A Meteor Surprise
The normally meek Ursid meteor shower could surprise
sky watchers with a powerful outburst on Dec 22nd when Earth
passes through a dust stream from periodic comet Tuttle.
December 18, 2000 -- Even among enthusiastic sky watchers, the prospect of the annual Ursid meteor shower rarely provokes more than mild interest. Normally, the shower's feeble maximum on Dec. 22nd produces no more than a few shooting stars per hour. It hardly seems worth the trouble to spend a frigid winter night outdoors just to see a meager half-dozen or so meteors.
But this year could be different.
According to NASA scientist Peter Jenniskens (NASA-Ames SETI Institute) and colleague Esko Lyytinen, our planet is now heading directly for a dusty debris stream shed by periodic comet Tuttle, the parent of the Ursids. Meteor rates could soar to more than 100 per hour next Thursday night and Friday morning when Earth plows through the stream of meteoroids.
Right: Artist Duane Hilton created this fanciful view of an Ursid meteor.
Comet 8P/Tuttle follows a 13.5-year elliptical orbit that stretches from just inside Earth's orbit at perihelion (closest approach to the Sun) to an aphelion point (greatest distance from the Sun) between Jupiter and Saturn.
Each time Tuttle swings past the Sun, it leaves behind a new trail of debris. These narrow, filamentary trails are regions with a high density of meteoroids. Until they disperse after some centuries, a strong meteor shower can happen whenever Earth passes through one.
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A second but weaker flurry of Ursids is possible at 0840 UT (0340 EST) on Dec. 22nd when Earth crosses a debris stream deposited in 1392. That filament lies about 3 times farther away than the1405 stream, so meteor rates will be lower.
Meteor enthusiasts might not wish to place too much confidence in the exact times of the predicted outbursts -- they could be wrong. Pinpointing the Ursid debris trails is a new science and observations are still needed to refine the models. Sky watchers should be alert for outbursts at any time after local sunset on Thursday, Dec. 21st, and before dawn on Friday, Dec. 22nd.
To see the Ursids, be sure to dress in warm clothing, then go outside and look north. Ursid meteors can appear anywhere in the sky, although their tails will point back toward the shower's radiant near the bright orange star Kochab in the bowl of the Little Dipper.
Because the radiant is so close to the north celestial pole, Ursid meteors are practically nonexistent south of the equator. At most northern latitudes the Ursid radiant will be above the horizon all night long. The likely timing of the outbursts favors North Americans.
Above: The Ursid radiant is near the "Little Dipper" (Ursa Minor), not far from the North Star, Polaris. This graphic shows the radiant about 40 degrees above the northern horizon, as it would appear to stargazers at mid-northern latitudes at 2 a.m. local time on Dec. 22, 2000.
The last known outburst of Ursids happened in Dec. 1993, when comet Tuttle was nearing perihelion. Robert Lunsford, Secretary General of the International Meteor Organization, saw a flurry that he estimates would have produced 75 meteors per hour under ideal observing conditions.
"It is possible that this shower produces a short-lived burst of activity every December," notes Lunsford. "The Holiday season combined with poor weather and bitterly cold temperatures at this time of year in the northern hemisphere may explain why Ursid outbursts are seldom seen."
"Lunsford's outburst occurred at perihelion of the comet," notes Jenniskens. "As we demonstrated in our recent paper (Possible Ursid Outburst on Dec. 22, 2000) the 1993 event was caused by a widely dispersed dust component that is dynamically different than the single dust trails we're heading for this year."
The strongest Ursid outburst on record happened in 1945, when European observers saw 120 meteors per hour. The shower was mostly ignored by sky watchers in the 50's, 60's and 70's, but thenobservers spotted another flurry of 90 per hour. Both outbursts, in '45 and '86, came approximately six years after comet Tuttle had passed perihelion. Jenniskens and Lyytinen believe they have an explanation for the curious six-year lag. Many of comet Tuttle's debris particles are dispersed by Jupiter's powerful gravity when they pass close to the orbit of the giant planet. But some are in a 12:14 orbital resonance with Jupiter; Jupiter completes fourteen orbits around the Sun in the same time it takes the meteoroids to complete twelve. The resonance prevents disturbing close encounters.
"Basically, when these resonant meteoroids pass Jupiter's
orbit, the planet is never there," explains Jenniskens.
"So, the particles are fairly safe for a period of time.
The stream as a whole can then be gently nudged [by planetary
perturbations] until the meteoroids become Earth-crossers. That
takes 6 centuries. Gradually the meteoroids fall behind the comet because they move in a wider orbit than the comet does. The lag accumulates and, after 6 centuries, it adds up to about 6 years."
Above: Earth is heading for a resonant stream of meteors on Dec. 22, 2000. The dashed line indicates Earth's path through space; colored ellipses denote the estimated cross sections of Ursid dust trails. [more information]
This year's Ursid meteor shower comes 6.5 years after comet Tuttle last reached perihelion in mid-1994, substantially increasing the odds of an outburst later this week.
Since astronomer William Denning first noticed the Ursids around the turn of the century, there have been plenty of Decembers falling six years after a perihelion of comet Tuttle when no one saw anything extraordinary. Is that because meteor watchers tend to congregate by a warm fireplace on Dec. 22nd, oblivious to the rain of shooting stars outside? Or, are the Ursids simply unpredictable -- powerful one year and meager the next?
That's what some astronomers are longing to find out and they may have their answer on Friday!
Stay tuned to SpaceWeather.com for updates about the Ursid meteor shower. Readers can learn more about the Ursid debris streams in the Dec. 10, 2000, issue of WGN, the Journal of the International Meteor Organization, in the paper "Possible Ursid Outburst on Dec. 22, 2000" by Jenniskens and Lyytinen.
PREDICTIONS FOR UPCOMING DEC. 22 URSID OUTBURST: ANOTHER LEONID-TYPE SHOWER! -- more information about the 2000 Ursids from Peter Jenniskens Leonid MAC web site
See the orbit of Comet Tuttle in 3D -- from JPL
Learn more about the Ursids -- compiled by Gary Kronk
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