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.
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.
 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.
The debris
stream that lies ahead was shed by comet Tuttle in the year 1405.
According to Jenniskens and Lyytinen, Earth will pass 0.52 lunar
distances (0.0013 AU) from the center of the stream at 0729 UT
(0229 EST) on Dec. 22nd -- that's the most likely time for a strong
Ursid outburst. From beginning to end, the encounter could last
for 3 to 4 hours, says Jenniskens.
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.
 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 then in December 1986
observers 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."
 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.
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!
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