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October meteors slow & fast: two contrasting meteor showers enrich mid-autumn's night skies.

As Earth wheels through the October portion of its orbit around the Sun, it passes through two reliable annual meteoroid streams: one fast, one slow, both long-lasting.

* The Taurids are slow and graceful. They originate from the unusually short-period comet 2P/Encke and other objects that seem to be ancient Encke breakup products, including the dormant, asteroid-like comet 2004 [TG.sub.10]. The Taurid stream is very broad, so the shower is exceptionally long-lasting; it runs throughout October, November, and into December. It has two distinct portions, the Northern and Southern Taurids, with radiants about 9[degrees] apart. Both remain active throughout these 2 1/2 months, though the Southern Taurids come to their full strength first.

The Taurids are not very exciting in terms of numbers. A careful counter might log about a half dozen per hour from late October through November under ideal observing conditions. But at least you don't have to go out at a pre-dawn hour; the radiants in Taurus reach a high altitude by late evening in October and mid-evening in November.

What's exciting about the Taurids is their occasional dazzling fireballs. Whatever happened to Comet Encke during its putative breakups in the past 20,000 years, the events left an unusual number of large chunks scattered amid the little debris bits of the kind that make up most meteoroid streams. If you see a grand fireball during these months, trace its path backward across the sky and see if the line intersects Taurus. That goes for daytime fireballs too, if you see them in the morning. Taurus doesn't set in the west until roughly 9 a.m. daylight-saving time in October, though sooner after sunrise in November. In late October and early November 2005, numerous "Halloween fireballs" made news around the world.

Some have suggested that the brightest meteor in recorded history--the several-megaton Tunguska explosion on June 30, 1908--was a Taurid. In

June Earth passes through a different part of the Taurid stream, causing a daytime shower. However, meteor expert Peter Jenniskens dismisses the idea. "The Tunguska fireball penetrated down to 8 km altitude," he says. "[Zdenek] Sekanina did a nice paper showing that you need relatively strong material to survive this deep into the atmosphere. Taurids have the strength of other cometary materials: they are fragile and fall apart high in the atmosphere."

Interestingly, however, an excess of Earth-crossing asteroids are reported to have Taurid-like orbits.

* The Orionid shower is more conventional. These fast-moving meteors reach an irregular peak from about October 20th through 24th, though some appear from mid-October into December. They radiate from the top of Orion's Club, which doesn't rise high until early-morning hours. By then the roughly first-quarter Moon this year will have set.

The Orionids are bits of Comet 1P/Halley, as are the Eta Aquariids of May--another case of one meteoroid stream crossing Earth's orbit in two places. Historically the Orionids have produced about 20 swift meteors per hour in a dark sky before dawn, but from 2006 to 2009 the shower was richer, with zenithal hourly rates of 40 to 70 per hour on two or three nights running. In more recent years the numbers have returned closer to normal.

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Title Annotation:Celestial Calendar
Publication:Sky & Telescope
Geographic Code:1USA
Date:Oct 1, 2012
Words:534
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