Re: Asteroid...
Geschrieben von jerrydalien am 24. Dezember 2000 11:32:51:
Als Antwort auf: Asteroid... geschrieben von Apollo am 24. Dezember 2000 07:13:08:
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A Meteoric Surprise - Ursid's to Appear on Dec. 22
December 19, 2000 08:02 CDTMost skywatchers normally consider the annual Ursid meteor shower somewhat average - with only a few shooting stars per hour. This year, however, there may be something to look forward to.
On Dec. 22, when Earth passes through a dust stream from periodic comet Tuttle, the Ursid meteor shower promises to put on quite a show!
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. The two scientists predict meteor rates could climb as high as 100 per hour - or more - next Thursday night and Friday morning for a period of roughly 3 to 4 hours.
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 - taking centuries to disperse. Until they do, 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. A second debris stream - from 1392 - will also be a possible second target.
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.
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 from the center of the stream at 2:29 EST on Dec. 22, that's the most probable time for a strong Ursid outburst. The encounter could last for 3 to 4 hours, says Jenniskens.
A second flurry of Ursids is possible at 3:40 EST on Dec. 22 when Earth passes through a debris stream deposited in 1392. That filament lies about 3 times farther away than the 1405 stream, so meteor rates will be lower.
Sky watchers shouldn't get too excited about this spectacular show since the science of pinpointing the Ursid debris trails is a relatively new science. Observations are still needed to refine the models.
Ursid meteors can appear anywhere in the sky, although north is the most probable direction. Their tails will point back toward the shower's radiant near the bright orange star Kochab in the bowl of the Little Dipper.
At most northern latitudes the Ursid radiant will be above the horizon all night long. The likely timing of the outbursts favors North Americans. Because the radiant is so close to the north celestial pole, Ursid meteors are practically nonexistent south of the equator.
Dec. 1993 is the last known date of an Ursids shower, when 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. European observers saw 120 meteors per hour. The showers were mostly ignored until December 1986, when 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 strong gravity when they pass close to the orbit of the giant planet. But some are in a 12:14 orbital resonance with Jupiter; as it 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."
Comet Tuttle last reached perihelion in 1994, 6.5 years ago, which increases the odds of a spectacular show later this week.
Source: Science@NASA