"We're predicting 20 to 30 meteors per hour over the Americas and as many as 200 to 300 per hour over Asia," said Bill Cooke, of NASA's meteoroid environment office. "Our forecast is in good accord with ... work by other astronomers."
"The Leonid shower is made of bits of debris from the Tempel-Tuttle comet, which streaks through Earth's inner solar system every 33 years.
It leaves a stream of debris in its wake. Forecasters, however, say it's hard to know exactly how many of the meteors will be visible."
According to NASA,The first stream will cross over Earth about 4 a.m. ET. That stream should produce about two or three dozen meteors per hour over North America.
According to the website: http://science.nasa.gov/headlines/y2009/10nov_leonids2009.htm,
"the first stream crossing on Nov. 17th comes around 0900 UT (4 a.m. EST, 1 a.m. PST). The debris is a diffuse mix of particles from several old streams that should produce a gentle display of two to three dozen meteors per hour over North America. Dark skies are recommended for full effect.
"A remarkable feature of this year's shower is that Leonids will appear to be shooting almost directly out of the planet Mars," notes Bill Cooke of NASA's Meteoroid Environment Office.
The following information will serve as an education for those of you interested in learning more about the Leonid meteor shower. According to the following website:
"The Leonids ([ˈli.əˌnɪdz] lee-uh-nids) are a prolific meteor shower associated with the comet Tempel-Tuttle. The Leonids get their name from the location of their radiant in the constellation Leo: the meteors appear to stream from that point in the sky. The 2009 display peaking on November 17 may produce more than 500 meteors an hour. This is not enough to rate it as a meteor storm which has over 1,000 meteors an hour"
"The Earth moves through the meteoroid stream of particles left from the passages of the comet. The stream comprises solid particles, known as meteoroids, ejected by the comet as its frozen gases evaporate under the heat of the Sun which begins to warm the comet as it comes within the orbit of Jupiter. A typical particle is no bigger than fine dust. The main source of light of a meteor is caused by the solar wind, which fragments and atomizes the dust,and the resulting spray of microscopic debris collides with individual atoms of the atmosphere ionizing the air. The air molecules recombine and cool by giving off photons. Larger particles leave a stream of smaller particles and form a tail, which can leave a glowing trail in the atmosphere. Leonids in particular are well known for having such bright meteors. Bolide is actually a generic term for any meteor, asteroid, comet, etc. The meteoroids left by the comet are organized in trails in orbits similar though different from that of the comet. They are differentially disturbed by the planets, in particular Jupiter."
"The Leonids are famous because their meteor showers, or storms, can be, and have been in a few cases, among the most spectacular. Because of the superlative storm of 1833 and the recent developments in scientific thought the Leonids have had a major effect on the development of the scientific study of meteors which had previously been thought to be atmospheric phenomena. The meteor storm of 1833 was of truly superlative strength. One estimate is over one hundred thousand meteors an hour, but another, done as the storm abated, estimated in excess of two hundred thousand meteors an hour over the entire region of North America east of the Rocky Mountains. It was marked by the Native Americans, slaves and owners, and many others. That same 1833 shower, near Independence, Missouri, was taken as a sign to push the growing Mormon community out of the area.
Other great Leonid storms were seen in 1866 and 1867. When the storms failed to return in 1899, it was generally thought that the dust had moved on and storms were a thing of the past. Then, in 1966 a spectacular storm was seen over the Americas. Leading up to the 1998 return, an airborne observing campaign was organized to mobilize modern observing techniques by Peter Jenniskens at NASA Ames Research Center. This resulted in spectacular footage from the 1999, 2001 and 2002 storms producing up to 3,000 Leonid meteors per hour. Initially, the exact location of the dust was unknown."
The following information about the Leonid Meteor shower was written by Joe Rao. Joe is an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
"The first cloud of comet dust was released from the nucleus of Tempel-Tuttle back in the year 1567. North America will be turned toward the constellation Leo when these particles begin pelting the upper layers of our atmosphere, some 80 to 100 miles (130 to 160 km.) above us. Earth's encounter with the comet dust is going to be brief – possibly no more than several hours long."
"Unfortunately, we won't be going directly through the center of cloud, but rather skim through its outer edge on Nov. 17, chiefly between about 4:30 and 10:30 GMT. As a consequence, the meteor rate is not expected to get much higher than 20 or 30 per hour (on average about one meteor sighting every two or three minutes). Still, this is about two to three times the normal Leonid rate."
"At the beginning of this window, it will still be dark across Europe and western Africa with Leo high up in the southeast sky, but within an hour the sky will be brightening as sunrise approaches, soon putting an end to meteor watching."
"North Americans – especially those living near and along the Atlantic Seaboard – will be able to watch for Leonids from after 1 a.m. local time right on until the first light of dawn, which comes soon after 5 a.m. local time.
Those in the eastern U.S. and Canada are especially favored because Leo will be high in the southeast sky between 3:30 and 5:30 a.m. EST, just before Earth is expected to exit the meteor cloud. For the West Coast, this translates to 12:30 to 2:30 a.m. PST, when Leo is much lower down in the eastern sky."
"Although the overall meteor numbers are expected to be modest at best, the particles that produce them might be larger than the usual flecks of dust that comprise the Leonid swarm. Recall that this cloud of comet particles was released into space in 1567. When such "comet bits" circle the sun for many hundreds of years, the tinier (dust grain) material tends to be pushed away from the sun and dispersed by the pressure of solar radiation. Conversely, because they are relatively unaffected by radiation pressure and leave the comet nucleus with less velocity than their smaller brethren do, the larger pebble-to-marble sized particles tend to linger for a much longer time."
"The Tempel-Tuttle comet was discovered by Ernst Wilhelm Liebrecht Tempel (Marseille, France) on 1865 December 19. It was then in the evening sky near the star Beta Ursa Majoris. He described it as a circular object, with a central condensation and a tail 30 arc minutes long. Horace Parnell Tuttle (Harvard College Observatory, Cambridge, Massachusetts) independently discovered this comet on 1866 January 6."
"The comet's best apparition was that of 1366 when it passed 0.0229 AU from Earth (2.1 million miles and 3.4 million kilometers)--marking the third closest approach of any comet to our planet in recorded history. Astronomers have suggested the total brightness may then have reached magnitude 3. The comet passed 0.0644 AU from Earth in 1699, which marked the 18th closest approach of a comet to Earth." (source:http://cometography.com/pcomets/055p.html )
"The Tempel-Tuttle, makes a fly-past of the Sun approximately every 33 years. During each close approach to the Sun, the comet sheds a stream of dust and small particles. These streams of particles can remain intact for centuries before dispersing. (source:http://star.arm.ac.uk/press/2003-Nov-7.html)
Bill Cooke can help us understand a meteor's energy. "To understand a meteors energy, Cooke explained, one needs to consider its mass and speed.
The faintest meteor that becomes visible to the average viewer on Earth is typically about 0.6 millimeters across (less than one-tenth of an inch or about the size of a sand grain). While such a speck is here and gone in a flash, the energy involved could light a 100-watt light bulb for about 2.5 seconds, Cooke said.
A slightly larger meteor, just 1 millimeter across and only moderately bright, packs the punch of a .22 caliber bullet.
Spectacularly bright fireballs, for which this annual event is known, dissipate far more energy during their plunge through the atmosphere. A typical fireball, which can briefly shine as bright as the planet Venus, is the size of a marble, about 9 millimeters in diameter.
"Such a critter has a striking power in excess of 1 million joules, or about the same punch as a VW moving at 60 mph," Cooke marveled, "from a particle just over one-third of an inch across!"
Behind this power is one simple fact: The Leonids scream.
The shooting stars of the Leonids are mere bits of dust left behind by a comet called Tempel-Tuttle, which orbits the Sun in the opposite direction as Earth travels. The comets dusty exhaust orbits the Sun, too.
|The Screaming Leonids |
The debris from comet Tempel-Tuttle is set up for a head-on collision with Earth.
"It's like two cars hitting head-on," Cooke says of a Leonid. "Greater speed equals greater energy to be dissipated, which equals a real short life in the atmosphere."
A Leonid crashes into Earths upper atmosphere at more than 160,000 mph (72 kilometers per second). A typical bullet from a rifle, moving at what seems like blinding speed, creeps along by comparison at just 2,240 mph (1,000 meters per second).
The comet grains become visible at around 60 miles up (100 kilometers) because they heat up as they plow through air, to the point that they glow. Most of them disintegrate high overhead.
Cooke uses basic physics to calculate the energy output of individual shooting stars. But with predictions for an historical flurry of activity in 2002, I pressed him to speculate on the total energy output of this years Leonids.
"You are putting me out on the limb," he said, but he attempted what he terms a crude answer anyway.
Most of the mass will come from big fireballs, Cooke reasons, because if you put 10,000 typical small Leonids together, they would weigh just 0.1 grams. A bright fireball, however, can weigh as much as 85 grams (3 ounces).
During the course of the peak hour over North America in the pre-dawn hours of Nov. 19, predictions of shower activity suggest that about 2.5 pounds (1 kilogram) of material will rain down over an area visible by a single observer. It will create about 650 kilowatts of energy, Cooke said, enough to run a small house for about a month if it could be harnessed.
"However, when one takes into account the entire hemisphere of Earth facing the Leonids, then we find that about 12-13 tons (12,500 kilograms) of Leonid stuff will hit our atmosphere at a screaming 45 miles per second," Cooke explained. "Which, taken in total, will produce around 4 kilotons of energy, or about 4.5 million kilowatt-hours, which could run about 7,000 houses for one month."
Making the light
The conversion of a meteors mass and momentum to heat and light is often said to be caused by friction with air molecules. This sounds logical, but its not true.
In fact, a meteor is moving so fast that it compresses the air in front of it, something like how a boat pushes water out of the way and makes a wake. The compressed air heats up by a phenomenon called ram pressure. Its the same thing that causes a hand-held air pump to get warm when you work it to fill a basketball.
The heated air, in turn, scorches the meteor. Temperatures can exceed 3,000 degrees Fahrenheit (1,650 Celsius).
A Leonids demise can look a bit like a fiery exclamation point in the sky, a long streak with, sometimes, a bright dot at the end. It is fitting punctuation to a journey that typically spans a stretch of time equal to a good chunk of American history.
The comet responsible for the Leonids, Tempel-Tuttle, orbits the Sun every 33.3 years on an elliptical trajectory that crosses Earths orbit, rounds the Sun and then swings out beyond the planet Uranus. On each pass through the inner solar system, the comet is bathed in heavier doses of solar radiation. Some of its outer shell boils off. This action explains why a few comets can sometimes be seen from Earth to glow dramatically and sport tails -- sunlight reflects off this fresh material.
Its also why we have meteor showers.
While Tempel Tuttle does not make itself visible to us, every orbit lays down a new trail of debris in a slightly different location. These trails really more like streams spread out over time as the individual dust particles continue to orbit the Sun, each taking a minutely different path.
This year, Earth will pass through two primary streams of debris. One, deposited in 1767, will generate an outburst of shooting stars over Europe. The other, from 1866, will fuel the North American burst of activity. These bursts will last less than a few hours, peaking perhaps in a flurry of activity that goes on for no more than 30 minutes.
The burst is caused by the streams area of greatest density. You can think of it as wading through a real stream the edges are shallow, and most of the water is in the deep middle.
There are many of these Leonid streams, many of them so ancient theyre no longer detectable as individual entities. For that reason, the total run of the Leonid meteor shower is from about Nov. 14-20. During this whole stretch, Earth will pick up Tempel-Tuttle dust thats been spreading for millennia.
Centuries from now, in fact, the two streams that caused this years storm will have become largely indiscernible, having spread out into the broader, overall river of dust that makes the Leonids a surefire annual event.
Here is a four minute video about meteors and the Leonid Meteor Shower: