[meteorite-list] The Orionid Meteor Shower Peaks on Thursday Morning, Oct 21

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Oct 20 17:26:13 2004
Message-ID: <200410202126.OAA28113_at_zagami.jpl.nasa.gov>


Magical Morning
NASA Science News
October 19, 2004

The Orionid meteor shower peaks this week on Thursday morning, Oct. 21st.

October 19, 2004: Would you like to see a piece of Halley's Comet streak
past a planet that looks like an exploding star? No problem. Just set your alarm.

It's going to happen, in plain view--no telescope required, on Thursday
morning, Oct. 21st.

Go outside before sunrise, around 5:30 a.m. is best, and look east. The
brightest object in that direction is the planet Venus. It looks like a
star going supernova. Above Venus lies Saturn, and below, near the
horizon, is Jupiter. Every 10 minutes or so you'll see a meteor streak
among these planets. The meteors are pieces of Halley's Comet.

"Every year around this time Earth glides through a cloud of dusty
debris from Halley's Comet," explains Bill Cooke of the NASA Marshall
Space Flight Center. "Bits of dust, most no larger than grains of sand,
disintegrate in Earth's atmosphere and become shooting stars."

"It's not an intense shower," he says, "but it is a pretty one."

Astronomers call it the "Orionid meteor shower," because the meteors
appear to stream out of a point (called "the radiant") in the
constellation Orion. The radiant is near Orion's left shoulder. But
don't stare at that spot, advises Cooke. Meteors near the radiant seem
short and stubby, a result of foreshortening. Instead, look toward any
dark region of the sky about 90 degrees away. The vicinity of Venus or
Jupiter is good. You'll see just as many Orionids there, but they will
seem longer and more dramatic.

Framing the scene are several bright stars: Sirius, Regulus, Procyon and
others. Pay special attention to Castor and Pollux in Gemini. They're
arranged in an eye-catching line with Saturn.

To sum it up in one word: "sparkling." Two more words: "early" and
"cold." Or how about "worth waking up for?" You decide.

More about the Orionids

The Orionids are related to the eta Aquarids, a 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
near the orbit of Neptune 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 layers of dust each

At first, the bits of dust 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. One thing is certain: "Orionid meteoroids
are old."

They're also fast. "Orionid meteoroids strike Earth's atmosphere
traveling 66 km/s or 148,000 mph," he continued. Only the November
Leonids (72 km/s) are faster. Sometimes fast meteors explode, and they
leave glowing "trains" (incandescent bits of debris in their wake) that
last for several seconds to minutes. These trains, blown by upper
atmospheric winds into twisted and convoluted shapes, can be even
prettier than the meteors themselves.

You never know what you might see, before sunrise, on a magical Thursday
Received on Wed 20 Oct 2004 05:26:09 PM PDT

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