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LEONID DAILY NEWS: December 18, 2000

Graph showing where various dust particles
ejected by comet Tuttle will cross the ecliptic plane

Figure showing where various dust particles ejected by comet Tuttle will cross the ecliptic plane six centuries later in the years 1994-2003.


Meteoroids ejected in 1405, before the birth of Columbus, are destined to hit Earth on December 22, 2000, at around 7:29 UT. The event is best seen from the USA and Canada. The shower will be 3-4 hours wide and have a peak rate of perhaps 1 per minute under good observing conditions, but many of these meteors may be faint. The peak time corresponds to 02:29 UT Eastern Standard Time (Dec. 22), 00:29 Mountain Time (Dec. 22) or 23:29 Pacific Standard Time (Dec. 21).

Meteor astronomer Peter Jenniskens of the SETI Institute, at NASA Ames Research Center, and his colleague Esko Lyytinen of Helsinki, Finland, applied the same tools used to predict the recent Leonid showers to another Halley-type comet, 8P/Tuttle, the source of the Ursids. They discovered why intense outbursts in 1945 and 1986 occurred six year's after passage of the comet. Moreover, they predict another outburst on December 21, 2000, when Earth will cross a dense trail of dust ejected during the comet's return in 1405. To alert meteor observers, a paper has been submitted to WGN, the Journal of the International Meteor Organisation.

Location of Tuttle trails

Graph showing the location of the Tuttle dust trails from ejecta from different returns of the comet as they are near Earth's orbit in the year 2000. Figure below shows an example Ursid meteor.

The explanation is in the fact that the particles are trapped in orbital resonances that determine the dynamical evolution of the shower. The meteoroids are released in an orbit that is completely outside of Earth's orbit. Once trapped, it takes 6 centuries for the meteoroid orbits to be perturbed enough to encounter the Earth. The particles are in a slightly wider orbit than the comet. The particle's orbital period is a factor of 7/6 times that of Jupiter. The comet's orbital period (13.62 years) is a factor 15/13 times that of Jupiter. The difference adds up to 600 x (7/6 divided by 15/13) - 600 = 6.7 years after 6 centuries. This explains why the dust arrives at the Earth about 6 years after the comet.

Ursid meteor

From many years of observations, Jenniskens discovered that the Ursid shower has two types of outbursts: broad outburst near perihelion and narrow outbursts near aphelion. The broad outbursts from what is now called the Ursid Filament, turn out to have a rather dispersed radiant and can not be from a single dust trail. The aphelion outbursts, however, are narrow and may well originate from a single dust trail.

Lyytinen calculated the position of the dust trails of comet 8P/Tuttle as created during each return 6 centuries ago, using the same approach as for earlier predictions of the Leonid shower. The trailet of 1378 was identified as the cause of the 1986 Ursid outburst, and that of 1392 as the cause of the 1945 outburst.

Now, in December 2000, the comet is again at aphelion. The figure to the right shows the position of the dust trailets during this year's encounter. The dust trailet of 1405 will be in the Earth's path. A meteor outburst is expected around 7:29 UT. The shower will be 3-4 hours wide. Rates may go up to 1 per minute under good observing conditions, but many of those will be fairly faint. Observers are recommended to go to a dark and clear-sky location. The Moon will not be disturbing.

The shower will not be as intense as recent Leonid outbursts, but may prove an important event to test some of physical processes observed for the Leonids. The meteoroids can be dated precisely and have the same dynamical background as the Leonids. However, they originate from a diffrerent comet, have spend six times longer in space before hitting Earth and come in at only half the velocity (Full paper (pdf); Full paper (HTML version) ).

Previous news items:
Dec. 18 - Dec 22 Ursid outburst
Nov. 20 - A bacterial fingerprint?
Nov. 15 - HCN disappears mysteriously
Nov. 14 - Meteor shower from space
Nov. 13 - Organic fingerprint
Nov. 12 - Train airglow chemistry
Nov. 11 - Hard bits and persisting glows
Nov. 10 - Meteoroid debris detected
Nov. 09 - New meteor picture
Nov. 08 - Spin city
Nov. 07 - Meteors affect atmospheric chemistry
Nov. 06 - Listen to this!
Nov. 04 - Fear of heights?
Nov. 03 - The pale (infra-red) dot
Nov. 02 - Twin showers
Nov. 01 - Leonids approaching Earth
Oct. 31 - Prospects for Moon Impact Studies
Oct. 30 - Comet dust crumbled less fine
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Results of this work were published in Icarus.

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