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(meteorobs) Fw: RASC List: RAS PN 99/09 1998 Leonid meteor surprise explained (fwd)
>
>ROYAL ASTRONOMICAL SOCIETY
>PRESS NOTICE
>
>Date: 15 April 1999
>For immediate release
>
>Ref. PN 99/09
>
>
>Issued by:
>
>Dr Jacqueline Mitton
>RAS Press Officer
>Office & home phone: Cambridge ((0)1223) 564914
>FAX: Cambridge ((0)1223) 572892
>E-mail: jmitton@dial.pipex.com
>
>RAS Web: http://www.ras.orgdot uk/ras/
>
>* * * * * * * * * * * * * * * * * * * * * * * * *
>
>CONTACTS FOR FURTHER INFORMATION ON THIS RELEASE:
>
>Dr David Asher, Armagh Observatory
>Phone: (0)1861-522928; Fax: (0)1861-527174; e-mail: dja@star.arm.acdot uk
>
>Prof. Mark Bailey, Armagh Observatory
>Phone: (0)1861-522928; Fax: (0)1861-527174; e-mail: meb@star.arm.acdot uk
>
>Mr John McFarland, Armagh Observatory (Public Relations Officer)
>Phone: (0)1861-522928; Fax: (0)1861-527174; e-mail: jmf@star.arm.acdot uk
>
>Professor Vacheslav Emel'yanenko, South Ural University, Chelyabinsk,
Russia
>Phone: 007 3512 399291; Fax 007 3512 655950;
>E-mail: emel@termeh.tu-chel.ac.ru
>
>* * * * * * * * * * * * * * * * * * * * * * * * *
>
>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.
>
>-end-
>
>
>
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