Blast from the Past

Armagh Observatory press release, 1999 April 21st.


Research by David Asher and Mark Bailey, of Armagh Observatory, and Vacheslav Emel'yanenko, of South Ural University, Chelyabinsk, Russia, has explained the unexpected, spectacular shower of bright meteors seen in the small hours of 17 November last year. The work is reported in a paper published in the 21 April 1999 issue of the Monthly Notices of the Royal Astronomical Society.

The Leonid meteor display occurs between 15 and 21 November each year, with peak activity on the night of the 17/18 November. The observed meteors are produced by the collision with the Earth's atmosphere of small dust particles ejected from the parent comet, Tempel-Tuttle. This object moves around the Sun in an elliptical orbit with a period of revolution approximately 33 years. The dynamical features of the orbit are similar to those of Halley's comet, and so comet Tempel-Tuttle is classified as a Halley-type short-period comet. Owing to the extreme inclination of the cometary orbit (162 degrees), the dust grains collide almost head-on with the Earth, at a relative velocity of about 71 kilometres per second. At such a speed, a centimetre-size particle has the same kinetic energy as a speeding truck on a motorway.

A meteor shower is seen every year, but every 33 years or so, owing to the much higher spatial density of dust grains close to the comet, the intensity of the display is greatly enhanced. In fact, strong meteor showers, known as `meteor storms', have been seen many times during the past thousand years, notable events being those of 1799, 1833, 1866 and 1966. The attached figure (JPEG, 340k) shows an engraving of the meteor storm seen from the United States in 1833, while the earliest record of Leonid activity dates back to the year 899.

November 1998 saw astronomers preparing for a possible meteor storm during the night of 17/18 November. (Another possible storm date is 17/18 November 1999.) However, although a moderately strong shower peak was observed as predicted, the meteor shower as a whole was dominated by the earlier appearance of hundreds of exceptionally bright meteors, known as fireballs. These were seen by observers at European longitudes during the previous night, 17 November, more than 16 hours ahead of the predicted peak.

In fact, meteor storms come in two types: a `normal' storm, comprising many visual meteors per second, and explained as the result of the Earth running into particles that have been recently released from the comet; and a `fireball' storm, in which the overall meteor count is lower but the event is dominated by hundreds of spectacularly bright meteors or fireballs.

The 1998 event was of the second kind, and 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, having sizes ranging up to several centimetres. The timing of the event, more than 16 hours ahead of schedule, suggested that these dust particles occupied an orbit somewhat different from that making up the main stream of small grains. The orbital difference showed that the meteoroids must have left the cometary nucleus many hundreds of years ago; but then, how could the stream have maintained its coherence and high spatial density for so long?

To solve the problem, David Asher and coworkers calculated the motion of large dust grains ejected from the comet at each of its last 42 perihelion passages. They checked each case to see whether any of the particles could explain the fireballs seen in 1998, and identified the perihelion passage of September 1333 as the time when most of the observed particles were released. These particles had avoided spreading out as a result of a dynamical process known as a resonance, analogous to the mechanism leading 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 metronome effect of Jupiter's period. Instead of spreading around the whole orbit, they instead occupy a rather short orbital 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 structure of the meteoroid stream as a whole.

The calculations by David Asher and coworkers 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. What proved that this really explained the observations perfectly was that, although an encounter with one or another particle strand might normally be possible at any time within a day or two of the predicted Leonid maximum, the calculated intersection of the 1333 arc with the Earth 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 the use of variations in the meteor rate close to the shower peak to infer the precise distribution in space of the meteor-producing strands, and correlating the variations in meteor rate with changes in the meteor brightness distribution to infer the history of cometary mass loss over many revolutions. The work points the way to making quantitative predictions of the timing and intensity of meteor showers, in contrast to the situation up to now, which has been largely based on broad-brush features of the cometary orbit and the trend in meteor activity from year to year.

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.
E-mail: jmf@star.arm.ac.uk, dja@star.arm.ac.uk, meb@star.arm.ac.uk.
Professor Vacheslav Emel'yanenko can be reached by e-mail at emel@termeh.tu-chel.ac.ru.


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