Saturn Workshop
November 2001

Saturn Workshop
Nov. 13, 2001
By Dave Gill
(Below are the text portions of my PowerPoint presentation)

What are meteors?
Meteors are better known as "shooting stars": startling streaks of light that suddenly appear in the sky when a dust particle from outer space evaporates high in the Earth's atmosphere.
We call the light phenomenon in the atmosphere a "meteor
the dust particle is called a "meteoroid".
How big are they?
Most visible Leonids are between 1 mm and 1 cm in diameter.
A Leonid meteor of magnitude +5, (barely visible with the naked eye in a dark sky), is caused by a meteoroid of 0.5 mm in diameter and weights only 0.00006 gram.

How fast do they travel?
That tiny particle can be seen over distances of hundreds of kilometers.
The reason is the astronomical speed of the meteoroids.
Just before they enter the Earth's atmosphere, Leonid meteoroids travel at 71 kps (44 mi/sec), and could travel around the Earth in 3.8 minutes!

Why are they bright?
When meteoroids enter the Earth's atmosphere, they collide with numerous air molecules. Those collisions sputter away the outer layers of the particle, creating a vapor of sodium, iron and magnesium atoms. In subsequent collisions, electrons are knocked into orbits at larger mean distances from the nucleus of the atoms. When the electrons fall back to their rest positions, light is emitted. This is the same process as in gas discharge lamps.

Are they colored?
The color of many Leonids is like the color of sodium discharge lamps (yellow).
Meteoroids contain traces of sodium. The color of a meteor is an indication of its composition and the excitation temperature:
sodium atoms give an orange-yellow light,
iron atoms a yellow light,
  magnesium a blue-green light,
calcium atoms may add a violet hue,
silicon atoms and molecules of atmospheric nitrogen give a red light.

Can I hear them?
Not usually
Sometimes a hissing noise is caused by VLF radio waves interacting with the environment
Rarely a bright fireball will cause a sonic boom

Do they leave a trail?
Wake - a brief glow behind the meteor head caused mainly by the green light of neutral oxygen atoms lasting 1-10 sec.
Afterglow/trail - the persistent metallic atom (Na, Fe,Mg) emission glow in the path of bright fireballs. The afterglow lasts a few seconds to many minutes in rare occasions

What are meteor showers?
Some days of the year show a larger number of meteors than usual that all have a common point of origin on the sky and originate from the same comet. The Leonids are such a meteor shower.

Where do they come from?
METEOROID STREAM: is the cloud of dust particles in the path of a comet. When Earth encounters a meteoroid stream, the particles evaporate high in the atmosphere in a brilliant light, causing a meteor shower.
Where do they come from?
Meteoroid streams are caused by the debris of comets.
The stuff of comets comes from interstellar space where the materials are assembled in the atmospheres of stars and in the dense molecular clouds of gas and dust between the stars.
The comets are build of that material and were formed in the outer parts of the solar system, in regions beyond Saturn's orbit, at the time of the birth of our solar system.
Where do they come from?
Comet nucleus is the mountain of ice and dust (mostly dust) that is at the center of a comet. This picture is the nucleus of comet 1P/Halley. It has a 2-3 times larger nucleus than 55P/Tempel-Tuttle.

The comet travels every 33.3 years between the orbits of Earth and Uranus.

Comets and Meteors
Dust trail is what the astronomers call the young meteoroid stream. The dust trail is found spread out along the orbit of the comet
Dust tail of a comet is caused by dust particles small enough to scatter sunlight. Those are very small smoke-like particles (less than 0.1 mm in size) and are ejected from the dust coma by solar radiation forces.

Comets and Meteors
Dust trails and dust tails represent large and small dust particles, respectively.
The large particles remain close to the comet and form a dust coma. However, small ejection velocities cause large differences in orbital period of these particles.
Hence, in the next return the slow particles will lag and the fast particles will proceed the comet.
The result is a trail like structure in the orbit of the comet. We see a meteor storm when the Earth crosses that trail of dust.
Meteor showers and storms
Meteor storm is seen when Earth crosses such dust trail. Officially, for a meteor storm rates have to increase to 1000 per hour, or 1 per second.
Annual shower is the name for a wider and more dispersed meteoroid stream after the major planets perturbed the dust away from the dust trails. Earth encounters this dust at about the same intensity every year. In most years, the Leonids are a rather insignificant annual meteor shower. Rates peak at 13 per hour on November 17. The main activity is between November 13 and 20, but rates are larger than 1 per hour in the period between October 31 and November 30 (in good dark skies early in the morning).
Meteor showers and storms
Meteor outburst is the common name for meteor storms and lesser non-annual showers. Only when the Earth travels through relatively fresh cometary matter will rates go up significantly above the normal level of annual activity. Then, and only then, can we see a meteor outburst. The Leonid storms are such meteor outbursts. There were no Leonid outbursts reported between 1970 and 1993. The first Leonid outburst of a new series associated with the upcoming return of P/Tempel-Tuttle was seen in 1994.

Why do they come from the same spot in the sky?
The Radiant is the point in the sky from where all meteors seem to radiate. The radiant of the Leonids is in the constellation Leo.
The radiant is a perspective effect. All particles move in about the same orbit (roughly that of the comet).
An observer in the middle of the stream sees the meteors fall left and right, above and behind him.
  However, they all seem to come from a certain direction. That direction is the radiant.

Leonids in History
1833 was probably the first recorded meteor storm in history
Leonids in History
Comet Tempel Tuttle was discovered in its 1865 pass, and its association with the Leonid meteors was made during that passage the first comet to be associated with a meteor shower/storm
Leonids in History
The 1966 pass of T-T brought another great meteor storm in the US

Predictions are based on trying to understand the 3-dimensional geometry of the dust trail
The earth passes through segments of the trail left in various past years
These parts of the trail have different densities
Predictions are based on our knowledge of the orbit of the comet, and the behavior of the meteor shower in past years, essentially using the Earth as a test probe for the richness of the trail
The Most Recent Pass
Tempel-Tuttle passed the inner solar system in 1998

The Most Recent Pass
In the past few years, there have been some sharp peaks seen from various parts of the world
The eastern US has not been favorably placed for these years
This year, it appears we are!

 Predictions for 2001
It appears that we will be passing through two heavy clumps of cometary dust
Predictions for 2001
Different astronomers have different predictions for the strength of the storm, but not whether we will have one!
Predictions for 2001
Two areas of earth are favored
Predictions for 2001
For us, that peak will be in the early morning of Sunday, November 18 (about 5a.m.)
The moon will not be a factor
The weather may be!

How to Observe
Find a dark observing site
The fainter the stars you can see, the more faint meteors you will see
Use your naked eye
Meteors move too quickly for telescopes or binoculars to be useful
Take a cot or chaise lounge

Where to Look
The meteors will appear to radiate from the southeast, in Leo
But they will be visible over a wide area of the sky
Point your chair to look high in the sky anywhere from northeast to west
Try to count the number of meteors you see
Try to trace them back and see if you can figure out the radiant point

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Last updated November 11, 2001 by Dave Gill