Editor's Note: Predicting meteor showers is challenging for astronomers. Skywatchers in dark-sky locations will see more meteors than those in suburban and urban areas, and rates will vary at different times during the overnight and early morning hours. Expect anywhere from six to 24 meteors per hour from the Orionid peak period, depending on conditions.
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.
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 time.
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 morning.
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