Saturday, 20. October 2007
Remains of Halley’s Comet to fall Sunday

Some astronomers think weekend Orionids will be flashier than usual.

A junior version of the famous Perseid meteor shower is scheduled to reach its maximum before sunrise on Sunday morning, Oct. 21. This meteor display is known as the Orionids because the meteors seem to fan out from a region to the north of Orion's second brightest star, ruddy Betelgeuse.

Weather permitting and under very dark skies away from light pollution, skywatchers could see several meteors per hour. Rates will be significantly lower in cities and suburban areas.

Interestingly, this year, brilliant Mars is nearby and the apparent source of these meteors, called the radiant, will be positioned roughly between Mars and Betelgeuse.


Location of the Orionids
For Northern Hemisphere Observers


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Skywatchers can start looking for Orion low in the eastern sky around 1 a.m. on any morning around the peak, Oct. 21. The setup is seen here from mid-northern latitudes. Even though the radiant of the shower is in Orion, meteors can appear far from the constellation.

When and where to watch

Currently, Orion appears ahead of us in our journey around the Sun, and has not completely risen above the eastern horizon until after 11:00 p.m. local daylight time. Expect to see few, if any Orionids before midnight – especially this year, with a bright waxing gibbous Moon glaring high in the western sky.

But moonset is around 1:30 a.m. local daylight time on Sunday, and that's a good time to begin preparing for your meteor vigil. At its best several hours later, at around 5:00 a.m. when Orion is highest in the sky toward the south, Orionids typically produce around 20 to 25 meteors per hour under a clear, dark sky.

"Orionid meteors are normally dim and not well seen from urban locations," said meteor expert Robert Lunsford, adding, ". . . it is highly suggested that you find a safe rural location to see the best Orionid activity."

According to Lunsford, Orionid activity has been increasing noticeably since Oct. 17 when they were appearing at roughly five per hour in dark-sky conditions. After peaking on Sunday morning, activity will begin to slowly descend, dropping back to around five per hour around Oct. 26. The last stragglers usually appear sometime in early to mid Nov.


Location of the Orionids
For Southern Hemisphere Observers


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Halley's Legacy

In studying the orbits of many meteor swarms, astronomers have found that they correspond closely to the orbits of known comets. The Orionids are thought to result from the orbit of Halley's Comet; some of the dust which has shaken loose from this famous object as it runs its gigantic loop from the Sun out to Neptune, ram our atmosphere to create the effect of these "shooting stars."

There are actually two points along Halley's path where it comes relatively near to our orbit.

One of these points corresponds to early May and causes a meteor display that emanates from the constellation Aquarius, the Water Carrier. The other point lies near the late October part of our orbit and produces the Orionids. In May we meet the "river of rubble" shed by the comet on their way outward from their nearest approach to the Sun, while in October we encounter the part of the meteor stream moving inward toward the Sun. The meteors are moving through space opposite or contrary to our orbital direction of motion. That explains why both the Aquarids and the Orionids hit our atmosphere very swiftly at 41 miles (66 kilometers) per second – only the November Leonids move faster.

Another distinguishing characteristic that the October Orionids share with the May Aquarids is that they start burning up very high in our atmosphere, possibly because they are composed of lightweight material. This means they likely come from Halley's diffuse surface and not its core.

What to expect

Last year, there was an unexpected surprise when the Orionids put on a display more worthy of the Perseids. Observers saw meteors falling at double the normal rate, or 40 to 50 per hour. In addition, many Orionids were much brighter than normal; a few even rivaled Venus in brilliance.

Two meteor researchers, Mikaya Sato and Jun-ichi Watanabe of Japan's National Astronomical Observatory, recently announced in a paper released by the Astronomical Society of Japan that the unusual concentration of large particles that produced last years Orionids, were probably ejected from Halley's Comet almost 3,000 years ago and are being held together by interactions with Jupiter about every 71-years.

Apparently, there may also have been unusual Orionid activity during the years 1933 through 1938, so perhaps after an absence of seven decades this concentration of comet material has returned, implying another rich Orionid display might be in offing this year.

The only way to know is to step outside just before the break of dawn on the morning of Oct. 21 (try the mornings of Oct. 20 and 22 as well). Almost certainly, you should sight at least a few of these offspring of Halley's Comet as they streak across the sky.

Source: Space.com

See also:
Guide to the Orionid Meteor Shower by Sapce.com.
Observing the Orionids and Orionids History by Meteor Showers Online.
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