Featuring perhaps the fastest meteors of any shower, the Leonids this week may be easy to locate but hard to actually see, thanks in both cases to the moon. (Get the scoop on last year's Leonids.)
"The quarter-phase moon"—when the lunar disk appears half dark—"will interfere with meteor watching this year," said Ben Burress, staff astronomer at the Chabot Space and Science Center in Oakland, California.
"But since [the moon] won't rise until after local midnight, and meteors can be seen in the earlier hours as well, there is a spell of time in the evening of the 17th when the sky will be moonless and darker, making for good conditions for viewing," Burress said.
For instance, sky-watchers worldwide can try looking up at the eastern sky during the predawn hours of Friday, when the Leonids' peak will produce about 20 visible shooting stars an hour.
Astronomers also expect as many as three outbursts during which there could be upward of 200 meteors an hour. However, because the particles involved will be exceptionally small—about a hundred thousandths of a millimeter across—these outbursts will most probably go unnoticed by observers, even in dark locations.
Mars, Moon Point to 2011 Leonids
All Leonid meteors appear to radiate from their namesake constellation, Leo, the lion, and this year skywatchers will have a couple of convenient guideposts to locate the phenomenon.
"At midnight the meteors, radiating from Leo, will be coming from the east point on the horizon, and since the moon and Mars are both inside the constellation Leo this year, they are perfect visual markers for the shower's radiant point," Burress said.
Like its more famous cousin, the August Perseids, the Leonid meteor shower occurs when Earth plows through a trail of debris left in the wake of a comet orbiting the sun—in this case, the 1.2-mile-wide (2-kilometer-wide) comet Tempel-Tuttle. (See asteroid and comet pictures.)
Every 33 years, as this icy visitor gets close to the sun, melting ice releases pieces of dust—most no larger than a grain of sand—and deposits them in clumps along the comet's orbit.
Earth annually crosses paths with these particle clouds, many of which burn up in our atmosphere and create fleeting shooting stars or meteors. Occasionally a larger object—more like a pebble or even a boulder—will produce a brilliant, slower moving fireball with smoke trails that can linger for a minute.
(Related: "Meteor Dust May Affect the Weather, Study Says.")
Because the Leonids are basically traveling toward us when Earth smashes into them, these meteors tend to be the fastest on record, with speeds of up to 130,000 miles (210,000 kilometers) per hour.
Leonids Changed Astronomy
People have known about the Leonids for centuries, but the meteor showers' claim to fame was the major Leonid outburst of 1833. Sky-watchers witnessed a storm of thousands of meteors per hour—an event that changed how people look at meteor showers, Burress said.
"It was [the] observation and study of the 1833 outburst that led to the understanding that meteors are a phenomenon coming from space, not originating, as previously speculated, solely from Earth's atmosphere," he said.