A comet flirts with Earth

Maybe you remember the spectacular comets Hyakutake and two-tailed Hale-Bopp gracing the sky throughout much of 1996 and 1997, respectively.

As my father and I observed them from our backyard in rural East Haddam when I was 13, I remember thinking how strange it was that they were speeding through space at thousands of miles an hour, yet seemed frozen in the sky. Their motion was apparent the very next night when they had quite obviously changed position relative to the stars.

On Oct. 20, 2010, comet Hartley 2 will come within 11 million miles of Earth. Its orbital period is 6.46 years and this is its fourth fly-by since 1986, when it was the second short-period comet discovered by astronomer Malcolm Hartley in Australia.

Short-period comets originate beyond Neptune's orbit in the Kuiper Belt. Studies of Hartley 2's travels show that it only recently settled into its current trajectory after encounters with Jupiter's gravity in 1947, 1971 and 1982. Until then, the comet never came closer to the sun than about 200 million miles and wasn't readily visible.

Hartley 2 spent September gaining speed in Andromeda and this month blazes across the northern sky to the bright star Capella in the constellation Auriga, the charioteer.

By the time you read this, the moon will be waning and the comet will be traveling through W-shaped Cassiopeia. On the night of Oct. 7, the moon will be new and Hartley 2 should be at magnitude 5 or 6-within naked-eye visibility-as it glides less than 1 degree south of the Double Cluster in Perseus, which is fairly easy to make out with binoculars.

Until mid-October, northern hemisphere observers can see the comet nearly all night long in the northeastern sky. After mid-October, it should be visible from 11:30 p.m. with optimal viewing just before dawn. Through a telescope, the comet may show more structure in the center and a faint tail.

NASA's Deep Impact spacecraft is scheduled to study Hartley 2 on Nov. 4 from about 600 miles away. Deep Impact's orbit around the sun was adjusted last May for this purpose. The craft is now on an extended mission and will gather photos and measurements of Hartley 2's half-mile nucleus.

The comet should be able to survive up to another 100 revolutions across approximately 700 years at its current rate of disintegration. It will pass us again around April 20, 2017.

localuniverse@msn.com

Sky calendar

OCT. 7 - New moon.

OCT. 20 - Hartley 2 makes its closest approach to Earth.

OCT. 21, 22 - Orionids meteor shower peaks, producing about 20 meteors per hour. Stray meteors may be visible a few days around the peak. Best viewed after midnight to the east.

OCT. 23 - Full moon

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