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.
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.
View the orbit of comet 8P/Tuttle in 3D:
http://neo.jpl.nasa.gov/cgi-bin/db?200504
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 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.”
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!
