dr Asher and colleagues have proved for the first time that meteoroid streams associated with Halley-like comets have complex braid-like structures within them. This work points the way to more precise predictions of the timing and intensity of meteor showers in the future. These results are reported in the 21st April 1999 issue of the Monthly Notices of the Royal Astronomical Society.
The Leonid meteor shower occurs between 15 and 21 November each year, with peak activity on the night of the 17/18 November. These meteors are produced when small dust particles ejected from Comet Tempel-Tuttle enter the Earth's atmosphere at high speed and burn up. Comet Tempel-Tuttle moves around the Sun in an elliptical orbit taking approximately 33 years for a complete revolution. Its orbit is similar to that of Halley's Comet, and so Comet Tempel-Tuttle is classified as a Halley-type short-period comet. Owing to the large angle between the Earth's orbit and the comet's (162 degrees), the dust grains collide almost head-on with the Earth, and hit the atmosphere at about 71 kilometres per second. At this speed, a one-centimetre particle carries the same amount of energy as a speeding truck on a motorway.
Every 33 years or so, when Comet Tempel-Tuttle passes near to the Earth, the intensity of the Leonid display is greatly enhanced because the stream of dust grains is more densely packed close to the comet. Meteor 'storms' have been seen many times during the past thousand years, notable events being those of 1799, 1833, 1866 and 1966. The earliest record of Leonid meteors dates back to the year 899.
November 1998 saw astronomers preparing for a possible meteor storm during the night of 17/18 November. Although a moderately strong peak was observed as predicted, the meteor shower as a whole was dominated by the appearance of hundreds of exceptionally bright meteors, known as fireballs, more than 16 hours ahead of the predicted peak.
The intensity and duration of this exceptional event indicated that the Earth must have passed through an extremely dense, narrow stream of large dust grains and particles, having sizes ranging up to several centimetres. The timing suggested that these particles occupied an orbit somewhat different from the main stream of small grains, and that they left the comet's nucleus many hundreds of years ago. But in that case, it is necessary to explain why the stream has held together so tightly for so long.
To solve the problem, Dr David Asher and his co-workers calculated the motion of large dust grains ejected from the comet at each of the last 42 occasions when it made its closest approach to the Sun. (Comets release very little dust, if any, when they are far from the Sun's heat.) They checked each case to see whether any of the particles could explain the fireballs seen in 1998, and identified September 1333 as the time when most of the observed particles were released. These particles did not spread out in space because of a dynamical process known as a resonance. (A similar process gives rise to the fine structure seen in Saturn's rings.)
Many comets and asteroids swing around the Sun in orbits that are simple multiples of the orbital period of Jupiter, the most massive planet in the solar system and the biggest disturbing influence on cometary orbits. Comet Tempel-Tuttle is no exception to this rule, having entered one of these 'resonant' orbits as long ago as the seventh century AD. For every fourteen revolutions of Jupiter, Comet Tempel-Tuttle makes five, and the same relation holds true for the largest dust particles gently released by the comet.
The large grains therefore have average orbital periods very close to that of the comet, and are kept in step by the influence of Jupiter. Instead of spreading around the whole orbit, they occupy a rather short arc, leading to the formation of a dense strand of large particles, distinct from the 'normal' storm strands of small particles, ahead of and behind the comet. The structure of the meteoroid stream close to the comet can be visualized as rather like a telephone wire, made up of many separate, narrow strands. These form a complex, braided structure of material within the broader, ribbon-like meteoroid stream.
The calculations by David Asher and co-workers showed that in November 1998 most of the resonant arcs missed the Earth by a wide margin, but the arc of particles released in 1333 cut right through the Earth's orbit, and the calculated time for when this happened matched the observed fireball maximum to the hour.
This remarkable result is the first observational demonstration of one of the most important dynamical features of meteoroid streams associated with Halley-type short-period comets. The work highlights the presence of fine structure *within* meteoroid streams, and suggests important new avenues for research. For example, by observing the variations in meteor rates close to the peak of a shower it may be possible to infer the precise distribution in space of the meteor-producing strands. Variations in meteor rates may be correlated with changes in the meteor brightness distribution to infer the history of mass loss by a comet over many revolutions around the Sun.
The researchers are not expecting a repeat performance of bright fireballs in November this year (1999). All the resonant strands in the meteoroid stream will be well past Earth in space. However, a strong 'normal' display is likely, peaking at about 2 a.m. on November 18th, due to meteoroids ejected from Comet Tempel-Tuttle in the years 1866, 1899 and 1932, which have not yet had time to disperse around the comet's orbit.