Posted on 10/18/2001 6:10:00 AM PDT by callisto
Oct. 17, 2001: Next Sunday, try doing something out of the ordinary: Wake up at 3 a.m. Put on warm clothes. Step outside. Look up.
If the skies are cloudy, go back to bed. Otherwise keep looking. Before long you'll spot something that makes the trip outside worthwhile: a bright shooting star -- and a genuine piece of Halley's comet!
"It's the annual Orionid meteor shower," explains Bill Cooke, a member of the Space Environments team at the Marshall Space Flight Center (MSFC). "Every year in October Earth passes through a stream of dusty debris shed long ago by Halley's comet." When bits of comet dust -- most no larger than grains of sand -- strike Earth's atmosphere and disintegrate, they become "shooting stars."
The Orionids -- so named because they appear to streak out of a point (called the radiant) in the constellation Orion -- will peak on Sunday morning, October 21st. Sky watchers north of the equator with dark clear skies will spot 15 to 20 meteors each hour before dawn. Observers south of the equator will see almost as many: 10 to 15 per hour.
Finding the Orionid radiant is easy. It lies near the left shoulder of Orion the Hunter, roughly centered within an eye-catching triangle consisting of Sirius -- the brightest star in the sky -- and the giant planets Jupiter and Saturn. (These stars and planets are in the southeastern sky before dawn, as viewed from mid-northern latitudes.)
But don't stare directly at the radiant, say experienced meteor watchers. Orionids that appear there will seem short and stubby -- a result of foreshortening. Instead, look toward any dark region of the sky about 90 degrees away. You'll see just as many Orionids, but they will seem longer and more dramatic. The tails of all Orionid meteors, no matter where they appear, will point back toward the radiant in Orion.
The October Orionids are cousins of the eta Aquarids -- a mostly southern hemisphere meteor shower in May. Both spring from Halley's comet.
"Earth comes close to the orbit of Halley's comet twice a year, once in May and again in October," explains Don Yeomans, manager of NASA's Near-Earth Object Program at the Jet Propulsion Laboratory. Although the comet itself is rarely nearby -- it's beyond the orbit of Saturn now -- Halley's dusty debris constantly moves through the inner solar system and causes the two regular meteor showers.
In 1986, the last time Comet Halley swung past the Sun, solar heating evaporated about 6 meters of dust-laden ice from the comet's nucleus. That's typical, say researchers. The comet has been visiting the inner solar system every 76 years for millennia, shedding dust each time.
At first, newly-liberated dust specks simply follow the comet -- which means they can't strike our planet. Earth's orbit and Halley's orbit, at their closest points, are separated by 22 million km (0.15 AU). Eventually, though, the dust spreads out and some of it migrates until it is on a collision course with Earth.
"Particles that leave the nucleus evolve away from the orbit of the comet for two main reasons," explains Yeomans. "First, gravitational perturbations caused by encounters with planets are different [for the dust and for the comet]. Second, dust particles are affected by solar radiation pressure to a far greater extent than the comet itself."
"The orbital evolution of Halley's dust is a very complicated problem," notes Cooke. No one knows exactly how long it takes for a dust-sized piece of Halley to move to an Earth-crossing orbit -- perhaps centuries or even thousands of years. However, one thing is certain: "Orionid meteoroids are old."
And fast. "These meteoroids strike Earth's atmosphere traveling 66 km/s or 148,000 mph," he continued. Only the November Leonids (72 km/s) are faster. Such meteors often leave glowing "trains" (incandescent bits of debris in the wake of the meteor) that last for several seconds to minutes.
Cooke and a group of his colleagues, led by Rob Suggs of the MSFC Engineering Directorate, will be observing the Orionids this weekend from Huntsville, Alabama, using an array of image-intensified cameras that can detect stars as faint as 8th magnitude. (For comparison, the unaided human eye can see stars of 6th magnitude against a very dark sky. An 8th magnitude star is 6.3 times dimmer than a 6th magnitude star.)
"This is our tune-up for the Leonid meteor storm next month," says Suggs. "We plan to station these cameras, which were developed at the University of Western Ontario, all around the world to monitor meteor activity on November 18th." That's when Earth will pass through a series of debris streams from periodic comet Tempel-Tuttle, perhaps unleashing a meteor shower of thousands of shooting stars per hour.
"The Orionids won't produce nearly as many meteors as the Leonids," added Suggs, "but, like the Leonids, the Orionids are fast, so they'll provide a good test for our system."
Next week Science@NASA will feature the results of Suggs' weekend meteor filming and explain more about the upcoming Leonid meteor shower. But why wait? You can enjoy a Leonid tune-up of your own this weekend. Simply go outside, look up, and watch as Halley's Comet returns ... in bits and pieces.
Dad gummit! I stepped out about 3:07 and waited for a minute and didn't see any more so I went on to bed! Oh well, I'll have to wait for the Leonids. But last night was so clear and beautiful I was even able to see the Pleides very well which I usually cannot do because of light pollution around here.
Many people have never observed Mercury but on Oct 30, Mercury makes its farthest elongation form the sun and is easy to view. What makes it special on this day is that Mercury resides less than one degree from Venus. To view, look to the eastern sky 45 minutes before sunrise. As you may know, Venus is the third brightest object in the sky (behind the sun and moon). For this reason, Venus will be very easy to sight and since Mercury is so close, you should be able to view both planets quite well. Actually, there are five days on either side of the 30th where Mercury is near enough to Venus to sight positively.
I’m really waiting for the Leonids, tho. In the 19th century, some Leonid meteor showers where so great that it looked as if an illuminated umbrella covered the sky!
Here's the link:
"If you missed the show Sunday morningt, try again Monday! The Orionid meteor shower has a broad maximum spread over several days. Sky watchers will likely spot 5-to-15 shooting stars per hour between 3 a.m. and dawn on Oct 22nd."
Bump for Mercury and Venus viewing tomorrow morning.
callisto, you began this thread as a notice for the Orionids and the remants of Halley’s comet. Another comet observation in November is Comet LINEAR (C/2000 WM1), a binocular event with a chance for unaided visibility by the end of the year, which starts early November in Perseus and ends November near the cord nexus in Pisces. See: Finding Charts ,the second chart down; notice Comet in Perseus in early November, progressing through Aries (almost ontop of Hamal, alpha Aries, on 11/27) and into Pisces in late Nov/early Dec.
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